Magnesium alloy is the lightest metal structural material, so the wide industrial application of magnesium alloy can effectively achieve the purpose of energy saving and emission reduction. However, the low strength and poor formability limit the wide application of magnesium alloys. The low strength of magnesium alloy is closely related to the low critical shear stress of the two deformation mechanisms of substrate slip and tensile twin. Scholars from all over the world have carried out a lot of research in an attempt to achieve magnesium alloy strengthening. Age hardening is an effective way to strengthen alloys, but its strengthening effect in magnesium alloys is not very ideal. Because the rare earth atoms have a large atomic size mismatch degree and strong binding force with magnesium atoms in magnesium alloys, a higher level of solution strengthening effect of magnesium alloys can be achieved. In addition, rare earth atoms have a higher solid solubility in magnesium alloys at high temperatures, and their solid solubility decreases significantly with the decrease of temperature, so that rare earth magnesium alloys often have more significant aging hardening ability. However, compared with age-hardening aluminum alloys, the age-hardening ability of rare earth magnesium alloys is still unsatisfactory. An in-depth understanding of the interaction between the precipitated phase and various deformation mechanisms in magnesium alloys can provide theoretical guidance for the preparation of high-performance magnesium alloys.
Recently, the research group of Prof. Wenchen Xu and Prof. Debin Shan of Harbin Institute of Technology, together with Maria Teresa Pérez-Prado et al. from the Institute of Advanced Materials in Madrid, Spain, used microscale mechanics experimental methods to study the interaction between the precipitated phase and the base dislocation and tensile twins in the MG-GD-Y-ZR alloy. The results show that during the compression process of the basal slip oriented single crystal, the flow instability and softening phenomenon occur due to the formation of shear bands by the shear precipitate phase of the basal dislocation. When compressed perpendicular to the single crystal c-axis, the yield of the solution single crystal is caused by the active cylindrical slip. After aging, the yield is dominated by the tensile twin due to the softening of the twin nucleation stress. When compressed along the C-axis of a single crystal, it is an age-hardening phenomenon.
Figure 1 shows mg-5gd-2y-03zr (wt.% transmission electron microstructure before and after alloy aging. Figure 1a shows that the homogenized alloy is a supersaturated solid solution without any precipitated phase in the alloy structure. After 200 aging and 80 h, a large number of fine and diffuse second phases were precipitated, with a width (W) of about 16 nm, a thickness of about 17 nm, and a length of about 55 N5 nm as shown in Figure 1B-C. The corresponding selective electron diffraction results show that the precipitated phase is'phase (mg7re). According to Equation (1):
The precipitated phase volume fraction vf can be estimated as 31%。where n a is the number density of the precipitated phase, and its value is 2075×1015 m-2；h is the thickness of the transmission specimen, which is about 100 nm.
Figure 1 (a) Homogenization of mg-5gd-2y-03ZR alloy TEM brightfield image; (b) and (c) are mg-5gd-2y-0Haddf image of 3Zr alloy after 200 aging for 80 h.
In this paper, the aging hardening behavior of a single crystal with a soft substrate orientation is first studied, and the experimental results are shown in Figure 2. The angle between the C-axis and the compression direction of the oriented single crystal is about 37°. As shown in Fig. 2a-b, the slip traces of both homogenized and aged single crystals are parallel to the base of the crystal after compression, indicating that the basal slip system is activated during the compression of the single crystal. A large number of basal slip traces appear on the homogenized single crystal compression surface, which are distributed in the deformation region with a width of about 1 m. However, only four slip traces appeared on the surface of the aging single crystal after compression, and the slip steps were clearer and smoother. Comparing the stress-strain curves of homogenized single crystal and aged single crystal compressive engineering (as shown in Fig. 2c-d), it can be found that aging precipitation has no obvious effect on the overall strength level of single crystal, but it has a significant effect on the flow stability of single crystal. There is a large number of instantaneous stress reduction-re-recovery phenomena in the compressive stress-strain curve of homogenized single crystals, which corresponds to the continuous formation of slip traces on the surface of single crystals. The yield stress of the single crystal is about 72 MPa, so the basal slip system CRSS is about 33 MPa. In the process of aging single crystal compression, the instantaneous stress reduction-re-recovery phenomenon is significantly reduced, and the flow curve has a wide stress oscillation, indicating that the flow instability has occurred.
In order to reveal the effect of precipitation of the slip system relative to the soft substrate, we further observed the transmission samples extracted from the compressed aging single crystal, and the experimental results are shown in Figure 3. After aging single crystal compression, a very narrow shear band with a width of about 30-40 nm is formed inside it, as shown in Figure 3a. The precipitate phase appears to have back-dissolved within the shear zone. However, the precipitated phase near the shear zone is obviously sheared by the base dislocation to form a shear step, as shown in Figure 3b-c. The results show that the base dislocations can be sheared during the compression of soft substrate-oriented single crystals'phase, which induces the softening of the slip path, resulting in the formation of shear bands and the occurrence of aging softening.
Fig.2 Secondary electron images of the compressed surface of a single crystal with a soft substrate orientation (37°) and the corresponding compressive engineering stress-strain curves: (a) and (b) are homogenized and aged secondary electron images of the single crystal surface, respectively. (c) and (d) are the stress-strain curves of homogenization and aging single crystal compression engineering, respectively.
