2024 Volume 65 Issue 2 Pages 109-118
In recent years, Hard Cyclic Viscoplastic Deformation has become a new effective SPD method for studying the evolution of the structure and properties of Cu-alloys without changing the dimensions of the workpieces up to their destruction during processing at room temperature. Linear compression-tension of the material in the viscoplastic region is carried out in the strain control mode in the range from ε = ±0.2% to ±3% at a constant frequency of f = 0.5 to 2 Hz with a total number of 20–40 cycles in each test series. This method can be used also to improve and stabilize the ultrafine grained microstructure, and also allows you to study changes in the mechanical, physical and functional properties of coarse grained, ultrafine grained, and nanocrystalline metallic materials. Can also to be used to study the stability and viability of metallic materials and predict their suitability over time in harsh environments such as space and military applications.
Written science in the field of SPD first appeared in the late 20th century,1,2) but archaeological research has shown that the process was known and used at least 2,700 years ago3) by ancient peoples to make bladed tools such as knives and swords. In a review of historical developments and recent advances3) written by many researchers from different countries, it is shown that the field of SPD is constantly being improved with new methods already developed and partially patented for more than a hundred names. Currently, more than 1,000 articles are published annually in the field of SPD, especially on the production of large metal parts for future technical and commercial applications.
Severe plastic deformation (SPD) techniques1–5) are popular due to their ability to modify the grain size of metals and alloys, with accompanying unique mechanical6–9) properties. It is known10–12) that the tensile strength and hardness of SPD-treated metallic materials are significantly better compared to their coarse-grained counterparts. Unfortunately, scientific works on SPD have so far mainly studied only changes in the microstructure and mechanical properties of materials, such as hardness and tensile strength. A number of collective scientific articles also show that in addition to changes in microstructure and properties the electrical conductivity also decreases,13–23) and phase transformations24–27) appear in materials during SPD.
In the book “Cyclic plasticity of metals” edited by Hamid Jahed and Ali A. Roostaei28) are described the response of metals and metallic alloys undergoing various loading conditions by number of authors - scientists from different countries. Is shown, that the models of cyclic plasticity, nonlinear kinematic herdening,29) viscoplasticity and constitutive modeling of viscoplasticity30) are presented in theory. The modeling of ratcheting,31) and Bauschiner effect32) in metals and structures are studied and compared with corresponding experimental results.33) The computational methods for cyclic plasticity are used for study of different aspects of strain- and energy-based fatigue damage methods and modeling of viscoplasticity.34) Unfortunately, all these works in this book don’t study the changes of metallic materials density, modulus of elasticity or Young’s modulus, the wear at friction under electrical current and so on.
Relatively little attention is paid to the study of functional properties by the methods of severe plastic deformation, which limits the widespread use of these materials in the industry of the modern world. The processing of materials by the method of hard cyclic viscoplastic deformation (HCVD) is described in a number of articles.35–44) The HCVD method has been developed and can be used to study the viscoplastic behavior of metallic materials, but is not yet widely popular due to its limited representation in the form of scientific papers in the scientific literature. Moreover, this method allows you to very quickly, simply and cheaply change and study the structure and properties of metallic materials in the desired direction, but unfortunately, in a very small volume of the workpiece. In this review, changes in the microstructure of materials, mechanical, physical and functional properties of coarse-grained, ultrafine-grained and nanocrystalline copper alloys were studied using this method.
The principles of HCVD were first presented in 2004 at the annual meeting of TMS Ultrafine Grained Materials III, Charlotte, North Carolina, USA45) and at the 4th DAAAM International Conference on Industrial Engineering - Innovation as a Competitive Advantage for Small Businesses and Medium-sized Businesses”, Tallinn, Estonia.46) Unfortunately, the HCVD as a new process has not yet become widespread in the study of the evolution of the structure and properties of metallic materials for using its in advanced industry and science. It is well known that “viscoplasticity” is a reaction of solids involving time-dependent and irreversible deformations.47–50) It has been established that this dependence increases at temperatures above one third of the melting point of the material. The HCVD is based partly on the Bauschiner effect.51–55) This study investigated the viscoplasticity of copper alloys and the accompanying changes in microstructure and properties at room temperature. The investigations of wear and tribological behavior of Cu-alloys at a dry sliding under electrical conduction are presented in Refs. 56, 57), respectively. The changes in the Youngs modulus is studied in human bone58) and in Cu-alloys in Refs. 9, 59), respectively. To do this in listed experimental works, we were used HCVD with a constant strain amplitude at each stage of the experiment.
