The AZ31 Mg alloy was hot torsion tested from 180 to 450 °C and from 0.01 to 1.0 s −1 . The flow curves showed a peak and a decline towards a steady-state regime which were lower as temperature T rose and strain rate declined; however, the fracture strain increased to about 1.9 at 0.1 s −1 . In transmission electron microscopy, twins were observed from 180 to 360 °C (in declining numbers). At low T , they had sharp walls and contrasting transverse bands; while the matrix showed indistinct linear streaks. As T rose, the twin bands developed cells with tangled walls and finally subgrains (∼360 °C), while the twin walls became tangles of dislocations and finally serrated boundaries. The matrix developed elongated dislocation walls and subgrains at higher T . The twin intersections at 180 and 240 °C consisted of diamond-shaped cells with a duplex set of orientations but at 300 and 360 °C, these had developed into polygonal cells with high misorientations in dark field. The first very small dynamically recrystallized grains were observed at these intersections, slightly larger than the cells. At 360–450 °C, as observed by optical microscopy, small dynamically recrystallized grains formed at the original grain boundaries, probably related to multiple slip. Since twinning and other features described at low T were also found at high ones, albeit with decreasing frequency, the microstructures showed severe heterogeneity which accounted for the limited ductility.

Twinning, dynamic recovery and recrystallization in hot worked Mg–Al–Zn alloy

Unique high-temperature deformation dominated by grain boundary sliding in heterogeneous necklace structure formed by dynamic recrystallization in HfNbTaTiZr BCC refractory high entropy alloy

The microstructure development of the near-α titanium alloy IMI834 displaying an initial bimodal α + β microstructure has been characterized with respect to dynamic and static β recrystallization, through quantitative metallography. The investigation was supplemented by interrupted compression testing, to evaluate the applicability of fractional softening in the determination of static recrystallization kinetics for titanium alloys. Micrographs are presented that indicate that dynamic recrystallization occurred under test conditions (temperatures of 975 °C, 1000 °C, 1025 °C, 1060 °C, and 1100 °C and strain rates of 0.01, 0.1, and 1 s−1), although complete grain refinement and homogeneity were only achieved following static recrystallization. Measurement obtained via image analysis revealed recrystallization kinetics that increased with temperature, for single-phase β predeformation microstructures (1060 °C to 1100 °C). However, bimodal α + β microstructures (1000 °C to 1025 °C) displayed more rapid recrystallization rates with decreasing temperature, which was attributed to a corresponding increase in α phase fraction. This increase yielded a more refined initial β grain size and an increase in favorable nucleation sites. In two-phase α + β microstructures (975 °C) with secondary α in the β matrix, recrystallized grains could not be directly observed with optical microscopy. However, interrupted compression testing indicated that static recrystallization may be comparable to that observed at 1000 °C. At all temperatures, an increasing strain rate accelerated recrystallization kinetics.

Recrystallization during Thermomechanical Processing of IMI834

Dynamic softening and substructural changes during hot deformation of a ferritic Fe-26Cr stainless steel were studied. The flow stress increased to reach a steady state in all the cases and the steady-state stress decreased with decreasing Z, the Zener-Hollomon parameter. A constant subgrain size was observed to correspond to the steady-state flow and the steady-state subgrain size increased with decreasing Z. Substructure examinations revealed that elongated, pancake-shaped subgrains formed in the early stage of deformation. Straight sub-boundaries and equiaxed subgrains developed progressively with strain, leading eventually to a stable substructure at strains greater than 0.7. During deformation at 1100 °C, dynamic recrystallization occurred by the migration and coalescence of subboundaries. Dynamic recovery dominated during deformation at 900 °C, resulting in the formation of fine equiaxed subgrains. Based on microstructural observations, the process of substructural changes during hot deformation was described by a schematic diagram.

Substructural changes during hot deformation of an Fe-26Cr ferritic stainless steel

By means of the measurement of the creep curve and the observation of SEM and transmission electron microscope (TEM), an investigation has been made into the microstructure evolution and deformation features of AZ31 Mg-alloy during high temperature creep. Results show that the deformation features of the alloy in the primary stage of creep are that significant amount of dislocation slips are activated on basal and non-basal planes, then these ones are concentrated into the dislocation cells or walls as creep goes on. At the same time, twinning occurs as an additional deformation mechanism in the role of the compatibility stress. During steady state creep, the dislocation cells are transformed into the subgrains, then, the protrusion and coalition of the sub-boundaries results in the occurrence of dynamic recovery (DRV). After the dynamic recrystallization (DRX), the multiple slips in the grain interiors are considered to be the main deformed mechanism in the later stage of the steady state creep. An obvious feature of creep entering the tertiary stage is that the cracks appear on the locations of the triple junction. As creep continues, the cracks are viscous expanded along the grain boundaries; this is taken for being the fracture mechanism of the alloy crept to failure. The multiple slips in the grain interiors and the cracks expanded viscous along the grain boundary occur in whole of specimens, that, together with the twins and dynamic recrystallization, is responsible for the rapid increase of the strain rate in the later stage during creep.

Microstructure evolution and deformation features of AZ31 Mg-alloy during creep

The body centered cubic (bcc) metals undergo a high level of dynamic recovery during elevated temperature straining so that the stress increases monotonically to a steady-state value σs. The strain rate and σs are related by means of the power, the exponential, or the sinh law with an Arrhenius temperature relationship. The activation energy for a iron has values of 250–280 kJ/mol, whereas for β titanium and β zirconium it is in the range 134–184 kJ/mol. The structure developed during hot working consists of elongated grains containing subgrains of dimension inversely proportional to σs. In warm working of α iron (limited to below 0.66T
 m), the textures are similar to those for cold working. In working β titanium and β zirconium which is limited to above 0.6T
 m except in β stabilized alloys or as matrix in α+ β processing, the bcc textures transform into α textures. The α iron relies principally on substructure strengthening in association with carbides. The β phases can be thermomechanically processed to provide equiaxed or lamellar a in a variety of dimensions and combinations, with or without substructure. Hot working of the bcc refractory metal alloys, principally molybdenum, is similar to hot working of α iron.

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David L Bourell

Papers

Thermomechanical processing of iron, titanium, and zirconium alloys in the BCC structure