A new mechanism for twin growth in Mg alloys

Abstract

Twinning is an important deformation mode in lightweight Mg alloys, but the mechanisms of nucleation and growth of twins and their interactions with solutes remain largely unresolved. Here, a new model for thermally-activated, stress-driven growth of twin boundaries is presented and the role of random and segregated solutes in controlling this growth is studied analytically and using direct molecular dynamics simulations. Twin growth occurs by the thermallyactivated nucleation and expansion of twin dislocation loops on a pre-existing twin boundary. Fluctuations in the local concentration of random solutes lowers the energy barrier for this process and thus facilitates twin growth. Segregation of solutes to the twin boundary strongly increases the energy barrier and suppresses twin growth. In random solid solutions at experimental strain rates, the thermally-activated nucleation process is sufficiently fast at low applied stresses so that growth of the twin is controlled by solute strengthening of the nucleated twin loop as it expands across the twin boundary. Annealing of a deformed sample leads to solute segregation and very strong pinning so that further twinning can only be accomplished by nucleation of new twins, consistent with experimental observations. The new mechanism also (i) operates for detwinning and rationalizes complex twin shapes observed in experiments and (ii) suggests a process for dynamic strain aging commonly observed in Mg alloys at elevated temperatures.

Average-atom interatomic potential for random alloys

“…This provides a way to understand how individual constituent elements behave, on average, in the alloy material and how the behavior varies with the overall alloy composition. This also forms a robust basis for developing predictive models of alloy properties 40,[44][45][46] : the average alloy material is a natural reference state from which one can compute all the necessary model parameters using the A-atom potential alone or combined with the elemental potentials.…”

Annealing strengthening of pre-deformed Mg–10Gd–3Y–0.3Zr alloy

“…Generally, twin nucleation includes homogeneous nucleation from a free surface [7], dislocation-twin interactions [1,8,9], twin-twin interactions [10] and precipitation/ void-induced nucleation [11]. Recently, a new model is proposed for thermally activated, stress-driven growth of twin boundaries [12]. Though these studies can provide more understanding on the twin nucleation and growth, it is still scarce on the effect of solutes on deformation twinning.…”

Observations on the intersection between 101̅2 twin variants sharing the same zone axis in deformed magnesium alloy

“…However, as hexagonal close-packed (hcp) materials, Mg and its alloys exhibit very limited formability at room temperature due to their strong plastic anisotropy, in which the activation of slip with components in the c direction requires very high applied stress [1][2][3]. As a consequence, twinning plays a crucial role during the plastic deformation of Mg and its alloys.…”