In the work under review, the microstructure and properties of the test specimens from copper alloys was modified beforehand in IEAP method with the number of pressings up to 6 passes along the BC route. The maximum von Mises degree of strain used was up to ∼6.7, respectively. For comparison, these same Cu-alloys (with the original structure) were used so that the results could be compared. The IEAP method produce processing steps is presented in Fig. 1.
Copper alloys of one melt and heat treatment at a temperature of 1000°C for 30 minutes (Fig. 2(a)) were subjected to pre-treatment by the IEAP method from one to six presses along route BC (Fig. 2(b)). The test materials, used in the present work were hot-rolled rods of copper alloys with the chemical compositions of 0.66%Cr, 0.03%Si, 0.02%Fe, 0.016%S, and Cu balance (in mass%) and 1.2%Cr, 0.2%Hf, and Cu balance. The technically pure CG copper chemical composition was (Cu–0.02%Fe–0.01%Al), respectively. The chemical composition was studied by SPECTROLAB (Spectro Analytical Instruments, Germany).
Initial heat-treated at 1000°C for 30 min and in cold water quenched sample for IEAP processing (a), after IEAP for 6 passes by BC route (b).38) HCVD sample (c) with stepwise cross-section (As-cast, A1, A2, A3, and A4), (d) showing mini-specimens (MS1, MS2, MS3, MS4, MS5, and MS6) cut from the HCVD sample diametric section (e) in the three layers (1, 2, and 3) by electrical erosion technique, and tensile test mini-specimen (f) with dimensions in mm.9,38)
The HCVD method is characterized by the generation of cyclic stress, the magnitude of which depends on the strength properties of the material at a given compression-tension deformation or strain amplitude. The evolution of the microstructure of metal materials is mainly studied from the deformation rate, the number of cycles, and the deformation stress amplitude. In present overview work these large strain amplitudes were step-by-step increased and were ε = ±0.2%, ±0.5%, ±1.0%, ±1.5%, ±2.0%, ±2.5%, and ±3.0% are used, respectively. At each degree of deformation, up to 20 ÷ 30 cycles are performed with a frequency of f = 0.5 to 2.5 Hz, respectively. The number of tests of the HCVD method starts from 10–20 cycles and up to 40 cycles per test for a series with the corresponding constant strain amplitude. The frequency in HCVD is usually between f = 0.5 Hz and 2.5 Hz with a single deformation amplitude. The tests were conducted on materials tester Instron-8516 (Germany). When studying the properties, the creep limit and tensile strength, the elastic modulus or Young’s modulus at tension or nanoindentation, creep, relaxation, etc. were measured by Instron-8516. To measure elongation, an extensometer with a base length of 10 mm is usually attached to a part of the workpiece with a minimum cross-section of a stepped HCVD sample (see Fig. 2(c)). The amplitude of deformation and is controlled by an extensometer using a computer program that controls the process and displays the corresponding on the computer screen.
The HCVD method can be used also as a new test method in materials science when it is necessary to determine the behavior of a material under stresses that can exceed the elastic limit (ε > 0.2%) and deform under extreme operating conditions. For example, such extremes may occur in aviation, space, or military technology because these devices have a minimum calculated strength or margin of safety compared to other devices. For example, the compressor blades of a turbojet engine for military fighters have a safety margin of no more than 3–5%.
