Hot Deformation Behaviour and Processing Maps of an as-Cast Mg-6.8Y-2.5Zn-0.4Zr Alloy

Abstract

Deformation behavior of an as-cast Mg-6.8Y-2.5Zn-0.4Zr alloy during plane strain compression was characterized in present work by high-temperature testing. Based on the experimental data, the values of strain rate sensitivity, efficiency of power dissipation and the instability parameter under the condition of various hot working parameters were investigated. Processing maps were established by superimposing the instability map over the power dissipation map, this being connected with microstructural evolution analysis in the hot deformation processes. Accompanied microstructure characterization of the binary α-Mg/ Long Period Stacking Ordered (LPSO) microstructure reveals that the flow behavior is related to the deformation mechanisms. At lower temperatures (350 – 400 °C) formation of kink bands is observed, which normally occur when deformation twinning is inhibited and other slip systems are strongly hindered by the complex LPSO structures. Dynamic recrystallization (DRX) was initiated at higher temperatures above 400 °C, influencing the softening behavior of the material significantly. DRX was the main softening mechanism when deformation takes place at 500 °C and the kink band deformation decreased.

Hot Deformation and Dynamic Recrystallisation Behaviour of Twin-Roll Cast Mg-6.8Y-2.5Zn-0.4Zr Magnesium Alloy

“…Flow instabilities caused premature material failure through the occurrence of cracks. Comparable hardening and softening behaviour is determined for a cast Mg-6.8Y-2.5Zn-0.4Zr alloy in [ 21 ]. However, comparison with the material investigated in the current study clearly shows that dynamic recrystallisation in the case of the twin-roll cast and heat-treated Mg-6.8Y-2.5Zn-0.4Zr alloy already commenced at lower temperatures.…”

“…However, comparison with the material investigated in the current study clearly shows that dynamic recrystallisation in the case of the twin-roll cast and heat-treated Mg-6.8Y-2.5Zn-0.4Zr alloy already commenced at lower temperatures. In addition to an earlier onset of DRX (low critical degree of deformation), the results in Figure 2 show that DRX was more likely to have been completed than would be the case for a conventionally cast initial material [ 21 ]. It was assumed that DRX was favoured in the twin-roll cast and heat-treated Mg-6.8Y-2.5Zn-0.4Zr alloy.…”

“…To avoid accumulation of internal energy, the processing maps are plotted for strains after strain hardening. The corresponding material parameters m , η and ξ are calculated according to [ 20 , 21 ]. Isolines of temperatures and strain rates are plotted on the two-dimensional plane; namely the power dissipation map and the instability map.…”

“…Processing maps are based on dynamic material models (DMM) and are used to optimise the hot-deformation parameters. The principles for creating processing maps can be found in [ 20 ] and [ 21 ], among others. The method takes into account the opposing relationship between the rate of heat generation induced by the forming process and the rate of energy dissipation.…”

Microstructure and Hot Deformation Behavior of Twin Roll Cast Mg-2Zn-1Al-0.3Ca Alloy

“…The preparation of the processing maps is based on thermodynamic analysis of the flow curves. Further information about processing maps based on DMM derivation can be found in [28]. The method considers the complementary relationship between the rate of heat generation induced by the forming process and the rate of energy dissipation associated with microstructural changes such as recovery and dynamic recrystallization (stability domains) and material damage (instability domains).…”