The three most common crystal structures encountered in group IV transition metals Ti, Zr, and Hf and alloys based on them, under different temperature, pressure and alloy concentration conditions, are hcp(α), bcc (β) and simple hexagonal (ω). Although the structural relations of α⇌β andβ⇌ω transformations are well understood, the same is not true for α→ω phase change, which occurs at high pressures. We have done high pressure experiments on Ti-V alloys, followed by electron diffraction to study this. These patterns from pressure treated foils of alloys Ti₉₅ V₅ and Ti₉₁ V₉ showed the presence ofβ-phase with fourω variants. Some of them showed the existence of all three phases, α,β andω, with the number of variants given by the lattice correspondence matrix, derived through the orientation relations of α →β andβ →ω. This is a clear evidence that the α →ω transformation proceedsvia theβ-phase. The atomic rearrangements required forα →ω are found to be much smaller if the path is via theβ-phase, rather than the earlier model of Silcock.

The shock Hugoniot curves of a large number of materials up to a few Mbar have been obtained experimentally. Metallurgical examination and physical measurements on metallic and other samples recovered after shock loading up to several 100 kbar indicate the existence of large concentrations of point, line and planar defects. Dislocation mechanisms have been invoked to explain shock wave propagation and the phenomena related to the quick homogenisation of stress and strain behind the shock front. Computer simulation models using molecular dynamics calculations have also been used to understand some aspects of shock wave propagation at an atomistic level. For very strong shocks, the material is expected to melt under shock heating, but the experimental evidence regarding this is inconclusive. A combination of shock temperature measurement and theory may be able to answer this question.

The systematics of the shock constants in shock velocity-particle velocity relations for metals have been examined by energy band theory methods. The causes of non-linearity of this relation at high pressure are discussed in terms ofs ⇌d electron transfer.

There are many fascinating areas of research related to the response of materials at high static and dynamic pressures. The experimental range of compression achievable in the condensed state under pressure is much larger than the range of expansion achievable before melting by variation of temperature. The advances in the experimental techniques have been matched by the developments in the first principles theories and in computational resources. Studies of equation of state and of phase transitions in materials have helped to increase our basic understanding of the condensed state of matter, with possible applications in many fields, including nuclear technology. The current status of high pressure research is briefly reviewed, taking examples mainly from the work of our group at Trombay.