2.3 Methods for other properties of materials testing after HCVDMaterial testing in corresponding sections (see Fig. 2) was also carried out using gas analysis,44) electrical conductivity measurements,18) dry sliding wear tests under electrical conductivity conditions,56,57) density measurements,43) determine the atomic interactions,42) etc. The mini-samples (see Fig. 2(d) and (e)) were tested on the MDD MK2 Stand test system manufactured in the UK.9,59) The microhardness was measured using a Mikromet-2001 tester after holding for 12 s at a load of 50 and 100 g.5,9) The tribological behavior of materials under dry sliding conditions56) was investigated before and after IEAP, followed HCVD and heat treatment to provide a comparison over a range of material properties as well as collected strain to understand their influence on the coefficient of friction and on the specific wear rate. Dry sliding wear was studied in a ball-plate system with a tribometer (CETR, Bruker, and UMT2) using an aluminum oxide (Al2O3) ball with a diameter of 3 mm as a counter surface.57) The coefficient of friction (COF) was obtained automatically. For wear volume calculations, the cross-sectional area of the worn tracks was measured by the Mahr Pertohometer PGK 120 Concept 7.21.9,56) The content of metals inclusions (in ppm) were studied according to MBN 58.261-14 (ICP-OES Agilent 730) and gases concentration according to method of MBN 58.266-16 (LECO ONH-836) and S according to method of MBN 58.267-16 (LECO CS-844), respectively.44) The electrical conductivity (MS/m and %IACS) of metal materials was determined with a measurement uncertainty of 1% for different orientations on flat samples by means of the Sigmatest 2.069 (Foerster), accordingly to NLP standards at 60 and 480 kHz on a calibration area of 8 mm in diameter.18) The electrical conduction was measured at room temperature of 23.0 ± 0.5°C and humidity of 45 ± 5% according to the international annealed copper standard (IACS) in the Estonian national standard laboratory for electrical quantities. To obtain one electrical conductivity data, 30 measurement tests were automatically performed and the result was displayed on the computer screen.
The micromechanical properties were measured by nanoindentation under load of 20 mN for 100 times using a nanoindentation device from the NanoTest NTX testing centre (Micro Materials Ltd.).21) The nanohardness and indentation (Young) modulus were performed by a Zwick ZHN instrument with a maximal load of 0.2 mN using a Berkovich indenter.38) The measurements were performed in fast mode with a loading time of 5 s, and an unloading time of 2 s inconformity with ISO 14577-1. The samples density after IEAP with different pressing number was measured by OHAUS Scout-Portable balances at room temperature.43) The dislocation density was calculated by the Rechinger method according to the results of the X-ray investigation by the D5005 AXS (Germany) and Rigaku (Japan) diffractometers.38) The X-ray photoelectron spectroscopy (XPS) system was used to determine the change in atomic and mass concentration of Cu and Cr during IEAP and HCVD treatment in copper alloys. To study the microstructure, the samples were mechanically polished with silicon papers up to 4000, and then with diamond paste on Struers grinder.6) After the grinding, the samples were etched by an ion polishing/etching facility using precision etching system at 30 kV for 30 min in an argon atmosphere.43) The microstructure of the samples was studied using an optical microscope (Nikon CX) and electron microscopes (Zeiss EVO MA-15 and Gemini Supra-35) equipped with an EDS apparatus.38,59)
The initial sample with coarse-grained microstructure and average grain size of 120 µm before processing is demonstrated in Fig. 3(a). A closer view using electron microscopy on the same sample in Fig. 3(b) shows that there are residual particles of Cr with a size of about one micrometer. During the IEAP processing after 6 passes, the particles partially dissolved in the matrix and hence, reduced in size down to 500 nm (Fig. 3(c)). Figure 3(d) shows the sample after IEAP followed by HCVD where Cr particles entirely disappeared. Further analysis by SEM-EDS confirmed this fact. The presence of Cr inclusions in the IEAP-processed sample is visible in Fig. 3(e), while after processing via IEAP + HCVD, the particles were fully dissolved in the Cu matrix (Fig. 3(f)).
Microstructural observations of Cu–0.66Cr–0.02S-alloy before and after processing by optical (a) and electron microscopy (b), (c), (d), (e), (f). Optical microscopy of the initial sample with coarse-grained microstructure (a); electron microscopy of the initial sample demonstrating the undissolved Cr particles in the Cu matrix (b); processed sample subjected to IEAP after six passes with fine particles of Cr (white spots) (c); IEAP-processed sample followed by HCVD (d); SEM–EDS microscopy of the IEAP-processed sample with the presence of Cr particles (e); SEM–EDS microscopy of IEAP + HCVD sample where Cr particles were fully dissolved and dispersed in the Cu matrix (f).9,38)
The results of materials microstructure characterization revealed that HCVD dissolved the Cr particles, reduce the micro strains, and transformed the elongated microstructure to an equivalent structure in the SPD-induced copper alloy (see Fig. 3). Results of mechanical testing showed that HCVD enhanced the elongation of SPD-processed samples at the cost of a slight decrease in strength and hardness. Further, aging treatment at different temperatures was conducted on the samples. Results showed that HCVD-processed samples retained the dislocation densities in the microstructure and possessed the highest hardness and electrical conductivity after aging. Such enhancement was attributed to the effect of HCVD on the relaxation of severely deformed grain boundaries as well as the construction of HCVD to the full dissolution of Cr residuals, and subsequently, the formation of Cr sub-precipitates after aging.
