Martensitic transitions and mechanical spectroscopy of Ni50.8Ti49.2 alloy containing hydrogen

Hydrogen additions [n(H) =H/(Ni + Ti) =0.003; 0.008; 0.013; 0.021; 0.029; 0.045 at.] to a Ni50.8Ti49.2 alloy produce several effects in the elastic and anelastic properties of the material. At temperatures 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) less than or equal to 0.008) the internal friction peak PAM (PRM) associated with austenite/martensite (A --> M) or R-phase/martensite (R --> M) transitions dramatically increases with increasing the H content, while the dip occurring in the Young's modulus (E) vs temperature curves becomes gradually wider and shallower. The enhancement of peak PAM (PRM) 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 (PH) appears and progressively grows becoming the only internal friction feature for nH = 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 PH is most likely associated with stress-induced motion of H in solid solution within the R-phase or within a hydride. (C) 1999 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved.