Rare Earth Texture Research Articles

Microstructure evolution and mechanical properties of the Mg-Sm-Gd-Zn-Zr alloy during extrusion

Mg-2.6Sm-1.3Gd-0.6Zn-0.5Zr (wt.%) alloy is prepared with metal mold casting and followed hot extruded at 400°C with the extrusion ratio of 25:1. Microstructure and mechanical properties of the as-extruded alloy are investigated using optical microscope (OM), scanning electron microscope (SEM) with electron backscatter diffraction (EBSD), X-ray diffraction (XRD) and transmission electron microscope (TEM). The results show that the as-extruded alloy is fully dynamically recrystallized, which develops rare earth (RE) texture component. The as-extruded alloy shows an age-hardening response at 200°C, in which there are numerous nano-scale plate strengthening phase β′ precipitated in the matrix. The as-extruded alloy in the peak-aged condition exhibits ultimate tensile strength, yield strength, and elongation of 332 MPa, 220 MPa and 22.7% at room temperature, respectively. The dominant strengthening mechanisms of the investigated alloy are fine-grain strengthening, solid solution strengthening and precipitate strengthening.

Unraveling the effect of deformation-induced phase transformation on microstructure and micro-texture evolution of a multi-axially forged Mg-Gd-Y-Zn-Zr alloy containing the LPSO phase

The effect of blocky to lamellar phase transformation on the microstructure and micro-texture of a Magnesium-Rare earth alloy containing long-period stacking order (LPSO) phases were examined comprehensively at the isothermal temperature of 400 °C via multiaxial forging. A very fine-grained microstructure with an average grain size of 1 μm was achieved after applying three multi-axial forging (MAF) passes. Particle stimulated nucleation (PSN) along with continuous dynamic recrystallizations (CDRX) due to the blocky LPSO phases were realized to be the main reasons for the achievement of such fine microstructure. The deformation-induced blocky to lamellar phase transformation began at 1/3 pass and expanded through the whole microstructure after the third pass. Such phase transformation was found to be initiated by the fragmentation of blocky phases. As a result of PSN and CDRX mechanisms, two new rare earth (RE) texture components of <10-11> and <2-1-10> || Transverse Direction (TD) were formed at the second MAF pass that led to a highly randomized deformation texture. Nevertheless, the continuous breakdown of blocky phases and their subsequent phase transformation to lamellar LPSO suppressed PSN at the third deformation pass. Hence, the formed RE texture components were disappeared which in turn increased the texture intensity at this deformation pass.

A study of high ductility Mg-2Gd-0.4Zr magnesium alloy under monotonic tension at room temperature

The present study aimed to explore the deformation behavior of a highly ductile Mg-2Gd-0.4Zr alloy at room temperature. Tensile tests were carried out at room temperature, aiming to explore reasons for the high ductility through investigating deformation modes by quasi-in-situ electron backscatter diffraction-assisted slip trace analysis, texture evolution analysis and microstructure characterizing by scanning electron microscope and transmission electron microscope studies. The results show that the rare earth texture benefitted basal slip, the higher proportion of the non-basal slip compared to pure magnesium, the high grain boundary strength and the synergy between the twinning and dislocation contribute to the high ductility of this alloy. In addition, some local strain compatibility may also play a positive role.

A statistical analysis of compressive deformation mechanisms in an extruded Mg–3Y sheet

