Thermally activated processes in microcrystalline Mg

Rapid degradation of the mechanical properties of conventional Mg alloys with temperature precludes their wide application in industry. The LPSO (long period stacking ordered) Mg alloys, on the other hand, show considerable potential to achieve excellent high-temperature mechanical performance. In this study, a Mg-5.1Y-2.4Zn (wt.%) extrusion alloy with the microstructure comprising of α-Mg matrix and LPSO phase was prepared. It is shown that the yield strength of the alloy at 200 °C retained ∼83% of the yield strength of 198 MPa at room temperature (RT). Importantly, the ductility also improved, owing to the increased non-basal slip activities at elevated temperatures. In contrast to RT and 200 °C, the alloy exhibited softening tensile behavior with an apparent loss of yield strength at 300 °C. In-situ lattice strain analysis revealed that load transfer from α-Mg to LPSO remains effective at temperatures up to 200 °C, governing the high yield strength of the alloy. The post-mortem analysis by means of electron microscopy confirmed that the LPSO phase is structurally stable at 200 °C. In particular, the high strain hardenability is rationalized by the sandwiched LPSO structures together with stacking faults serving as effective obstacles to the motion of non-basal dislocations. The load transfer effect disappears at the temperature of 300 °C, resulting in the fast deterioration of strength of the alloy. The cracking of the LPSO phase along with the recrystallizing of α-Mg grains were found to be the main causes for the softening behavior of the alloy at 300 °C.

In the present study, we have systematically investigated how the influence of silver (Ag) addition (0.3 wt%) on the microstructural and mechanical properties of Mg-6Gd-3Y-1Zn-0.4Zr (wt%) alloy. The results indicate that the Ag addition enhances the age-hardening response, especially at the early stage of aging. The improved age-hardening response of the alloy is associated with dense nanosized basal γ'' precipitates formed upon the Ag addition and the β’ phase. Furthermore, the presence of Ag also improves the tensile properties of the peak-aged alloy. The Ag-containing alloy exhibits higher ultimate tensile strength, yield strength, and elongation to failure than the Ag-free alloy at room and elevated temperatures. We suggest that the improved strength and ductility of the Ag-containing alloy originate from the coprecipitation of the nanosized basal γ'' precipitates and the β’ phase.

The effect of 0–1.0 wt% Y addition on the microstructures and mechanical properties of as-aged Mg-6Zn-1Mn-4Sn alloy was investigated. The results showed that α-Mg, α-Mn, MgZn₂, Mg₂Sn and MgSnY phases were found in the as-aged Y-containing alloys. The MgSnY was a high temperature phase and could promote the age-hardening response. The as-aged ZMT614-0.5Y alloy exhibited excellent mechanical properties was attributed to the high number density of ${\beta }_1^{\prime }$ rods. Its yield strength (YS), ultimate tensile strength (UTS), elongation was 371 MPa, 376 MPa and 7.7%, respectively.

The microstructure–property relation of an extruded Mg–11Gd–4.5Y–1Nd–1.5Zn–0.5Zr (wt%) alloy was investigated by conducting hot compression and high temperature creep at temperatures upto 250 °C. The alloy exhibits an average compressive yield strength (${\sigma }_{\mathrm{CYS}}$) of 363±1 MPa and an average elongation to failure (${\epsilon }_{\mathrm{CF}}$) of 10.5±0.2% at room temperature, 301±13 MPa and 12.8±1.1% at 200 °C. In creep the minimum creep strain rate (${\dot{\epsilon }}_{\min }$) is 1.94×10⁻⁹ s⁻¹ at 175 °C/160 MPa and 6.67×10⁻⁹ s⁻¹ at 200 °C/100 MPa. The obtained stress exponent n is in the range of 3.7–4.7, suggesting that the creep is controlled by the dislocation climb mechanism. The improvement in compressive strength and creep resistance is attributed to the fine recrystallized grains, SFs in the grain interior, Mg₅RE and LPSO phases at grain boundaries. The alloy exhibits a bimodal texture with 〈0001〉 and 〈$10\overline{1}0$〉 components. Its strengthening effect is determined by the competition between these two texture components. In compressive deformation, the textural evolution from 〈$10\overline{1}0$〉 to 〈0001〉 is mainly attributed to the operation of basal 〈a〉 slip and {$10\overline{1}2$}〈$10\overline{1}1$〉 tensile twinning. This texture evolution is not seen in creep.

Influence of aluminium content on the deformation mechanisms in Mg–Al binary alloys has been studied using in-situ neutron diffraction and acoustic emission technique. It is shown that the addition of the solute increases the critical resolved shear stress for twinning. Further, the role of aluminium on the solid solution hardening of the basal plane and softening of non-basal planes are discussed using results of the convolutional multiple peak profile analysis of diffraction patterns. The results indicate that the density of both prismatic 〈a〉 and pyramidal 〈c + a〉 dislocations increases with increasing alloying content.

The effects of Nd addition on the microstructure and mechanical properties of Mg–6Zn–1Mn–4Sn alloy have been investigated. The results show that α-Mg, α-Mn, Mg₇Zn₃, Mg₂Sn and MgSnNd phases are found in the as-cast Nd-containing alloys. The MgSnNd phase is a high temperature phase which can enhance the age hardening response of the heat-treated alloys. The ZMT614–1.5Nd alloy under the extruded and aged states exhibits higher strength. This is mainly attributed to the fine grains after extruding and high number density of rod-shaped ${\beta }_1^{\prime }$ precipitates in the α-Mg matrix after aging, respectively.