Effect of local stress fields on twin characteristics in HCP metals

Abstract

a b s t r a c tWe study the effect of nearest neighboring grains on the propensity for {1012} twin growth in Mg and Zr. Twin lamellae lying within one grain flanked by two neighboring grains with several orientations are considered. The fields of resolved shear stress on the twin system are calculated in the multicrystal using a three-dimensional full-field crystal plasticity Fast Fourier Transform approach. The calculations were carried out for Mg and Zr using slip threshold stresses corresponding to 300 K and 76 K, respectively, where twin activity is important. We show that the neighboring grain constraint tends to oppose further growth and that the critical applied stress needed to overcome this resistance depends on neighboring grain orientation, more strongly in Zr than in Mg. We also present results for a pair of adjacent and parallel twins at various spacings. It is found that their paired interaction increases the resistive forces for twin growth above that for an isolated twin. The critical spacing above which this enhanced resistance is removed is smaller for Zr than Mg. Our analysis reveals that these two disparate responses of Zr and Mg are both a consequence of the fact that Zr is elastically and plastically more anisotropic than Mg. Additional calculations carried out on Ti support this conclusion. These findings can help explain why, for the same grain size, more twins per grain form in Zr than in Mg, twins in Zr tend to be thinner than those in Mg, and the relationship between the thickness of the twin and its Schmid factor in Zr is not as strong as in Mg.Published by Elsevier Ltd on behalf of Acta Materialia Inc.

Shifting the Shear Paradigm in the Crystallographic Models of Displacive Transformations in Metals and Alloys

“…In addition, it was experimentally noticed that some twinning modes in magnesium have "abnormal", i.e., "non-Schmid" behavior [18]. The reason was attributed to the fact that the local stresses differ from the global ones [19], but to our knowledge, there are no convincing quantitative experimental results that can confirm this explanation. Non-Schmid behavior was also observed at least for slip deformation in bcc metals.…”

Deformation twinning and grain partitioning in a hexagonal close-packed magnesium alloy

“…Past research concentrated mostly in characterizing these processes with transmission electron microscopy (TEM) on post-deformation specimens 21–24 . Most recently, in situ experiments 7,25–29 , full-field crystal plasticity modeling 30–35 and atomistic simulations 5,36,39 have expanded the understanding of twin formation and its effect on material behavior. There is an interrelation between twin domain, twin interface, crystallographic defects and local stress fields that conditions twin propagation and hardening.…”

“…There is an interrelation between twin domain, twin interface, crystallographic defects and local stress fields that conditions twin propagation and hardening. A twin transformation (~13% shear for {} tensile twin in Mg) induces local stress fields in the vicinity of twins 35–40 . The magnitude of these local stresses is determined, in part, by the ability of neighboring grains to accommodate the transformation.…”

“…The magnitude of these local stresses is determined, in part, by the ability of neighboring grains to accommodate the transformation. Recently, experimental and modeling efforts were devoted to characterize the twin local stresses and the role of neighbors 7,29,32,35,40–47 . Among all the experimental literature, only the work of Balogh et al 44  focuses on the local elastic strain within and in the vicinity of a twin inside a bulk polycrystalline material.…”

Strong grain neighbour effects in polycrystals

“…While recent experimental measurements and modeling studies have revealed large localized stress fields near the tips of twins 10 – 12 , it is generally believed that twin transformation strain is one of the key factors that can relax grains resolved stresses in HCP alloys 13 . For instance, with the use of neutron diffraction it was shown that formation of twins results in stress relaxation in the texture component selected by the diffraction of an incident beam 14 , 15 .…”