As is shown in SEM pictures (Fig. 4(a), (b), (c)) formed in measured by extensometer zone the diameter of nanopillars is about 80 nm and the length is about 800 nm. The nanopillars are situated parallel to each other in the UF grains (see Fig. 4(d)).
Moreover, the effect of IEAP and HCVD on the microstructural refinement was studied by TEM. Figure 5 shows the micrographs in bright and dark fields obtained by TEM as well as the Selected Area Electron Diffraction patterns (SAED). TEM images demonstrated the UFG microstructures formed by IEAP (Fig. 5(a), (b), and (c)), IEAP followed by HCVD (Fig. 5(d), (e), and (f)), and HCVD (Fig. 5(g), (h), and (i)). A high dislocation density was observed in dark-field micrographs (Fig. 5(b) and (e)) inside the grains of the Cu phase obtained after IEAP and IEAP + HCVD. The size of these grains is about 600 nm. The microstructure after IEAP is strongly oriented and consists of a cell-granular microstructure with a high density of dislocations. The distance between the elongated boundaries of the cells is about 50–300 nm. Applying HCVD led to the reduction of dislocation densities and thinning of the boundaries (Fig. 5(d)). As can be seen in the pictures, HCVD after IEAP smoothened the grain boundaries and developed an equiaxed microstructure.
Bright (a), (d), (g), (h) and dark (b), (e) field TEM micrographs with corresponding electron diffraction patterns (c), (f), (i) of processed specimens. The first row (a), (b), and (c), the second row (d), (e), and (f), and the third row (g), (h), and (i) correspond to IEAP, IEAP + HCVD, and HCVD only, respectively.38)
However, a low dislocation density was observed in the grains after HCVD (Fig. 5(g), and (h)); large scale shear bands with coarse microstructure are shown in Fig. 5(g) by arrows. The grain boundary width is about 100 nm, and the distance between dislocation layers is about 67 nm. The electron diffraction patterns of the specimens are presented in the right column of the image (Fig. 5(c), (f) and (i)). The SAED patterns of the IEAP + HCVD specimen (Fig. 5(f)) refer to a significant fraction of small grains with different orientations as well as large grains. The ring patterns indicate the accumulation of small grains with different orientations, and the spots pertain to the coarse-grained texture. Weak reflections in Fig. 5(c) are referred to the diffractions from chromium.
3.2 Properties evolution at the HCVD processing of Cu-alloysResults of tensile testing of samples of 0.66Cr, 0.03Si, 0.02Fe, 0.016S, and Cu balance (in mass%) are demonstrated in Fig. 6(a). The maximum tensile strength in the samples was measured as 488 MPa in the IEAP-processed sample after 6 passes by the BC route of extrusion. A UFG microstructure in this sample with high dislocation densities was disclosed by TEM investigations.38) The sample after IEAP + HCVD lost strength to 387 MPa, but the elongation slightly increased to 7.6%. The sample with only HCVD processing demonstrated a moderate strength in the tensile strength of 266 MPa, and a further elongation rate at 15%. The initial sample showed the largest elongation rate up to 33% with the maximum strength of 210 MPa. The mechanical response of the samples of 0.66Cr, 0.03Si, 0.02Fe, 0.016S, Cu balance (in mass%) for 6 passes by BC route to cyclic tension and compression loads are shown in Fig. 6(b). These results were achieved by imposing a strain amplitude of ε = ±1% for 20 cycles during 40 s at a frequency of f = 0.5 Hz and measuring the corresponding stress during the cycles. The initial sample demonstrated a hardening behavior during HCVD in both tensile and compressive cycles as the strength increased from ±110 to ±180 MPa. This hardening behavior of metals under cyclic tension compression is consistent with the isotropic hardening model when the loading cycles strengthen the material homogeneously in all directions of compression and tension. In contrast, the kinematic hardening or Bauschinger effect is the other hardening counterpart model53,54) by which the compressive flow stress decreases when the loading direction is recurrently reversed.55) The IEAP-processed sample showed a cyclic softening behavior as the initial stress value of 600 MPa decreased to 410 MPa in tension and to 530 MPa in compression, respectively (see Fig. 6(b)). In contrast, cyclic loading of the heat-treated IEAP sample during HCVD showed a stable behavior at a tension of 450 MPa and at a compression of 490 MPa, respectively.
