In iron, the stable low-temperature phase (the a phase) has a body-centered-cubic (bcc) structure that transforms to the face-centered-cubic (fcc) y phase at 1185 K. Since the fcc crystal is close-packed whereas the bcc lattice is more open, the specific volume change of the a o y transition, Ava-Y, is negative. In addition, Aha-Y is positive, so Eq (5.15) shows that the transition temperature decreases with increasing pressure. The y o L phase-change (i.e., melting) temperature increases with increasing pressure because, according to Eq (5.15), this is the behavior expected when both Avtr and Ahtr are positive. For either the solid-solid phase change or melting, the pressure effect is small because the volume changes associated with the transitions are small.
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