Microstructure in the cubic to monoclinic transition in titanium-nickel shape memory alloys

Microstructure in the cubic to monoclinic transition in the technologically important Ti-Ni shape memory alloys is considered. Using a geometrically n onlinear theory of martensitic transformations, the twinned martensite, aus tenite-martensite, wedge; triangle, and diamond microstructures are studied . Specifically, compound, type I, and type II twins are possible for any ch oice of the lattice parameters; while, non-standard twins may exist with sp ecial lattice parameters only. In addition, 192 habit planes are found for a particular Ti-Ni alloy, but only 24 have been unambiguously observed in e xperiments. Further, the twinned wedge is possible in this alloy, but the t riangle and diamond are not. These latter three are special microstructures , which provide a mechanism through which a specimen can easily transform a nd are possible only for alloys with specific lattice parameters. A complet e enumeration of the various microstructures is given, and algorithms are p resented so that the calculations can be repeated with different lattice pa rameters. (C) 1999 Published by Elsevier Science Ltd. On behalf of Acta Met allurgica Inc. All rights reserved.