Tensile strength and elongation of Cu-alloy (a), and experimental values of stress amplitudes obtained during HCVD testing for 20 cycles at a frequency of f = 0.5 Hz for a test time of 40 s, and a strain amplitude of ε = ±1.0% (b). The tested samples include the initial specimen (without IEAP processing), IEAP-processed samples (IEAP), and IEAP-processed samples followed by heat treatment at 450°C (IEAP + HT), respectively.38)
The evolution of the hardness and corresponding electrical conductivity of the copper alloy specimens with respect to the processing method is shown in Fig. 7.
Evolution of the hardness of Cu–0.7 mass% Cr–0.2S mass% 0.02S mass% alloy versus the aging temperature (a); and evolution of the electrical conductivity vs. the aging temperature (b) depending on the processing modes.38)
As shown in this graph, the processing of Cu-alloy by any technique noticeably decreased the electrical conductivity in the samples. A post-processing heat treatment (aging) at different temperatures ranging from 20°C to 900°C was used to investigate the impact of aging on the hardness and electrical conductivity of the samples. As shown in Fig. 7(b) an increase in hardness occurred by increasing the aging temperature up to 450°C for the IEAP and up to 550°C for HCVD and IEAP + HCVD samples. Thereafter, increasing the temperature rapidly decreased the hardness. Similar behavior can be observed in the electrical conductivity, i.e., increasing the aging temperature to 550°C significantly enhanced the conductivity in all samples (Fig. 7(b)). It is also noticeable that heat treatment of the IEAP + HCVD sample at 550°C, increased the conductivity from 42% IACS to 100% IACS. Another point that should be noted in this figure is that increasing the aging temperature after 550°C sharply lowered the hardness, even lower than the initial value before aging (150 HV at 20°C versus 60 HV at 900°C; 60% of drop). This sharp drop in hardness is attributed to the grain growth at elevated temperatures, thus, degrading the mechanical properties.9,38,57) A similar trend can be found in the electrical conductivity after aging at higher temperatures up to 900°C.
The measurement of nanohardness and indentation modulus (Young’s modulus) conducted on the cross section of the samples is shown in Fig. 8(a), (b), and (c), and the results of measurement are plotted in Fig. 8(d), (e), and (f). The selected area with the dimensions of 26 ÷ 26 µm, included 36 points where 6 points in 6 rows were located with an equal distance of 5 µm with respect to each other. Akin to tensile strength, the average hardness and indentation modulus in the IEAP sample is higher than that of HCVD and IEAP + HCVD. The nanohardness distribution by contour plots38) green and blue areas in the contours represent the regions with the highest and lowest values, respectively.
Nanohardness testing: Measurement of nanohardness versus indentation modulus by applying 36 indents on the surface of the samples (a). The arrows in the micrographs are referring to the footprint of a Berkovich nano-indenter applied by a load of 0.2 mN (b), (c). Variation of hardness and indentation modulus with respect to the process in the samples after IEAP (d), IEAP followed by HCVD (e), and HCVD (f).38)
The specific wear rate and COF measurements show their dependence from sample material chemical composition, sample (surface) hardness as well material wear track surface softening/hardening37) during wear testing (Fig. 9). Results show that HCV deformed sample in surface was hardened from 77HV0.05 to 90HV0.05 and on the wear track surface from 115 HV0.05 to 126HV0.05, respectively. In this case the surface hardening was induced by cyclic straining and wears track hardening as result of sliding. During HCV deformation of Cu–1.0 mass% Cr–0.1Hf alloy the specific wear rate (Fig. 9(b)) was decrease from 0.3 mm3/min to 0.07 mm3/min, respectively. Microhardness evolution of wear test sample (from Cu–1.0 mass% Cr–0.1Hf alloy) on surface and on wear track surface increase during HCV deformation and IEAP + HCV deformation and influence microhardness evolution during wear testing on the specific wear rate. The results of tribological tests show that the coefficient of friction (COF) and wear track C-S area increases are directly dependent on the load (15, 50, 100 and 150 g) applied. According to these data, the grain size as well the hardness have little influence on the COF, although the specific wear rate at the time was increased.
