Rare Earth Texture Component Research Articles

Dynamic and Postdynamic Recrystallization Behavior of GWZ Magnesium Alloy Under Double‐Hit Hot Compression

Herein, dynamic and postdynamic recrystallization behaviors of GWZ magnesium are investigated. Toward this end, the single‐hit and double‐hit hot compression tests are conducted under strain rate of 0.001 s−1 at 400 °C. The prestrains of 0.1 and 0.5 are considered to investigate the effect of interpass time (5–300 s) on the compressive strength level. At the low strain level of 0.1, the contribution of Hall–Petch effect is considerable due to the occurrence of static recrystallization. In addition, the rare earth texture component is eliminated during interpass annealing. This causes increasing the strength of the material during second pass of hot compression. In contrast, at higher imposed strain, the strength level decreases with increasing the interpass time of annealing. The high amount of strain is completely consumed and the remaining stored energy is not high enough to trigger the occurrence of static recrystallization. The occurrence of metadynamic recrystallization and subsequent growth are characterized. In addition, the texture does not change in respect of the intensity or numbers/types of components. Accordingly, the observed decreasing trend of the strength is justified relying on the occurrence grain growth.

Effect of heat treatment on the microstructure and mechanical properties of a multidirectionally forged Mg-Gd-Y-Zn-Zr-Ag alloy

Multidirectional forging (MDF) was successfully applied to fabricate large-size Mg-Gd-Y-Zn-Zr-Ag alloy in this work and effects of T4, T5 and T6 treatments on the microstructure and mechanical properties of the as-forged alloy were analyzed. Results show that dynamic recrystallization (DRX) occurs and second phase particles precipitate along the grain boundary during the MDF process. After annealing treatment (T4), the volume fraction and size of dynamic precipitates slightly increase at a lower temperature (430 °C) compared with those of MDFed sample, while they are dissolved into the α-Mg matrix at a higher temperature (450 °C). At the meantime, short plate-shaped long-period stacking ordered (LPSO) phases are observed in the DRX grains of the MDFed sample and then dissolved into the α-Mg matrix during annealing at both temperatures. Typical basal texture is identified in the MDFed sample, but the basal pole tilts away from final forging direction and rare-earth texture component with <112¯1> orientation parallel to penultimate forging direction becomes visible after annealing. The T6 sample annealing at 430 °C for 4 h and ageing at 200 °C for 34 h exhibits the superior strength and ductility in this study. The ultimate tensile strength, tensile yield strength and elongation to failure, which is 455 MPa, 308 MPa and 7.7%, respectively, are overall improved compared with the directly-aged (T5) sample. This paper provides a superior heat treatment schedule to manufacture high-performance large-scale Mg-Gd-Y-Zn-Zr-Ag components for industrial production.

Cold rollability improvement by twinning and twin–slip synergy in an Mg–Zn–Gd alloy with rare earth texture

In this study, the Mg–2.0Zn–0.8Gd (wt%) alloy with rare earth (RE) texture was successfully cold-rolled in a single-pass rolling with 35% reduction without edge cracks. The microstructure and texture evolutions were minutely examined to reveal the deep reason for the higher cold rollability than commercial AZ31 alloy. Several {10−12} extension twins were found to be activated due to the RE texture. The {10−12} extension twinning that occurred in parent grains with orientation tilted toward one side of the transverse direction (TD) could reverse the orientation to another side of TD, i.e., from +TD to −TD or from −TD to +TD, which partially resulted in the stability of RE texture component. Complex twin types, including {10−12}-{10−12} secondary extension twin, {10−11}-{10−12} double twin, and even the coexistence of extension twin with contraction twin or double twin were found. Basal and prismatic slips, were extensively activated during the cold rolling process due to the RE texture and Gd alloying. The synergy effect of various types of twinning and multiple dislocation slip, which postponed the shear banding, was the major cause of the high cold rollability.

