Shape Memory Effect Induced by Stress-induced α′. Martensite in a Metastable Fe-Cr-Ni Austenitic Stainless Steel

It is not clear whether a shape memory effect (SME) can be realized by stress-induced alpha ' martensite in metastable austenitic stainless steels although the stress-induced epsilon 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 e and alpha ' martensite in a 304 stainless steel was investigated. The results show that the stress-induced alpha ' 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 epsilon martensite and alpha ' martensite, respectively. After deformation at room temperature, the alpha ' martensite produced can result in the SME only when directly holding at 1073 K. The intrusion of more dislocations before the formation of the alpha ' martensite at room temperature than at 77 K is the reason that the alpha ' martensite induced at room temperature cannot result in the SME in the case of slow heating. The recovered strains resulting from the stress-induced epsilon and alpha ' martensite are proportional to the amounts of their reverse transformation, respectively.