Elsevier

International Journal of Plasticity

Volume 123, December 2019, Pages 121-132
International Journal of Plasticity

Towards high ductility in magnesium alloys - The role of intergranular deformation

https://doi.org/10.1016/j.ijplas.2019.07.014Get rights and content

Highlights

  • Most of slip traces were formed in pairs across grain boundary.

  • Intergranular deformation ability was measured by combining Schmid factor and geometric compatibility factor m’.

  • Improving strain accommodation around the grain boundary interfaces can improve the ductility of Mg alloys.

Abstract

This paper investigates the intergranular deformation behavior and its effect on the overall ductility of a Mg-Gd-Y alloy, using in-situ tension in scanning electron microscopy (SEM) combined with electron backscattered diffraction (EBSD) and digital image correlation (DIC) techniques. At regions surrounding grain boundaries, the majority of activated dislocation slip traces were found to form in pairs across grain boundary, and the basal-to-basal (B-B) slip pair is the dominant type. Strain accommodation around the grain boundary interfaces is affected by both Schmid factor in the adjacent grains and the m’ value between these main slip systems. A mk value was proposed in this paper as the maximum value of the product of the Schmid factor and the m’ value of the adjacent grains to measure strain accommodation around the grain boundary interface, where a higher mk value would mean a larger local strain. It is suggested that the overall ductility of a Mg alloy could be evaluated by measuring its average mk value. As an evidence, the Mg-Gd-Y alloy tubes with higher mk value generally show higher elongations during tensile tests in this study.

Introduction

Magnesium is the lightest structural metal and has received increasing attention as a promising lightweight material in the transportation industries (Luo, 2002; Wang et al., 2018). However, the application of wrought magnesium alloys is restricted due to their poor ductility and workability at room temperature, caused by strong basal texture developed during processing and the limited deformation modes at room temperatures (Yoo, 1981). Magnesium, with a hexagonal close-packed (HCP) crystal structure, exhibits a number of deformation modes according to previous studies (Ma et al., 2012; Yoo, 1981): (0001) <11-20> basal <a> slip; {10-10} <11-20> prismatic <a> slip; {11-22} <11-23 second-order pyramidal <c+a> slip; {10-12} <10-11> extension twinning and {10-11} <10-12> contraction twinning.

Polycrystalline magnesium alloys develop internal strains/stresses characteristic of three length-scales, i.e., macro-, intergranular and intragranular (Jin et al., 2015). At intergranular scale, strain mismatch between grains with different orientations due to the elastic and plastic anisotropy at the grain level will result in stress relaxation in individual grains and then the activation of various intragranular deformation modes (Jin et al., 2015; Orozco-Caballero et al., 2017). Thus, the deformation behavior of Mg alloys depends largely on the compatibility of neighboring grains, and then the interactions between neighboring grains would significantly affect the mechanical properties.

Intergranular deformation behavior of magnesium alloys has been a subject of recent research (Bieler et al., 2014; Chelladurai et al., 2019; Martin et al., 2013, 2014; Stanford et al., 2011; Wang and Agnew, 2016; Wang et al., 2014b). Martin et al. (2013) found there was a lack of correlation between local strain and crystallographic orientation in a Mg-Zn-RE-Zr alloy based on digital image correlation (DIC) analysis and full-field viscoplastic Fast Fourier Transform (VPFFT) crystal plasticity simulations. The activation of basal slip adjacent to grains that are deformed predominantly by non-basal slip would accommodate high local strains. Wang et al. (2014b) also identified the so called Non-Schmid behavior of extension twinning in Mg alloys. In their in-situ compression tests, the activation of extension twinning is affected by the local deformation condition. Recently, the integrated crystal plasticity-phase field model for the twinning behavior in Mg alloys shows that strain transfer across grain boundaries plays an important role in twin transmission than the nominal Schmid factor effect(Liu et al., 2018).

