Deformation mechanisms of mechanically induced phase transformations in iron

Abstract

Effects of uniaxial tensile directions on the phase transformations in a nano-sized face centred cubic (fcc) crystal in iron were studied by molecular dynamics simulations. The results show that externally applied loading induced phase transformations from fcc phase to body centred cubic (bcc) phase, and variant selection took place depending on the tensile direction relative to the crystallographic orientation of the fcc crystal. For the loading along the close packed fcc plane, the uniaxial tensile direction perpendicular to the close packed direction led to the formation of flat-ellipsoid bcc nuclei composed of {0 1 1} twinning martensite, while that parallel to the close packed direction resulted in zigzag bcc plates with the {1 1 2} twinning sub-structure. When the tensile loading was perpendicular to the close packed fcc planes, a number of stacking faults as a result of the movement of Shockley partial dislocations appeared temporarily in the shape of “funnels”, which distorted the fcc lattices severely before it transformed into the bcc phase. The deformation mechanisms of the mechanically induced phase transformations through twinning or dislocations revealed in the present study may provide clues to develop iron or iron-based shape memory alloys with both high strength and good ductility.