Superplasticity of a nano-grained Mg–Gd–Y–Zr alloy processed by high-pressure torsion

Abstract

While most of the reports on Mg-Gd-Y-Zr alloys report superplasticity after extrusion or friction stir processing, it is important to investigate superplasticity in these alloys after other severe plastic deformation processes having greater grain refinement capability. Accordingly, superplasticity was studied in an Mg-9Gd-4Y-0.4Zr (GW94) alloy after different high-pressure torsion (HPT) conditions. The HPT was performed at room temperature under an applied pressure of 6.0 GPa for up to 16 turns. TEM microstructural characterization revealed that the grain size was reduced from an initial value of ~8.6 m in the extruded condition to ~95 ± 10 and ~85 ± 10 nm after 8 and 16 turns, respectively. A shear punch testing method was used for evaluation of superplasticity at 573, 623, 673 and 723 K. Maximum strain rate sensitivities of ~0.51 ± 0.05 and ~0.48 ± 0.05 were obtained at 623 K for the material processed through 16 and 8 turns, respectively. This strain rate sensitivity and an activation energy of ~100 ± 5 kJ mol -1 suggests the occurrence of grain boundary sliding in the superplastic region.

Microstructural evolution and superplasticity in an Mg–Gd–Y–Zr alloy after processing by different SPD techniques

“…Also, there exists a saturation strain after which the grain refinement essentially ceases [38,39]. In an earlier investigation of Mg-Gd-Y-Zr alloys, it was found that the saturation strain of these alloys was ~4 corresponding to 4 passes of ECAP [24] and about ~72 corresponding to 8 turns of HPT [25] and therefore these optimum values were used in the present investigation.…”

“…However, only limited reports are available to date documenting superplasticity in these alloys after processing by ECAP [23,24] or HPT [25].…”

“…Shear punch testing (SPT) is an example of a localized method that introduced recently as an appropriate technique for measuring the SRS in different materials processed by SPD and a summary of the results available to date was given in an earlier report [25]. It should be noted that the validity of the SPT-tensile correlation was demonstrated several years ago [26].…”

“…Although the main features and effects of ECAP and HPT on the microstructure and superplasticity of Mg-Gd-Y-Zr alloys was discussed in earlier publications, a clear comparison between ECAP and HPT cannot be made based on these published data, since they were obtained for different alloys having different concentrations of Gd and Y (for example, the ECAP and HPT were performed on GW50 [24] and GW94 [25] alloys, respectively). This is because the Gd and Y elements can affect the grain size of Mg alloys and this is beyond the scope of the present paper.…”

“…Therefore, the overall aim of this research was to investigate and compare the microstructural evolution and superplasticity in an Mg-Gd-Y-Zr alloy after processing by extrusion or by extrusion followed by either ECAP or HPT. It has to be mentioned that the optimum ECAP and HPT temperatures and strains were chosen so as to achieve the highest degree of grain refinement which can be obtained by each processing method, based on our earlier experiments on Mg-Gd-Zr [24] and Mg-Gd-Y-Zr alloys [25].…”