Rustam Kaibyshev

Correlation of plastic deformation and dynamic recrystallization in magnesium alloy ZK60

Abstract The mechanisms of plastic deformation and dynamic recrystallization (DRX) in a Mg–5.8% Zn–0.65% Zr alloy were studied by compression tests at temperatures between 423 and 723 K and at strain rates ranging from 10−5 to 10−1 s−1. It was shown that the mechanisms of DRX depended on the operating deformation mechanisms which changed with temperature. Low-temperature DRX (LTDRX below 473 K) was associated with the operation of twinning, basal slip and (a+c) dislocation glide. In the intermediate temperature range (473–523 K) continuous DRX (CDRX) was observed and associated with extensive cross-slip due to the Friedel–Escaig mechanism. At temperatures ranging from 573 to 723 K both bulging of original grain boundaries and subgrain growth were the operating DRX mechanisms and controlled by dislocation climb.

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.

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.

Grain refinement in coarse-grained 7475 Al alloy during severe hot forging

Grain refinement taking place in a coarse-grained 7475 Al alloy was studied in multidirectional compression at 490°C and at a strain rate of 3 × 10−4 s−1. The integrated flow curve displays significant work softening just after yielding and an apparent steady-state plastic flow at high strains. The structural changes are characterized by the development of deformation or microshear bands in coarse-grain interiors, followed by homogeneous evolution of new grains at high strains. The new grains are considered to be developed by a kind of continuous reaction through grain fragmentation that is similar to continuous dynamic recrystallization (cDRX). The mechanism of fine grain production and the factors controlling grain refinement during hot multidirectional deformation are discussed in detail.

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.

On the possibility of producing a nanocrystalline structure in magnesium and magnesium alloys

Abstract The mechanical properties and structural evolution of pure magnesium and Mg6%Zn0,65%Zr alloy during cold plastic deformation has been analysed. It has been shown that dynamic recrystallization (DRX) occurs in both the materials. Specific features of the recrystallized structure are considered. The influence of chemical and phase composition of the material on the formed structure is analysed as well. An attempt is made to determine the conditions necessary for obtaining a nanoscale grain size in the material.

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.

Effect of Grain Refinement on Jerky Flow in an Al-Mg-Sc Alloy

The influence of microstructure on the manifestations of the Portevin–Le Chatelier (PLC) effect was studied in an Al-Mg-Sc alloy with unrecrystallized, partially recrystallized, and fully recrystallized grain structures. It was found that the extensive grain refinement promotes plastic instability: the temperature–strain rate domain of the PLC effect becomes wider and the critical strain for the onset of serrations decreases. Besides, the amplitude of regular stress serrations observed at room temperature and an intermediate strain rate increases several times, indicating a strong increase of the contribution of solute solution hardening to the overall strength. Moreover, the grain refinement affects the usual sequence of the characteristic types of stress serrations, which characterize the dynamical mechanisms governing a highly heterogeneous unstable plastic flow. Finally, it reduces the strain localization and surface roughness and diminishes the difference between the surface markings detected in the necked area and in the region of uniform elongation.

Σ3 CSL boundary distributions in an austenitic stainless steel subjected to multidirectional forging followed by annealing

The effect of processing and annealing temperatures on the grain boundary characters in the ultrafine-grained structure of a 304-type austenitic stainless steel was studied. An S304H steel was subjected to multidirectional forging (MDF) at 500–800°C to total strains of ~4, followed by annealing at 800–1,000°C for 30 min. The MDF resulted in the formation of ultrafine-grained microstructures with mean grain sizes of 0.28–0.85 μm depending on the processing temperature. The annealing behaviour of the ultrafine-grained steel was characterized by the development of continuous post-dynamic recrystallization including a rapid recovery followed by a gradual grain growth. The post-dynamically recrystallized grain size depended on both the deformation temperature and the annealing temperature. The recrystallization kinetics was reduced with an increase in the temperature of the preceding deformation. The grain growth during post-dynamic recrystallization was accompanied by an increase in the fraction of Σ3n CSL boundaries, which was defined by a relative change in the grain size, i.e. a ratio of the annealed grain size to that evolved by preceding warm working (D/D0). The fraction of Σ3n CSL boundaries sharply rose to approximately 0.5 in the range of D/D0 from 1 to 5, which can be considered as early stage of continuous post-dynamic recrystallization. Then, the rate of increase in the fraction of Σ3n CSL boundaries slowed down significantly in the range of D/D0 > 5. A fivefold increase in the grain size by annealing is a necessary condition to obtain approximately 50% Σ3n CSL boundaries in the recrystallized microstructure.

Effect of liquid phase on superplastic behavior of a modified 6061 aluminum alloy

Abstract It is shown that a 0.15%Zr+0.7%Cu––modified 6061 aluminum alloy with an initial grain size of about 11 μm exhibits a maximum elongation-to-failure of 1300% at 590 °C and an initial strain rate of 2.8×10−4 s−1 in a partially melted state.