Path dependency of phase transformations

At high pressures titanium and zirconium are known to undergo a phase transformation from the hexagonal close packed (HCP), alpha-phase to the simple-hexagonal, omega-phase. Under conditions of shock loading, the high-pressure omega-phase can be retained upon release. It has been shown that temperature, peak shock stress, and texture can influence the transformation. Moreover, under these same loading conditions, plastic processes of slip and twinning are also affected by similar differences in the loading path. To understand this path dependency and it role on microstructural evolution, in-situ velocimetry measurements along with post-mortem metallographic and neutron diffraction characterization of soft recovered specimens have been utilized to qualitatively understand the kinetics of transformation, quantify volume fraction of retained omega-phase and characterize the shocked alpha and omega-phases. Together the work described here can be utilized to understand phase and structure for these metals and lend insight into the partitioning of plastic processes between phases far from equilibrium conditions. Experiments discussed here, reveal that the transformation from alpha to omega phase is more difficult in Ti than in Zr metals. This result is discussed in terms of both differences in shear modulus and free space within the lattice for these systems.