ALPHA-]OMEGA TRANSITION IN SHOCK-COMPRESSED ZIRCONIUM - A STUDY ON CRYSTALLOGRAPHIC ASPECTS

In 1973, Usikov and Zilbershtein proposed that the alpha(hcp)-->omega (a three atom hexagonal) transformation in Zr and Ti proceeds via the beta(bcc, a high temperature phase) intermediate. Based on this they d erived two non-equivalent orientation relationships (OR) between alpha and omega phases. Their transmission electron microscopy (TEM) study carried out on these elements, that were alpha-->omega-transformed und er static high pressure, revealed only one of the two proposed ORs. Va rious TEM studies done thereafter on these elements and their alloys ( omega transformed under static pressures) conform to either one of the se ORs. In a recent TEM study by Song and Gray on Zr, omega-transforme d under shock compression, a new OR has been observed which according to them is different than those given by UZ and they put forth the dir ect alpha-->omega transformation mechanism. In the present study, we h ave generated additional TEM data on shock compressed Zr samples and h ave reconciled the above conflicting results. We find all our ORs (whi ch contain the OR of SG also) to be described by the OR reported by UZ . The latter OR (i.e. of SG) is shown to be a subset of the former. Th ese observations show that the same type of mechanism of transformatio n is operative both, under static and shock compression. Mechanism of the transition is discussed in terms of the required strains.