Andrey Belyakov

Grain refinement in copper under large strain deformation

Abstract Structure evolution taking place in pure polycrystalline copper was studied in multiple compressions at room temperature. Rectangular samples were compressed with consequent change in the loading direction from pass to pass. The deformation behaviour at high strains of above 2 shows an apparent steadystate flow following a rapid rise in the flow stress at an early stage of deformation. The structural changes are characterized by the evolution of many mutually crossing subboundaries at low to moderate strains, finally followed by the development of very fine grains with medium- to large-angle boundaries at large strains. These new grains are concluded to be evolved by a kind of continuous reaction, that is continuous dynamic recrystallization (DRX). The grains developed under continuous DRX are much finer than expected from the extrapolation of discontinuous DRX data for hot deformation. An average grain size of about 0.2μ evolved at room temperature is roughly similar to that for subgrains developed at preceding strains.

Effect of initial microstructures on grain refinement in a stainless steel by large strain deformation

Abstract Initial grain size effect on submicrocrystalline structure evolution was studied in multiple compressions of a 304 stainless steel at 873 K (0.5Tm). Four sets of specimens with different initial microstructures were used, i.e. annealed samples with grain sizes of D0=15 and 2.2 μm, and dynamically recrystallised ones with D0=3.5 and 1.5 μm. The new ultra-fine-grains ( D=0.25 μm) develop as a result of a continuous increase in the misorientations between the subgrains that evolved during deformation. In the samples with D 0 ≤3.5 μm, the fraction of the strain-induced high-angle boundaries increases rapidly to more than 60% with a straining to about 1.5. On the other hand, their fraction does not exceed 20% at e=1.5 in the sample with D 0 =15 μm. The latter needs much more straining to around 6 to obtain 60% of high-angle (sub)grain boundaries.

Continuous recrystallization in austenitic stainless steel after large strain deformation

Static restoration mechanisms operating during annealing were studied in a 304 steel with strain-induced submicron grain structures. The initial microstructure with an average grain size of about 0.3 μm was developed by large strain deformation at 873 K. Early annealing leads to a full relaxation of high internal stresses associated with non-equilibrium strain-induced grain boundaries, while their boundary misorientations and the average grain size barely change. Further annealing results in a transient recrystallization followed by a normal grain growth. The average grain boundary misorientation increases up to around 45° in the former and becomes constant in the latter. This is associated with the change in the grain boundary misorientation distribution from a characteristic strain-induced one to a near random distribution corresponding to a fully recrystallized state. The annealing processes operating in the strain-induced fine grains take place homogeneously in the whole matrix and can be called continuous recrystallization.

Dynamic recrystallization under warm deformation of a 304 type austenitic stainless steel

Abstract Warm (and hot) deformation of a 304 type austenitic stainless steel was studied in connection with microstructural developments in compression at temperatures of 873–1223 K (0.5–0.7 Tm) under strain rates of 10−4–10−1s−1. The two deformation domains can be categorized due to their different mechanical and microstructural behaviors. In the region of flow stresses lower than around 400 MPa, the deformation behaviors are typical for hot working accompanied with dynamic recrystallization (DRX). New grains are evolved mainly by dynamic bulging mechanism, which can be accelerated by the development of serrated grain boundaries and strain induced dislocation subboundaries. The relationship between dynamic grain sizes ranged from 2 to 7 μm and peak flow stress can be expressed by a power law function with a grain size exponent of −0.72. In contrast, in the region of flow stresses higher than 400 MPa, the deformation behaviors hardly depend on strain rate and temperature and so can be in the region of athermal deformation. The stress–strain curves under such warm deformation are similar to those affected only by dynamic recovery. The microstructures evolved at high strains are mainly characterized by the dense dislocation walls evolved in pancaked original grains, while grain boundary serration also takes place even at such warm deformation. Mechanisms of this microstructural evolution are discussed in combination with analysis of deformation mechanisms operating under warm deformation.

Substructures and internal stresses developed under warm severe deformation of austenitic stainless steel

Ultrafine-grained materials having grain sizes of tens and hundreds of nanometers, that offer much improved mechanical and physical properties, recently have aroused great interest among researchers in the materials science. The aim of this work is to study submicron-scale substructure evolution in a 304 type stainless steel caused by severe warm deformation at 0.5 T{sub m}. The effects of strain-induced grain boundaries on the internal stresses and the related lattice distortions evolved in these grain interiors are discussed in detail.