A statistical analysis of plastic deformation mechanisms was performed on an extruded Mg-3 wt%Y sheet during room temperature uniaxial compression along the extrusion direction. Grain-scale slip trace analysis and twin variant analysis were performed grain-by-grain to reveal the active slip/twinning systems based on quasi-in-situ SEM and EBSD. The initial microstructure was fully recrystallized exhibiting typical rare-earth texture characteristics. The true stress-true plastic strain curve showed a sigmoidal shape indicating a profuse twinning activity which was consistent with the twin analysis. After 10% strain, there were 78 out of ~700 grains exhibited slip trace. In addition to the dominated basal <a> slip (79.5%), considerable non-basal slips including 2nd pyramidal <c + a> slip (11.5%) following prismatic <a> slip (9%) were active. For twinning activity, among the ~250 grains analyzed at 10% strain, all the identified ~150 twins were tension twins with an area fraction of 19%. The twin variant selection did not solely follow the Schmid law, i.e. 42.3% twins exhibited low Schmid factor values (m<0.2) and 5.4% m values was negative. However, the proposed normalized m (mnor) can correlate the variant selection more effectively, and over 80% twins exhibited large mnor values. Twin transmission at grain boundary was observed for over 50% twins, and only 43.6% of them had high Luster-Morris parameter values (m’>0.75). However, the transmitted pairs with both large mnor sum and normalized m’ were preferred, and the fraction was over 64%. The twined number fraction was closely related to the maximum m values of prismatic slip. The present work implied that the twinning behavior is greatly dependent on the local conditions, and slip and twinning are closely related.

The electronic origins of the \u201crare earth\u201d texture effect in magnesium alloys

Although magnesium alloys are lightweight, recyclable and relatively cheap, they suffer from poor ductility. This can be improved by the addition of rare earth (RE) elements, and this is now a well-established criterion for wrought alloy design. It is notable that this behavior is largely restricted to the lanthanides, but no hypothesis is yet available to explain why other elements do not have the same effect. To answer this question, ab initio simulations of crystallographically complex boundaries have been undertaken to examine the electronic origin of the RE effect. While the electronic structure provided strong bonding between the RE elements and their Mg surroundings, local disruption in atomic arrangement at the grain boundaries was found to modify this effect. This work shows quantifiable changes in electronic structure of solutes resulting from grain boundary crystallography, and is suggested to be a contributing factor to the RE texture effect.

Enhanced Degradability of Mg-2Gd Alloy by Alloying Cu

To obtain alloys with both high degradability and plasticity used in the petroleum industry, 0.25, 0.5, 0.75 and 1.0 wt.% Cu were added into Mg-2Gd alloy. With Cu addition, finer grain size, precipitation of Mg2Cu, and variations of crystal orientation were observed. Regardless of the good mechanical properties gained by alloying Cu, the degradation rate was obviously increased in this study. The corrosion rate and I corr of Mg-2Gd were 13.0 ± 0.1 mm·y−1 and 1.04 × 10−4 A·cm−2, respectively, while those value of Mg-2Gd-1Cu increased to 226.4 ± 6.1 mm·y−1 and 3.42 × 10−4 A·cm−2, respectively. We confirmed by scanning Kelvin probe force microscopy and scanning electron microscope with electron backscatter diffraction detector that profuse second phases (mainly Mg2Cu), refinement of grain size, and weakened rare earth texture contributed to the lower corrosion resistance.

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.

Microstructural Stability of Extruded Mg-Mn-Ce Hollow Profiles with Weld Seams

Despite aluminum profiles, magnesium profiles have not been well developed due to the low formability. Furthermore, extruded magnesium profiles show a strong dependence on the mechanical properties, according to the loading direction. This is caused by a strong basal texture, which is directly dependent on the process parameters during the extrusion and the subsequent aging. Thus, the present paper focuses on the analysis of the microstructure and its evolution of extruded magnesium hollow profiles, which were subjected to a series of heat treatments at 475 °C up to one hour. The hollow profiles were extruded through a porthole die, thus, containing longitudinal weld seams. These were formed by material that underwent heavy shearing along the tool surface based on the friction conditions in the porthole die. Three extrusion ratios (ER = 8:1, ER = 16:1, ER = 30:1) were applied, resulting in three different wall thicknesses of the profiles. The microstructure of the profiles was analyzed using light-optical microscopy (LOM) and scanning electron microscopy (SEM) coupled with electron backscatter diffraction (EBSD). The analysis revealed no change of the microstructure of the profiles extruded at the two higher extrusion ratios within the time frame of the heat treatment. In contrast, the microstructure and, thus, the micro-texture of the profile with the lowest extrusion ratio (ER = 8:1) has been affected to a great extent. While only small changes in microstructure in the weld-free area were observed, the initial microstructure in the weld seam was transformed from fine recrystallized grains into a significantly bimodal microstructure mainly due to an abnormal grain growth (AGG). These changes were accompanied by a promotion of the rare-earth (RE) texture component for the weld-free material and a change of the overall texture from RE to a typical non-RE double fiber texture for the weld seam due to the intense AGG within the short-time heat treatments. In addition, the influence of the extrusion ratio on particle size and distribution as well as the character of the microstructure governing the behavior during heat treatments was analyzed and discussed.

