Compressive strength and hot deformation mechanisms in as-cast Mg-4Al-2Ba-2Ca (ABaX422) alloy

Abstract

The behaviour of an as-cast ABaX422 Mg alloy has been evaluated with regard to its compressive strength in the temperature range 25–250 °C and hot working characteristics in the range 260–500 °C. The microstructure of the as-cast alloy has intermetallic phases Mg₁₇Ba₂ and (Al, Mg)₂Ca at the grain boundaries and is fine grained. The alloy has compressive strength better than AZ31 with Ca and Zn, which was attributed to the finer grain size. A processing map developed to characterize its hot working behaviour revealed two dynamic recrystallization domains in the temperature and strain rate ranges of (1) 300–390 °C/0.0003–0.001 s⁻¹ and (2) 400–500 °C/0.0003–0.5 s⁻¹. In the first domain, basal + prismatic slip occurs along with recovery by climb while in the second domain, second-order pyramidal slip dominates and recovery occurs by cross-slip. The apparent activation energy estimated in Domains 1 and 2 are 169 and 263 kJ/mol respectively, both being higher than that for self-diffusion suggesting that the intermetallic particles in the matrix cause considerable back stress. Bulk metal working of this alloy may be done in Domain 2 which ensures high workability while finish working may be done in Domain 1 in order to achieve a fine grained component. The alloy exhibits flow instability regimes at higher strain rates, in both the lower and higher temperature regions of the processing map, the manifestation being adiabatic shear band formation and flow localization respectively.

Keywords:

• Mg-Al-Ba-Ca alloy
• microstructure
• hot working
• thermomechanical effects
• dynamic recrystallization
• processing map

Acknowledgement

The work presented in this paper has been supported by a Strategic Research Grant (Project #7002744) from the City University of Hong Kong. The authors would like to acknowledge the help provided by Dr H. Dieringa, Helmholtz-Zentrum Geesthacht, Germany, in the preparation of cast alloy used in this study.