Pressure Dependence of theβ-Brass Order-Disorder Critical Temperature

The order-disorder critical temperatures $T_c$ of $\beta$-brass samples of varying compositions were measured at pressures $P$ of up to 10 kbar by the differential thermal analysis technique. $T_c$ increases linearly with pressure, and $\frac{\delta T_c}{\delta P}$ varies with composition. The values of $\frac{\delta \ln T_c}{\delta \ln a}$, where $a$ is the lattice constant, estimated from the measured $\frac{\delta T_c}{\delta P}$, 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.