Factors for Controlling Martensitic Transformation Temperature of TiNi Shape Memory Alloy by Addition of Ternary Elements

Abstract

ABSTRACTCorrelations between the changes in martensitic transformation start temperature (Ms) by addition of ternary elements X and several factors of the ternary additions were investigated for TiNi shape memory alloy. The change of Ms by addition of 1mol%X is referred to as ΔMs (in K/mol%), and ΔMs was systematically evaluated by differential scanning calorimetry experimentally using (Ti, X)50Ni50 solution-treated at 1273K for 3.6ks where the Ni content was kept constant to be 50mol%. The ternary additions X investigated are the transition metal (TM) elements selected from 4th period group (Zr, Hf) to 10th period group (Pd, Pt). The factors investigated are (1) the number of total outer d- and s-electrons (Nele), and electron hole number (NV), (2) electronegativity (EN), (3) atomic volume (VX) and (4) Mendeleev number (NM). It was found that the values of ΔMs are different even in a same period group; ΔMs of 6th period group are -133K/mol%Cr (3d-TM), -152K/mol%Mo (4d-TM) and -64K/mol%W (5d-TM) for example. The results found in the correlations between ΔMs and those factors are summarized as follows. (1) ΔMs depends on Nele and NV. However, the data are scattered because same Nele and NV are often given in a same period group. Then, other factors than Nele and NV are required for clear understanding of ΔMs. (2) ΔMs seems to become lowered slightly with increasing EN. (3) ΔMs weakly depends on atomic volume VX. Ternary addition with large VX increases ΔMs slightly, and with small VX decreases ΔMs largely. Since the stress field must be formed by substitution due to size mismatch, the type of stress field, tension/compression, may be an important role to determine the sign of ΔMs. (4) ΔMs shows a good correlation with NM as -9.4Kmol%−1/ΔNM where ΔNM is the difference in NM. This suggests that a ternary alloying element with smaller (larger) NM stabilizes the B19’ martensite (B2 parent) phase. Effect of site occupancy on Ms is also discussed only for Cr.