Morphological and crystallographic aspects of C11_b-type precipitates nucleated in martensitic and parent phase matrices in Ti-rich Ti-Pd shape memory alloys

Ti₂Pd 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// (101̄)_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.