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%.

Adjustment of the Mechanical Properties of Mg2Nd and Mg2Yb by Optimizing Their Microstructures

The deformation behavior of the extruded magnesium alloys Mg2Nd and Mg2Yb was investigated at room temperature. By using in situ energy-dispersive synchrotron X-ray diffraction compression and tensile tests, accompanied by Elasto-Plastic Self-Consistent (EPSC) modeling, the differences in the active deformation systems were analyzed. Both alloying elements change and weaken the extrusion texture and form precipitates during extrusion and subsequent heat treatments relative to common Mg alloys. By varying the extrusion parameters and subsequent heat treatment, the strengths and ductility can be adjusted over a wide range while still maintaining a strength differential effect (SDE) of close to zero. Remarkably, the compressive and tensile yield strengths are similar and there is no mechanical anisotropy when comparing tensile and compressive deformation, which is desirable for industrial applications. Uncommon for Mg alloys, Mg2Nd shows a low tensile twinning activity during compression tests. We show that heat treatments promote the nucleation and growth of precipitates and increase the yield strengths isotopically up to 200 MPa. The anisotropy of the yield strength is reduced to a minimum and elongations to failure of about 0.2 are still achieved. At lower strengths, elongations to failure of up to 0.41 are reached. In the Mg2Yb alloy, adjusting the extrusion parameters enhances the rare-earth texture and reduces the grain size. Excessive deformation twinning is, however, observed, but despite this the SDE is still minimized.

Influence of extrusion conditions on microstructure and mechanical properties of Mg-2Gd-0.3Zr magnesium alloy

In general, different extrusion conditions will affect the microstructure of magnesium alloys and further determine the mechanical properties. The effects of extrusion parameters and heat treatment processes such as extrusion speed, pre-forging, annealing time, extrusion ratio and cooling rate on the microstructure, texture evolution and tensile properties of Mg-2Gd-0.3Zr alloys were investigated in this study. Compared with the as-cast alloy, the extrusion process significantly refines the grains and exhibit the rare earth texture. With the increase of extrusion speed and annealing time, the growth of recrystallized grains is accelerated, leading to the increase of elongation. Large pre-forging deformation achieves to refine the grains by promoting recrystallization nucleation, resulting in increased strength of Mg-2Gd-0.3Zr alloy. Decreasing the extrusion ratio or increasing the cooling rate will introduce coarse un-DRXed grains, which transformed the texture into the basal texture. In particular, the effect of rapid cooling on refining the recrystallized grains is also obvious. Different extrusion conditions influence the mechanical properties of the Mg-2Gd-0.3Zr alloy through the grain size, proportion of non-recrystallization region and texture type.

Quasi-in-situ observing the rare earth texture evolution in an extruded Mg-Zn-Gd alloy with bimodal microstructure

The static recrystallization and associated texture evolution were investigated in an extruded Mg-Zn-Gd alloy with bimodal microstructure based on a quasi-in-situ electron back-scatter diffraction (EBSD) method. The typical rare earth (RE) texture formed during annealing, evolving from the bimodal microstructure with [10 1 ¯ 0] basal fiber texture that consisted of fine recrystallized (RXed) grains and coarse unrecrystallized (unRXed) grains elongated along the extrusion direction. In both RXed and unRXed regions, the RXed nucleation produced randomized orientations without preferred selection and the RXed grains with RE texture orientation had more intensive growth ability than those with basal fiber orientation, thereby leading to the preferred selection of RE texture orientation during grain growth. The relationships between stored strain energy, solute drag, grain growth and texture evolution are discussed in detail. This study provided direct evidence of the RE texture evolution in an extruded Mg-RE alloy, which assists in understanding the formation mechanisms for RE texture during extrusion and better developing wrought Mg alloys with improved formability.

