Anomalous plastic behavior of fine-grained MP35N alloy during room temperature tensile testing

In this paper, results of an investigation on the strain hardening responses of superalloy MP35N with two average grain sizes of 38 and 1 μm, during room temperature tensile testing are reported. The microstructural evolution of the deformed samples was studied using optical and transmission electron microscopy (TEM) techniques. The strain hardening behavior of the 38 μm material was rather similar to that previously reported for low stacking fault energy (SFE) fcc alloys. The plastic behavior of the fine grain material, however, was unexpected. In the strain range of 0.1–0.4, the work hardening rate of the fine grain size sample was evidently lower than that of the large grain size material. Based on the TEM results obtained in this study, it is suggested that the unusual strain hardening response of MP35N alloy of 1 μm grain size is related to the difficulty of formation of deformation twins in these samples. Results obtained in this study further support the effectiveness of deformation twins in hindering crystallographic slip in low SFE fcc polycrystals and the need to develop new plasticity models to include the role of deformation twinning in these materials.