Inhomogeneous microstructure and mechanical anisotropy of multi-directional forged Mg-Gd-Y-Zn-Ag-Zr alloy

Abstract In this paper, through multi-directional forging with strain controlled multi-passes under the conditions of initial forging temperature of 480 °C and an accumulative strain of 4.4 after 27 forging passes, a bulky Mg-Gd-Y-Zn-Ag-Zr alloy with a size of 460 mm × 260 mm × 160 mm is prepared. The microstructure, texture and mechanical properties of different regions (the center, 1/2R and edge regions) in the alloy bulk are discussed. The results show that the material has undergone significant grain refinement during the multi-directional forging process. Due to the uneven strain and temperature distributions in different regions, inhomogeneous microstructures with different levels of dynamic recrystallization are formed. The average grain size of the center, 1/2R and edge regions is 2.91 ± 1.77, 3.83 ± 2.22, and 17.78 ± 12.24 μm, respectively. The center region exhibits a double-peak basal texture, while the 1/2R and edge regions additionally show a “rare earth texture component”. The tilt of the basal poles in some specific direction causes obvious mechanical anisotropy of the bulk. However, the non-basal rare earth texture component weakens the mechanical anisotropy. The optimum comprehensive mechanical properties are obtained in the center region, with the ultimate tensile strength of 364 MPa, the yield strength of 295 MPa, and the elongation to failure of 4.8%.

Microstructure, mechanical properties, and texture evolution of Mg–Zn–Y–Zr alloy fabricated by hot extrusion–shearing process

In this study, ultrafine-grained Mg–Zn–Y–Zr alloys were obtained via a novel extrusion–shearing (ES) deformation process, which combined initial forward extrusion with subsequent shearing. The experimental results showed that the composition of the main phase was converted from the α-Mg, I (Mg3Zn6Y), and W (Mg3Zn3Y2) phases to the α-Mg and W phases as the Zn/Y mass ratio of the as-cast alloys decreased from 4 to 0.67. Needle-shaped YZn5 and micron-sized Mg7Zn3 phases were also observed in the as-homogenized Mg–4Zn–1Y–0.6Zr and Mg–3Zn–2Y–0.6Zr alloys. The quantitative electron backscatter diffraction results demonstrated that the dynamic recrystallization (DRX) and the growth of fine DRX grains had a strong dependence on the equivalent strain and increased temperature caused by the die corner. The abundance of W precipitates resulted in the gradual evolution of the sub-grains into fresh DRX grains for the ES alloy III. Additionally, reduced strength anisotropies were related to the decreased pole density caused by the increasing addition of Y; a weakened basal texture with a rare-earth texture component was also achieved at 3 wt% of Y addition. Finally, the influences of the grain size and basal texture on the ductility and strength of the ES alloys were systematically investigated.

Microstructure and texture characterization of Mg–Al and Mg–Gd binary alloys processed by simple shear extrusion

Microstructural and textural evolutions of pure Mg, Mg–2wt% Al, and Mg–2wt% Gd were investigated after extrusion and simple shear extrusion (SSE). Microstructural studies revealed that the grain size of all extruded samples decreased after 4 passes of SSE at 553K (280°C). In the fine-grained Mg–2Gd alloy, however, a duplex structure consisting of fine recrystallized grains and coarse unrecrystallized patches was formed. Although the 2.5μm size of the recrystallized grains did not experience a significant change, the volume fraction of the coarse 15μm-wide unrecrystallized patches decreased, and the overall microstructural homogeneity was improved after 4 passes of SSE. In Mg–2Gd, fragmentation of large grains and unrecrystallized patches encouraged the formation of HAGBs, while Mg–2Al did not experience significant changes in the fraction of HAGBs. Contrary to pure Mg and Mg–2Al alloy, Mg–2Gd alloy developed a new “rare earth texture component” with the 〈1 1 2¯ 1〉 direction parallel to the extrusion direction in the extruded condition. However, all three materials developed the conventional “extrusion texture” after SSE.

