Materials Science Forum Vols. 715-716

Abstract: The paper discusses the microstructural evolution during dynamic recrystallisation (DRX) of a titanium-modified austenitic stainless steel (alloy D9). Isothermal hot compression tests were conducted in a Gleeble thermo-mechanical simulator in the temperature range 1173-1373K to various strains at a constant strain rate of 0.1 and 1 s^-1. The extent of DRX increased with increase in strain and temperature. Nucleation of new DRX grains was found to occur by bulging of parent grain boundary. A continuous sub-grain rotation around the original grain boundaries, which would lead to the formation of DRX nucleus in sub-grain structures, could not be confirmed from the present study. Fractions of Σ3 boundaries increased almost linearly with increase in area fraction of DRX. The generation of this Σ3 boundary was accounted for in the formation of annealing twins during DRX. The possible role of annealing twins on DRX in alloy D9 is also discussed.

Abstract: The selective abnormal grain growth (AGG) of Goss grains in Fe-3%Si steel was investigated using a parallel Monte-Carlo (MC) simulation based on the new concept of sub-boundary enhanced solid-state wetting. Goss grains with low angle sub-boundaries will induce solid-state wetting against matrix grains with a moderate variation in grain boundary energy. Three-dimensional MC simulations of microstructure evolution with textures and grain boundary distributions matched to experimental data is using in this study.

Abstract: Dynamic recrystallization mechanisms have been studied after 5182 aluminum Friction Stir Spot Welding (FSSW) and dissimilar friction stir spot welding of 6016 aluminum alloy to IF-steel using EBSD measurements. Moreover, welds have been ice quenched after welding to state on the post-dynamic microstructure evolution after the tool removal. For the Al/Al welds, fine recrystallized grains of the stir zone result from a continuous dynamically recrystallization mechanism followed by a post-dynamic recovery that reduces the low angle boundary fraction in the periphery of the pin. As far as the dissimilar Al/Fe welds are concerned, steel grains of the base metal were fragmented into sub-grains in the thermomechanically affected zone. Nevertheless, recrystallized grains of the stirred zone were about three times larger than these sub-grains. In this case, the continuously recrystallized grains undergo a post-dynamic grain-growth during friction stir welding cooling. In the upper aluminum sheet, the recrystallization mechanisms are the same as in the Al/Al welds.

Abstract: The effect of cold rolling reduction on shear band formation and crystal orientation within shear bands and annealing texture were investigated in Fe-3%Si {111}<112> single crystals. Several types of shear bands were observed with different angles to rolling direction, dependent on rolling reduction. As for shear band formation, those with smaller angles were formed earlier and those with larger angles were formed later. Regarding crystal orientation along shear bands after rolling reduction, orientation distribution from the initial became large in accordance with reduction and even exceeded Goss orientation when rolling reduction became larger than 40%. After annealing, however, recrystallized grains along shear bands were mainly Goss grains regardless of reduction. The speculated reason for the dominance of Goss after annealing is that Goss subgrains with less density of dislocations were surrounded by largely deformed areas.

Abstract: The dynamic recrystallisation (DRX) behaviour of magnesium AZ31 is investigated using a plane strain compression (PSC) testing machine at 450°C. The variables included strain rate, double hit including intermittent anneal and double hits with different strain rate at each hit. The alloy shows higher peak stress and strain with increasing strain rates. Predominant basal texture with different intensities are observed at different strain rates. The annealing treatment between double tests leads to strong basal texture. Reversal of strain rate during double hit results in similar flow curves. This shows that in AZ31 alloy, DRX mechanism is independent of the initial microstructure and only depends on the test condition viz. temperature, strain rate and total equivalent strain.

Abstract: Multi-layered materials have been made from Cu-Fe with approximately equal volume fractions using the Accumulated Roll Bonding (ARB) technique with less than 1 μm thickness of the individual layers. The so-obtained multi-layers have been subjected to deformation by cold rolling to 25, 50, 75, 87 and 93% reduction in thickness. A detailed characterization has been carried out using X-ray diffraction (line profile analysis and texture measurement) and electron (scanning and transmission) microscopy. It has been found that Fe layers are disintegrated whereas Cu retains its continuity within a layer. Microstructural Characterization from X-Ray Line profile Analysis (XRDLPA) through Variance Method reveals that large amount of strain is initially carried by Cu layers during deformation. In the Cu-Fe layer, the texture is comparatively weaker in Cu layer and strong in Fe layers. Brass Component increases up to 75% reduction and then decreases, while the ratio of Cu/S and Bs/S remains almost constant through out the deformation. After 50% reduction, dynamic recovery is predominant as indicated by the increase in the amount of low angle grain boundaries and decrease in dislocation density. The presence of R component indicates continuous dynamic recovery and recrystallization (CDRR) at the advanced stage of deformation.

Abstract: There has recently been renewed interest in magnesium alloys from both the automotive and aerospace industries. Due to the low density (approximately 35% lower than aluminium) and high specific strength, these alloys can introduce significant weight savings, and consequently fuel savings, to both vehicles and aircraft. Elektron™ 675 is a new alloy based on the magnesium-yttrium-gadolinium ternary system, developed by Magnesium Elektron Ltd. for wrought applications. Elektron™ 675 has superior mechanical properties relative to the current commercially available wrought alloys AZ31B, WE43, and ZK60.

Abstract: A study has been made of the evolution of the microstructures and textures in three kinds of low-carbon steel sheets (MAFE, BH and IF) having well developed <111>//ND texture that were rolled by low reductions and annealed at 780 °C in Ar atmosphere. The steel sheets developed different microstructures and textures, even though their initial textures and thermomechanical treatments were similar. MAFE steel showed an unusual behavior that grains with high Taylor factors survived and grew very rapidly. This unusual behavior and the differences in microstructure and texture have been discussed.

Abstract: Grain boundary properties depend on both composition and structure. To test the relative contributions of composition and structure to the grain boundary energy, we calculated the energy of 388 grain boundaries in four elemental, fcc metals: Ni, Al, Au and Cu. We constructed atomic-scale bicrystals of each boundary and subjected them to a rigorous energy minimization process to determine the lowest energy structure. Typically, several thousand boundary configurations were examined for each boundary in each element.

Abstract: The substructure and crystallographic texture characteristics of both the deformed matrix and dynamically recrystallized (DRX) grains were investigated in a Ni-30%Fe austenitic model alloy subjected to hot torsion. Deformation was performed at a temperature of 1000°C using strain rates of 1, 0.1 and 0.01 s^-1, which produced a range of DRX grain sizes. Electron back-scattered diffraction and transmission electron microscopy were employed in the investigation. Both the deformed matrix and DRX grains revealed the texture components expected for simple shear deformation by crystallographic slip. The texture of DRX grains was dominated by low Taylor factor components as a result of their lower consumption rate during growth of these grains. There was a marked difference in the substructure characteristics between the deformed matrix and DRX grains regardless of the grain size and orientation. The deformed matrix substructure was largely characterized by organized, banded subgrain arrangements with alternating misorientations. By contrast, the substructure of DRX grains was generally more random in character and displayed complex, more equiaxed subgrain/cell arrangements characterized by local accumulation of misorientations. Based on the experimental observations, a mechanism of the distinct substructure development within DRX grains has been proposed.