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