Influence of Hydrostatic Pressure on the t → o Transformation in Mg‐PSZ Studied by In Situ Neutron Diffraction

The influence of hydrostatic pressure on the t → o transformation in Mg‐PSZ has been studied using a series of neutron diffraction measurements on samples cooled to cryogenic temperatures. A hydrostatic pressure of 400 MPa led to the t → o onset temperature being depressed by 75 K, and the yield of o phase at the base temperature (40 K) being halved. This lends support to the notion that the t particles in this system are predisposed to undergo the t → o transformation by thermal expansion mismatch stresses. It is estimated from thermal expansion data that the mismatch stresses could be as high as 760 MPa along the c ‐axis of the tetragonal precipitates and that the mismatch strains are qualitatively very similar to the transformation strains for the t → o transformation. It is proposed that the critical factor in determining the transformation path ( t → o vs t → m ) is the presence, in multiphase systems, of a coherent interface between the t phase and the surrounding phases.