Effect of Pd in NiTi on the martensitic transformation temperatures and hysteresis: A first-principles study

In this paper phase stability and functional properties of Pd-doped NiTi with different Pd concentrations (Ni_24-nPd_nTi₂₄, n = 2, 3, 4, 5, 6, 9, 12; C_Pd = 4.2 at.%, 6.3 at.%, 8.4 at.%, 10.4 at.%, 12.5 at.%, 18.8 at.% and 25 at.%) are calculated by first-principles method. Results show that B19′ is the most stable when C_Pd is less than 10.4 at.%, whereas B19 is the most stable for C_Pd is equal to or larger than 10.4 at.%. The formation energy decreases with increasing Pd concentration. With increasing C_Pd, the energy difference between austenite and martensite decreases slightly and increases for C_Pd < 10.4 at.% and ≥ 10.4 at.%, respectively. This indicates that the phase transition temperature decreases slightly and increases evidently for C_Pd < 10.4 at.% and ≥ 10.4 at.%, respectively, which is consistent with the experimental results. A geometric model is used to evaluate the hysteresis: when the ratio of the lattice constants of the two phases is close to 1, the hysteresis tends to zero. For Pd-doped NiTi, the lowest hysteresis is achieved at C_Pd = 10.4 at.%, which agrees well with the experimental results.