Search

The present work deals with emphasizing the role of substructure development in fragmentation of the cast structure in Mg-Sn-0.3Li-3Zn alloy during thermomechanical processing. In this respect, a new micro-mechanism and a conceptual model were proposed for dendrite fragmentation, which was supported by 3D-tomography, electron back scattered diffraction, and angle selective back-scattered imaging analyses. Balancing of the dihedral angle of the non-equilibrium triple junctions led to the formation of grooves at the dendrite arms, followed by penetration of α-Mg matrix through the grooves and finally resulted in fragmentation of the specified dendrite arm. In fact, despite the conventional expectation regarding the high contribution of brittle fracture and shearing in fragmentation of the dendrites at low temperature regime, it was observed that the substructure development in a medium-to-high stacking fault energy material greatly participated in thermally-activated disintegration of the dendritic cast-structure.

This study offers the effect of dendrite orientation selection on the defamation mechanisms and their subsequent effects on the homogenization of the dendritic structure during hot deformation processes. The results indicated that the solidification macro-texture has controlled by the dendrite growth pattern, which has effectively influenced the dominant deformation mechanisms (slip/twin activity). Interestingly, the boundary of the exhausted twins has been preferred for solute diffusion and has effectively contributed to the homogenization of the dendritic structure during hot deformation processes.

This study was conducted to examine the correlation between the microstructure and textural evolution and isothermal grain growth behavior of a friction stir processed Mg-Y-RE magnesium alloy. To this end, the friction stir processing (FSP) was applied under the constant rotational speed of 630 rpm and two traverse speeds of 90 mm/min and 30 mm/min. The continuous dynamic recrystallization (CDRX) was identified as the dominant restoration mechanism. The strong preference of [0001]-Taylor axes in CDRX grains, suggested the significant role of prismatic 〈a〉 slip on the lattice rotation during CDRX. The new grains with nearly 〈11-20〉//PD orientations and 30°[0001] grain boundaries (GBs) preferentially nucleated in grains with nearly 〈10-10〉//PD orientations via CDRX. Consequently, the typical {0001}〈uvtw〉 texture component of FSP-Mg alloys with the single orientation of 〈10-10〉//PD was changed to a double orientation of 〈10-10〉-〈11-20〉//PD, due to the further development of recrystallization texture in the lower traverse speed. Further, the isothermal grain growth study of the processed materials revealed the growth preference of grains with specific orientations, which have emerged during recrystallization. This was attributed to the formation of high mobile Ʃ13a GBs, as well as their orientations with respect to the shear stress which rendering them favorable for preferred growth and reducing the total energy of system. It was generally observed that the thermal stability of FSPed microstructures during subsequent annealing was depended upon the processing conditions due to the different extent of microstructure and textural developments. These findings create a processing window where microstructure instability or i.e. abnormal grain growth (AGG) could be prevented via manipulating processing parameters.

The recrystallization behavior of a well-known non-equiatomic high entropy alloy by the nominal chemical composition of Fe–27Mn–10Cr–10Co, at.% was investigated. The compression tests were performed in the temperature range of 400–1000 °C by the interval of 200 °C under the strain rate of 0.001 s−1. Initial microstructure consist of face centered cubic and hexagonal phases by the average grain size of 90, and 11 μm, respectively. Interestingly, the occurrence of dynamic recrystallization was observed during compression testing at warm temperature regime (400 and 600 °C). The appreciable grain refinement, the corresponding flow curve softening, and the detailed analysis of the hardening curves according to the Poliak and Jonas method, verified the occurrence of dynamic recrystallization at such warm temperature regime. The required stored energy was provided through the shear reversion of thermally induced martensite before any straining. In contrast, the softening mechanisms was not strong enough to compensate the strain hardening of the single phase microstructure. This further approved the significant role of the martensite phase to trigger the occurrence of dynamic recrystallization. The intensified substructure development and the random microtexture of the recrystallized grains well indicate the contribution of continuous recrystallization mechanism. The microstructure was also considerably refined (by the mean grain size of ∼1.7 μm) through compressive deformation at 1000 °C. In fact, the deformation temperature was high enough to compensate the inhibitory effects of sluggish diffusion and severe lattice distortion.