Hot compression tests of Mg-Gd-Y-Zr alloy at different deformation temperatures and strains were carried out with Gleeble-1500 simulator at strain rate of 0.002 s−1. The microstructure evolution of 400 °C/0.002 s−1 sample under different strain was analyzed emphatically by transmission electron microscopy and electron backscatter diffraction technology. Dynamic precipitation characteristics and nucleation-expansion mechanism of dynamic recrystallization (DRX) were discussed in detail. The results indicated that the dynamic precipitation takes place prior to DRX, and the morphology, size and distribution of precipitates change with increasing strain. At the later stage of deformation, the large-size hard phase β-Mg5(Gd,Y) can induce the particle-stimulated nucleation (PSN) mechanism under large strain and act as an effective nucleation site for DRX. In addition, we constructed a schematic diagram of the DRX nucleation-expansion mechanism of Mg-Gd-Y-Zr alloy under high temperature deformation. The first layer of DRX grains is formed by discontinuous dynamic recrystallization (DDRX) mechanism characterized by grain boundary bulge out, and the precipitates distributed at the original grain boundary promote DDRX nucleation; The expansion of necklace structure depends on continuous dynamic recrystallization (CDRX) mechanism; Subgrain division can further refine DRX grains.

Dynamic precipitation and recrystallization mechanism during hot compression of Mg-Gd-Y-Zr alloy

We analyzed the deformation behavior of Mg-Gd-Nd(-Zn)-Zr alloys under high temperature uniaxial compression, and reported two special nucleation mechanisms of dynamic recrystallization (DRX): Nucleation and expansion of DRX based on reticular shear bands (Shear band dynamic recrystallization, SBDRX) and long chain twins (Twin dynamic recrystallization, TDRX). The reticular shear bands in Mg-Gd-Nd-Zr alloy are mainly caused by the crossing of strain gradients from different directions, which is related to the high activity of pyramidal slip at high temperature. The addition of Zn makes the DRX mechanism mutate, and TDRX is dominant in Mg-Gd-Nd-Zn-Zr alloy. The nucleation of a large number of {10–12} twins is abnormal, which may be related to the decrease of I 2 stacking fault energy caused by Gd/Nd–Zn compound addition. Based on the mechanical properties and microstructure, it can be concluded that the addition of Zn can promote the DRX, reduce the deformation resistance and improve the workability.

Deformation behavior characterized by reticular shear bands and long chain twins in Mg-Gd-Nd(-Zn)-Zr alloys

Hot deformation behavior and microstructure evolution of Mg-Gd-Y(-Sm)-Zr alloys

The tensile properties and microstructure of hot extruded GW84 alloy stretching at 200 °C, 250 °C and 300 °C were investigated. The results showed that the alloys had excellent high temperature mechanical properties, when stretching at 250 °C, the ultimate tensile strength of alloy reached peak value, 393 MPa, and the tensile strength stretching at 300 °C reached 354 MPa. During high temperature tensile process, the microstructure markedly changed, which is not recognised in previous researches. Stretching at 200 °C, the precipitate phase was still metastable β′ phase. However, when stretching at 250 °C, the nano β′ precipitate transformed into nano β phase (β′ → β), and the precipitation mechanism is mainly inoculation nucleation of β phase. When stretching at 300 °C, the number of β precipitates increased significantly at triangular grain boundaries, which grew up to sub-micron particles, and precipitation mechanism is mainly growth and coarsening.

Investigation of hot extruded GW84 alloy on high temperature tensile properties and microstructure evolution

We investigated the effect of Sm on microstructures and mechanical properties of Mg–Gd (–Sm)–Zr by hot extrusion and aging treatment. The dynamic recrystallization (DRX) and precipitation behavior were analyzed by the electron backscattering diffraction (EBSD) and transmission electron microscope (TEM). For as-extruded alloys, the addition of Sm promotes DRX and dynamic precipitation, and the dynamic precipitation can activate the particle-stimulated nucleation (PSN) mechanism to promote the nucleation of DRX. The DRX grains of the as-extruded Mg–Gd–Zr show a random texture, but the as-extruded Mg–Gd–Sm–Zr shows an abnormal texture (<0001>Mg was parallel to the extrusion direction). For the extruded-aged alloys, the matrix precipitates of the Mg–Gd–Zr and Mg–Gd–Sm–Zr were all β′. But the size of β′ in Mg–Gd–Sm–Zr was smaller. The grain boundaries precipitates of the two alloys were all β. The β phase of Mg–Gd–Zr was granular and distributed discontinuously, while that of Mg–Gd–Sm–Zr was continuous distribution. Furthermore, there was an obvious precipitation-free zone of extruded-aged Mg–Gd–Sm–Zr. The high-temperature mechanical properties of extruded-aged Mg–Gd–Sm–Zr were obviously improved by the addition of Sm. The tensile strength, yield strength and elongation at 200 °C were 336 MPa, 271 MPa and 7.4%, respectively. Finally, the strengthening mechanism of extruded-aged Mg–Gd–Sm–Zr was analyzed in detail. 

Effect of Sm on microstructures and mechanical properties of Mg-Gd(-Sm)-Zr alloys by hot extrusion and aging treatment

The microstructure evolution and dynamic precipitation behavior of Mg–10Gd–4Y–1Sm–0.5Zr alloy during backward hot extrusion are studied. The results show that the equiaxed grains become polygonal grains and fine DRX grains are generated which form a “necklace structure” along the initial deformed grain boundaries. With further increase of deformation, DRX grains volume fraction significantly increases and finally almost fully DRX extruded microstructure is obtained in the completely hot-extruded sample. The extrusion deformation results in increase of dislocation density and formation of dislocation cells. The subgrain boundaries continue to absorb dislocations and finally change from the low angle grain boundaries to the high angle grain boundaries. Hot extrusion deformation results in formation of dynamic precipitated phase, β phase particle, i.e., Mg5(Gd,Y) phase, with diameter of 45–145 nm, which is incoherent relationship with the matrix. In process of increasing deformation, the shape of β phase changes from spindle shape to disc shape and the distribution of β phase transforms from strip distribution along grain boundary to discrete distribution.

Microstructure Evolution and Dynamic Precipitation Behavior of Mg–10Gd–4Y–1Sm–0.5Zr Alloy During Backward Hot Extrusion

Investigation on β′ phase inversion of peak-aging hot extruded GW84 alloy after short high temperature secondary aging

The invention relates to a creep-resistant rare-earth magnesium alloy and a preparation method thereof, and belongs to the technical field of magnesium-based alloy materials. The creep-resistant rare-earth magnesium alloy comprises, by mass, 5.5-7.5% of Ho, 1.5-4.5% of Sm, 0.6-1% of Sr, 0.35-0.55% of Zr, the balance Mg and inevitable impurities. According to the creep-resistant rare-earth magnesium alloy, by optimizing alloy element proportion, solid solution and aging process regulation and control, the magnesium alloy matrix and the grain boundary are obviously reinforced, and the creep resistance of the alloy at a high temperature is improved.

Lipeng Yan

Papers

Investigation of hot extruded GW84 alloy on high temperature tensile properties and microstructure evolution

Microstructure Evolution and Dynamic Precipitation Behavior of Mg–10Gd–4Y–1Sm–0.5Zr Alloy During Backward Hot Extrusion

Investigation on β′ phase inversion of peak-aging hot extruded GW84 alloy after short high temperature secondary aging

Creep-resistant rare-earth magnesium alloy and preparation method thereof