Materials Science Forum Vols. 715-716

Abstract: The present work provides a summary of the recent findings obtained from the experimental investigation of the grain structure, crystallographic texture and dislocation substructure evolution in an austenitic Ni-30%Fe model alloy during dynamic recrystallization (DRX) and post-dynamic annealing. It has been found that the DRX texture characteristics become increasingly dominated by the low Taylor factor grains during DRX development, which presumably results from the preferred nucleation and lower consumption rates of these grains. The substructure of DRX grains is random in character and displays complex, hierarchical subgrain/cell arrangements characterized by accumulation of misorientations across significant distances. The stored energy within DRX grains appears to be principally consistent with the corresponding Taylor factor values. The changes observed within the fully dynamically recrystallized microstructure during post-dynamic annealing have provided a basis to suggest a novel mechanism of metadynamic softening for the current experimental conditions. It is proposed that the initial softening stage involves rapid growth of the dynamically formed nuclei and migration of the mobile boundaries. The sub-boundaries within DRX grains progressively disintegrate through dislocation climb and dislocation annihilation, which ultimately leads to the formation of dislocation-free grains, and the grain boundary migration gradually becomes slower. As a result, the DRX texture largely remains preserved throughout the annealing process.

Abstract: Qualitative and quantitative texture changes are investigated in a severely deformed 5xxx series aluminum alloy. Unusual recrystallization textures are observed after a rolling strain of more than 97% in thickness reduction. The influence of both strain amplitude and strain mode on the development of the deformation and recrystallization textures is discussed based on experimental data and results of crystal plasticity calculations.

Abstract: The structural changes in a 304-type austenitic stainless steel during large strain cold rolling and subsequent annealing were studied. The severe deformation resulted in the development of highly elongated grains/subgrains aligned along the rolling axis. The transverse grain/subgrain size rapidly decreased to its minimal value of about 50 nm at relatively small strains of ~1 and then hardly changed upon following deformation. Such a structural response on cold working was associated with multiple twinning resulting in fast grain subdivision. The processing was accompanied by a partial martensitic transformation resulting in a decrease of austenite volume fraction to about 0.35 after straining to ε = 4.0. Isochronal annealing for 30 min was characterised by a gradual coarsening of grains, the average size of which increased to about 200 nm after heating to 800°C. The high elongation of ferrite grains facilitated simultaneous homogeneous nucleation of austenite grains throughout the matrix upon heating; and, therefore, promoted the development of ultrafine grained structure with the size of structural elements well below 1 micron.

Abstract: Mesoscale experiment and simulation permit harvesting information about both geometric featuresand texture in material microstructures. The grain boundary character distribution (GBCD) is an em-pirical distribution of the relative length (in 2D) or area (in 3D) of interface with a given lattice misori-entation and grain boundary normal. During the growth process, an initially random texture distribu-tion reaches a steady state that is strongly correlated to the interfacial energy density [9]. In simulation,it is found that if the given energy depends only on lattice misorientation, then the steady state GBCDand the energy are related by a Boltzmann distribution. This is among the simplest non-random dis-tributions, corresponding to independent trials with respect to the energy. Why does such a simpledistribution arise from such a complex system.

Abstract: Development of bulk ultra-fine grained (UFG) materials by severe plastic deformation to attain improved mechanical properties is becoming more attractive and extensively studied nowadays. Equal channel angular extrusion (ECAE) is one of technique used effectively for obtaining bulk UFG materials. Novelty of this technique is one can build up significant amount of plastic strain by increasing the number of passes without much dimensional change. In present investigation dynamic recrystallization at deformation zones around the non-shearable second phase particles in Al-Li based alloy processed by ECAE is reported. Transmission electron microscopy technique involving imaging the regions of such deformation zones with different specimen tilt conditions is used. It is shown that the dynamic recrystallisation occurring in the proximities of second phase particles during the deformation at room temperature, leads to very fine grained microstructure. Observation of multiple active nucleation sites around even sub-micrometer sized non-deformable particles in the as-processed material indicates that the system exhibits efficiency >1 based on the concept of particle stimulated nucleation (PSN). Crystallites of ultra-fine/nanocrystalline size ranges are formed in the deformation zones around the non-deformable particles during deformation itself. Effect of short term post deformation annealing to understand the recovery and recrystallization was undertaken. Based on these results effect of optimal post deformation heat treatment conditions on the thermal stability of the microstructures is emphasized. It is suggested that with significant fraction of non-shearable particles it might be possible to get grain size in the nanocrystalline or ultra-fine range with relatively low effective strain levels using ECAE.