Microstructure and properties of biodegradable Mg–Zn–Y-Nd alloy micro-tubes prepared by an improved method

In this study, Mg-Zn-Y-Nd alloy micro-tubes with an outer diameter of φ2.46 mm and a wall thickness of 0.14 mm were successfully prepared using an improved technique including hot extrusion with water cooling, multi-pass cold drawing and final annealing treatment. The outer diameter error, inner diameter error and wall thickness error of the cold drawn micro-tubes were 0.58%, 0.36% and 3.43%, respectively. After final annealing at 350 °C for 60 min, the micro-tubes showed refined microstructure with an average grain size of 8.6 μm, a weak texture of the rare earth texture or recrystallization texture, and homogeneously distributed fine second phase particles. The annealed micro-tubes exhibited significantly improved mechanical properties with a yield strength of 240.1 ± 6.0 MPa, an ultimate tensile strength of 319.9 ± 6.0 MPa and a breaking elongation of 22.2 ± 1.0%. Meanwhile, the typical characteristic of ductile morphology could be observed for the fractured specimens. In addition, the annealed micro-tubes also had better corrosion resistance and showed a uniform corrosion morphology.

Effects of Ca addition on the microstructures and mechanical properties of as-extruded Mg–Bi alloys

Abstract The microstructure and mechanical properties of as-extruded Mg–1.3Bi–xCa (x = 0, 0.4, 0.9, and 1.3 wt%) were investigated. All samples exhibit full dynamic recrystallization; the mean grain size first increases slightly and then decreases significantly. Additionally, both Mg2Bi2Ca and Mg2Ca phases are formed with increasing Ca content. In particular, the dynamic precipitation of nanoscale Mg2Ca is enhanced remarkably. The typical fiber basal texture in Ca-free binary alloy transformed into a rare-earth texture with the components of [ 1 ¯ 2 1 ¯ l] and [01 1 ¯ l]//extrusion direction in Ca-bearing ternary alloys. In addition, the tensile elongation increases significantly with a trace amount of Ca addition (0.4 wt%), but an overdose of Ca addition (1.3 wt%) results in the formation of a large number of microscale Mg2Bi2Ca phases. The Mg-1.3Bi-0.9Ca alloy demonstrates the optimal balance of ductility and strength with an excellent elongation of 40% and yield strength of 158 MPa, owing to texture modification and grain refinement. This study may provide insights into the development of high-ductility, low-cost Mg–Bi-based alloys using microstructure tuning and texture modification.

Multi-microalloying mediated grain growth and texture evolution during the high-temperature static recrystallization of AZ80 alloys

Abstract Y, (Y + Nd), and (Y + Ca) multi-microalloying, which has potential to mediate the grain growth and texture evolution, played an important role in the mechanical properties of the studied extruded AZ80 sheet. Quasi-in-situ electron backscattered diffraction results revealed that grain boundary migration and grain rotation simultaneously occurred during the grain growth. All these three microalloying combinations effectively lowered the grain growth rate. In particular, the AZ80 + 0.2Y+0.15Ca maintained the lowest growth rate and the most uniform fine grain structure. The rare-earth (RE) texture components, which temporarily formed in AZ80 + 0.2Y and AZ80 + 0.1Y+0.1Nd, weakened their basal texture intensity. In contrast, continuously enhanced basal texture, accompanied by grain growth, dominated the texture evolution in AZ80 + 0.2Y+0.15Ca. As a result, all microalloyed combinations exhibited lower Vickers hardness than that of AZ80. The microalloying also reduced the hardness anisotropy of the sheets.

