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Linkage between mechanical properties and phase transformations in a 301LN austenitic stainless steel Maréchal, David


In this work, the deformation mechanisms of an austenitic stainless steel (grade 301LN) have been investigated with particular attention on the strain-induced phase transformations from austenite to ε and α’ martensites. The average grain size of this alloy was varied in the range 0.5-28 µm, and two strain paths, namely uniaxial tension and simple shear, were analyzed. At the macroscopic level, the work-hardening response was examined in relation to the formation of ε and α’ martensites, followed by X-ray phase quantification and Feritscope measurements. At a microscopic level, the microstructures after deformation were investigated using electron back-scatter diffraction, energy-dispersive X-ray spectroscopy and transmission electron microscopy. It was found that the grain size refinement was responsible of a change in nucleation mechanisms of α’-martensite, thereby affecting the macroscopic volume fraction of α’-martensite. The switch from tension to shear was not found to affect the mechanisms of formation of ε and α’ martensites, but significantly reduced the work-hardening, an effect too large to be attributed to the slight reduction of the kinetics of α’ volume fraction. The stresses borne in the α’-martensite were quantified using a novel method based on the magnetomechanical effect. These stresses, together with the determination of the intrinsic constitutive laws of austenite and α’-martensite, were used to design a one-dimensional physically-based model of the work-hardening in this alloy. This model, based on the “dynamic composite" effect of the formation of fresh α’-martensite in austenite, successfully predicted the measured stress-strain behaviour in tension, as well as the tensile instabilities encountered in this class of materials.

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