Effects of transformation twin on Hall–Petch relationship in MnCu alloy

Abstract

MnCu high damping alloys bear the transforms from face centred cubic (FCC) phase to face centred tetragonal (FCT) phase with formation of the {1 0 1} twin boundaries. In the present work, a commercial M2052 (Mn–20Cu–5Ni–2Fe) high damping alloy was adopted, in order to characterize the features of transformation twins in the specimens with different grain sizes. For this purpose, the yield strengths, the phase transformation points (Tp) and the axes ratio (c/a) of M2052 with different grain sizes were determined by tensile tests, dynamic mechanical analysis (DMA) and X-ray diffraction (XRD), respectively. The results indicated that the effects of twinning structure on the yield strength of the MnCu alloy can be divided into two terms; a grain-size-independent term and a grain-size-dependent term. The grain-size-independent term influences the nominal friction stress (σ∞) and the grain-size-dependent term influences the Hall–Petch (HP) slope (k) of the specimens significantly. Analysis of the experimental data shows that the formation of twins in M2052 is affected by not only aging treatment prior to phase transformation, but also the grain size of the FCC phase. Higher nominal friction stress observed in the specimens with longer time aging treatment would be due to the higher twin density, and the decrease of HP slope would be due to the grain-size dependence of twin density and axes ratio. It indicated that even under the same aging condition, the twin densities in larger grain-sized specimens would be higher than those in smaller grain-sized specimens.