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 an d anelastic properties of the material. At temperatures between 100 and 150 K hydrogen atoms act as fixed pinning points for dislocations, as they can cel a newly observed dislocation relaxation. At low H contents (0 < n(H) le ss than or equal to 0.008) the internal friction peak PAM (PRM) associated with austenite/martensite (A --> M) or R-phase/martensite (R --> M) transit ions dramatically increases with increasing the H content, while the dip oc curring 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 m odes (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 o f 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 o ccur ring over the coexistence region of the A and M (R and M) phases decre ases 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 solut ion within the R-phase or within a hydride. (C) 1999 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved.