Surface Severe Plastic Deformation for Improved Mechanical/Corrosion Properties and Further Applications in the Bio-Medical and Hydrogen Sectors

Abstract

Various techniques enabling surface severe plastic deformation (SSPD) have been developed or optimised over the past years. This manuscript presents a broad overview of recent developments in the field of SSPD and its application that take advantages of having a deformed gradient surface with both (i) higher strength and (ii) an improved “reactivity”. First, the principle and technological advantages/disadvantages of several SSPD technologies, involving either guided or non-directional mechanical impacts, are recalled. Then, after a short recall on the nature of the structure the modified surface formed under SSPD, the effects of the processing parameters and temperature of deformation on the surface roughness and subsurface microstructure modifications are illustrated with a particular emphasis on the surface mechanical attrition treatment (SMAT) and ultrasonic shot peening (USP) techniques deriving from the traditional pre-strain shot peening. The effect of these surface and sub-surface modifications on the mechanical properties, and in particular the fatigue response, are recalled with a special emphasis on the surface integrity and potential over shot peening. Then, the manuscript concentrates on the effect of the surface enhanced diffusion of chemical species induced by the presence of structural defects to modify the corrosion behaviour and enhance the potential assisted SSPD + thermo-chemically “duplex” treatments. In these cases, in addition to induce grain refinement and dislocations, the importance of controlling some potential surface contamination is stressed. Finally, the manuscript terminates by illustrating some new research studies on potential applications for the challenges in the hydrogen sector for its solid-state storage and the protection of mechanical infrastructures as well as for bio-medical applications with biocompatible Ti-based alloys and biodegradable Mg-based ones.

1. Introduction

NanoCrystalline (NC) materials (generally associated by metallurgists to materials having an average grain size lower than 100 nm) and Ultra Fine Grain (UFG) materials (having a sub-micrometric grain size) have received increasing attention.¹⁾ In terms of mechanical properties, they have an inherent high strength compared to standard coarse grained (CG) materials together with low temperature superplasticity or improved creep resistance.^2–4) Their functional properties are also appreciated for potential applications in a wide variety of technological areas such as corrosion resistance, catalysis, biotechnology, photocatalytic CO2 conversion, superconductivity, thermoelectric performance, hydrogene storage, etc….^1–4) In this context, processes have been developed to produce - using a top-down approach in which the microstructure of a bulk material is refined by Severe Plastic Deformation (SPD) - NC or UFG structures within components.^2–4) However, because of the load required to deform the parts containing a bulk refined microstructure and the associated reduction in ductility, some of these techniques can be rather difficult to implement in industry.

As the failure of engineering materials is often initiated from the surface as a result of local overload, corrosion, fatigue, and wear, deformation treatments focusing on the modification of the outer part of a work piece have also been developed. Even if the process of pre-strain Shot Peening (SP) was considered as rather unpredictable during the early 19th century, continuous research has been conducted from 1920s onwards on SP that is now introduced into the product design specifications of numerous industrial components.⁵⁾ The use of intense plastic deformation through Surface Severe Plastic (SSPD) is more recent to treat some specific zones or critical parts of engineering materials and, thus, enhance their performances. SSPD can be imparted either (i) directly, by mechanical shocks,^6–12) continuous contact^13,14) or (ii) indirectly, by using pulsed laser^15,16) as well as pulsed ion or electron beam treatments.^17–19)

The present contribution will essentially focus on SSPD imparted by mechanical shots using processes deriving from the traditional pre-strain SP but involving much longer treatment durations and often higher velocity of the shots. After a short description of some of these techniques (Section 2) a description of the gradient microstructure thereby generated and the effect of the processing parameters (impact energy as well as treatment temperature) will be treated in Section 3. Then, after a short recall of the effect of these hardened gradient surfaces on mechanical properties (Section 4), the interest of having an improved surface “reactivity” produced by SSPD will be illustrated in Section 5 for different purposes such as corrosion resistance, “duplex” surface treatments as well as for potential applications and challenges in the hydrogen sector and population ageing for bio-medical applications.

2. Shot Peening and Derived Techniques of Surface Severe Plastic Deformation Techniques

The following paragraph intends to described different methods used to introduce properties gradients at the surface of material with a focus on shot peening and its derivatives. A more exhaustive lists of mechanical surface treatments can be found in the article of Schulze et al.²⁰⁾

Figure 1 shows a schematic representation of the most common mechanical surface treatments. On top are presented processes based on directional and guided impacts. Two configurations are shown where i) the deformation tool is in continuous contact and rolls on the surface to be treated (Fig. 1(a)) or ii) numerous located impacts are generated by the tool (Fig. 1(b)). The first one consists in applying a force to a tool in continuous contact with the surface to be deformed and having a spherical tip or a pressure roller. One of the most common denomination for these processes is deep rolling²¹⁾ but can also be found as Surface Mechanical Rolling Treatment (SMRT).²²⁾ This type of treatment presents the advantages to induced deep gradient modifications without a substantial increase in surface roughness. Concerning the second one, instead of applying a constant load on the surface to be deformed, localized controlled impacts are used to deform the surface. This can be done with a conventional hammering device²⁰⁾ but also, on a smaller scale, by using smaller ultrasonic impacting tips in the Ultrasonic Surface Nanocrystallization Modification (USNM) process.²³⁾ One limit of such treatments, weather they are in continuous contact or intended to repeatedly impacting the surface, is that they are more particularly adapted to revolution parts and plates. A setup where the rolling is assisted by ultrasonic vibration can also be found.²⁴⁾

Fig. 1

Schematic representations of different mechanical surface treatments: (a) deep-rolling or SMRT, (b) hammering or USNM, (c) RASP, (d) ABSP, (e) SMAT, (f) USP and (g) VP.

In order to process part with more complex shapes, other means of surface deformation have been developed based on directional non-guided impacts. These include treatments based on the projection of hard particles, the most popular being the shot peening processes.^25,26) Examples are schematized in Figs. 1(c) and (d). They consist of projecting medias having a hardness higher than the part to be treated and are usually in the shape of spherical particles or cut-wire (cylindrical particles) with a single size distribution. It has to be noted that mechanical milling apparatus are sometimes used to produced impacts and referred, for example, as Surface Nanocrystallization and Hardening (SNH).²⁷⁾ They will not be described in this article. Usually, the shots are set in motion either by a centrifugal blast wheel as in the Rotationally Accelerated Shot Peening (RASP, Fig. 1(c)) or blasted using compressed air as in the Air Blast Shot Peening (ABSP, Fig. 1(d)). The deformation is imparted by the kinetics energy of the projected medias. The selection of the medias (shape, size and nature) and the control of their velocities (by controlling the rotating speed or air pressure) are the main factor influencing the gradient properties induced to the treated part.

