The tension-compression asymmetry of martensite phase transformation in a metastable Fe40Co20Cr20Mn10Ni10 high-entropy alloy

亚稳Fe40Co20Cr20Mn10Ni10高熵合金中马氏体的拉伸压缩不对称性

The microstructural evolution of a metastable face centered cubic(FCC)Fe40Co20Cr20Mn10Ni10high-entropy alloy(HEA)under both tension and compression is systemically investigated.The results show much higher level of martensite phase transformation from FCC structure to hexagonal closed packed(HCP)structure under compression than tension,indicating a distinct tension-compression asymmetry.The compressive tests underwent higher true stresses,which further provided stronger driving forces to trigger the phase transformation than those in tensile tests.Except for the martensite phase transformation,dislocation planar slip prevails in both tension and compression,along with the occasional formation of mechanical twins.Dislocation slip dominates the whole tensile deformation,while both dislocation motions and martensite phase transformation play critical roles in the compressive deformation.The martensite phase transformation is preferred to nucleate at grain or subgrain boundaries due to a medium stacking fault energy(SFE)of^20 m J m^-2.The formation of HCP phase via partial dislocation emission from low-angle grain boundaries offers additional pathways for martensite phase transformation.Our study thus remarkably benefits the understanding of the deformation mechanisms of metastable HEAs.

本文系统研究了具有面心立方结构的亚稳态Fe40Co20Cr20-Mn10Ni10高熵合金在拉伸和压缩变形下的微观结构演变.结果表明,从FCC到HCP的马氏体相变在压缩条件下比在拉伸条件下要高得多,具有明显的拉伸-压缩不对称性.与拉伸试验相比,压缩试验的真应力更大,为马氏体相变的开启提供了更大的驱动力.除马氏体相变外,位错的平面滑移以及少量的变形孪晶在拉伸和压缩过程中均有发现.位错滑移在整个拉伸变形过程中起主导作用,而在压缩变形中位错运动和马氏体相变都起到了关键作用.由于该高熵合金具有中等的层错能(20mJm^-2),因此马氏体相变更倾向于在晶界或亚晶界等高能区域处成核.我们发现了六方马氏体相可以通过从小角晶界发射不全位错来形成,这为马氏体相变提供了新的路径.本研究对于理解亚稳态高熵合金的变形机制具有重要的意义.