Morphological and crystallographic aspects of Cl1b-type precipitates nucleated in martensitic and parent phase matrices in Ti-rich Ti-Pd shape memory alloys

Abstract

Ti 2 Pd phase with C11 b -type structure which precipitates in Ti-rich Ti-Pd shape memory alloys may nucleate either in martensitic phase or parent phase depending upon thermal treatment conditions, since the transformation temperatures are around 800 K. In the alloy aged above A f , three C11 b -type precipitate variants are always observed by TEM. The orientation relationship between those three variants and the B19 structure, which was transformed from B2 structure after the precipitation, is as follows: Variant I (001) Ti2Pd //(101) B19 ; Variant II (001) Ti2Pd //(101) B19 ; Variant III (001) Ti2Pd //(010) B19 . The shape of the precipitates was deduced to be disk-like and/or elliptic. On the other hand, only the variants I and II are observed when the precipitates are nucleated in martensitic phase. Since the difference between the lattice spacing of (010) B19 and (001)/3 Ti2Pd is about 20%, the formation of a third variant in the martensitic matrix is energetically unfavorable.