Composition and temperature dependence of twinning stress in non-modulated martensite of Ni-Mn-Ga-Co-Cu magnetic shape memory alloys

We study compositional and thermal dependences of lattice parameters and twinning stress sigma(TW) in Ni50-xMn25-yGa25-zCoxCuy+z magnetic shape memory alloys with non-modulated tetragonal crystal structure. The sigma(TW) was found to decrease with increasing temperature with a slope from -0.0076 to -0.0181 MPa/K. The sigma(TW) = 0.47-0.81 MPa values measured near composition-specific austenite transformation temperatures T-A were discovered to depend linearly on the c/a ratio with a slope of 14 MPa. We predict that the further decrease of c/a ratio to the level of 1.085-1.104 can result in extremely low sigma(TW) approximate to 0.1 MPa in non-modulated martensite. (C) 2017 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.