Abstract:

It is not clear whether a shape memory effect (SME) can be realized by stress-induced α′ martensite in metastable austenitic stainless steels although the stress-induced ε martensite in these materials can result in the SME. To clarify this problem, the relationship between the shape recovery and the reverse transformation of the stress-induced ε and α′ martensite in a 304 stainless steel was investigated. The results show that the stress-induced α′ martensite can result in the SME when heating above 773 K. After deformation at 77 K and step heating or directly holding at 1073 K, two-stage shape recoveries below 440 K and above 773 K can be obtained due to the reverse transformation of the stress-induced ε martensite and α′ martensite, respectively. After deformation at room temperature, the α′ martensite produced can result in the SME only when directly holding at 1073 K. The intrusion of more dislocations before the formation of the α′ martensite at room temperature than at 77 K is the reason that the α′martensite induced at room temperature cannot result in the SME in the case of slow heating. The recovered strains resulting from the stress-induced ε and α′ martensite are proportional to the amounts of their reverse transformation, respectively.

Key words: Austenitic stainless steel, Shape memory effect, Alpha prime martensite, Epsilon martensite, Two-stage recoveries