Superplastic deformation of micro-specimens of duplex stainless steel

Microstructural development in micro tensile test specimens having the gage width and thickness of 0.6 and 0.2 mm, respectively, of a (γ+α) duplex Fe–25Cr–7Ni–3Mo alloy was examined in order to investigate role of interphase boundaries in the superplastic deformation. An elongation of more than 800% was obtained at a strain rate of 5×10−2 s−1 at 1323 K, and the strain rate sensitivity, m-values, were found to be 0.5 at the same strain rate. It was clearly recognized, by SEM observation on the cross section of the specimen, that several γ-phase grains were arranged in diagonal direction. It was found that cooperative interphase boundary sliding involving a number of grains occurred on the preferential plane composed of interphase boundaries. A model of superplastic deformation of two-phase materials constructed from a stochastic approach, which was reported previously, can be described quantitatively under the premises that (1) several tens of grains are involved in a single sliding event at the interphase boundaries, and (2) the rate of sliding at the γ–α interphase boundary is intrinsically much higher than those of γ–γ and α–α grain boundaries as was demonstrated by the present authors employing bicrystal specimens previously.