Étienne Martin

Texture weakening and static recrystallization in rolled Mg–2.9Y and Mg–2.9Zn solid solution alloys

The microstructure and texture evolution in the Mg–2.9Y and Mg–2.9Zn solid solution alloys were investigated following rolling and subsequent isothermal annealing. The Mg–2.9Y alloy was hot rolled, and the Mg–2.9Zn alloy was rolled at room temperature in order to evaluate the possibility of attaining texture weakening by the suppression of dynamic recrystallization (DRX) and promotion of static recrystallization (SRX). It was found that texture weakening can be attained in Mg even in the absence of Y when there is no DRX, and SRX occurs during annealing. In solid solution, Y suppresses DRX during hot rolling, and retards the kinetics of SRX and grain coarsening in Mg. In the two alloys, the orientation of statically recrystallized grains at bands/twins (TSRX grains) is close to that of double and compression twins, exhibiting a much more evenly distributed and slightly wider orientation than that of basal parent grains and twins. In both Mg–2.9Zn and Mg–2.9Y alloys, a continuous texture weakening is observed with the progress of SRX, which results in a bimodal microstructure consisting of small TSRX grains and larger ones. With the increase in grain size during coarsening, the maximum intensity of basal pole figures rises linearly, with the slope of the lines being nearly identical in the two alloys. This texture strengthening was ascribed to the consumption of small TSRX grains by larger ones.

Evaluation of strain rate sensitivity by constant load nanoindentation

Constant load measurements by nanoindentation offer the potential for measuring strain rate sensitivity from individual features and defects on a submicron scale. However, recent reports reveal a conflicting load dependence (both increasing and decreasing strain rate sensitivity with load) which has yet to be fully explained. In this study, constant load measurements on five materials (Zn, Al, Cu, Ti, and SiO2) were conducted over a range of peak loads, and then compared with both constant strain rate results and conventional values in the literature. The load dependence was found to be caused by the increasing contribution of drift errors throughout the test. A proposed framework, involving higher loads, shorter hold and loading times, and a physically sound fitting method, was found to produce unambiguous results free from load dependencies, with improved correlations to conventional values and reduced standard deviations.

Effect of twinning on recrystallisation textures in deformed magnesium alloy AZ31

The evolution of texture during the tensile deformation of Mg–3.1%Al–1.05%Zn was studied by microtexture and macrotexture techniques. The deformation texture is shown to be the direct consequence of the variant selection that occurs during primary (contraction) and secondary (extension) twinning. During subsequent annealing, the secondary twins have a higher rate of nucleation than the primary twins and expanded within the latter rather than in the matrix grains. As a result, the primary twins did not contribute significantly to the annealing texture.

The combined effect of static recrystallization and twinning on texture in magnesium alloys AM30 and AZ31

Abstract The potential for decreasing the texture intensity generated during the bulk deformation of Mg alloys was investigated using a combination of contraction twinning, double (secondary) twinning and static recrystallization. A large number of twins was induced by tensile deformation at room temperature. Their volume fraction and the variants selected during straining were found to be largely responsible for the changes evident in the deformation texture. Recrystallization of the twins generated a fine-grained microstructure, although no growth into the matrix grains was observed. In this way, annealing of the deformed samples did not lead to significant further texture changes.

Deformation Structures and Recrystallization in Magnesium Alloys

Magnesium, being a hexagonal close-packed metal, has limited ductility and poor formability at room temperature. These characteristics hinder the production of magnesium wrought products, which represent a mere 1% of the total annual magnesium usage (Polmear, 1995). Formability can be significantly increased when additional slip systems are thermally activated. As such, hot deformation is extensively used to produce wrought magnesium products such as sheet metal or extruded bars and tubes (Barnett, 2001). Besides ductility limitations, Mg alloys also suffer from strong mechanical anisotropy (Kleiner & Uggowitzer, 2004). Depending on the crystallographic texture, different combinations of deformation systems may be activated. Since the deformation modes available for displacements parallel to the c-axis are limited, the crystallographic orientation has a strong influence on the deformation of Mg. In many metals, dynamic recrystallization leads to the randomization of initial textures and can therefore be of practical interest with regards to subsequent forming (Humphreys & Hatherly, 2004). According to the literature, recrystallization is not usually accompanied by sharp changes in the crystallographic texture. Yi et al. (Yi et al., 2006) have reported, for example, that the ODF (orientation distribution function) intensities in AZ31 were similar in the large deformed and small dynamically recrystallized (DRX) grains. (Nevertheless, they did observe a slight shift in the location of the main texture component.) Other investigators have reported that most of the newly recrystallised grains in Mg alloys have orientations similar to those of the matrix grains, but with slightly weaker intensities (Backx et al., 2004; del Valle et al., 2005; Jager et al., 2006). Such recrystallization appears to promote 30o rotations that preserve the basal texture or, at the very least, delay its decomposition (Gottstein & Al Samman, 2005; Beausir et al., 2007). However, these investigations were mostly performed on highly deformed samples, so that it is difficult to separate the influence of the different recrystallization and deformation mechanisms on the texture. In magnesium, basal glide invariably leads to a basal texture (i.e. c-axis aligned with the compression axis (CA)) or perpendicular to the tensile axis (TA)). However, different twinning and slip systems can also be activated under different deformation conditions (i.e. temperature, strain rate, texture, etc.). It is well known that the deformed state has a strong influence on recrystallization, and several studies have linked the different deformation

Work hardening and kinetics of dynamic recrystallization in hot deformed austenite

The work hardening behaviors of 4 steels deformed under dynamic recrystallization (DRX) conditions were analysed. The h and r parameters necessary to describe the work hardening or dynamic recovery (DRV) curves in the absence of DRX were determined from the experimental flow curves. The fractional softening X attributable to DRX, defined as the difference between the calculated DRV and experimental DRX curves, was used to derive the Avrami kinetics of DRX. Some conclusions are drawn about the effects of deformation temperature, strain rate and steel composition on the kinetics of DRX.