Microstructural characteristics developed by inhomogeneous plastic flow in fcc metals and alloys have been reviewed. The aspect of microstructural evolution due to plastic deformation in fcc materials is markedly influenced by the values of SFE. Particular emphasis is laid on the formation of shear bands in a cold-rolled low SFE metal such as 310S austenitic stainless steel. In cold-rolled 310S steel, deformation twinning easily occurs to form fine lamellar structure due to twin (T) and matrix (M). Further deformation causes shear band formation. In shear bands, fine-grained structures are formed with large orientation scattering. The multiplication of shear bands destroys the fine T-M lamellae to form fine-grained structures being similar to those due to severe plastic deformation. Shear bands are considered to be a microstructural instability where intense shear deformation occurs due to the local collapse of fully work-hardened structure.
Formation mechanisms of ultrafine grained structures in severe plastic deformation (SPD) of metallic materials are overviewed and discussed on the basis of experimental results. It is concluded that the formation of ultrafine grained structures in SPD can be understood in terms of grain subdivision. The ultrafine grained structures in the as-SPD materials are essentially deformation structures, although they can be simultaneously considered as grain structures from a viewpoint of large misorentation to each other. When the SPD processed materials are annealed, a continuous change in microstructures can happen, which might be called continuous recrystallization. The phenomenon is fundamentally discussed after determining the nucleation events in recrystallization.
The evolution processes of new high-angle boundaries as well as ultrafine grains processed by severe multidirectional forging (MDF) are studied in ferritic, austenitic steels and copper at low temperatures below 0.5Tm (Tm is the melting point), and aluminum and magnesium alloys at high temperatures above 0.5Tm, where dynamic recovery operates mainly as a restoration process. The structural changes can be characterized by the evolution of deformation bands such as microshear or kink bands at moderate strains. MDF promotes the multiple shearing, which results in the formation of spatial net of mutually crossed bands subdividing the original grains. This is a mechanical induced event and so an athermal process. On the other hand, in large strain the fast operation of recovery processes accelerates the kinetics of ultrafine grain evolution with increasing temperature at low temperatures below 0.5Tm. In contrast, the volume fraction of new grains decreases in large strain through hard development of deformation bands with increasing temperature at elevated temperatures above 0.5Tm. The misorientations between (sub)grains evolved gradually increase with increase in strain, finally leading to the development of a new fine-grained structure. Temperature effect and the mechanism of strain-induced grain formation are discussed in detail.
Equal-Channel Angular Pressing (ECAP), High Pressure Torsion (HPT) and Accumulative Roll Bonding (ARB) are typical processing procedures of severe plastic deformation. They can reduce the grain size to well within the submicrometer range. This paper reviews principles and characteristic features of the three major processes and summarizes recent studies reported on pure Fe and steels. Microstructural features and related mechanical properties are introduced for each process. Important aspects in conducting each process are described, and practical applicability and future development are discussed in terms of scaling-up and continuous operation.
Mechanical milling is one of the severe plastic deformation processes and can give a heavy deformation to a metallic powder. Using such a mechanical milling process nano crystalline material can be produced with relatively simple equipment, low cost and short time. The mechanical milling process leads to finer grain than other severe plastic deformation for bulk materials and the grain size produced by mechanical milling is less than 10 nm. Nano grain structure by mechanical milling is finally formed with nano layered structure though the grain refinement rate depends on the milling method and the milling condition. The minimum grain size produced by mechanical milling is related to many factors such as melting point and stacking fault energy. The nano grain tungsten powder demonstrates a good workability and its sintered compact demonstrates the superplastic deformation at 1473K. When the milling intensity is suitable, mechanically milled powder consisting of shell and core can be produced. The compacts produced from such mechanically milled powder have the nano/meso hybrid microstructure and demonstrate advanced mechanical properties.
The change in microstructure and mechanical properties of machined steel surface was reviewed. Nanocrystalline ferrite structure layer can be produced by high energy shot peening. Nanocrystalline layer consists of equiaxed grains of about 20 nm and distinguished from the adjacent heavily deformed region with a clear boundary. Nanocrsrystallne ferrite structure showed extremely high hardness and substantially slow grain growth. White etching layer can be produced by drilling, hard-turning and sliding wear. The white etching layer thus produced was re-austenitized region due to the deformation heat. White etching layer has clear boundary (transfrmation boundary) between the matrix. White etching layer consist of submicron grained martensite which is much finer than conventional martensite. The white etching layer produced by drilling and sliding contains nanocrystalline layer at the top surface. The thickness of nanocrystalline layer is about 20% of whole white etching layer. The thickness of white etching layer and also nanocrystalline layer increases with the increase in drilling or cutting speed and feed rate. The formation mechanism of white etching layer and nanocrystalline layer were discussed. The thermal and mechanical influence for such layer formation was presented. The influence of white etching layer on the mechanical properties of machined components was introduced.
Abrasion induced subsurface deformation and work hardening along with related microstructural changes of the pearlitic steel has been presented. The abraded surfaces were examined with a nanoindentation appratus to evaluate the variation of nanohardness and elastic modulus with depth below sliding contact on a nanometer scale. The varing nature of influence of interlamellar spacing and the vanadium addition on the wear response of the pearlitic steel has been discussed in terms of the ratio between hardness and elastic modulus of the abraded surface which corresponds to a plasticity factor. The application for the crankshaft of the automobile is also presented.
It is well known that three types of white-colored microstructural changes occure in rolling contact fatigue (RCF). Those are called WB (White band), Butterfly and WEA (White etching area) respectively. As the conditions of bearing use have become severer, recently these phenomena are observed more frequently. However, their formation mechanisms and countermeasures have not been clarified, because very complicated factors are involved in these phenomena. Authors remark the fact that white-colored microstructural changes consist of ultrafine grains. Although the formation of such ultrafine grains are generally found in SPD (Severe Plastic Deformation), it is expected that the knowledge in this field would be useful in the field of RCF, as well. From this point of view, microstructural changes in RCF were reviewed in this report, referring previous studies and recent authors studies.