[SY-E1] Symposium E-1

[SY-E1] Mechanism of Enhanced Ductility in Mg Alloys

Invited

^○W. Curtin¹, Z. Wu², R. Ahmad¹, B. Yin¹ (1.EPFL, Switzerland, 2.IHPC, Singapore)

Pure Mg has low ductility due to strong plastic anisotropy and due to a transition of pyramidal dislocations to a sessile basal-oriented structure [1]. Alloying generally improves ductility; for instance, Mg-3wt.%RE (RE=Y, Tb, Dy, Ho, Er) alloys show relatively high ductility [2], and typically larger than most commercial Mg-Al-Zn alloys at similar grain sizes. Possible concepts for ductility in alloys include the reduction of plastic anisotropy due to solute strengthening of basal slip, the nucleation of from basal I1 stacking faults, the prevention of the detrimental transformation to sessile structures, and the weakening of strong basal texture by some solute/particle mechanisms. Experiments and modeling do not strongly support these concepts, however. Here, we introduce a new mechanism of pyramidal cross-slip from the lower-energy Pyr. II plane to the higher energy Pyr. I plane as the key to ductility in Mg and alloys [3]. Certain alloying elements reduce the energy difference between Pyr. I and II screw dislocations, accelerating cross-slip that then leads to rapid dislocation multiplication and alleviates the effects of the undesirable pyramidal-to-basal dissociation. A theory for the cross-slip energy barrier is presented, and first-principles density functional theory (DFT) calculations, following methods in [4], are used to compute the necessary pyramidal stacking fault energies as a function of solute type for many solutes in the dilute concentration limit. Predictions of the theory then demonstrate why Rare Earth solutes are highly effective at very low concentrations, and generally capture the trends in ductility and texture evolution across the full range of Mg alloys studied to date. The new mechanism then points in directions for achieving enhanced ductility across a range of non-RE alloys.

[1] Z. Wu, W.A. Curtin, Nature 526 (2015) 62-67
[2] S. Sandlobes, et al., Acta Materialia 59 (2011) 429-439; Acta Materialia 70 (2014) 92-104
[3] Z. Wu, R. Ahmad, B. Yin, S. Sandlobes, and W. A. Curtin, Science 359, 447-452 (2018).
[4] B. Yin, Z. Wu, and W. A. Curtin, Acta Materialia 136 (2017) 249-261.