When comparing our results with the results presented in previous works, the mass loss decreased remarkably as the number of IEAP passes and Cr content increased, being affected more by the sliding distance than by the applied load under the experimental conditions. From these data, it has been shown that the wear mechanism was observed to be adhesive and delaminating initially, and an abrasive mechanism appeared as the sliding distance increased. In our experiments, the abrasive wear mechanism did not show any dependence on sliding distance. UFG microstructure in Cu–0.68%Cr–0.02%S as energy alloy, designed for use in electrical energy industry, was successfully processed by BC route of IEAP for 6 passes.57) At follows the suitable mechanical and physical properties were received on UFG samples by ageing heat treatment at temperatures from 250°C up to 750°C with heating step of 100°C for 1 h. The wear rate and wear mechanism at sliding pare alloy with graphite disc dependence from testing parameters and material mechanical and physical properties were analyzed.
The wear rate of the UFG Cu–0.68Cr–0.02S energy alloy was minimal for samples with maximal electrical conductivity (95% IACS) and with maximal Vickers microhardness (160 HV0.1), respectively. The wear rate dramatically increased via electric spark erosion (friction surface micro piercing) with lowering of pressure stress (by normal load decrease) and increase of sliding speed.57) Spark erosion at sparking leads to accelerated wear of the contact layer. Friction surface micropiercing is differed for adhesion and delamination. The worn surfaces of samples were covered by graphite (carbon) layer for both the CG and UFG microstructure alloy even if at optimal wear test parameters. Minimal wear rate shows the IEAP sample with maximal hardness and best electrical conductivity after heat treatment at 450°C for 1 h. CG CuCrS alloy is seen as the main advantage and a promising property of ECAP for 6 passes by BC route and at follows heat treated at 450°C for 1 h, which makes it attractive for high temperature (up to 600°C) electrical engineering application in power plants. The challenge of the science of tribology is materials with nano-columns inside the ultrafine grains as a result of extreme grain refinement by SPD methods.
In the present work, HCVD was implemented as a processing method after IEAP to improve the properties and microstructure of Cu-alloys. It is known that micro stresses are decresed and the properties of such metals as copper and copper alloys change during HCVD. The results of nanoindentation and tensile testing showed that HCVD after IEAP provides somewhat lower hardness and strength. The results of TEM and XRD analysis and microstructure evaluation are also consistent with this fact, demonstrating that HCVD after IEAP the changed grain size and reduced lattice deformation as well as dislocation density. The tensile test results showed that the IEAP treatment increased the yield strength of the material and reduced the electrical conductivity by passes number icrease.9) By this, the HCVD after IEAP reduced the strength. An earlier study showed that two main mechanisms can contribute to strength enhancement with IEAP of CG metals: grain refinement and work hardening. Work hardening mainly affects the earlier passes of IEAP, sacrificing elongation, while the grain refinement process dominates the hardening mechanism after further passes (reportedly after four), increasing strength without sacrificing ductility. Based on this theory, it can be concluded that the main part of the IEAP hardening mechanism of the copper alloy during six pressing passes is due to work hardening. Evaluation of the XRD patterns by the Williamson-Hall method showed that IEAP processing imposed large amounts of dislocations densities in the order of 1014 m2 on the material.38) The same order of magnitudes has also been reported in former studies on IEAP processing of copper by which the enhancement in strength and hardness was achieved.
