Precipitation Behavior and Its Effects on Mechanical Properties in a Pre-Twinned Mg–6Al–1Zn Alloy

Abstract

In the present study, an extruded Mg6Al1Zn alloy was pre-compressed along extrusion direction to generate sufficient tensile twins. Afterward, the pre-twinned samples were annealed at 170°C for aging. Precipitation of the second-phase particles was found to preferentially occur in twins and twin boundaries. The precipitation phase is proved to be Al 12 Mg 17 . Moreover, precipitation behaviors in the two twin boundaries on both sides of every twin were very different: a considerable number of second-phase particles precipitated in one twin boundary but only a few second-phase particles were present in the other twin boundary. The atomic structures or internal stresses of the two twin boundaries are assumed to be different. A theoretical model of atomic structure was constructed to analyze the difference between the two twin boundaries. This model shows that atomic densities of the two twin boundaries are different, thereby resulting in varying precipitation behaviors. Owing to precipitation in twins and twin boundaries, an obvious increase in yield stress was observed when the material was reloaded along the same direction. [

Strengthening of Mg-6Al-1Zn Alloy via Simultaneous Loading and Aging

“…In magnesium alloys, it is found that the existence of pre-strain is beneficial for precipitation of the second-phase particles. If a sample without pre-strain is annealed at 170 • C or a slightly higher temperature, no obvious strengthening effect is observed [14][15][16]. It is deduced that the inner stress induced by pre-strain may have effects on precipitation of the second-phase particles.…”

“…It is reported that pre-deformation has an obvious influence in precipitation behavior in magnesium alloys. Our previous work [14] had found that there were a few precipitates appearing in the AZ61 alloy during annealing if the alloy was not subjected to pre-deformation. However, if the alloy was subjected to pre-deformation and then annealed under a relatively low temperature, a higher quantity of second-phase particles, for instance, Al 12 Mg 17 , would appear in the alloy.…”

“…The rearrangement of the dislocation structure activated by annealing processes and the blocking of dislocations were responsible for that strengthening phenomenon. In addition to these, the orientation change caused by pre-deformation and subsequent annealing under relatively high temperature [12], grain refinement by recrystallization after cold plastic deformation [13] and precipitates generated during annealing after deformation [14][15][16] all affect the mechanical behavior of magnesium alloys. Among all the possible strengthening mechanisms for annealing hardening, the occurrence of precipitates to prevent dislocation gliding and twin expanding, thus leading to a strengthening effect, is important for magnesium alloys.…”

Improve the Mechanical Properties of Mg–3Al–1Zn Alloy via Simultaneous Annealing and Loading

“…This indicates that the solute atoms will be more active during the heat treatment combined with loading, thus leading to enhanced precipitation during aging. Our previous study [17] also reported that the pre-strain could promote the second-phase nucleation during the subsequent annealing owing to the internal stress in the pre-strained alloy. As the applied stress was lower than the yield stress in this study, no dislocation gliding and twin nucleation occurred in the alloy during the simultaneous annealing and loading.…”

“…This indicates that the solute atoms will be more active during the heat treatment combined with loading, thus leading to enhanced precipitation during aging. Our previous study [17] also reported that the pre-strain could promote the second-phase nucleation during the subsequent annealing owing to the internal stress in the pre-strained alloy.…”