Atomic-scale observation and characterization of deformation twins in uniaxial tensile-deformed 120Mn13 steel

•Deformation twins with zero net macroscopic strain were frequently distributed in coarse-grained 120Mn13 steel under different engineering stresses, which revealed the fact that the zero-strain twin is ubiquitous in coarse-grained fcc metallic materials.•A new 9R phase with different stacking sequences was first discovered and defined as 9R-Ⅱ to distinguish it from the classical 9R phase.•Three types of precursors were discussed for the zero-strain twin formation.•A new formation and growth mechanism of zero-strain twins based on the 9R phase transformation was proposed. High-resolution transmission electron microscope was used to observe and characterize the different formation stages of zero-strain twins in uniaxial tensile-deformed 120Mn13 steel at the atomic scale, which revealed the growth process of zero-strain twins in coarse-grained austenitic manganese steels. Here, plenty of zero-strain twins are observed in deformed 120Mn13 steel and they can be formed by three types of precursors. Their processes of expansion and connection, thickening and thinning can be carried out through the transformation of 9R phase with the matrix and the twin. A new 9R phase with different stacking sequences is firstly discovered and defined as 9R-Ⅱ to distinguish it from the classical 9R phase (9R-Ⅰ). 9R-Ⅰ and 9R-Ⅱ have the same lattice structure but different internal atomic arrangements and satisfy a twin-like relationship. In addition, a new formation mechanism of zero-strain twins based on the 9R phase transformation is proposed.