Susceptibility of a Fe-Mn-Al-Ni-Cr shape memory alloy to environment-assisted corrosion cracking

In recent years, iron-based shape memory alloys (SMA) have become increasingly interesting for application in civil engineering structures. Promising candidates in the field are iron-based Fe-Mn-Al-Ni-X (X = Ti, Cr) SMAs. These alloys have demonstrated the potential for various types of applications. While the fully recoverable martensitic phase transformation can be used for structures where high damping capacities are required, i.e. for high rise buildings in earthquake prone areas, it has been successfully demonstrated that the material can also be used as prestressing element in concrete structures. However, with the application in civil engineering structures the problem of stress corrosion cracking or environment-assisted cracking has to be considered. The simultaneous occurrence of tensile stresses and unfavorable environmental conditions, such as carbonation of concrete or the penetration of salt, can lead to spontaneous failure of the building structure. In the present study, the susceptibility of Fe-Mn-Al-Ni-Cr to stress corrosion cracking was studied in neutral chloride containing solutions. For that purpose, tensile tests under constant load control at different stress levels have been conducted. The pitting corrosion that occurred during immersion in the electrolytic solution has a decisive influence on the martensitic phase transformation and the crack evolution. The high density of dislocations at the martensite-to-austenite interfaces promotes the formation of transgranular cracks as well as stress-induced corrosion cracks originating from corrosion pits. Further investigations of the fracture surfaces revealed a dominating intergranular crack growth most likely induced by hydrogen embrittlement of the grain boundaries. The detrimental mechanisms lead to failure of the specimens well before the targeted test time of 1000 h.