Electron energy-loss spectroscopy and first-principles calculation studies on a Ni–Ti shape memory alloy

Electron energy-loss spectroscopy (EELS) investigations and first-principles calculations were carried out on different aspects of a Ni–Ti shape memory alloy. The composition of lens-shaped precipitates is determined to be Ni4Ti3 by model-based EELS quantification and the Ni-depleted zone in the B2 matrix surrounding those precipitates is quantified. EELS spectra show that the intensity of the normalized Ni L3 peak of both the Ni4Ti3 precipitates and Ni-depleted zones is higher than that of matrix regions with the nominal composition, which is confirmed by first-principles simulations. The total amounts of 3d electrons, however, are the same for both elements in the different Ni–Ti structures and regions. The Young’s and bulk moduli of the B2 matrix with 51 at.% Ni and the Ni4Ti3 precipitates were evaluated and the precipitates are found to be harder than the matrix, while first-principles calculations indicate that the bulk modulus for the Ni4Ti3 precipitate is higher by 5% than that for the equiatomic NiTi B2 phase.