Formation of a shock deformation induced ω phase in Zr20Nballoy

The formation of a plate shaped omega phase in Zr-Nb alloy after shock deformation has been studied with a view to ascertaining the nature of this transformation. The orientation relationship between the beta and the omega lattices was {1 1 1}(beta)//(0 0 0 1)(omega), and <1 1 0>(beta)//<1 1 1 0>(omega) which is identical to that seen in case of omega phase forming in this alloy on thermal treatment. The experimentally determined habit plane of the plate shaped w phase has been compared with that predicted from the phenomenological theory of martensite formation. A mechanism of transformation involving shear on the (1 1 2) planes has been considered. The importance of mechanical instability of the beta phase in bringing about this transformation has been investigated. The mechanism of initiation of w transformation has been ascertained by high resolution electron microscopy (HREM) of the beta lattice and compared with that happening during omega formation in the same alloy by thermal treatment. (C) 2004 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.