The effect of initial tempers with different characteristics of precipitates and contents of supersaturated solute atoms, including the homogenized, peak–aged and over–aged conditions, on the tensile creep behavior of WE43 alloy has been investigated at 523 K. Results show that the peak–aged alloy at 523 K obtained superior creep resistance than the homogenized, peak–aged at 498 K and over–aged at 523 K alloy. A uniform dispersion of β precipitates was dynamically formed within steady–stage creep microstructure of the homogenized WE43 alloy after creep deformation of 200 h. It is found that the precipitate size and distribution is similar with the alloy aged equal time without the applied stress. In addition, the WE43 alloy in all tempers obtains similar precipitate size and distribution in their steady–stage creep microstructures. Therefore, it is inferred that the various initial tempers mainly affect the primary creep stage. Furthermore, numerous dislocations were detected between precipitates and the stress exponent n is 4.5, which is close to 5. Thereby, dislocation climb is suggested to be the creep mechanism. The reason for the peak–aged alloy at 523 K obtained superior creep resistance is that the initial uniform dispersion of β″ and β′ precipitates have smaller precipitate size and higher precipitate density than that of the homogenized and over–aged alloy, which are more effectively to hider dislocation climb. However, a deterioration of creep resistance was occurred in the peak−aged alloy at 498 K due to precipitate recovery when crept at 523 K. As a consequence, WE43 alloy in peak–aged temper at 523 K achieves the highest creep resistance.

The effect of cold plastic deformation by multi-directional forging prior to artificial aging treatment on the microstructure and mechanical properties of WE43 alloy was investigated. The results show that the pre-deformation produced high density of dislocations and deformation twins. After peak-aging treatment at 200 °C, the pre-deformation reduced the average size of fine β″ and β′ precipitates and promoted the formation of β1 phase. Additionally, spheroid–shaped precipitates formed at twin boundaries and the deformation twins were thermally stable during the aging treatment. Mechanical properties reveal that the tensile yield and ultimate strength is significantly enhanced from 191±3 MPa to 270±15 MPa and 278±5 MPa to 320±18 MPa, respectively.

The Mg-2.0Zn-0.8Gd (wt%) sheet with double peak transverse direction (TD) tilted non-basal texture was cold rolled, and obtain a new texture, named as elliptical annular texture, rather than the traditional basal texture. In addition, the maximum texture intensity of the cold rolled samples decreased with the increased accumulated reductions. The formation of this unusual texture was attributed to the initial non-basal texture and multiple deformation modes.Download high-res image (203KB)Download full-size image

•Dilute Mg–Zn–Ca–Mn alloy was newly developed as low-cost wrought Mg alloy.•Excellent balance of strength and ductility was achieved via extrusion process.•Weakened basal texture with RE texture component was formed after almost full DRX.•Fine Mg2Ca and α-Mn phases dynamically precipitated against DRXed grain growth.Dilute Mg–0.21Zn–0.30Ca–0.14Mn (wt.%) alloy was newly developed as low-cost wrought Mg alloy. Excellent balance of strength and ductility was achieved via extrusion process by grain refinement and texture modification. The dilute alloy extruded at 300 °C exhibited a tensile yield strength of 307 MPa and an elongation of 20.6% due to a bimodal microstructure consisting of fine dynamically recrystallized (DRXed) grains of ∼2.3 μm and strongly textured coarse unDRXed grains. After extrusion at 350 °C, almost fully DRXed microstructure (DRX fraction of ∼0.96) and significantly weakened basal texture (maximum intensity of 3.2) with rare-earth texture component were achieved, consequently resulting in a high compression/tension yield ratio of ∼0.8 with tensile elongation of 30.0% and tensile yield strength of 164 MPa. Besides, fine spherical Mg2Ca and α-Mn phases dynamically precipitated during extrusion, acting as effective pinning obstacles against the DRXed grain growth.

Four kinds of Mg alloys with different Zn and Ca concentration were selected to analyze the effect of Zn and Ca concentration on the microstructure and the mechanical properties of Mg–Zn–Ca alloys. It was found that Zn and Ca concentration has a great influence on the volume fraction, the morphology and the size of second phase. The Mg–1.95Zn–0.75Ca (wt%) alloy with the highest volume fraction, continuous network and largest size of Ca2Mg6Zn3 phase showed the lowest elongation to failure of about 7%, while the Mg–0.73Zn–0.12Ca (wt%) alloy with the lowest volume fraction and smallest size of Ca2Mg6Zn3 phase showed the highest elongation to failure of about 37%. It was suggested that uniform elongations of the Mg–Zn–Ca alloys were sensitive to the volume fraction of the Ca2Mg6Zn3 phases, especially the network Ca2Mg6Zn3 phases; post-uniform elongations were dependent on the size of the Ca2Mg6Zn3 phase, especially the size of network Ca2Mg6Zn3 phase. Reduction in Zn and Ca concentration was an effective way to improve the room-temperature ductility of weak textured Mg–Zn–Ca alloys.