The effect of slip transfer at grain boundary on heterogeneous deformation of polycrystals has been of recurring interest. Experimental characterizations of slip transfer in Ti alloys can be found in previous studies(Bieler et al., 2014; Luster and Morris, 1995). The analysis of slip transfer in these studies show that many observations can be rationalized in terms of the geometrical alignment of activated slip systems in neighboring grains.

However, fundamental understanding on the interactions between neighboring grains in Mg alloys is still not clear. Furthermore, the effect of intergranular deformation behavior on mechanical properties is still lacking. In-situ tensile test has been used to characterize the microstructure evaluation process (Jin et al., 2015; Khosravani et al., 2015; Wang et al., 2016). With the combination of electron backscatter diffraction (EBSD) and micro-scale digital image correlation (DIC), the relationship between local strain and grain orientation can be fully studied (Sun et al., 2016). Therefore, it is believed that these methods can be used to investigate the intergranular deformation mechanisms and their effects on mechanical properties of Mg alloys, providing important understanding towards improving the ductility of Mg alloys.

This paper focuses on the intergranular deformation behavior of magnesium alloys, which is critical to their ductility. Experiments were performed on an extruded Mg-Gd-Y alloy, in which the discrete grain orientation distribution and the grain boundary misorientation distribution have been assessed. In-situ tensile tests were used in combination with EBSD and DIC to correlate the activation of deformation modes and micro-scale strain distribution to the microstructure. Interactions between grains during deformation were carefully analyzed and effects of grain orientation and grain boundary compatibility are discussed.

Section snippets

Experimental procedure

The alloy used in this paper with a chemical composition of Mg–8.0Gd–3.0Y (wt%) (GW83) was prepared by direct-chill casting. Tubes were prepared by extrusion using pre-extruded bars as billets. The pre-extruded bar has a diameter of 100 mm and was extruded from cast billets with a diameter of 180 mm. The pre-extruded bars were solution-treated at 773K for 8 h and preheated to about 673K before extrusion. The corresponding extrusion ratio for tubes was about 20 and the extrusion speed was

Initial and deformed microstructure

Fig. 2 shows the initial microstructures of all tubes, where fully recrystallized grains can be observed. The average grain size of tubes A1-A4 is about 25 μm and the average grain size of tubes W1-W3 is about 7 μm. Textures of these extruded tubes are shown in Fig. 3. With the addition of rare-earth elements, textures were weakened in all tubes. The maximum texture intensity in these tubes is only about 5.7 which is significantly lower than that in most of the current wrought Mg alloys. The

Effect of grain orientation and grain boundary on the deformation behavior of Mg alloys

Grain orientation is thought to have the most important effect on deformation behavior of Mg alloys (Dong et al., 2017). In single crystal Mg, mechanical behavior shows severe anisotropy during loading along different directions (E.W. Kelley and W.F. Hosford, 1968). In strong textured polycrystalline Mg alloys, mechanical properties also show an obvious anisotropy (Agnew et al., 2004; Bohlen et al., 2007). Because basal slip system is the main deformation mode during plastic deformation

Conclusions

Intergranular deformation behavior was found to have a significant effect on the deformation behavior and mechanical properties of Mg alloys:

  • 1.

    The vast majority of the activated dislocation slip traces across grain boundaries were found to be basal to basal (B-B) slip pairs. Slip transfer was also observed across grain boundary pairs with basal to prismatic (B-P) slip pairs and basal slip to {10-12} twinning (B-T).

  • 2.

    At regions surrounding grain boundary, a higher Schmid factor in the adjacent

Acknowledgements

The authors gratefully acknowledge the financial supports of the National Key Research and Development Plan (Grant No.2016YFB0701201, No. 2016YFB0301103) and the National Natural Science Foundation of China (Grant No. 51771109, No. 51631006). The first author would also like to express his gratitude to China Scholarship Council for supporting his stay at The Ohio State University as a visiting scholar.

References (30)

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