Elsevier

Journal of Alloys and Compounds

Volume 692, 25 January 2017, Pages 898-902
Journal of Alloys and Compounds

Activations of stacking faults in the calcium-containing magnesium alloys under compression

https://doi.org/10.1016/j.jallcom.2016.09.128Get rights and content

Highlights

  • SFs activations in Mg alloys are closely associated with twining migrations.

  • Increasing both Ca concentration and strain rate promotes deformation twinning.

  • The enhanced twinnability contributes to the higher density of SFs formations.

Abstract

Stacking fault (SF) is critically important for both the strength and ductility improvement in Mg alloys, whereas understanding the controlling factors of inducing deformation SFs is insufficient due to the complexity. In this study, deformation mechanism of the calcium-containing Mg alloys is examined under different Ca concentrations and strain rates. It is found that the SFs activations in Mg alloys are closely associated with twining migrations. Increasing both Ca concentration and strain rate promotes deformation twinning, which in turn contributes to the higher density of SFs formations.

Introduction

Magnesium alloys with low density and high specific strengths exhibit great potential of reducing weight of vehicles [1], [2], [3], [4], [5], [6]. However, Mg-based alloys also face challenge of poor formability at room temperature (RT) due to the limited number of easy slip systems, i.e., basal <a> slip (0001)1¯21¯0, while non-basal slip systems are harder to be activated [7], [8]. {101¯2}101¯1 extension twinning is an important deformation mode in accommodating strain along the c-axis [9], [10]. For example, Zhang et al. improved formability of Mg alloys by controlling the texture components and activating extension twining [11], [12]. Recently, <c+a> pyramidal slip of {{112¯2}1¯1¯23} is effectively enhanced in some RT deformed Mg alloys by either alloying or raising strain rate [13], [14], [15], [16], which can accommodate strains along c-axis of α-Mg matrix and enhance the ductility [17].

Additionally, another crystal defect, known as stacking fault (SF) is also found to be activated in Mg under some particular conditions [18], [19], [20]. For example, Sandlöbes et al. [18] observed both profuse SFs and <c+a> dislocations in cold-rolled Mg-Y alloy at RT, but no SF is found in pure Mg. In contrast, Li et al. [19] present observations of SFs in compressed pure Mg alloy, while the deformation rate is not mentioned. Recently, Zhang et al. [20] reported that SFs can be readily formed in both as-extruded pure Mg and AZ31 alloy under initial strain rate of 10−3/s at RT. In other word, both the composition and loading rate/orientation can affect the SFs formation in Mg alloys. That is, understanding the controlling factors for inducing deformation SFs is far from enough for guiding the alloying design of Mg alloys [21], [22], [23]. To address the issues, the representative as-extruded Mg-2Sn-1Ca (TX21) Mg-2Sn-2Ca (TX22) and Mg-2Ca (X2) alloys were compressed along extrusion direction (ED) to strain value of ∼7% under initial strain rates of both 10−3/s and 0.667/s, aiming to clarify the composition or strain rate dependence of activations of SFs. The results would be beneficial for both strength and ductility improvement in Mg alloys.

Section snippets

Experimental

The Mg-2Sn-1Ca, Mg-2Sn-2Ca and Mg-2Ca alloy ingots were prepared by casting, homogenization and hot extrusion. For detail, please refer to [15]. In the following sections, it is noted that TX21, TX22 and X2 represent Mg-2Sn-1Ca, Mg-2Sn-2Ca and Mg-2Ca alloy, respectively. The compression deformation was conducted on the Shimadzu AG-XPlus250 kN at strain rates of both 0.667/s and 1 × 10−3/s along the extrusion direction. The developed textures of as-compressed samples were measured via X-ray

Results and discussion

The initial microstructures of above Ca-containing samples are similar after processing, including the average grain size of 1–2 μm and the same typical fiber textures with comparable maximum intensities. The only difference is the Ca contents dissolved in α-Mg matrix, i.e., TX21 < TX22 < X2. For detail, please refer to [15]. After ∼7% of compression under the strain rate of 0.667/s, a basal texture with c-axis paralleling with ED is readily formed (Fig. 1d–f). In fact, macro-textures of the 3%

Conclusions

In summary, the activation of SFs in Mg alloys is found to be closely associated with deformation twinning, and both composition and strain rate could be the controlling factors. Increasing Ca concentration in our samples may enhance deformation twinning, which in turn shift the dominant deformation mode from basal slipping in TX21 alloy to activations of the stacking faults in X2 alloy under the same lower strain rate. Increasing strain rate can also promote deformation twinning, and

Acknowledgement

The authors acknowledge the financial support from National Natural Science Foundation of China (No. 51525101, No. 51501032, and No. 51371046), and Fundamental Research Funds for the Central Universities (No. N130510002, No. N141003001, and No. L1502047).

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