On the interaction of precipitates and tensile twins in magnesium alloys

Abstract

Although magnesium alloys deform extensively through shear strains and crystallographic re-orientations associated with the growth of twins, little is known about the strengthening mechanisms associated with this deformation mode. A crystal plasticity based phase field model for twinning is employed in this work to study the strengthening mechanisms resulting from the interaction between twin growth and precipitates. The full-field simulations reveal in great detail the pinning and de-pinning of a twin boundary at individual precipitates, resulting in a maximum resistance to twin growth when the precipitate is partially embedded in the twin. Furthermore, statistically representative precipitate distributions are used to systematically investigate the influence of key microstructural parameters such as precipitate orientation, volume fraction, size, and aspect ratio on the resistance to twin growth. The results indicate that the effective critical resolved shear stress (CRSS) for twin growth increases linearly with precipitate volume fraction and aspect ratio. For a constant volume fraction of precipitates, reduction of the precipitate size below a critical level produces a strong increase in the CRSS due to the Orowan-like strengthening mechanism between the twin interface and precipitates. Above this level the CRSS is size independent. The results are quantitatively and qualitatively comparable with experimental measurements and predictions of mean-field strengthening models. Based on the results, guidelines for the design of high strength magnesium alloys are discussed.

Solving Material Mechanics and Multiphysics Problems of Metals with Complex Microstructures Using DAMASK—The Düsseldorf Advanced Material Simulation Kit

“…The studies presented here span the scale of individual dislocations (Section ) over single grains (Section ) up to grain aggregates (Section ). Especially for studying the interaction of individual grains, it is a common approach to use experimentally obtained microstructures as input for the simulations .…”

“…This model allows the concurrent description of dislocation plasticity and heterogeneous twinning behavior including twin nucleation, propagation, and growth in Mg alloys and was implemented in DAMASK. In the following, the essential aspects of the model have been compiled from Liu et al and its capabilities are demonstrated with the help of three application examples …”

“…As a last example, the influence of precipitates on the twinning behavior—see Liu et al for further details—is studied. Precipitation hardening is a well‐known mechanism for increasing the yield strength.…”

“…c) Evolution of the CRSS increment (), average basal ( ρ basal ), prismatic ( ρ prismatic ), and pyramidal ( ρ pyramidal ) dislocation density in the twinned region at a shear strain of 8.3% as a function of precipitate size for a precipitate volume fraction of 4%. Adopted under the terms of the CC BY 4.0 license . Copyright 2019, C. Liu et al…”

“…The simulations also reveal that the mean back‐stress in the twin band is only slightly influenced by the precipitate size (see Figure c). Based on these observations, Liu et al concluded that the hardening effect of smaller precipitates is governed by an Orowan‐like mechanism while the back‐stress mechanism is dominant for larger precipitates.…”