alpha omicro Crystallography of the transition in shock-loaded zirconium

The orientation relations (ORs) between the alpha (hcp) and the omicro (three-atom hexagonal) structures have been determined for Zr, in which alpha transformed to omicro under shock loading. The ORs obtained are found to belong to the variant I OR given by Usikov and Zilbershtein in 1973 for statically compressed samples and also consist of an OR that is the same as that reported by Song and Gray in 1995 in recent dynamic experiments on Zr. The latter OR is shown to be a subset of the former. These observations show that the same type of mechanism of transformation is operative under both static pressure and shock compression. The crystallography of the transition is discussed in terms of the required strains.