Furthermore, IEAP appeared to be effective in promoting the phase transformation by means of partial dissolution of the residuals in the base metal.24,25,44) Nevertheless, it was found that SPD-induced phase transformations and the dissolution of particles cannot always be completed because a dynamic equilibrium may happen during SPD which acts as annealing at an effective temperature, leading to the dissolution of precipitates and at the same time, decomposition of the solid solution, producing a secondary phase of precipitates, thereby achieving a steady-state condition in the concentration of the solid solution. It should also be noted that the results displayed imply the fact that when studying the effect of HCVD on the mechanical behavior of materials, the initial state of the material and the processing history (isotropic hardening, stable state, or softening) should be considered. Generally, cyclic loading can promote the slip of dislocations, and this movement will lead to the shearing and dissolving of the particles when encountering the gliding dislocations. Nevertheless, the presence of such defects will lead to the scattering of electrons at the grain boundaries and reduce the electrical conductivity. In principle, heat treatment can sacrifice the strength in the dissolution of precipitates in the base metal, and a decrease in hardness takes place. On the other hand, it was shown that heat treatment of IEAP samples relieved the microstrains and decreased the dislocation densities, hence fewer interactions occurred between the dislocations and precipitates; thus, the persistence of Cr particles stabled the material against the grain growth and softening. In principle, heat treatment can sacrifice the strength in strain-hardened materials by reducing the dislocation densities and lowering the hardness. However, earlier studies proved that the presence of alloying elements such as chromium, zirconium, or calcium can enhance the thermal stability of alloys owing to their contribution to entangling the dislocations as well as the formation of the precipitates and stacking faults after the heat treatment and thereby, improving the strength of materials. This study demonstrated that increasing the content of Cr had a positive influence on the hardening because of the effect of Cr atoms on decreasing the average grain size of the material and increasing the dislocation densities and stacking faults. In the final analysis, high conductivity and strength are the two phenomena that usually tend to head in opposite directions. In order to gain high strength in highly conductive metals without sacrificing the electrical conductivity, it is advised to utilize the grain size hardening mechanism as well as nanosized precipitate hardening to minimize the scattering of electrons. HCVD as a new technique can provide relaxed grain boundaries and promote the solubility of residual particles leftover from casting. Post-heat treating of the material leads to the decomposition of solute atoms and the formation of nanosized precipitates, which are in favor of electrical conduction and can stabilize the thermal sensitivity of the grain size. Results of this study proved that HCVD followed by aging can improve the mechanical properties and electrical conductivity, and modify the microstructure of severely deformed materials by virtue of decreasing the microstrains, relaxation of the grain boundaries, and dissolving the residual particles in the base metal. These outcomes may open the way to implement this technique for the development of complex concentrated alloys or high entropy alloys toward the enhancement of their functional properties which are in the growing demand for industrial applications. Microstructure, wear rate, wear mechanism, hardness, and electrical conductivity of tested energy alloy after IEAP, and heat treatment with and without no HCVD were investigated. The wear rate of the UFG CuCrS alloy was lower than that of the CG and it increased with the decreased hardness. The wear rate dramatically increased via electric spark erosion (friction surface micro piercing) with lowering of normal load and increase of sliding speed. Friction surface micropiercing is different from the adhesion and delamination. The worn surfaces of samples were covered by graphite (carbon) layer for both the CG and UFG microstructure alloy even if at optimal wear test parameters. Microstructure, wear rate, wear mechanism, hardness, electrical conductivity of tested energy alloy after IEAP, heat treatment with and with no HCVD were investigated also.
This overview study evaluated the impact of a new processing method so called as HCVD on microstructure, mechanical, physical, chemical, functional, performance, tribological, etc. properties of metallic materials such as Cu-alloys. To expand the capabilities of the new processing process, metal materials with various structures were tested, such as single-crystal, coarse-grained, ultrafine-grained, and nanocrystalline.
With this new test method, it is possible to initiate and study the processes occurring in the microstructure and properties of materials during HCVD before their destruction. HCVD is based on the application of a cyclic tensile and compressive load by controlled strain amplitude on materials at a constant frequency at a given strain level. In this test method, the main parameters are the strain amplitude of compression-tension in the range from ε = ±0.2% to ε = ±3.0% with the number of cycles from 20 to 40 for one level of strain amplitude and with a frequency of f = 0.5 Hz to 2.5 Hz. The strain rate depend on the materials main parameters. The rest of the process parameters are set automatically depending on the strength properties of the tested metal material in general.
The microstructure of Cu-alloys processed by HCVD method is significantly different from the microstructure produced by other SPD techniques. The electrical conductivity in the SPD processes decreases with increasing hardness and dislocation density and increases when HCVD is combined with heat treatment.
Using the HCVD method, it is also possible to study the viability of metallic materials during operation in aviation, space, and defense under conditions of high load, and close conditions before failure, when the margin of safety is only few percent.
Accordingly, this overview article provides a brief overview of the structure and properties of metallic materials that change as a result of HCVD and thereby extending materials science with new relationships.
This overview work was supported by the Estonian Research Council institutional research funding by Project PRG1145, and the European EU 7-FP, ERA.Net Rus STProjects-219, Nano Phase. The help and advise of Dr. B.O. Shahreza, V. Mikli, R. Traksmaa, and M. Viljus from TalTech are greatly appreciated. The author would like to extend own sincere thanks to Mr. M. Heier from Zwick/Roell for nanoindentation experiments, to Prof. S.V. Dobatkin for TEM investigations, and Dr. A. Pokatilov for the measurements of electrical conductivity.