The most interesting property of the shot peening treatments is the generation of a compressive residual stress gradient at the surface required to delay crack initiation and propagation during cyclic loadings. The Conventional Shot Peening (CSP) can also be used to shape thin parts by deformation accommodation, it is referred as peen forming.²⁸⁾ The creation of compressive residual stress on the treated surface and tensile residual stresses on the opposite surface will force the sheet part to bend. It can also be used to correct part dimension/tolerance of parts during processing. However, one drawback of the SP treatments is the substantial modification in surface roughness induced after numerous impacts. As a rough surface can significantly contribute to surface crack initiation via stress concentrations sites, efforts have been made to associated the beneficial effect of the compressive residual stress state generated by impacts with a reduction of the roughness. One approach consists of a multi-step SP, also found as re-peening.²⁹⁾ The principle is to firstly peen the surface with rather high peening intensity and then re-peen it with smaller medias and/or a lower intensity in order to smooth the roughness generated at the first step.

The last category of processes presented in Fig. 1 is based on non-directional non-guided impacts. In this configuration the medias are set in motion in a confined chamber. The shots are then set in motion either by a translating plate driven by an eccentric (Fig. 1(e)) or by a high frequency resonating sonotrode (Fig. 1(f)). These techniques are referred as Surface Mechanical Attrition Treatment (SMAT) or Ultrasonic Shot Peening (USP).^11,30) The major difference with the CSP treatments resides in the fact that the shots have a wide variety of incidence angles when colliding onto the surface. In this way, the multi-directional impacts bring the advantage of generating more efficiently a thicker UFG layer at the surface (see Section 3). The amplitude and frequency are the parameters influencing the initial shot speed (when in contact with the moving/vibrating part). However, the number of shot and the volume of the chamber are also a determining parameters since they influence the final impact speed (collision to the surface) due to several interactions dissipating gradually the kinetics energy through shot/shot or shot/chamber interactions.³¹⁾ Finally, the treatment duration will be directly linked with the total accumulated energy/deformation imparted to the surface. In the case of the Vibratory Peening (VP, Fig. 1(g)), the whole vibrating chamber is filled with medias in which the part is completely immersed. This process combines the effects of the CSP by introducing compressive residual stresses to the surface and the ones of the vibratory finishing by reducing the surface roughness. It has been documented that the impact velocity component normal to the surface is responsible of the deformation and the creation of compressive residual stresses whereas the tangential component generates friction and smoothen the treated surface.³²⁾ This treatment can produce compressive residual stress gradient comparable to those generated with a CSP treatment.³³⁾

The SSPD technics based on the projection of shots have a clear advantage when treating parts with complex shapes like gears, wheels or turbine blade. Due to the plastic deformation capacity of metals and the resulted deformation gradient (strain hardening, compressive residual stress, …), these techniques are usually applied on metallic materials but can also be used on ceramics^5,34) or composites.³⁵⁾ They can be easily implemented at an industrial scale, can be automatized^36,37) and produce a small amount of waste in comparison to thermochemical surface processes. The affected depth depends on the contribution of all the treatment parameters (shot and material initial hardness, temperature, duration …) - as will be detailed in section 3 - and the gradient can be produced, both in terms of microstructure and residual stresses, along several tens to some hundreds of microns depending on the material and the exact processing parameters.

As will be illustrated is the following sections, the SSPD techniques can have some drawbacks and some limitations have to be carefully considered. As the peening treatments are based on impacts with the surface, the surface integrity can be deteriorated when intense conditions are used. This phenomenon is reported as an Over Shot Peening (OSP)¹²⁾ and will be highlighted along with potential solution in Section 4. Additionally, inclusions from the processing media or local contaminations by abrasion of the treatment chamber are also important factors affecting the local surface reactivity. The presence of surface contamination is now clearly established and depends on the exact processing rig and conditions.³⁸⁾ Fe and Cr can be introduced from the stainless steel balls and chamber walls,³⁹⁾ Al and Zr from the ceramic shots,⁴⁰⁾ while Ti (+ Al and V) come from the sonotrode when an ultrasonic device (generally steel or Ti alloy) is used.^40,41) In particular the nature of these contaminations can affect significantly the corrosion behaviour, as will be recalled in Section 5.

3. The Gradient Microstructure, How to Modify it Efficiently

3.1 Nature of the gradient microstructure

For any application, the metallurgical and microstructural states of the deformed surfaces have to be tailored - by modifying the processing parameters - in a reproducible manner to form a controlled gradient microstructure. Taking into account the structure of the grains and their misorientation, the gradient microstructure generated by any SSPD is generally schematically depicted as the succession of three (or four if we include the NC zone) different zones: (i) the UFG zone - which also contains the NC zone present at the extreme top surface - containing randomly oriented grains separated by the high angle grain boundaries, (ii) the “transition zone” where grains were fragmented under the heavy plastic deformation and, finally, (iii) the “deformed zone” where the initial grains are simply deformed.^42–44) A schematic representation of these different zones (layers) is given in Fig. 2(a) while, applied to SMAT, Fig. 2(b) gives an estimate of the different zone thicknesses in a stainless steel deformed by different magnitude of the processing parameters (see comments in Section 3.2). It must be recalled however that the transition between these different schematic layers is quite continuous and that there is no distinct and clear transition between them.

Fig. 2

(a) Representation of a gradient produced by SSPD (from Ref. 11)) and (b) Thickness evolution of the different layers depending of the SMAT conditions for the 316L stainless steel (from Ref. 42)).

