Stress corrosion cracking of twinning induced plasticity (TWIP) steel

Metals and alloys with low stacking fault energy (SFE) undergo twinning within their grains. Twinning Induced Plasticity (TWIP) steels possess an exceptional combination of tensile strength and ductility, and therefore, they offer enormous potential in applications where impact resistance is critically important, such as the critical components of automotives that experience sudden impact in accidents. Because of their high toughness as well as manganese as the major alloying element (instead of Ni that is the major alloying element in common austenitic stainless steels), the TWIP steels are also a less expensive alternative for the toughness aspect of the common Cr-Ni austenitic stainless steels. However, alloys with attractive mechanical properties when exposed to corrosive environment must also possess the desired resistance to environment assisted cracking (EAC) that can be caused as a result of the synergistic action of tensile stress and corrosion, e.g., stress corrosion cracking (SCC). Previous corrosion studies on twinning induced plasticity (TWIP) steels in the open literature have generally focused on electrochemical corrosion and hydrogen embrittlement. In this research, the stress corrosion cracking susceptibility of an austenitic Fe-Mn-Al-Si TWIP steel was determined at different strain rates in acid, neutral, basic aqueous solutions at different temperatures. SCC was investigated using slow strain rate test (SSRT) and constant load (CL) testing. Roles of strain rate, temperature, pH and electrolyte concentration in SCC of TWIP steel has been investigated. In order to estimate the extent by which the strain rate was changing within the specimens gauge length during the SSRTs, a finite element simulation (using ABAQUS STANDARD 6.9) was employed to calculate the gradient of strain rate developed in the necking area. This research demonstrates the role of the types of twins (annealing twins and mechanical twins) in stress corrosion cracks propagation. This research discusses in particular the role of severe and frequent localized deformation (i.e., twinning) in SCC resistance.