Microstructural Evolution of Lean Duplex Stainless Steels under Different Loading Conditions

Abstract

Lean Duplex Stainless Steels 2101 and 2404 (LDX 2101 and LDX 2404) are relatively new dual phase steel alloys with equal volume fractions of ferrite and austenite phases. Their attractive mechanical properties (high yield stress, ultimate tensile strength, and ductility), and corrosion properties make them suitable for many applications, such as offshore platforms and in the nuclear industry. Engineering components in such applications are likely to be subjected to different stress states and strain rates. However, the available literature shows a limited number of studies on the mechanical response and microstructural changes of LDX 2101 and LDX 2404. Thus, the aim of this study is understanding the mechanical response of LDX 2101 and LDX 2404 and microstructural changes in their constituent phases under uniaxial stress and uniaxial strain conditions. Compressive uniaxial stress testing at a strain rate of 10-3 s-1 showed that flow stress of both alloys is anisotropic. This anisotropy resulted from strain partitioning between the constituent phases, phase boundaries morphology, grain size, and austenite deformation mechanisms. Compressive uniaxial stress testing at different strain rates showed that LDX 2404 has a higher flow stress. LDX 2101 showed a higher working rate at 10-3 s-1 because of the martensitic transformation and subsequent mechanical twinning within the martensite. Also, increasing strain rate had a higher effect on the flow stress of LDX 2101 due to deformation localization within the hard austenite as strain rate increases. Shock-compression testing of both materials under peak stresses (9-19 GPa) showed that the reversible ferrite to martensite transformation occurs above 18 GPa, the fingerprint of this transformation is the development of {332}<113> twins. Quasi-static compressive testing of post-shocked samples showed that samples shocked above 18 GPa experience a considerable increase in their flow stresses likely because of the aforementioned reversible transformation. Examining the spall response of both material under different peak stresses and deformation histories showed that LDX 2404 has a higher spall strength because it has a higher fraction of the phase boundaries. Incipient spall damage in both materials preferentially occurred within ferrite. Spall damage varied with peak stress from quasi-cleavage fracture in the incipiently spalled samples to brittle fracture in the fully spalled samples. This study showed that the mechanical responses of LDX 2101and LDX 2404 depends on the deformation mechanisms of both constituent phases and the interactions between them in a manner that differs from classical duplex stainless steel alloys.