The grain refinement process depends on the deformation mechanisms (twining, dislocation activities, …) involved under the SSPD loading coupled with potential dynamic recrystallisation/recovery processes. In the case of steels for example, the relevant material property commonly used to determine the plastic deformation accommodation mechanisms is the stacking fault energy (SFE). When the SFE is high, dislocation activity and rearrangement are formed during deformation leading to dense dislocation walls and tangles to refine the material microstructure, as observed in pure Fe (∼200 mJ·m⁻²).⁴⁵⁾ Medium SFE promotes twinning to accommodate the deformation – as observed in Fe–Mn–C alloys⁴⁶⁾ (12 to 35 mJ·m⁻²) - and the structural refinement is driven by twin/twin interactions or twin fragmentation. When the SFE becomes low, a deformation induced martensitic transformation can be triggered like in metastable austenitic stainless steels^47–49) (<20 mJ·m⁻²). While for face-centred cubic (FCC) and body-centred cubic (BCC) metals, the grain subdivision to accumulate strains takes place by the formation of dislocation cells and dense dislocation walls at high SFE, deformation induced twining is always prevalent for hexagonal close packed (HCP) metals because of the limited number of slip systems, at least in the early stages of deformation.^50,51)

In addition to the modification of the grain size through the depth of the samples, the gradient microstructure can be in some cases the superimposition of different gradients: one in terms of grain size, the other one in terms of phase distribution for example.⁴⁹⁾ A detailed analysis of the effect of various processing parameters - including the use of cryogenic temperature for processing - on the 304L steel as revealed that, in such TRansformation Induced Plasticity (TRIP) steels, the maximum amount of martensite was never found in the vicinity of the extreme surface, where the finest grains were present, but at 50 to 100 µm below the surface depending on the processing conditions. This aspect is important to interpret correctly the hardening in SSPD materials in which a phase transformation can take place. The modification of the exact nature and the depth of the gradient structure can be done by changing either “conventional” processing parameters which will modify the energy imparted by the impacting media (pin, ball, harmer …) and/or the processing temperature.

3.2 Effect of the impact energy through “conventional” processing parameters

The energy imparted to the surface depends on the impact density and the shot velocities. They are the result of a combination of various processing parameters such as the shot characteristics (quantity, diameter and nature), the vibrating amplitude that set the shots in motion and the duration of the treatment. Several research works have investigated the effects of the processing parameters. Chen et al.⁵²⁾ used two sets of SMAT processing parameters (vibrating frequencies, nature and diameter of the balls) to treat the 304 stainless steel under low (0.5 m·s⁻¹) and high (10 m·s⁻¹) speeds of the shots. For similar surface hardness, different sub-surface hardness gradients were obtained resulting in different mechanical behaviour of thin (1 mm) plates treated on both sides. Several pioneer results,^52–54) have shown that increasing the energy imparted to the surface allows to increase the material hardness deeper in the material.

However, the control of this evolution, and the separate effect of the processing parameters, remains generally quite qualitative. In order to characterize quantitatively the thickness of the different layer of the graded structure, Samih et al.⁴²⁾ have established an automated procedure - based on an analysis of Geometrically Necessary Dislocations (GNDs) coupled with indexing and size criteria - obtained from various Electron BackScattered Diffraction (EBSD) maps carried out at different magnifications. As illustrated in Fig. 2(b), under their tested processing conditions - varying the amplitude of vibration (in µm) and treatment duration (in min) - these authors have established that the UFG and transition zones were more significantly modified than the overall affected thickness for a 316L stainless steel. Also, trying to increase the treated depth by using an increase of the intensity of the peening parameters has a limit because the top surface will lose its integrity and result in crack initiation under too severe loading. As it will be illustrated hereafter, an effective way to modify the affected depth and the evolution of the sub-surface hardness (and residual stress gradient) is to modify the processing temperature.

3.3 Effect of the shot peening temperature

One of the important processing parameters that has received so far little attention is the temperature at which the shot peening is carried out. Processing by SPD of bulk material at Cryogenic Temperature (CT) prevent dynamic recovery and stimulate mechanical twinning in order to, in both cases, enhance the grain refinement. To reduce further the size of the surface microstructure, cryogenic surface treatments have also been developed such as cryogenic burnishing,^55,56) cryogenic laser shot peening,⁵⁷⁾ cryogenic impact,^58,59) as well as the SMAT/USP processes applied at low temperature.^49,60–65) It is also interesting to change the peening temperature in order to modify the deformation modes in alloys for which twinning or martensitic transformation can compete with conventional slip systems to accommodate plastic deformation. In a comparative analysis of the SMAT behaviour of two austenitic stainless steels having very different stabilities of their austenitic phases, Novelli et al.⁶⁰⁾ have shown that, in addition to surface hardening, the most interesting result was a significant increase in the subsurface hardness for the steel in which the strain induced martensitic transformation deeper in the subsurface. This is illustrated in Fig. 3. At CT, for the 304L steel (Md₃₀ = 21°C), the martensitic transformation is promoted for lower stress/strain and, thus, deeper towards the depth of the sample (single arrow). Comparatively, the 310S steel, which is extremely stable against the martensitic transformation (Md₃₀ = −169°C), is harder to deform plastically and its subsurface hardness reduces (double arrow). Thus, peening at CT can be justified for this kind of TRIP steels if additional subsurface hardenings are targeted.

Fig. 3

Effect of the ultrasonic SMAT temperature on the cross-section hardness gradient (RT = 20°C, CT = −130°C). Dotted lines represent the hardness measured after SMAT under CT.⁶⁰⁾

As just stated, because the strength of the material increases at CT, in the absence of martensitic transformation, it becomes more difficult to impart plastic deformation at the depth of the treated sample. In this case, to improve the sub-surface hardness, processing must be done at higher temperature. Thereby, as the yield limit of a metal decreases when heated, higher subsurface hardening and larger maximum compressive residual stresses can be achieved. This is illustrated for hardness in Fig. 4 taken from the work of Wang et al.⁶⁶⁾ The higher compressive residual stress gradients also have an improved stability, as observed by Wick et al.⁶⁷⁾ Therefore, as proved by results on steels^66,67) and Mg alloys,⁶⁸⁾ increasing slightly the peening temperature can be a good idea to improve the fatigue life or fatigue strength. The increase in peening temperature generally generates higher subsurface hardening and larger maximum compressive residual stresses. Therefore, for an optimum Almen intensity, the surface of the warm shot peened specimen is more plastically deformed but less “damaged” due to the increase in plastic deformation ability.⁶⁸⁾ Also, the dislocation structure is likely to be modified towards a higher stability.⁶⁷⁾

Fig. 4

Cross-section hardness of a 304 stainless steel treated by electropulsing-assisted ultrasonic surface rolling process for different temperatures ranging from 25 to 180°C (higher current corresponds to higher temperature).⁶⁶⁾

