Landau Theory of the Martensitic Phase Transition in NiTi Shape Memory AlloyTuesday (15.05.2018) 20:06 - 20:09 Part of:
The B2 -> B19’ phase transition from the cubic space group Pm3m to monoclinic P21/m breaks rotational as well as translational symmetry (amon = acub, bmon= bcub - ccub, cmon= bcub+ccub). Symmetry analysis shows that six irreducible representations are involved in the structural distortion, three at the Γ point (including the very small totally symmetric volume change) and three at the M-point of the Brillouin zone, critical wave vector k = ½(011)cub . The latter describe the breaking of translational symmetry which is associated with transverse atomic displacements from the positions in the cubic phase ((‘shuffle’). Neutron diffraction shows that the deviation of the monoclinic angle γ from 90° behaves like expected for a classical Landau order parameter for a first-order transition: it shows a step at the phase transition temperature Ttr and a continuous to increase as temperature drops further below Ttr. This distortion mode breaks rotational symmetry only (from cubic to monoclinic) and it is associated with the Γ5+ irreducible representation. However, the atomic displacements of the M-point modes show a strong first-order jump at Ttr and remain fairly independent of temperature below Ttr. This is unusual, as classical Landau theory of ferroelastic systems leads one to expect an improper ferroelastic behavior, with the M-point mode being the primary, driving order parameter, and the Γ5+ distortion amplitude couples to the square of the driving M-point amplitudes. The observed temperature dependences rule out this possibility. This leads us to suggest a scenario where the Landau coefficients of the M-point mode are independent of temperature, while the coefficient of the second-order Landau term of the Γ5+ mode is temperature dependent and drives the transition, and both modes are coupled biquadratically. This scenario is able to explain the observed data. The soft-mode corresponding to the monoclinic angle triggers the first-order transition from the B2 topology into the M-point topology due to the energetically favorite coupling between the monoclinic strain and the M-point distortions.