Neutron diffraction study of temperature-dependent elasticity of B19′ NiTi—-Elinvar effect and elastic softening

The temperature-dependent elasticity of the B19′ NiTi is unknown today. To gain insights into the lattice-level temperature-dependent elasticity of the B19′ crystal, we present results of in-situ neutron diffraction experiments performed on polycrystalline martensitic specimens in the temperature range of 300 down to 50 K. The experimental results are compared with the density functional theory molecular dynamics (DFT-MD) and Quasi Harmonic Approximation (QHA) calculations. The results confirm that the temperature-dependent Young's modulus (TDYM) of the B19′ crystal is strongly anisotropic. For different crystallographic orientations, the change in Young's modulus over the temperature range of 300–50 K (ΔE_(hkl)=E_(hkl) ^50K−E_(hkl) ^300K), ranges from ΔE_(102¯) = 2.8 ± 3.5 GPa (extremely weak dependence) to ΔE₍₁₀₃₎ = 59.6 ± 9.1 GPa (strong dependence). Moreover, it is found that the orientation-specific TDYM and thermal expansion (TE) of the B19′ crystal are correlated. The crystallographic orientations with weak and negative TE responses exhibit a weaker TDYM than the orientations with positive TE. The DFT-MD and QHA results capture qualitatively the above experimental observations and further show that there are orientations in a B19′ crystal exhibiting elastic softening (ΔE_(hkl)<0) and ideally no change in Young's modulus (ΔE_(hkl)= 0) with cooling. This is found to originate from the strong negative temperature dependence of c₃₅ stiffness constant. The experimental results along with the first-principles calculations confirm that the Elinvar and Invar are two confluent properties in NiTi SMAs and can be tailored by texturing B19′ crystallographic orientations.