MARTENSITIC TRANSITIONS AND MECHANICAL SPECTROSCOPY OF Ni_50.8Ti_49.2 ALLOY CONTAINING HYDROGEN

摘要

Hydrogen additions [n_H = H/(Ni + Ti) = 0.003, 0.008, 0.013, 0.021, 0.029, 0.045 at.] to a Ni_50.8Ti_49.2 alloy produce several effects in the elastic and anelastic properties of the material. At tempera- tures between 100 and 150 K hydrogen atoms act as fixed pinning points for dislocations, as they cancel a newly observed dislocation relaxation. At low H contents (0 ＜ n_H ≤ 0.008) the internal friction peak P_AM (P_RM) associated with austenite./martensite (A → M) or R-phase/martensite (R → M) transitions dramati- cally increases with increasing the H content, while the dip occurring in the Young's modulus (E) vs tem- perature curves becomes gradually wider and shallower. The enhancement of peak P_AM (P_RM) can be accounted for in terms of a mechanism involving the excitation of collective vibration modes (dyadons) of twin boundaries interacting with H or the stress-induced motion of parent/product interfaces. The widening of the dip in the Young's modulus is due to the introduction by H of a two-step transition (A → R → M). With increasing the H content n_H from 0.008 to 0.045 the height of peak P_AM (P_RM) decreases and a higher temperature peak (P_H) appears and progressively grows becoming the only internal friction feature for n_H = 0.045. With increasing H content the thermal hysteresis in the E(T) curves occur- ring over the coexistence region of the A and M (R and M) phases decreases due to the inhibition caused by H of the martensitic transition. Peak P_H is most likely associated with stress-induced motion of H in solid solution within the R-phase or withi