Stability of Zr-Al alloys

The Zr-Al alloy system has one of the more complicated binary transition-metal aluminide phase diagrams: ten reported intermediate phases belonging to four different crystal systems. In order to understand the competition among different structures and concentrations, the heats of formation of a number of different possible structures were calculated using both all-electron and pseudopotential methods. The Zr5Al3 (tI32) and Zr5Al4 (hP18) phases are predicted to be high-temperature phases. The existence of the large number of observed phases is attributed to the fact that the heats of formation for ZrxAl1-x for x = 0.25-0.75 fall on a nearly straight line, suggesting that these phases should have narrow composition ranges. In addition, a simple procedure to modify and test the pseudopotentials is presented and shown to yield calculated metallic alloy properties in good agreement with the all-electron results.