Tensile deformation characteristics and high-plasticity mechanism of directionally solidified pure magnesium

Abstract

The pure Mg with columnar crystals was prepared by directional solidification, and the effect of process parameters on the crystal orientation and tensile properties was studied. Moreover, the microstructure evolution during tensile deformation was analyzed by electron backscatter diffraction (EBSD) technology. Furthermore, the slip within adjacent grains and grain boundary strain were discussed using the bicrystal model theory. The results show that the microstructure of the pure Mg at a pulling rate of 200 μm/s is columnar polycrystal with growth orientation concentrated in < 022ˉ5 >, and no transverse grain boundaries can be seen. In addition, the Schmid factors (SFs) of basal slips in columnar crystals are higher than 0.43 under tensile stress. Moreover, the geometric compatibility factor of slip systems on both sides of grain boundaries is greater than 0.7, showing good strain coordination ability of grain boundaries. Therefore, the elongation of the directionally solidified pure Mg is as high as 53% at room temperature.

1. Introduction

Casting magnesium alloys often show poor strain compatibility of grain boundaries and low plasticity because there are only two independent basal slip systems at room temperature, while at least 5 independent slip systems are required to coordinate grain boundary strain during deformation of equiaxed crystals. Therefore, it is very important to optimize the grain morphology and crystal orientation, and to improve strain compatibility of grain boundaries for developing high-performance Mg alloys.

Directional solidification is a technology making metals grow along the required direction during solidification to form columnar grains with preferred orientation. LIAN et al prepared Mg-6.3Gd alloy with growth orientation concentrated in <21ˉ1ˉ0> and Mg-8.2Gd alloy with growth orientation concentrated in <32ˉ1ˉ4> by directional solidification, and found that the Mg-6.3Gd alloy had high yield strength (176 MPa), and the Mg-8.2Gd alloy had good plasticity (elongation of 22%). ZHANG et al investigated the tensile properties of the directionally solidified Mg-0.8Ca alloy, and the results showed that the mechanical properties of the alloy were significantly improved, especially for the elongation (over 20%). CHAI et al studied the directionally solidified Mg-Gd-Y alloy and found that the alloy had excellent tensile plasticity (elongation of 32.8%) and good strain hardening ability. ZHAO et al reported that the elongation of the directionally solidified Mg-4.5Zn-0.5Y-0.1Zr alloy was as high as 30% at room temperature, and the yield strength was up to 178 MPa simultaneously. Furthermore, the researches on Mg-4.0Zn, Mg-3.0Nd-1.5Gd, Mg-2.35Gd, and Mg-1.5Mn-Al also indicated that the mechanical properties of the alloys can be improved by directional solidification. WANG et al, WANG et al, YANG et al, and LUO et al found that the grain morphology and the primary dendrite arm spacing of columnar crystals can be adjusted by controlling temperature gradient (GL) and pulling rate (R), thereby improving the mechanical properties of the Mg-Gd alloys. GENG et al studied directionally solidified Mg-xGd (x=4 wt.% − 12 wt.%) alloys, and found that the Gd content influenced the mechanical properties by changing the growth orientation of the columnar crystals. Similarly, FANG et al stated that the growth orientation of the columnar crystals in the directionally solidified Mg-Gd-Y-Zr alloy greatly affected deformation mode and strain compatibility of grain boundaries.

At present, there are few reports on the relationship of the growth orientation of columnar crystals, deformation mode, and strain compatibility of grain boundaries. In this study, pure Mg with columnar crystals was prepared by directional solidification. One of the aims is to study the effect of process parameters on the growth orientation of columnar crystals. Another aim is to investigate the relationship of the crystal orientation, strain compatibility of grain boundaries and mechanical properties. On this basis, it is expected to reveal the mechanism improving the plasticity.