Introduction: The ultra-high strength and toughness as-cast Mg-10Gd-1.7Y-1Zn-0.5Zr(wt.%) alloy was prepared by special ultrasonic melt treatment, and its peak aging (200°C, 48h) ultimate tensile strength was studied. The yield strength (UTS), yield strength (YS) and elongation (EL) at room temperature were 430 MPa, 324 MPa and 13.6%, respectively. Ultrasonic treatment in semi-solid conditions refines the grains and hinders the separating eutectic phase (α-Mg+Mg) during the casting process
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Gd). During solution treatment, the refined Mg 3Gd phase inhibited the formation of the cubic GdY phase and the bulk 14H long-period stacked structure (14H-LPSO) phase, and further increased the solute concentration in the matrix. More solute atoms promote the growth of β' plates with an aspect ratio of 8:1, which can increase the critical resolved shear stress (CRSS), YS, and work hardening rate. It was found that the aspect ratio of the β′ plate was the main reason for improving the strength of the alloy under the same composition and heat treatment conditions.
As lightweight structural metal materials, magnesium rare earth alloys such as Mg-Gd-Zn and Mg-Gd-Ag alloys have better mechanical properties than other magnesium alloys, so they are very suitable for use in aerospace, munitions and other fields. At present, the elongation (EL) of high-strength Mg-Gd-based casting alloys with tensile strength (UTS) exceeding 350 MPa is generally <7%, and the elongation (EL) of ultra-high-strength Mg-Gd-based casting alloys is generally <7%. UTS over 400 MPa is usually < 3%. The β plate formed on the (110) prism plane of α-Mg is the most effective barrier to dislocation slip in the dislocation-particle interaction mechanism. However, the γ plate formed on the substrate of (0001) with a large aspect ratio is also considered to be a key strengthening phase that hinders the formation and coarsening of the β phase. More rare earths are typically added to magnesium alloys to increase strength by using higher density nanoprecipitates to pin and store more dislocations, while the introduction of the brittle phase inevitably reduces ductility. In the Exponential Strain Hardening (ESH) model, increasing the work hardening rate is the most important factor for the simultaneous increase in strength and uniform elongation (SISUE) effect. In addition, when the Fe content exceeds 100 ppm, the corrosion resistance and plasticity of the magnesium alloy will be severely reduced. It is important to develop new melting and casting processes to achieve high purity, homogeneous chemical composition, and fine grains. Ultrasonic has a good application in melt purification and grain refinement, which is expected to solve these problems.
To this end, Chongqing University of Technology and the team of Professor Jiang Bin of Chongqing University reported the progress of Mg-10 Gd-1.7Y-1Zn0.5Zr alloy, and further developed the as-cast Mg-10Gd-1.7Y-1Zn-0.5Zr magnesium alloy, and studied the effects of ultrasonic treatment on the microstructure and mechanical properties of the as-cast and aging alloys.
相关研究成果以“An ultra-high strength and toughness as-castMg-10Gd-1.7Y-1Zn-0.5Zr alloy”发表在Journal of Magnesium and Alloys上
Link: https://www.sciencedirect.com/science/article/pii/S2213956724000240
Figure 1. Schematic diagram of the machining routes of the three alloys.
Figure 2. (a) Engineering strain-stress curves for UT3 and (b) work hardening curves for UT3.
Figure 3. (a) XRD patterns of as-cast alloys of UT1, UT2 and UT3, SEM micrographs showing the following as-cast microstructures: (b) UT1, (c) UT2, (d) UT3, and (e) OM micrographs of UT1, UT2 and UT3 under the same processing (480 °C*12 h + 200 °C*48 h).
Figure 4.
SEM micrographs of peak aging tissues (480°C×12 h+200°C×60 h), EDS points of magnesium matrix: (a) UT 1; SEM micrographs of peak aging tissues (B) UT 2, (c) UT 3; (d) XRD patterns of peak aging alloys for UT 1, UT 2 and UT 3, respectively; (e-f) BF-TEM and SAED of the cuboid-shaped phase, photographed with an incident electron beam parallel to [0001]; (g) Brightfield TEM and Fast Fourier Transform (FFT) images of a block of LPSO, taken with an incident electron beam parallel to [110].
Figure 5.
BF-TEM micrographs of the matrix at peak aging (z=[100]α-Mg):(a) β' phase in UT 1, (b) β' phase in UT 3; (c-d) HR-TEM and FFT micrographs of β' phases; The incident electron beam is parallel to [100]; (e) HAADF-TEM and EDS plots of the β' phase in UT 3. (f) BF-TEM, FFT and EDS points of the solid solution matrix under an incident electron beam parallel to [0001]α-Mg.
Figure 6.
TEM image (z=[0001]α-Mg) in UT 1:(a) BF-TEM and EDS plot at solid solution, (b) BF-TEM plot at early aging (200°C×24 h), (c) BF-TEM and EDS plot at peak aging, (d) BF-TEM precipitated in UT 3, (e) HR-TEM at the interface between the intergranular precipitation and matrix, (f) FFT image in Figure 6 e, (g) Mg Schematic diagram of the orientation relationship between the 5Gd phase and the Mg matrix.
Figure 7. Schematic diagram of the microstructure between UT1 and UT3.
Conclusion: Mg-10 Gd-1.7Y-1 Zn-0.5Zr alloy was treated by two-step sonication. The yield strength, tensile strength and electroluminescence strength of the alloy were 324 MPa, 430 MPa × 13.6% after solution treatment at 480°C×12 h and 200°C 48 h, respectively. In the process of alloy purification, the composite purification process combining ultrasonic and low-temperature treatment can increase the sedimentation rate of iron particles, so that the iron content in the alloy can be reduced to less than 40 ppm. In the microstructure, the grain size was reduced to 24 μm by ultrasonic treatment at semi-solid temperature, and the growth of the dissimilar eutectic phase (α-Mg+Mg3Gd) during the casting process was inhibited. Sonication does not change the composition of the phases, but significantly changes their distribution and size. The refinement of the Mg3Gd phase can reduce the solute segregation and increase the solute concentration of Mg during heat treatment, so that the volume fraction and area fraction of the 14H-LPSO phase and the cuboid GdY phase of UT3 are significantly lower than those of UT1 and UT2. More solute atoms in the UT 3 matrix compared to UT 1 and UT 2 will further increase the length-diameter ratio and size of the β' sheet and reduce the number density of the β' sheet. In addition, the increase of the axial ratio c/a and the growth of the Mg5Gd phase at the grain boundary during the aging process will seriously reduce the plasticity. Based on the microstructure and mechanical properties of the three alloys, grain refinement can improve the work hardening rate in the strain hardening stage, and increase UTS and EL, but there is little improvement in YS. With the same composition and heat treatment conditions, the aspect ratio of the β' plate is the main reason for improving the strength of the alloy.
Reprint: MaterialsScience Network