It is also important to mention that, additionally, in-situ diffusion phenomenon can take place during warm treatments. Ye et al.⁶⁹⁾ have observed the formation of more nanoprecipitates in an AA6061 aluminum alloy and Wick et al.⁷⁰⁾ the precipitation of fine carbides in the microstructure of an AISI 4140 deformed by warm peening. These fine solutes contribute to pin and impede the movement of dislocations, thus raising the mechanical resistance. Dynamic recrystallization is also a mechanism to consider during warm surface treatment. Due to the energy stored by plastic deformation, microstructure restoration and/or recrystallization can take place, as observed by Pan et al.⁷¹⁾ and Kikuchi et al.⁷²⁾

These kinds of treatment are usually referred as heat-assisted surface mechanical treatment.⁷³⁾ Different ways can be found in the literature to heat the sample like a pre-heat in oven,⁷⁴⁾ heating plate with electric resistances,⁷⁵⁾ the Joule’s effect⁷⁶⁾ or infrared radiation.⁷⁷⁾ Only two articles have reported on the effect of heat-assisted SMAT. Wei et al.⁷⁸⁾ used a pre-strain warm SMAT/USP to deform the surface of a NiTi alloy. They managed to increase the bending fatigue by 13 times using a combination of a finer surface microstructure, a deeper hardness gradient and higher compressive residual stresses. Comparatively, Kong et al.⁷⁹⁾ applied a temperature-assisted USP on a two phases Mg–Li alloy. They reached a deeper hardening under warm condition but this was associated with a decrease in hardness. The smallest surface grain size was achieved with a peening of 100 s at 100°C.

As pointed out in several contributions under warm conditions^66–68) or CT,⁴⁹⁾ the peening temperature has a strong effect on the surface roughness evolutions. Because the material becomes softer, the surface roughness increases when the peening temperature is raised. For ultrasonic shot peening, the amplitude of the sonotrode, that is inducing the kinetic energy to the flying shots, is also a factor affecting strongly the surface roughness. Thus, for a given type of shots (size and weight), the highest roughness is generally obtained for the highest amplitudes of the sonotrode and/or higher peening temperature. On the other hand, it is possible to lower the roughness by using a “moderate” sonotrode amplitude, increasing the treatment time and, more significantly, by lowering the peening temperature. Adapted from Ref. 49), Fig. 5 illustrates, for two types of steels, the direct correlation between the surface roughness and top surface hardness for different sets of processing parameters. Thus, the use of temperature is an additional parameter during ultrasonic SMAT that allows to tailor the surface properties towards tribological applications by controlling the surface hardness and roughness rather independently.

Fig. 5

Effect of the SMAT temperature on the surface hardness as a function of the surface roughness R_q of the 304L (flat samples) and 316L (round samples) stainless steels. Numbers closed to markers correspond to the treatment temperatures given in Celsius degrees (from −130°C to 500°C). Adapted from Ref. 49).

4. Gradient Structures and Improved Mechanical Properties

The overall material properties can be significantly enhanced through the formation of gradient-structures with refined surface microstructures.⁸⁰⁾ Considering steels for example, besides a conventional use to harden the surface and improve wear,^81,82) tensile properties of the 316L⁸³⁾ and the 304^53,84) stainless steels were also improved while multi-layered laminate composites were created by SMAT and subsequent roll bounding.⁸⁵⁾ To this respect, in addition to the contributions of the finer grains in the surface layer and the high dislocation density produced by SMAT, the presence of residual stresses has also been demonstrated to contribute substantially to the improved tensile properties.⁸⁶⁾ While the compressive residual stress (CRS) into the surface layer directly contribute to the enhancement in yield strength, the tensile/compressive layers contribute also by developing strong hetero-deformation induced (HDI) hardening. The hetero-zones deform inhomogeneously, generating back stresses in the soft zone and forward stresses which, together, produce this HDI strengthening that increases yield strength and enhance strain hardening, thus and helping to retaining ductility.^87,88) The HDI strengthening, which comes from the mutual constraint of the hard and soft zones, is active in many types of heterostructured materials containing for example laminate, gradient or core-shell structures and possessing superior combinations of strength and ductility.⁸⁸⁾

The major mechanical response targeted through SSPD techniques derived from SP remains the optimisation of the fatigue properties. Several review papers dealing with the fatigue behaviour of metals subjected to different types of SSPD treatments have been published recently.^89,90) Factors affecting the fatigue life of a component can be roughly divided into two main categories. First, the surface roughness or the presence of stress raisers, influence essentially the initiation stage of crack growth.^91,92) Second, the presence of residual stresses and structural defects which affects the crack propagation mechanics.^88,93–95) The association of a superficial refined microstructure and a high compressive residual stress gradients produced by SSPD, and in particular SMAT/USP, has been shown to delay the crack initiation and impede its propagation on a wide range of materials such as Fe,⁹⁶⁾ Ti⁹⁷⁾ or Mg⁹⁸⁾-based alloys. If the use of mechanical surface treatment allows to push the initiation site under the peened surface and that deeper compressive residual stresses are known to improved fatigue properties, an excess of peening energy can also affect the surface integrity which can impede the fatigue resistance of the components.^97,99,100) Concerning crack initiation, for precipitation hardenable Al-based alloys, the effect of SMAT before or after precipitation aging has been investigated by Maurel et al.⁹⁹⁾ It was shown that a high notch sensitivity (7075) should not be processed by SMAT as the generated low surface integrity is always detrimental to fatigue performances. On the contrary, for a less notch sensitive alloy (2024), SMAT prior to aging formed smaller and denser precipitates - resulting in a high hardened depth - and microstructures more resistant to residual stress relaxation than after conventional shot-peening. SMAT after aging resulted in a significant improvement of fatigue performance with only subsurface crack nucleation sites for the same 2024 alloy.⁹⁹⁾ Concerning the relaxation of the compressive residual stresses, Dureau et al.¹⁰⁰⁾ have compared the fatigue performance of a SMATed austenitic stainless under two fatigue load ratios: fully reversed Tension-Compression (TC) R_TC = −1 and Tension-Tension (TT) R_TT = +0.1. After identical SMAT processing conditions, the fatigue limit was enhanced by +17% for R_TC while it was reduced by −7% for R_TT. The residual stress measurements have revealed that, while the stress gradient was simply smoothed after the R_TC fatigue loading, it was completely reverted for the R_TT one. The fatigue crack initiation sites and propagation were significantly modified between the R_TC and R_TT loading conditions.¹⁰⁰⁾ Finally, considering the phenomenon of Over Shot Peening (OSP), a novel shot peening treatment called gradient Severe Shot Peening (SSP) has been introduced recently by Maleki et al.¹²⁾ to mitigate the problem of losing surface integrity and weakening the fatigue properties. Instead of using constant projection pressure of the shots as during conventional SP, this treatment implements gradually increasing or decreasing pressures. Significant fatigue life improvements were obtained for SSP treated specimens because SSP avoided the detrimental effects of over-peening while maintaining the beneficial effects of surface nano-crystallization, surface hardening and compressive residual stresses.¹²⁾

