Understanding the influence of yttrium on the dominant twinning mode and local mechanical field evolution in extruded Mg-Y alloys
📝 Abstract
Twinning is a primary deformation mechanism in Mg alloys. This study focuses on tension twins during uniaxial compression of Mg-Y alloys, with three key aspects: the orientation specificity of twin grains, the relative evolution of CRSS with increasing Y content, and the local stress and strain evolution at twin sites. Experimental characterization and crystal plasticity modeling were performed. In Mg-7wt.%Y, TT2-{112-1} tension twins were observed in addition to the common TT1-{101-2} twins. Increasing Y suppressed TT1 formation while promoting TT2 activity. A previously unreported group of crystallographic orientations with a higher global Schmid factor for <c+a> slip was identified, which exhibited TT1 twinning with increasing compression strain. To elucidate Y effects on twin activity and local mechanical fields, both TT1 and TT2 tension twin modes were incorporated into PRISMS-Plasticity, an open-source, finite element-based crystal plasticity solver. Four binary Mg-Y alloys were modeled under compression, and statistical analysis was conducted to correlate initial orientations, stress-strain distributions, and twin activities as functions of Y concentration. The plasticity analysis revealed that increasing Y decreases the CRSS ratio of prismatic and pyramidal slip relative to TT1 twinning, while the slip-to-twin CRSS ratio for TT2 increases, thereby serving as a potential indicator of differential twin activity with Y addition in Mg alloys. Additionally, despite their small volume fraction, TT2 twin sites were predicted higher local strain accumulation locally, relative to the representative volume element and TT1 twins, suggesting their potential influence on localized phenomena such as recrystallization or twin nucleation. These findings provide insight into local mechanical behavior in Mg alloys and support alloy design for advanced engineering applications.
💡 Analysis
Twinning is a primary deformation mechanism in Mg alloys. This study focuses on tension twins during uniaxial compression of Mg-Y alloys, with three key aspects: the orientation specificity of twin grains, the relative evolution of CRSS with increasing Y content, and the local stress and strain evolution at twin sites. Experimental characterization and crystal plasticity modeling were performed. In Mg-7wt.%Y, TT2-{112-1} tension twins were observed in addition to the common TT1-{101-2} twins. Increasing Y suppressed TT1 formation while promoting TT2 activity. A previously unreported group of crystallographic orientations with a higher global Schmid factor for <c+a> slip was identified, which exhibited TT1 twinning with increasing compression strain. To elucidate Y effects on twin activity and local mechanical fields, both TT1 and TT2 tension twin modes were incorporated into PRISMS-Plasticity, an open-source, finite element-based crystal plasticity solver. Four binary Mg-Y alloys were modeled under compression, and statistical analysis was conducted to correlate initial orientations, stress-strain distributions, and twin activities as functions of Y concentration. The plasticity analysis revealed that increasing Y decreases the CRSS ratio of prismatic and pyramidal slip relative to TT1 twinning, while the slip-to-twin CRSS ratio for TT2 increases, thereby serving as a potential indicator of differential twin activity with Y addition in Mg alloys. Additionally, despite their small volume fraction, TT2 twin sites were predicted higher local strain accumulation locally, relative to the representative volume element and TT1 twins, suggesting their potential influence on localized phenomena such as recrystallization or twin nucleation. These findings provide insight into local mechanical behavior in Mg alloys and support alloy design for advanced engineering applications.
📄 Content
Magnesium alloys, with properties such as low density, high specific strength, and high elastic modulus, can provide economical and environmentally friendly alternatives in transportation, aerospace, and energy applications. [1,2]. To further improve the strength, Mg alloys with rare-earth elements (RE) additions have been developed [3,4]. RE alloying additions, like yttrium (Y), weaken the strong basal texture formation during manufacturing and improve the formability of Mg [5,6]. It was also reported that the Y addition benefits the alloy’s ductility [7,8], creep resistance [9,10] and corrosion resistance [11].
While the most notable effect of Y addition in Mg is activating non-basal slip, such as pyramidal <c+a> dislocations [12][13][14], the influence of Y on twinning behavior is also of great interest.
Twin formation significantly affects the mechanical behavior of Mg by accommodating deformation along the c-axis. Figure 1 presents a schematic of two types of tension twins, TT1 twins -{101 # 2} <101 # 1 # > and TT2 -twins {112 # 1} 〈1 # 1 # 26〉. Both twin types accommodate tensile strain along the
TT1 twins are commonly observed in Mg alloys. Extensive characterization has been conducted to understand the nucleation, propagation, and twin interactions of the TT1 twins during deformation.
Although the TT2 twin is observed in HCP metals like Co [18], Ti alloys [19], and Zr alloys [20,21], it is less commonly observed in the Mg alloys. Nonetheless, observations of the TT2 twins have been reported in various Mg-RE alloys such as WE54 [22], WE43 [23], Mg-Gd-Y-Zr [24], and Mg-17wt.%Gd [25].
Specifically with Y addition, the TT2 type twinning was observed in Mg-9wt.%Y [26] and Mg-10wt.%Y [27].
With the emerging research interest in TT2 twin formation, many state-of-the-art experimental techniques, as well as modeling methods, have been employed to understand TT2 twinning in the Mg alloys. For example, using TEM/STEM analysis, Chen et al studied the twin-twin interactions of the TT2 twin variants in WE43 [23], while Zhang et al studied the TT2 activity at a higher strain rate in Mg-Gd-Y-Zr alloy [24]. Li et al. observed TT2 twin activation prior to the activation of TT1 twins in solutionized Mg-Gd-Y alloy; however, the sequence of twin occurrence was reversed in alloys subjected to additional aging treatment [28]. Gengor et al. calculated the Generalized Planar Fault Energy (GPFE) for TT2 twin variants in HCP materials using ab-initio simulations and emphasized the nonzero shuffle distortions normal to the shear plane for the TT2 twin [29]. The authors reported a nonlinear positive correlation between the elastic stiffness coefficient C44 and twin boundary energy. Thus, most of the recent research has been focused on understanding twin energetics or twin activation in different heat-treated or deformation conditions. However, a detailed analysis of the TT2 twinning activity and its influence during deformation in Mg alloys is still lacking in the literature. Hence, the current study employs detailed experimental characterization and crystal plasticity analysis to gain further insights into correlations between the TT2 twin activity and micromechanical field evolutions during the deformation. Very few studies in the literature have employed crystal plasticity for Mg-Y alloys. For example, Li et al. conducted crystal plasticity finite element (CPFE) analysis for Mg-0.8wt.%Y, which showed higher non-basal slip activity and lower twin activity at higher strains [30]. Huang et al utilized CPFE analysis to understand the anomalous TT1 twin activity in Mg-2wt.%Y alloy [31], while Su et al. utilized nanoindentation and CPFE of Mg-2wt.%Y alloys for analyzing orientation dependence during the deformation [32]. Kula et al. illustrated a monotonic increase in the critical resolved shear stresses (CRSS) for active deformation modes with increasing Y addition [33]. These investigations all involved alloys with relatively low Y concentrations; thus, the
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