On the interaction of precipitates and tensile twins in magnesium alloys

C Liu,P Shanthraj,J.D Robson,M Diehl,S Dong,J Dong,W Ding,D Raabe

doi:10.1016/j.actamat.2019.07.046

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.

Highlights

Wrought magnesium (Mg) and Mg alloys usually exhibit significant plastic anisotropy due to their strong crystallographic textures and the large differences in the critical resolved shear stress (CRSS) for the various deformation modes, i.e. basal a, prismatic a, pyramidal a + c slip systems, and tensile twins [1,2,3,4,5,6,7,8,9]
For strongly textured Mg alloys, the relative increase of the CRSS for twin growth compared to that of prismatic slip due to precipitation strengthening has been suggested to play an important role for the yield asymmetry [10]
Plate-shaped precipitates are generally expected to be effective strengtheners against tensile twin growth since they produce high incompatibility stress and remain unsheared in the twinned region. β − Mg17Al12 basal plate precipitates are commonly observed in Mg-Al alloy systems and have a body-centered-cubic (BCC) structure

Summary

Introduction

Wrought magnesium (Mg) and Mg alloys usually exhibit significant plastic anisotropy due to their strong crystallographic textures and the large differences in the critical resolved shear stress (CRSS) for the various deformation modes, i.e. basal a , prismatic a , pyramidal a + c slip systems, and tensile twins [1,2,3,4,5,6,7,8,9]. Based on in situ neutron diffraction experiments and elastoplastic self-consistent (EPSC) modeling, it was found that prismatic plate-shaped precipitates in Mg-RE alloys harden basal slip, leading to an CRSS increase from 12 to 37 MPa, i.e. an enhancement of over 200 % [11]. This effect is much less pronounced in the case of prismatic slip, where the CRSS value was found to increase from 78 to 92 MPa, i.e. by merely 18 % [11]. In that context the interparticle spacing has been identified as a critical microstructural parameter for controlling the hardening effect against dislocation slip [21, 23]

Results

Discussion

Conclusion