5. Gradient Microstructure and Improved Surface Reactivity

The high fraction of structural defects (i.e. High Angle Grain Boundaries (HAGB), Low Angle Grain Boundaries (LAGB), dislocations …) induced by SSPD can lead to a significant increase in stored energy which, inevitably, increases the material reactivity and the diffusion of chemical species. In addition to modify the corrosion or oxidation properties, these refined surface structures have been proved to activate the kinetics of chemical reactions and several “duplex” mechanically assisted thermo-chemical treatments have been developed or improved. In addition, some potential applications of SSPD are raising in the hydrogen sector and for bio-medical applications.

5.1 Corrosion resistance

Several initial studies on the effect of SPD on the corrosion response have revealed either beneficial of deleterious effects (see for example Ref. 101)) indicating also that the literature was insufficient to find general trends.¹⁰²⁾ There is at present a significant amount of research concerning the corrosion behaviour of metals treated by shot peening or its derivative SMAT and USP types of technology indicating that other factors than the grain size and/or the amount of structural defects must be taken into account.¹⁰¹⁾ Obviously, the higher reactivity may not be suitable or lead to a dual behaviour depending on the material environment. Indeed, improvements or deleterious effects can be obtained for the same material depending on the location on the EH-pH Pourbaix diagram. SPD improves the corrosion resistance in a passive environment whereas it increases the dissolution rate in a non-passive environment.¹⁰³⁾ In the case of stainless steels for example, a decrease in corrosion resistance is reported in depassivating electrolytes while the higher reactivity of the SMATed surface helps to create faster a passivating (i.e. protective) film in many aqueous environments.^104,105) The corrosion resistance can also depend on the processing route used to generate the structure.¹⁰⁶⁾ Indeed, these processing routes generally affect other parameters than the grain size such as the chemical homogeneity, size and distribution of inclusions, dislocation densities or solid solubility; parameters which are equally important for corrosion properties. Strain induced precipitate coarsening was observed in a Ti–Nb–Sn orthopedic alloy after SMAT, which increased marginally its corrosion rate.¹⁰⁷⁾ Comparatively, on a 301 stainless steel, the effect of SMAT was to facilitate the diffusion of Cr on the surface to form a stronger and better passivation layer.¹⁰⁸⁾

As mentioned in Section 2, contamination can be introduced by the peening medias that are also able to transfer chemical elements from both chamber and sonotrode to the sample surface, modifying thereby locally the chemical composition of the treated surface, affecting the corrosion behaviour. Wen et al.¹⁰⁹⁾ have demonstrated that the corrosion resistance of a 2024 Al alloy was directly affected by the nature of the shot peening media. Indeed, while the NC layer fabricated by SMAT with ceramic balls improved the corrosion resistance because of the formation of a dense passive film, the Fe containing layer induced by SMAT with the steel shots led to galvanic corrosion reaction between Fe and Al.¹⁰⁹⁾ In accordance with this result, a recent analysis of the corrosion in two aluminium alloys (AA 2024 and AA 7150) has shown the dual effect of such treatment: the dissolution of the nano-sized strengthening precipitates in the Al matrix and grain refinement generated under the USP treatment have improved the intergranular corrosion resistance while the overall corrosion rate was increased by the Fe contamination.¹¹⁰⁾ To counterbalance, or at least mitigate, the potential effect of the surface contamination, Murdoch et al.¹¹¹⁾ have introduced a pre-processing step (pre-coating of the shot media and chamber) to reduce the resulting impact on corrosion properties.

5.2 Use of increased surface reactivity for “duplex” treatments

SSPD processes such as SMAT and USP, thanks to the high quantity of structural defects generated at the surface and associated enhancement in diffusion, several industrial thermochemical treatments can be optimized. These mechanically assisted thermochemical “duplex” treatments are for example nitriding,¹¹²⁾ pack boronizing,¹¹³⁾ aluminizing,¹¹⁴⁾ chromizing¹¹⁵⁾ as well as plasma electrolytic oxidation.¹¹⁶⁾

Among these “duplex” treatments, nitriding has been the most widely investigated one. Indeed, for industrial applications, nitriding can benefit in different ways from the presence of these NC or UFG surface and gradient structure produced at the deformation stage. First, the enhanced atomic diffusion and enhanced chemical reaction kinetics can be used either (i) to produce thicker layers or (ii) to lower the nitriding temperatures.^112,117,118) This latter option is very interesting for austenitic stainless steels. Indeed, carrying the nitriding treatment at temperature as low as 300°C is of primary importance for the quality of the corrosion properties in austenitic stainless steels because it keeps a single-phase structure of expended austenite and avoids the precipitation of Cr-rich nitrides. Second, the duplex SMAT + nitriding treatment also has the advantage to produce a much harder surface nitrided layer that forms on a sub-surface hardened substrate.¹¹⁾ The top surface is hardened by a combination of both plastic hardening due to the severe deformation and solute hardening due to the presence of the N-enriched austenite while the subsurface hardening induced by the SMAT is maintained after nitriding.¹¹⁾

To avoid the contamination produced by SMAT which is acting as a barrier to the nitrogen flux and leads to the formation of reduced or discontinuous nitriding layers,¹¹⁹⁾ chemical etching¹²⁰⁾ or mechanical polishing^40,121) must sometimes be used as an intermediary stage during such duplex treatment in order to reduce the surface contamination and, thereby, improve further the quality and thickness of the nitrided layers.

5.3 Use of SSPD for the hydrogen sector

For hydrogen to make a significant contribution to clean energy transitions, it needs to be adopted in sectors where it is almost completely absent, such as transport, buildings and power generation. However, clean, widespread use of hydrogen in global energy transitions faces several challenges. In addition to produce “green” hydrogen, two key issues are also its storage under safe conditions and its transportation.