Correlation between dynamic recrystallization and formation of rare earth texture in a Mg-Zn-Gd magnesium alloy during extrusion

The trace addition of rare earth (RE) elements in Mg alloys can modify the extrusion texture, leading to the formation of RE texture and thus improved formability. The interrupted extrusion experiment as well as electron back-scatter diffraction (EBSD) characterization was conducted in Mg-1.5Zn-0.5Gd (wt.%) alloy to unveil the dominant dynamic recrystallization (DRX) mechanism and its correlation with the formation of RE texture during extrusion. The results indicate that continuous DRX (CDRX) dominated the microstructural development. Fresh DRXed grains with 30° [0001] grain boundaries preferentially nucleated in unDRXed grains with [10bar{{bf{1}}}0] basal fiber orientation via CDRX, showing preferred selection of [2bar{{bf{1}}}bar{{bf{1}}}0] basal fiber orientation rather than RE texture orientation. Consequently, CDRX contributed to the weakening of [10bar{{bf{1}}}0] basal fiber texture and had a more significant effect on the formation of [2bar{{bf{1}}}bar{{bf{1}}}0] basal fiber component than that of RE texture component. Besides, the preferred growth of recrystallized grains with RE texture orientation was confirmed to occur during static annealing after extrusion, which is inferred as the key reason for the formation of RE texture in dilute Mg-RE alloys.

Micro and macro texture evolution during multiaxial forging of a WE43 magnesium alloy

The deformation texture evolution of a WE43 magnesium alloy was investigated by imposing consecutive multiaxial forging at 400 °C. Obtained results implied that although a strengthened basal texture was achieved after one multiaxial forging pass with basal planes of most grains perpendicular to the final forging direction, a random basal texture was attained after three deformation passes which was attributed to the formation of a new <5-4-13> texture component parallel to the transverse direction. The contribution of both particle stimulated nucleation mechanism, as well as continuous dynamic recrystallization, were proposed as determining factors in the creation of the new rare earth texture component. The later findings confirmed that the strain induced partial dissolution of eutectic phases contributed to the counteracting the particle stimulated nucleation mechanism which thereby resulted in the disappearance of the novel rare earth texture component formed at three deformation passes along with the increase in the maximum texture intensity at fifth passes of deformation.

Effects of Zn addition on the mechanical properties and texture of extruded Mg-Zn-Ca-Ce magnesium alloy sheets

Mg-x Zn-0.2Ca-0.2Ce (x = 0.5, 1.0, 1.5 and 2.0, wt%) alloys were subject to hot extrusion, for the purpose of evaluating the effects of Zn addition on the mechanical properties and texture characteristics in this current paper. A spread of basal poles in extrusion direction (ED) was found in all four extruded alloys, beyond that, the rare earth texture component existed in the alloys. It is interesting that both the ED-split texture and rare earth texture components reduced with the increasing Zn addition; furthermore, the increasing Zn addition leaded to a broader spread of basal poles toward transverse direction (TD). Both the strength and plasticity were improved with the increasing Zn addition, by solid solution hardening, solid solution softening, and the enhanced cohesion of grain boundaries; while further Zn addition deteriorated the plasticity, because of the dominant solid solution hardening. Mg-Zn-Ca-Ce magnesium alloys received the best comprehensive mechanical properties when Zn addition reached 1.5wt%, with tension yield strength (TYS) and tension fracture elongation (TFE) about 131.0MPa and 42.4%, respectively, in ED.

Development of dilute Mg–Zn–Ca–Mn alloy with high performance via extrusion

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.

