STRESS-CORROSION BEHAVIOR OF NI-TI SHAPE- MEMORY ALLOYS IN HIGH-TEMPERATURE WATER

The effects of composition, heat treatment, test temperature and disso lved oxygen (DO) concentration on the stress corrosion cracking (SCC) behavior of Ni-Ti and Ni-Ti-Cu shape memory alloys in high temperature pure water were examined. The tests were conducted by using slow stra in rate test machine. The 50 similar to 50.9 at%Ni-Ti and 40 similar t o 41.5 at%Ni-Ti-10 at%Cu shape memory alloys were subjected to SCC in air saturated water of 561 K, except for the annealed 50 at%Ni-Ti allo y. The annealed 50 at%Ni-Ti was immune to SCC, but the SCC susceptibil ities of the other Ni-Ti alloys were riot affected by nickel concentra tion and heat treatment. In the case of Ni-Ti-Cu alloys both low nicke l concentration and aging promoted SCC. SCC of Ni-Ti and Ni-Ti-Cu allo ys initiated from the scratches and propagated transgranularly. At 473 K, SCC occurred and the susceptibilities were not largely different f rom those at 561 It. In the case of a stainless steel shape memory all oy, the martensite phase specimens were susceptible to SCC in deaerate d low temperature water. On the contrary, the martensitic Ni-Ti alloys did not crack in the deaerated water at 287 K. The susceptibilities t o SCC of Ni-Ti and Ni-Cu-Ti alloys in deaerated water of 561 It were t he same as those in aerated water of the same temperature. Dissolved o xygen did not accelerate nor prevent SCC in high temperature water.