5.3.1 Hydrogen solid-state storage

A considerable amount of research has been devoted to develop advanced H-storage media in solid-state materials by forming metal hydrides.¹²²⁾ Magnesium and its alloys have attracted significant interest in the field of hydrogen storage owing to their high hydrogen-storage capacity, low-cost and abundance in Earth’s crust. However, the high thermodynamic stability of Mg–H bond as well as sluggish sorption kinetics limit their practical applications.¹²³⁾ Microstructure refinement using various processes such as High Energy Ball Milling (HEBM) of powders or severe plastic deformation have been used to lower the hydrogenation/dehydrogenation temperature and increase the reaction rates in Mg-based materials.¹²⁴⁾ Despite significant output, the HEBM process is prohibitive because it is time- and energy-consuming as well as because handling pyrophoric powders raises severely safety concerns. Other metallic systems based on Ti–V–Cr or FeTi have a lower storage capacity but authorises the reversible transformation near room temperature. However, while materials based on these systems can thermodynamically store hydrogen at RT, their hydrogenation requires a first “activation” process at high temperature and/or under high pressure to react with hydrogen in the first hydrogenation cycle.^125,126) Edalati et al. reported that SPD applied of TiFe by High Pressure Torsion (HPT) can activate the material to reversibly absorb and desorb hydrogen at RT without any conventional activation heat-treatment; and this even after 400-day storage in air.¹²⁷⁾ It was suggested that the main mechanism for the HPT-induced activation was the formation of large fraction of grain boundaries acting as pathways for hydrogen. Comparatively, the room-temperature hydrogenation capability of alloys based Ti–V–Cr was possible but their reversibility was degraded by the presence of structural defects.^128,129) Avoiding the introduction of structural defects and fine gains within the overall sample as is the case for HPT, a gradient microstructure produced by SMAT was proved to be an effective way to activate the material so that it could subsequently reversibly absorb and desorb hydrogen at RT.^129,130) As illustrated in Fig. 6, the gradient structure created by SMAT has its own advantages. First, the nanostructure and cracks present at the surface of the SMATed sample could act as a pathway for hydrogen transport through the oxide layer and activate the material for H-storage. Second, the H-atoms could be stored in the defect-free subsurface were more reversibility is expected. Thus, processes inducing surface gradients can be regarded as having a high potential for elaborating industrially H-storage materials.¹³⁰⁾

Fig. 6

Representation of a gradient produced by mechanical surface treatment and the corresponding mechanics on chemical diffusion and hydrogen absorption.¹²⁹⁾

5.3.2 Hydrogen protection

When hydrogen diffuses into a metal, it can decrease its mechanical resistance by a localization of the hydrogen concentration at microstructural defects. This phenomenon is known as Hydrogen Embrittlement (HE) and several HE theories have been documented to explain this behavior like Hydrogen-Induced Phase Transformation (HIPT), Hydrogen-Enhanced DEcohesion (HEDE), Hydrogen-Enhanced Localized Plasticity mechanism (HELP) or Hydrogen-Enhanced Strain-Induced Vacancies (HESIV).¹³¹⁾ As the hydrogen is absorbed and diffuse within a material from the surface, surface functionalization has been tested in order to prevent or reduce phenomenon liked HE. It can take the form of surface chemistry modifications like coatings but also mechanical surface treatments.¹³²⁾

Most of the articles about HE reduction using mechanical surface treatment deal with the use of conventional SP applied essentially on steels. An et al.¹³³⁾ reported good tensile and fatigue properties after hydrogen charging on a shot peened X80 steel. They suggested that the combined effect of grain refinement, the high amount of dislocation density and compressive residual stresses decrease the hydrogen diffusion. More precisely, they proposed that the high density of defects associated to the accumulation of hydrogen in the shot peened layer (Figs. 7(a) and (b)) is responsible for creating a mismatch between the treated surface and the non-deformed core leading to the formation of sub-surface cracks (Fig. 7(c)) which will then propagate towards the surface (Fig. 7(d)). Kawamori et al.¹³⁴⁾ also reported that the HE resistance was improved whether the hydrogen charging was carried out by cathodic charging or by cyclic corrosion tests. They also reported the beneficial effect of compressive residual stress to reduce hydrogen absorption and thus HE. Wang et al.¹³⁵⁾ have studied the effect of SP coverage and intensity on the HE of a hypo-eutectoid steel. The hydrogen diffusivity was decreased for all peening coverage in three steps. In the CSP regime (<400%), the hydrogen diffusivity rapidly decreases by an increase of the hydrogen trapping sites. Then followed a plateau regime in the SSP range (1000 to 7000%) due to a limitation of dislocation generation to the benefit of subgrain structure formation. Finally, the hydrogen diffusivity decreased again when the sample was OSP (>12000%) by the formation of a fully refined microstructure, further increasing the probability of the hydrogen to be trapped. The authors suggested that increasing the number of hydrogens trapping site by plastic deformation is more critical than the presence of compressive residual stress in order to reduce hydrogen diffusivity and HE in this hypo-eutectoid steel.

Fig. 7

Representation of the HE-resistance mechanism developed in a SP layer.¹³³⁾

A more limited amount of work of research has been devoted so far to the use of the less conventional SSPD techniques. Kim et al.¹³⁶⁾ used an USNM treatment at RT and elevated temperature (300°C) on a high-Mn steel. Both samples treated by USNM showed less hydrogen content than the untreated sample due to the presence of compressive residual stress gradients. The creation of ε-martensite at RT was the main reason of the mechanical property degradation after hydrogen charging, even compared to the initial sample. As the strain-induced martensitic transformation was suppressed at higher temperature of deformation, the sample treated at 300°C showed the lowest decrease in mechanical properties. Thus, tailoring the deformed microstructure by controlling the phase in presence can increase the HE resistance as well as the presence of refined grains and compressive residual stress. Safyari et al.¹³⁷⁾ processed a 7075-T6 aluminum alloy by USP. They reported that the production of a fine-grained surface layer allows to increase the HE resistance of the material. Production of smaller grains increases the grain boundaries surface fraction which then decreases the hydrogen trapping sites per unit length of grain boundary.

More recently, to achieve high yield strength and high ductility in the presence of hydrogen, Mohammadi et al.¹³⁸⁾ used low and mild intensity controlled SMAT conditions to generate gradient structures containing different amount of surface nanotwins in a CrMnFeCoNi high-entropy alloy. They compared the samples behaviour with its coarse- and bulk nano-structured counterparts produced by high-temperature homogenization and HPT, respectively. In the presence of hydrogen, gradient samples showed a good combination of high yield stress of 500–700 MPa, which was 2–3 times higher than the yield stress of coarse-grained material, coupled with a high level of plasticity (15–33%). This study introduced gradient-structured high-entropy alloys as new high-strength materials with high resistance to HE, particularly when the hydrogen was kept under a critical level to prevent the HELP mechanisms.

5.4 Use of SSPD for bio-medical applications

A considerable amount of research has been devoted to the biocompatibility of the SSPD treated materials. When a prosthesis is placed in a body, the cells react with the surface of the prosthesis according to its chemical and mechanical properties. Thus, over the past three years, several studies have characterised the effects of SSPD on cell viability. Two major families of base-metal alloys are essentially investigated. The first family of alloys for the manufacture of prostheses is titanium because it is perfect biocompatible and, for the β-Ti alloys, because their Young’s modulus (30–60 GPa) are rather close to bone ones (15–30 GPa).¹³⁹⁾ The second family is magnesium alloys, again because of the good compatibility of Mg but also because more and more medical studies have focused on resorbable alloys. The advantage of degradable implants is that they provide mechanical support to the fractured bone for a short time while the tissue repairs and remodels. Some recent studies focusing on the effect of plastic deformation of the surface on bio-compatibility are reviewed hereafter for these two families of alloys.

5.4.1 Titanium alloys

Agrawal et al. have performed short time USP treatments on pure titanium and showed that both the cell viability and the surface corrosion resistance were significantly improved.¹⁴⁰⁾ The beneficial effect of USP on cell proliferation was attributed to the nanocrystallisation of the surface coupled with the increase in positive potential at the treated surface. The improvement in corrosion resistance was due to the preferential formation of TiO₂ that tended to grow from the high density of grain boundaries. It should also be noted that performing a vacuum stress relief treatment after USP further improved the cell viability. The effect of SMAT on pure titanium has also been studied by Luo et al.¹⁴¹⁾ The in vitro study on MG63 cells showed that SMAT promoted the adhesion effect and inhibited cell apoptosis. The authors explain this by an improved mineralisation capacity, improved protein adsorption capacity and hydrophilicity due to the nanostructuring. In order to clarify which is the predominant parameter acting on biocompatibility between chemistry, roughness and nanostructuring, Weiss et al. performed SMAT on TiMo and TiNb chemically functionally graded materials (FGM). The overall range of composition for the FGM span from 100% Ti to 100% Mo (or Nb)¹⁴²⁾ and several interesting result were obtained. Firstly, from a practical experimental point of view, this study has demonstrated that the use of FGMs allows the study of the effect of severe plastic deformation on several materials in one go (saving on the number of samples) while ensuring that all the biological tests are performed under the same conditions. This is illustrated in Fig. 8 that compares the effect of 3 surface conditions (polishing, SMAT and SMAT + polishing), varying by their roughness and degree of deformation, on cells adhesion and proliferation. Figure 8 indicates that, from a biocompatibility point of view, the increase in roughness induced by SMAT improve cell adhesion (human mesenchymal stem cells) but do not alter the proliferative capacity of the cells. Clearly, the results show that the refinement of the microstructure and the presence of structural defects induced by the surface severe plastic deformation have an effect on the distribution of cells during the early stages of proliferation. However, in the long term, it is the alloy chemistry that remains the most important factor in ensuring cell proliferation; with here niobium been a better alloying element than molybdenum for this purpose.¹⁴²⁾

Fig. 8

Effect of polishing, SMAT and SMAT + polishing on cells adhesion (human mesenchymal stem cells) and subsequent proliferation along the FGM for different amount of Nb or Mo. (a) 0%; (b) 25%; (c) 50%; (d) 75%; (e) 100%.¹⁴²⁾

When applied on β-Ti alloys, SMAT can generate unfamiliar deformation mechanisms such as kink-bands.¹⁴³⁾ A limited surface hardening was observed for an orthopedic Ti–Nb–Sn alloys after SMAT but SMAT enhanced the cell proliferation rate and, more importantly, the osteogenic differentiation of stem cells.¹⁰⁷⁾ SMAT treatment followed by calcium ion implantation has been tested on the Ti–25Nb–3Mo–2Sn–3Zr β-Ti alloy by Huang et al.¹⁴⁴⁾ The SMATed samples showed improved hydrophilicity compared to the untreated samples. Subsequent Ca ion implantation further improved wettability. In vitro cell experiments indicated that the SMAT samples promoted cell adhesion, cell proliferation, osteogenic differentiation, collagen secretion and extracellular matrix mineralization of Mesenchymal Stem Cells (MSC). The addition of Ca further enhanced MSC adhesion and osteogenic functions. Thus, following the strategy implemented for more technical issues (see Section 5.2), this kind of duplex treatment opens up a very interesting avenue of research.

Laser Shot Peening (LSP) also improved biocompatibility as shown in the study by Vishnu et al. on Ti22Nb alloy.¹⁴⁵⁾ The laser-peened surfaces showed increased cell spreading and anchorage, attributed to the presence of micro-topography and associated nanoscale features to enhance cell-surface interactions. Similar features were observed by Zhang et al. on NiTi.¹⁴⁶⁾ In addition to an increase in scratch resistance, electrochemical tests showed an increase in corrosion resistance. Immersion tests in simulated body fluid indicated that the initial release of Ni ions was inhibited by LSP, especially during the first week. In addition, LSP improved calcium deposition. The in-vitro study of adipose-derived stem cells indicated that the Live/Dead ratio, adhesion and propagation were improved by LSP. This process also allowed Shen et al. to improve the wettability of the Ti–6Al–7Nb orthopaedic alloy.¹⁴⁷⁾ Yang et al. to increase the resistance of the Ti–3Cu alloy to Staphylococcus aureus bacteria¹⁴⁸⁾ as shown in Fig. 9. This figure illustrates that the antibacterial efficiency increases from 70.7% on an untreated sample to 98.2% on a sample treated by LSP.