Comparative Study of Hot Deformation Behavior and Microstructure Evolution of As-Cast and Extruded WE43 Magnesium Alloy

Under compressive testing at 400 °C and a strain rate range of 0.05–5 s−1, the hot deformation behavior and microstructure evolution of an as-cast (AC), as-extruded (with a bimodal grain structure (named as Ex-1) or a relatively uniform fine grain structure (Ex-2)) WE43 alloy have been investigated and compared. The results indicate that the AC sample exhibits the highest peak stress, while the Ex-2 sample has the lowest value. Within the AC material, fine grains were firstly formed along the pancake-like deformed grains (as a necklace structure). The necklace structure was also formed within the Ex-1 and Ex-2 materials at high strain rates of 0.5 and 5 s−1. However, a lamellar structure that the coarse elongated grains divided by parallel boundaries was formed within the Ex-1 material. A relatively more homogeneous fine grain structure is achieved after a true strain of 1.0 within the Ex-2 material at a low strain rate of 0.05 s−1. In addition, a discontinuous dynamic recrystallization mechanism by grain boundary bulging is found to occur. After a true strain of 1.2, a (0001) fiber texture, a typical rare earth (RE) texture, and a relatively random texture are formed within the AC, Ex-1, and Ex-2 WE43 alloy material, respectively.

Effect of Cooling Path on Precipitation Behavior, Microstructure, and Texture Evolution of Micro-alloyed Magnesium Alloy

Alloying with rare-earth (RE) elements enhances the formability of magnesium (Mg) alloys, especially for the lower elevated temperatures. The beneficial effect brought by RE addition is related to texture modification. However, the complexity of the precipitation behavior in RE-containing alloy and the often excessive RE additions has impeded the understanding of the mechanism for RE texture modification. The current study is based on three micro-alloyed Mg-1Zn-xNd (wt.%) alloys with reduced levels of Nd, i.e., x = 0.1, 0.2, and 0.5 wt.%. Two different cooling paths were employed after solutionizing to achieve different precipitation conditions. The alloys were then subjected to hot rolling at 450 °C to a total reduction of 45% in two passes, and post-deformation annealing was also conducted. The precipitation behavior of the current alloy was discussed based on experimental observations and thermodynamic simulations. It was found that “dilute” Nd in solid solution is more effective in suppressing both dynamic and static recrystallization compared to Nd in the form of precipitates. Ultimately, a non-basal texture with basal poles splitting toward rolling direction and transverse direction was obtained for the Mg-1Zn-0.5Nd alloy after annealing. The mechanism of texture weakening effect was attributed to the presence of Nd in solid solution.

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.

Twinning and slip behaviors and microstructural evolutions of extruded Mg-1Gd alloy with rare-earth texture during tensile deformation

An extruded Mg-1Gd alloy with a non-basal texture, whose basal poles are tilted 60.9° from the extrusion direction (ED), exhibits a high room-temperature tensile elongation of 37.2%. This study investigates the active deformation mechanisms and microstructural evolutions of this highly ductile extruded alloy during tensile deformation through in situ electron backscatter diffraction analysis. When tensile strain is applied along the ED, a small amount of {10-12} twins is formed at the early stage of deformation, and the twins gradually grow with increasing applied strain. However, {10-11} twins and {10-11}-{10-12} twins, which are generally observed in extruded Mg alloys with a typical basal fiber texture, are not formed during deformation. The tilted initial texture is favorable for basal slip, and therefore, basal slip is vigorously activated during tension. As a consequence, the average angle between the c-axis of the grains and the ED increases from 60.9° to 66.3° as the applied tensile strain increases from 0% to 30%. Prismatic slip is also activated during tension, which is confirmed by the gradual increase in the rotation angle of the prismatic poles around the <0001> axis during deformation. The activation of prismatic slip causes a significant reduction in the texture intensity of the alloy during deformation. The high ductility of the Mg-1Gd alloy is attributed mainly to the activation of {10-12} twinning, which is beneficial to elongation, instead of the activation of {10-11} twinning, which is detrimental to elongation, and the promoted activation of basal and non-basal slips. Furthermore, microstructural characteristics such as the fully recrystallized structure, large average grain size, and absence of second-phase particles also contribute to the high ductility of the alloy.

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.