Three dilute Mg-Zn-Ca-Mn alloys were successfully extruded at 24 m/min and the alloy with lowest Zn content (0.21 wt%) can even be extruded at 60 m/min without any surface defects, which was ascribed to the thermally stable Mg2Ca phase and high solidus temperature (∼620 °C). The alloys extruded at die-exit speed ≥6 m/min showed a fully dynamically recrystallized (DRXed) microstructure and weak rare earth (RE) texture at the position between [21̅1̅4] and [21̅1̅2] parallel to the extrusion direction. Besides, fine Mg2Ca and α-Mn particles dynamically precipitated during extrusion, acting as effective pinning obstacles against the DRXed grain growth via Zener drag effect. Due to the deformation temperature rise with increasing extrusion speeds, the grain size increased gradually, which can be understood from the relationship between DRXed grain size and Zener-Hollomon parameter. The RE texture contributed to high uniform elongation of ∼23%, but the increased grain size (>30 µm) deteriorated post-uniform elongation due to the prevalence of {101̅1} contraction and {101̅1}-{101̅2} double twins during post-uniform deformation.

Mg-1.58Zn-0.52Gd (wt%) alloy was successfully extruded at high extrusion speed of 60 m/min, suggesting the much better extrudability than commercial AZ31 alloy. After high-speed extrusion (die-exit speed ≥24 m/min), the Mg-Zn-Gd alloy exhibited a fully recrystallized microstructure with fine Mg3Zn3Gd2 phase at grain boundaries (GBs) and within grain interiors and rare earth (RE) texture at the position between [21̅1̅4] and [21̅1̅2] parallel to the extrusion direction. The RE texture favored the operation of both basal slip and {101̅2} extension twins, thus leading to a highly improved ductility of ∼30%, which was twice than that of the AZ31 alloy. It is hypothesized that the segregation of Gd solutes at GBs greatly influences the recrystallization behavior and thus contributes to the formation of RE texture.

Multi-directional impact forging (MDIF) was applied to a Mg-7Al-2Sn (wt.%) Mg alloy to investigate its effect on the microstructural evolution. MDIF process exhibited high grain refinement efficiency. After MDIF 200 passes, the grain size drastically decreased to 20 µm from the initial coarse grains of ~500 µm due to dynamic recrystallization (DRX). Meanwhile, original grain boundaries remained during MDIF and large numbers of fine spherical β-Mg17Al12 particles dynamically precipitated along the original grain boundaries with high Al concentration, acting as effective pinning obstacles for the suppression of DRXed grain growth. Besides, micro-cracks nucleated during MDIF and propagated along the interface between the remained globular or cubic Al-Mn particles and Mg matrix.

High temperature tensile–creep behavior of Mg–4Y–2.3Nd–1Gd–0.6Zr (wt%, WE43(T6)) alloy at 523–573 K was investigated. The creep stress exponent is equal to 4.6, suggesting the underlying dislocation creep mechanism. The activation energy is (199 ± 23) kJ/mol, which is higher than that for self-diffusion in Mg and is believed to be associated with precipitates coarsening or cross slip. The creep mechanism is further suggested to be dislocation climb at 523 K, while a cross slip at 573 K is possible. The metastable β′ and β1 phases in the WE43(T6) alloy were relatively thermal stable at 523 K and could be effective to hinder the dislocation climb, which contributed to its excellent creep resistance. However, at 573 K it readily transforms into equilibrium βe phase and coarsens within two hours, thereby causing a decrease of creep resistance. In addition, precipitate free zones approximately normal to applied stress direction (directional PFZs) developed during the creep deformation, especially at 573 K. Those zones became preferential sites to nucleate, extend and connect microcracks and cavities, which lead to the intergranular creep fracture. Improving the thermal stability of precipitates or introducing thermally stable fine plate-shaped precipitates on the basal planes of Mg matrix could enhance the high temperature creep resistance.