Fig. 9

Typical Staphylococcus aureus colonies after 24 h incubation on (a) CP Ti, (b) Ti–3Cu before LSP, Ti–3Cu after LSP (c) 5J-3, and (d) 7J-3, (e) antibacterial rate of alloys against S. aureus relative to CP Ti.¹⁴⁸⁾

5.4.2 Magnesium alloys

Vasu et al. showed that a Friction Stir Process (FSP) treatment results in the formation of a nanostructured layer in the ZE41 alloy without changing the corrosion resistance properties.¹⁴⁹⁾ The prosthesis would therefore have enhanced surface mechanical properties without losing its biocompatibility. The same process was used by Badisha et al. to alloy the surface of a Mg sample with Zn powder.¹⁵⁰⁾ They manage to obtain a refined surface structure having a lower corrosion rate than the pure Mg in a simulated body fluid.

Manivasagam et al. studied the surface of a WE43 alloy deformed by SMAT.¹⁵¹⁾ They found an increase in hardness, roughness, surface oxide layer and hydrophilicity, as well as a 90% increase in corrosion resistance. Human fetal osteoblast (hFOB) cells showed higher biocompatibility on SMAT-treated substrates, with the bone matrix depositing significantly more Ca²⁺. In addition, a significant reduction in MRSA (methicillin-resistant Staphylococcus aureus) adhesion was observed on SMAT-treated substrates compared to untreated substrates. The beneficial effect on corrosion remains debatable as, at the same time, Singh et al. measured a lower corrosion resistance on AZ91D alloy after SMAT.¹⁵²⁾

6. Conclusions

A broad overview is given here on different aspects related to applications of surface treatments involving severe plastic deformation with a more specific focus on recent research derived from shot peening such as the SMAT or USP for example. Considering the structure of the grains and their misorientation, any SSPD treatment generates a gradient of microstructure with different transitions ranging from surface NC or UFG highly misoriented grains to large deformed grains towards the core of the material. Due to the plastic deformation capacity of metals and the resulted deformation gradient (strain hardening, compressive residual stress, …), these techniques are usually applied on metallic materials but can also be used on ceramics or composites.

The SSPD techniques are rather easy to implement in industry and produce a small amount of waste in comparison with conventional surface thermochemical modification techniques. The short presentation of the principle of some of these SSPD techniques (Section 2) highlights the interest of having multidirectional impacts of the shots in a confined chamber, instead of directional ones, in improving the grain refinement process. As underline in section 3, the grain refinement process depends on the deformation mechanisms (twining, dislocation activities, …) activated on loading, which are directly related to the crystallographic nature of the impacted material and its stacking fault energy, coupled with potential dynamic recrystallisation/recovery processes. The control of the peening parameters and the processing temperature are important parameters to tailor the surface roughness and the sub-surface properties of the treated parts while avoiding over peening.

The affected depth depends on the contribution of all the treatment parameters (shot and material initial hardness, temperature, duration …) and the gradient can be produced, both in terms of microstructure and residual stresses, along several tens to some hundreds of microns depending on the material and the exact processing parameters. In addition to the contributions of the finer grains in the surface layer and the high dislocation density produced by SMAT or USP, the presence of residual stresses contributes also substantially to the improved yield strength and enhance strain hardening which helps to retain a good ductility. The effect of the processing parameters on the surface integrity and the importance of the subsurface residual stress relaxation on loading are highlighted for understanding the fatigue response of mechanical parts. Over Shot Peening (OSP) must be avoided.

In general, strengthening through grain refinement by SSPD does not sacrifice the corrosion resistance (Section 5.1). On the contrary, if properly located on the EH-pH Pourbaix diagram, the presence of structural defects modifies the surface reactivity and ease the formation of a passive film. However, as the peening treatments are based on impacts with the surface, inclusions from the processing media or local contaminations by abrasion of the treatment chamber are also important factors that can create local galvanic cells and encourage corrosion by pitting.

Due to the imparted severe plastic deformation and the high quantity of generated structural defects which enhance the surface reactivity, SMAT has been used within “duplex” treatments to assit the kinetics of diffusion for several thermochemical industrial processes such as nitriding, aluminizing, plasma electrolytic oxidation …. Here again, the presence of surface contamination can be an issue (Section 5.2).

The recent results reviewed in section 5.4 show that having a rough surface with improved “reactivity” towards human cells as generated a new active field of research for SSPD. Indeed, bio-medical applications now in development both towards biocompatible Ti-based alloys and biodegradable Mg-based alloys.

Finally, some new research studies using SSPD for potential applications in the hydrogen sector are reviewed (section 5.3). Both hydrogen solid-state storage via the easier formation of metal hydride after SSPD and the protection of mechanical infrastructures via improvement of hydrogen embrittlement are considered.

Overall, SSPD techniques are generally cost-effective and chemical-free mechanical modification routes that are bringing viable solutions to different material sciences issues.

Acknowledgements

The authors would like to thank the support of the French State through the program “Investment in the future” operated by the National Research Agency (ANR) and referenced by ANR-11-LABX-0008-01 (Labex DAMAS).

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Appendix

ABSP

Air Blast Shot Peening

CG

Coarse Grain

CRS

Compressive Residual Stress

CSP

Conventional Shot Peening

CT

Cryogenic Temperature

EBSD

Electron BackScattered Diffraction

FSP

Friction Stir Process

GND

Geometrically Necessary Dislocation

HAGB

High Angle Grain Boundary

HE

Hydrogen Embrittlement

HEBM

High Energy Ball Milling

HEDE

Hydrogen-Enhanced Decohesion

HELP

Hydrogen-Enhanced Localized Plasticity

HESIV

Hydrogen-Enhanced Strain-Induced Vacancy

HDI

Hetero-Deformation Induced

hFOB

Human Fetal Osteoblast

HIPT

Hydrogen-Induced Phase Transformation

HPT

High Pressure Torsion

LAGB

Low Angle Grain Boundary

LSP

Laser Shot Peening

MRSA

Methicillin-Resistant Staphylococcus Aureus

MSC

Mesenchymal Stem Cells

NC

NanoCrystalline

OSP

Over Shot Peening

RASP

Rotationally Accelerated Shot Peening

RT

Room Temperature

SMAT

Surface Mechanical Attrition Treatment

SMRT

Surface Mechanical Rolling Treatment

SNH

Surface Nanocrystallization and Hardening

SP

Shot Peening

SPD

Severe Plastic Deformation

SSP

Severe Shot Peening

SSPD

Surface Severe Plastic Deformation

TC

Tension-Compression

TRIP

TRansformation Induced Plasticity

TT

Tension-Tension

UFG

Ultra Fine Grain

USNM

Ultrasonic Surface Nanocrystallization Modification

USP

Ultrasonic Shot Peening

VP

Vibratory Peening