Nanohardness and Elastic Modulus of TiNi-TiFe Single and Polycrystals

The paper reports on a nanoindentation study of the hardness H and Young’s modulus E in the B2 phase of quasi-binary TiNi-TiFe single crystals and Ti₄₉Ni₅₁ single and polycrystals with and with no thermoelastic martensite transformations. The study shows that the elastic properties of the alloy single crystals depend on the concentration of Fe atoms and decrease gradually with a decrease in the Fe content and with a gradual decrease in the B2-phase stability to martensite transformations. In Ti₅₀Ni_50–xFe_x, the dependence of the hardness H on the Fe content reveals a peak at equal Fe and Ni concentrations (25.0 at %), which is likely because the alloy at this Fe/Ni ratio is involved in its maximum solid solution hardening. The experimental results are compared with numerical data obtained by the Voigt averaging of elastic constants, showing a mean deviation of 11.55% between them. An analysis of the H and E correlation and the H/E ratio in the alloys as they lose the B2-phase stability to martensite transformations suggests that the correlation coefficient of E and H in TiNiFe measures 0.42, which corresponds to the range of moderate statistical values. In TiNi-TiFe with martensite transformation, the ratio H/E is higher than 0.035, and hence, higher than the values typical of metals and alloys. In our opinion, this is because the elastic moduli of the alloys are “softened” as they get close in concentration to the points of B2 → R → B19′ transitions. The H/E ratio in the alloys can be considered as a criterion of B2-phase stability loss with respect to martensite transformations.