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In situ neutron diffraction measurements during tensile testing at room temperature and low temperature of a duplex stainless steel consisting of BCC and FCC phases have been performed. The deformation behavior, phase stresses and contributions of the constituent phases to the strength were analyzed and discussed. The steel showed good balance of strength and ductility at 200 K, and the phase stresses of the constituent phases at 200 K are larger than those at room temperature. Although the contributed stress to the strength of BCC phase was higher than that of FCC phase, the contribution portion to the strength of each constituent phases remained nearly constant as the test temperature changed. The strengthening with decreasing test temperature occurred in both BCC and FCC phases enhancing the work-hardening rate of the steel at low temperature.

The effects of grain size and strain rate on the low-temperature tensile properties of ferrite-austenite duplex stainless steel were investigated. The coarse- and fine-grain specimens exhibited a grain size of approximately 20 and 8 μm, respectively. At 77 K, the fine-grain specimen exhibited a higher strength and elongation than the coarse-grain specimen. The work-hardening rate at 77 K in both specimens leveled off or increased slightly at late stage of deformation although the work-hardening rate was lower than the true stress. This characteristic work-hardening behavior is caused by the deformation-induced martensitic transformation of metastable austenite. Grain refinement stabilized austenite phase and enhanced the elongation of the material, resulting in the better low-temperature tensile properties of fine grain. The elongation decreased remarkably with an increasing strain rate at 77 K independent of grain size. The characteristic work-hardening was not detected at a high strain rate, indicating that the deformation-induced martensitic transformation did not occur. The strain rate affects the frequency of deformation-induced martensitic transformation, resulting in a change in elongation. Grain refinement effectively improves the low-temperature tensile properties of duplex stainless steels; however, these properties are strongly influenced by the strain rate, where a high strain rate causes a low elongation.