A shape memory alloy (SMA) is one that can be shaped when cold, but then returns to its previous, "remembered", shape when heated. For the cobalt-nickel-gallium (Co-Ni-Ga) alloys, this transition is between an austenite structure and a tetragonal martensite structure. During the transformation, the austenite crystal lattice undergoes shearing in a particular direction. The ‘martensite variants’ are the different potential orientations of the shearing.
Co-Ni-Ga alloys have potential for higher-temperature applications due to their promising functional properties at elevated temperatures and good formability. To understand the morphology of martensite variants as they form, and their influence on the functional properties, this study used in situ optical microscopy and neutron diffraction on SXD to study the stress-induced martensitic transformation of Co49Ni21Ga30 single crystals whilst they were undergoing compression cycles.
As well as studying a solution heat treated crystal, they studied an aged equivalent containing nano-sized particles, and discovered a big difference in the phase transition behaviour. In the solution heat treated crystal, the transition occurred via a single set of stress-induced martensite variants, whereas the presence of nano-particles caused the formation of two types of internally twinned variants. On applying further strain, they found that one was forming at the expense of the other: a process known as detwinning.
Related publication: “Effect of nanometric γ′-particles on the stress-induced martensitic transformation in <001>-oriented Co49Ni21Ga30 shape memory alloy single crystals” Scripta Materialia, 168, 42-46 (2019)
Funding: Deutsche Forschungsgemeinschaft and the Ministry of Science and Education of Russian Federation.
Authors: C Lauhoff (Universität Kassel), A Reul (Ludwig-Maximilians-Universität), D Langenkämper (Ruhr-Universität Bochum), P Krooß (Universität Kassel), C Somsen (Ruhr-Universität Bochum), M Gutmann (ISIS), I Kireeva, Y Chumlyakov (Tomsk State University), W Schmahl (Ludwig-Maximilians-Universität), T Niendorf (Universität Kassel)