Dynamic recrystallization of copper polycrystals with different purities

Abstract The hot deformation behaviour and associated structural changes were studied by means of compression tests and metallographic observations of polycrystalline copper with three different purities of 99.99 (4N), 99.9999 (6N) and 99.99999 (7N) mass percent of copper. The samples were strained up to 1.2 at temperatures ranged from 523 to 773 K under various true strain rates of 10−4–10−1 s−1. The deformation behaviours are characterised by multiple peaks or a single peak flow, followed by a steady state flow stress and a new grain evolution by dynamic recrystallization (DRX) at high strains. The steady state stresses and their temperature and strain rate dependencies for 6N and 7NCu are almost the same, but clearly lower and higher than those for 4NCu, respectively. The activation energy for hot deformation is affected by materials purity, i.e. about 210 kJ mol−1 for 6N or 7NCu and 245 kJ mol−1 for 4NCu. On the other hand, the DRX grain sizes, represented by a unique function of steady state flow stress, strongly depend on the solute concentration for all of samples studied. The effect of impurity atoms on the deformation and DRX grains is discussed in such high purity materials.

Ultrafine grain development in copper during multidirectional forging at 195 K

Strain-induced evolution of ultrafine grains in pure copper was studied in multidirectional forging (MDF) at 195 K. The stress–strain behaviour was characterized by rapid strain hardening during early processing and the rate of strain hardening gradually decreased with straining, leading to an apparent steady-state flow at large cumulative strains of more than 5. The structural changes were associated with the development of high-density microshear bands crossed by MDF. The new fine grains 0.16 µm in size, which was smaller than the subgrain size evolved during early deformation, were evolved primarily at microshear band intersections, and then the new fine grains filled out the whole sample as the number of microshear band intersections increased at large strains. This is essentially similar to continuous dynamic recrystallization. The size of new grains can be expressed by a power law function of flow stress with a grain size exponent of about –0.3. The kinetics of the strain-induced grain evolution is analyzed and the mechanisms are discussed.

Strain-induced submicrocrystalline grains developed in austenitic stainless steel under severe warm deformation

Strain-induced grain evolution in a 304 type austenitic stainless steel has been studied in multiple compression with the loading direction being changed in each pass. The tests were carried out to total strains above 6 at 873 K (0.5 T m) at a strain rate of about 10-3 s-1. Multiple deformation promotes the rapid formation of many mutually crossing subboundaries because various slip systems operate from pass to pass. The gradual rise in misorientations across dislocation subboundaries with increasing strain finally leads to the evolution of very fine grains with large-angle boundaries. It is concluded that a new grained structure can result from a kind of continuous reaction during deformation, namely continuous dynamic recrystallization. Such deformation-induced grains are characterized by relatively low densities of dislocations, and considerable lattice curvatures developed in their interiors. The latter observations suggest that high elastic distortions are developed in the grain interiors and so such strain-induced grain structures are in a non-equilibrium state.

Microstructure and deformation behaviour of submicrocrystalline 304 stainless steel produced by severe plastic deformation

Abstract The deformation behaviour and structural changes in a 304-type stainless steel were studied in uniaxial compression at temperatures of 873–1073 K (0.5–0.6 T m ) and under strain rates from 10 −4 to 10 −2 s −1 . The starting material, with an initial grain size of ≈0.3 μm, was produced by multiple warm deformation passes, changing of the loading direction from pass to pass. At high temperature, the resulting fine-grained steel has a low dislocation density and steady-state flow stresses lower than that of an annealed coarse-grained steel. Moreover, the strain rate sensitivity of the fine-grained material reaches high values, up to 0.3, which clearly differs from that of ≈0.1–0.2 for the coarse-grained steel. At room temperature, the initially fine-grained steel has higher hardness after warm deformation than the conventional steel.

The crystallography of M23C6carbides in a martensitic 9% Cr steel after tempering, aging and creep

The orientation relationships of M23C6 carbides in a martensitic creep resistant steel were studied. Almost all M23C6 carbides were located at (sub)grain boundaries after tempering and aging. The carbides were slightly elongated along the boundary planes and obeyed the Kurdjumov-Sachs, Nishiyama-Wassermann, and Pitsch orientation relationships as well as two new orientation relationships, that is and , with α-Fe matrix. On the other hand, the M23C6 particles in the neck portion of crept specimen lost their orientation relationships with α-Fe.