Deformation-Induced Martensitic Transformation and Mechanical Properties of Duplex and Austenitic Stainless Steels : A Synchrotron X-Ray Diffraction Study

Abstract: Metastable austenitic and duplex stainless steels are widely used materials in industrial anddomestic applications, owing to their attractive characteristics such as good corrosion resistanceand favorable mechanical properties. Both types of steel experience enhanced mechanicalproperties during plastic deformation due to the formation of the martensite phase from theparent austenite phase, this is called deformation-induced martensitic transformation (DIMT).It is therefore of technical interest to study the transformation mechanism and its impact onmechanical properties for a better understanding and ultimately for developing new materialswith improved performance in certain applications. In the present thesis, two austenitic stainless steels (201Cu, HyTens® 301) and two duplexstainless steels (FDX25®, FDX27®) were investigated. Samples were tensile tested during insitusynchrotron radiation experiments performed at the Cornell High Energy SynchrotronSource (CHESS), Ithaca, USA. Tests were performed at both room temperature and at elevatedtemperatures. The collected diffraction data were then processed by software such as Fit2D andMATLAB. Quantitative phase fraction analysis based on the direct comparison method wasperformed successfully. Microstructural analysis of samples before deformation and after thefull tensile testing was also performed using electron microscopy. The deformation induced martensitic transformation took place in HyTens 301, FDX25 andFDX27, but in 201Cu the austenite was stable during the tensile tests conducted here. The a’-martensite formed in a significantly higher fraction than the ε-martensite in all alloys. At roomtemperature, the critical stress levels for martensitic transformation were 490 MPa, 700 MPaand 700MPa for HyTens 301, FDX25 and FDX27, respectively.