Twinning-like lattice reorientation without a crystallographic twinning plane

Abstract

Twinning on the plane is a common mode of plastic deformation for hexagonal-close-packed metals. Here we report, by monitoring the deformation of submicron-sized single-crystal magnesium compressed normal to its prismatic plane with transmission electron microscopy, the reorientation of the parent lattice to a ‘twin’ lattice, producing an orientational relationship akin to that of the conventional twinning, but without a crystallographic mirror plane, and giving plastic strain that is not simple shear. Aberration-corrected transmission electron microscopy observations reveal that the boundary between the parent lattice and the ‘twin’ lattice is composed predominantly of semicoherent basal/prismatic interfaces instead of the twinning plane. The migration of this boundary is dominated by the movement of these interfaces undergoing basal/prismatic transformation via local rearrangements of atoms. This newly discovered deformation mode by boundary motion mimics conventional deformation twinning but is distinct from the latter and, as such, broadens the known mechanisms of plasticity.

Interface structures and twinning mechanisms of twins in hexagonal metals

“…Such a limited height step or the tip of a thin twin has been observed in atomistic simulations [50,51,88] and experiments [63,65]. Figure 12 is a plot of the variation of PB or BP step/facet heights in Mg and Co [60,[62][63][64][65]93,94]. Significantly, many have heights other than the stress-free height: they have Burgers vector content.…”

“…In the hcp system, arrays of TDs often relax into a PB or BP facet, as demonstrated in numerous experimental observation and MD simulations [21,22,30,31,[58][59][60][61][62][63][64][65][66][67]. These facets correspond to disclination dipoles [26,30,59].…”

“…Twin nuclei, whether formed homogeneously or heterogeneously, are bounded by CPB and CBP interfaces [32,60,94] with attendant large coherency stresses (Figure 6(d)) partitioned to the nucleus because of the stiffness of the matrix. In addition, there are local stresses associated with domain disclinations at the corners, characterized in Figure 6(a).…”

“…The basis for the proposal is the observation of a PB facet with no other defects present [49]. The TEM micrographs and analysis in [60,[62][63][64][65][93][94][95] show PB facets with varying sizes, Figure 12, many of which are not pure steps, i.e. not 15 or 30 planes high.…”

Twinning effects on strength and plasticity of metallic materials

“…62 , 63 Under some circumstances, the migration of BP/PBs can dominate twin growth, resulting in signifi cant deviations of the TB away from the {10 1 2} plane. 63 Crystal plasticity-based approaches for modeling the constitutive behavior of hcp alloys have proliferated over the past decade. The crux of the problem has involved correctly accounting for the behavior of twinning and detwinning mechanisms in twin nucleation and propagation.…”