Abstract: Non-isothermal austenite grain growth kinetics has been studied in a microalloyed linepipe steel with complex precipitates containing Ti, Nb and/or Mo. The goal of these experimental studies is to provide the basis for the development of a grain growth model to predict the austenite grain size evolution in the weld heat affected zone (HAZ). Detailed electron microscopic investigations of the as received steel proved the presence of Ti-rich, Nb-rich and Mo-rich precipitates. The steel was subjected to austenitizing heat treatments to selected peak temperatures of 950, 1150 and 1350 °C at heating rates of 10, 100 and 1000 °C/s, respectively. Thermal cycles have been found to have a strong effect on the austenite grain size. Austenite grain sizes increase with peak temperature and decreasing heating rate. However, the increase in heating rate from 100 to 1000 °C/s has a negligible effect on the austenite grain size. The observed austenite grain growth kinetics can be explained taking into account the potential dissolution of Nb-rich precipitates.

Abstract: Deformation structures and annealing behaviour have been analysed in the centre layer of two AA1050 samples cold-rolled to von Mises strains of 3.6 and 6.4. During annealing at 270-300°C structural coarsening and discontinuous recrystallization occurred in both samples. In the coarsened microstructure, the fraction of high angle boundaries was slightly lower than that in the as-rolled conditions. Recrystallization textures of both samples contained significant fractions of the rolling texture components. The fraction of the retained rolling texture was however greater in the strain-6.4 sample. The {001}<310> and {110}<566> components were also pronounced in this sample. The size of recrystallized grains having orientations of the rolling texture was considerably smaller than the size of grains having other crystallographic orientations. This may be attributed to orientation pinning that hinders growth of grains with orientations of the rolling texture.

Abstract: The present paper investigates the potential application of Strain Induced Boundary Migration mechanism on the two different types of surface textures developed after α-γ-α phase transformation annealing, one with preferred cube and Goss orientation at the surface and the other with random surface texture without preferred orientations. It has been demonstrated that these surface texture components grow in across the thickness of the sheet after an appropriate combinations of a critical amount of rolling reductions and an annealing treatment at the recrystallisation temperature.

Abstract: Microstructure evolution during annealing of a Ni-20%Cr alloy subjected to high-pressure torsion (HPT) at ambient temperature was examined. It was shown that discontinuous static recrystallization (DSRX) occurs in non-uniform manner under subsequent annealing in the alloy strained to ε<4. The material strained to ε6 or higher exhibits continuous grain growth (GG) under subsequent annealing. It is attributed to the fact that HPT led to the formation of nanoscale grains with an average size of 50 nm. Increasing fraction of these grains with strain leads to transition from DSRX to continuous GG under subsequent annealing. It was found that the main feature of recrystallization behavior of the Ni-20%Cr alloy subjected to HPT is simultaneous occurrence of short-range ordering and recrystallization processes under annealing conditions. As a result, despite the formation of recrystallized structure with an average grain size of 340 nm after annealing at 600°C (0.52 T_m), the material exhibits very high microhardness of 3.6 GPa.

Abstract: Employing x-ray diffractometry and electron microscopy, we have investigated thermally induced microstructural evolution in ball-milled nanocrystalline Fe. At low annealing temperatures, the early-stage growth of the area-weighted and volume-weighted average grain sizes deviates strongly from the parabolic behavior expected for normal grain growth. Analysis of the ratio of these two averages indicates that the width of the grain-size distribution changes with time. This result is more consistent with the occurrence of a transient stage of abnormal grain growth than with a grain-size-dependent change in the rate-limiting mechanism for grain-boundary migration.