Fig.3 Ageing of a single crystal transverse TEM with a soft substrate orientation with a compressive strain of 15%: (a) TEM brightfield phase, (b) and (c) high-resolution STEM phase near shear. Ribbon axis z = [11-20].
Secondly, the deformation behavior of a single crystal when compressed perpendicular to the c-axis of the crystal is studied, and the experimental results are shown in Figure 4. In this study, the angle between the C-axis and the compression axis of the single crystal is 85-88°, and the tensile twin and cylindrical slip systems have a large Schmidt factor (SFTWIN>043, sfprism>0.48）。Secondary electron images of the homogenized single crystal surface after 6% compressive strain in Figure 4a and B are 15% and 15%, respectively. As shown in Figure 4a, when the compressive strain is 6%, only a cylindrical slip trace appears on the surface of the single crystal, and no tensile twin trace appears. The corresponding transmission structure further confirms that the tensile twins are not activated during the deformation process. When the compressive strain of the homogenized single crystal reaches 15%, the single crystal pillar is distorted, as shown in Figure 4b. The corresponding TKD results show that the lattice rotation occurs in the kink region, which activates the basal slip system in this region. In addition, from the TKD results, we can also find that at 15% strain, a smaller tensile twin is activated in the homogenized single crystal, but the twin propagation is inhibited. By analyzing the surface of the aged single crystal with a compressive strain of 15% (Fig. 4c), we can find that the (0-112)[01-11] tensile twin and the basal slip system within the twin are activated during the deformation of the single crystal. Through the observation of the transmission sample extracted from the compressed aged single crystal, we can find that the tensile twin expands to the entire micropillar volume after the compression of the aged single crystal, and although the orientation of the precipitated phase in the twin region is rotated by nearly 90°, the basal slip system can still effectively shear the precipitated phase, as shown in Fig. 4D-E.
Fig. 4f and g are the stress-strain curves of the homogenized single crystal and the aged single crystal compression engineering, respectively. As shown in Figure 4f, the yield is followed by some degree of strain hardening when homogenizing single crystal compression, followed by significant softening at 6%-9% strain. Combined with the microstructure observation, we can deduce that the orientation homogenization single crystal yield and strain hardening are due to cylindrical slip, while the significant softening is due to the micropillar kinking, and the small strain jump on the curve is due to the small volume of twin nucleation. Based on the yield stress, the homogenized cylindrical slip CRSS is about 108 MPa, while the tensile twin CRSS should be greater than 97 MPa. Compared with homogenized single crystals, the yield stress of aging single crystals is significantly reduced, which is mainly due to the fact that aging reduces the concentration of solution atoms in the matrix, thereby reducing the critical shear stress of twin crystal nucleation. Based on the yield strength, the CRSS of the tensile twin in the aged single crystal can be estimated to be 62 MPa.
Fig.4 Quadratic electron diagram (a-c) and transmitted electron structure (d-e) of a single crystal compressed perpendicular to the c-axis of the crystal and the corresponding stress-strain curves of compressive engineering: (a) and (b) are 6% strain and 15% strain homogenized single crystal secondary electron diagram, (c) aging single crystal secondary electron diagram with 15% strain, (d) and (f) are 15% aged single crystal transmission electron microstructure, and (f) and (g) are homogenized and aged single crystal compressive engineering stress-strain curves, respectively.
In this study, the deformation behavior of single crystals when compressed along the C-axis of the crystal was also explored, and the experimental results are shown in Figure 5. In this study, the angle between the C-axis and the compression axis of the single crystal is about 5-7°. In this orientation, although the Schmidt factor of the basal slip system is smaller, about 01, however, due to the small substrate slip CRSS, the oriented single crystal is still dominated by the substrate slip, as shown in Figure 5a-b. The yield stress of the homogenized single crystal is about 425 MPa, so the critical shear stress of the basal slip is about 40 MPa, which is higher than the marginal shear stress calculated for the soft basal oriented single crystal. It is generally accepted that the critical shear stress is constant for a particular deformation mechanism, independent of orientation. However, it has been shown that the normal vertical partial stress applied to the dislocation core structure will affect the measured critical shear stress of this mechanism, so the same deformation mechanism will also exhibit different critical shear stresses with different single crystal orientations. In this orientation, the critical shear stress of the aging single crystal is 52 MPa, so the aging induces the hard-oriented basal slip strengthening.
Fig.5 Secondary electron images (a-b) of a single crystal compressed along the C-axis of the crystal and the stress-strain curves (c-d) :(a) and (b) of the surface of a homogenized single crystal and an aged single crystal with a strain of 15%, respectively, and (c) and (d) are the stress-strain curves of the compressive engineering of a homogenized single crystal and an aged single crystal, respectively.
In summary, the experimental method of microscale mechanics was used to study the essential causes of the low-aging hardening of MG-GD-Y-ZR alloys. The results show that the softening of the slip path caused by the shear precipitation phase of the soft substrate oriented single crystal during the deformation process due to the dislocation of the substrate, resulting in flow instability and stress softening. When compressed perpendicular to the c-axis of a single crystal, the nucleation stress of the tensile twin crystal decreases due to the desolubilization of solid solution atoms induced by aging. Along the single.