High Temperature Strength and Hot Working Technology for As-Cast Mg-1Zn-1Ca (ZX11) Alloy

Cast Mg-1Zn-1Ca alloy (ZX11) has been tested to evaluate its compressive strength between 25 degrees C and 250 degrees C, and workability in the range of 260-500 degrees C. The ultimate compressive strength of this alloy is about 30% higher than that of creep-resistant alloy Mg-3Sn-2Ca (TX32) between 25 degrees C and 200 degrees C, and exhibits a plateau between 100 degrees C and 175 degrees C, similar to TX32. This is attributed to Mg2Ca particles present at grain boundaries that reduce their sliding. The processing map, developed between 260 and 420 degrees C in the strain rate limits of 0.0003 s(-1) to 1 s(-1), exhibited two domains in the ranges: (1) 280-330 degrees C and 0.0003-0.01 s(-1) and (2) 330-400 degrees C and 0.0003-0.1 s(-1). In these domains, dynamic recrystallization occurs, with basal slip dominating in the first domain and prismatic slip in the second, while the recovery mechanism being climb of edge dislocations in both. The activation energy estimated using standard kinetic rate equation is 191 kJ/mol, which is higher than the value for lattice self-diffusion in magnesium indicating that a large back stress is created by the presence of Ca2Mg6Zn3 intermetallic particles in the matrix. It is recommended that the alloy be best processed at 380 degrees C and 0.1 s(-1) at which prismatic slip is favored due to Zn addition. At higher strain rates, the alloy exhibits flow instability and adiabatic shear band formation at <340 degrees C while flow localization and cracking at grain boundaries occurs at temperatures >400 degrees C.