Pressure Dependence of the β -Brass Order-Disorder Critical Temperature

The order-disorder critical temperatures Tc of β-brass samples of varying compositions were measured at pressures P of up to 10 kbar by the differential thermal analysis technique. Tc increases linearly with pressure, and δTcδP varies with composition. The values of δlnTcδlna, where a is the lattice constant, estimated from the measured δTcδP, vary by about 9% with 3% variation in composition. It is shown that this result implies that a single volume-dependent nearest-neighbor ordering interaction is insufficient to describe the order-disorder transition over the composition range in which it is observed. The comparison between these experimental results and various theories of the order-disorder interaction is presented.