Berlin 2015 – scientific programme
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MM: Fachverband Metall- und Materialphysik
MM 16: Functional materials III: Sensors and Actuators
MM 16.7: Talk
Monday, March 16, 2015, 17:30–17:45, TC 010
Density functional investigations on the effect of Ni excess in binary Ni-Ti shape memory alloys — •Ingo Opahle1, Jan Frenzel2, André Wieczorek2, Burkhard Maaß2, Gunther Eggeler2, and Ralf Drautz1 — 1ICAMS, Ruhr-Universität Bochum, Bochum, Germany — 2Institut für Werkstoffe, Ruhr-Universität Bochum, Bochum, Germany
The martensite start temperature Ms of binary Ni-Ti shape memory alloys depends strongly on the alloy composition and decreases by about 100 K within 1 at.% of excess Ni. We present density functional calculations for binary Ni-Ti alloys close to the stoichiometric composition. In agreement with experimental results it is shown that the heat of transformation Δ H decreases as the Ni concentration increases, which in turn results in a lower martensite start temperature. The strong decrease of Δ H is caused by a stabilization of the B2 austenite phase by structural relaxations around Ni antisite atoms together with a gradual destabilization of the B19’ martensite phase. In contrast, contributions from the valence electron count or magnetism are shown to be unable to explain the experimentally observed changes in Ms.