Effect of Ca Addition on the Intensity of the Rare Earth Texture Component in Extruded Magnesium Alloys

Extruded Mg alloys usually display strong basal fiber textures, which are related to poor formability. Inspired by the texture weakening effect of the addition of rare earths (RE) to Mg alloys, two Mg-Ca alloys were designed and a range of extrusion conditions were explored in attempts to reproduce this effect. Constant strain rate and strain rate change compression tests were performed on the specially prepared Mg-0.5Ca and Mg-1Ca alloys. Dynamic strain aging (DSA) was observed to take place in both alloys over a well-defined temperature and strain rate range. Seven extrusion trials were then carried out on the Mg-0.5Ca alloy within and external to the DSA range of conditions. From the resulting textures and microstructures, it is concluded that the 〈11−21〉 (RE) texture component is formed when extrusion is carried out while DSA is taking place. The occurrence of this type of texture weakening is attributed to the promotion of recrystallization within the shear bands formed when the rate sensitivity is negative. The latter in turn arises because of the DSA taking place in the presence of solute Ca.

Grain growth and texture evolution during annealing in an indirect-extruded Mg–1Gd alloy

Abstract RE (rare earth)-texture component has been reported to be responsible for improved formability and ductility in RE-containing Mg alloys. Grain growth and texture evolution during annealing of Mg–1Gd alloy was investigated after indirect extrusion and quenching to reveal the formation mechanisms of the RE texture component. In the as-extruded Mg–1Gd sample, both the recrystallized grains and large elongated grainsare oriented mainly with 〈 1 0 1 ¯ 0 〉 orientation parallel to the extrusion direction (ED). The preferred growth of the 〈 2 1 ¯ 1 ¯ 1 〉 grain with its 〈 2 1 ¯ 1 ¯ 1 〉 orientation parallel to ED has been observed during recrystallization annealing, leading to the gradual strengthening of 〈 2 1 ¯ 1 ¯ 1 〉 RE-texture component at the expense of the 〈 1 0 1 ¯ 0 〉 component with increasing annealing temperature. The driving force for the preferred grain growth is the difference in stored energy between the 〈 2 1 ¯ 1 ¯ 1 〉 grains and the 〈 1 0 1 ¯ 0 〉 grains. Abnormal growth of the 〈 1 0 1 ¯ 0 〉 grain takes place during long-time annealing, resulting in the strengthening of 〈 1 0 1 ¯ 0 〉 component texture, which can be explained by a texture effect.

Effect of dynamic strain aging on the appearance of the rare earth texture component in magnesium alloys

Abstract Binary Mg alloys were prepared containing Zn, Ce and Gd. These were extruded and the resulting mechanical properties were determined. The intensities of the rare earth (RE) texture components were measured and linked to the extrusion conditions. Tension and compression testing was carried out on samples taken from extruded bars and a Mg–0.5Ce cast alloy. Over particular temperature and strain rate ranges, dynamic strain aging (DSA) was observed. The ranges over which DSA occurred during testing are compared with the conditions under which the RE texture components were produced during extrusion. It is concluded that formation of the RE texture components can be enhanced by extruding when the rate sensitivity is negative, i.e. under conditions where DSA is taking place.

Microstructure, Texture and Mechanical Properties of Mg-Zn-Ce Alloy Extruded at Different Temperatures

The microstructure, texture and mechanical properties of Mg-1.5 mass%Zn-0.2 mass%Ce alloys extruded at different extrusion temperatures were investigated. A ‘‘rare earth (RE)’’ texture component, which laid at about 20 � from h2� 1� 1i in an inverse pole figure, appeared in the as-extruded specimen, only when the extrusion temperature was set to 703 K. It is suggested that a high extrusion temperature of around 723 K is essential for the formation of RE texture component. Electron back-scatter diffraction (EBSD) measurements revealed that the RE texture component originated from dynamic recrystallized grains in the band microstructure around the unrecrystallized grains, in contrast to the results of previous studies, in which it originated from shear bands in the unrecrystallized grains. The specimen extruded at 703 K and subsequently annealed showed a lower yield stress and higher ductility than that extruded at 573 K and subsequently annealed, owing to the wider spread of texture originating from the RE texture component in the as-extruded specimen. [doi:10.2320/matertrans.MC201008]