unDRXed Grains Research Articles

Comparative study on contribution of bimodal microstructure and heterogeneous microstructure to mechanical properties of ZK60 alloy

In this work, we compared the effects of bimodal microstructure and heterogeneous microstructure on the mechanical properties and strengthening mechanisms of ZK60 alloy. The bimodal microstructure is composed of coarse un-dynamically recrystallized (un-DRXed) grains with strong basal texture and fine dynamically recrystallized (DRXed) grains with weak texture, while the heterogeneous microstructure is composed of coarse DRXed grains and fine DRXed grains. The result suggests that high-density dislocations and high-density precipitates have significant contribution to the strength. The high-density dislocations in un-DRXed grains cause the formation of an uneven distribution of precipitates in the ZK60 alloy which results in the high yield strength of 305 MPa, ultimate tensile strength of 344 MPa and elongation of 12% in the bimodal-structured ZK60 alloy. In comparison, the heterogeneous-structured ZK60 alloy lacks the strengthening effect from high-density dislocations and precipitates, and a lower YS of 226 MPa, and UTS of 305 MPa are achieved. The result also indicates the weak texture, low density of dislocations, low density of precipitates, and stronger ability of storing dislocations in coarse grains for heterogeneous microstructure are helpful to the ductility. Thus, HE sample exhibits higher elongation of 23% than BI sample. The finding shed light on the microstructure designing of high-performance Mg alloys.

Microstructures, mechanical properties and damping performance of wrought Mg–Cu–Ca alloys

Mg–Cu–Ca alloy sheets show good room temperature formability and excellent thermal conductivity; however, their strength is insufficient for load-bearing applications. In this study, Mg–Cu–Ca ternary alloys were subjected to a conventional extrusion process, and the microstructures and tensile properties were systematically evaluated. The addition of extremely small amounts of Cu and Ca to pure Mg (Mg–0.03Cu–0.05Ca, wt%) led to significant refinement of dynamically recrystallized (DRXed) grains while preserving a certain fraction of un-DRXed grains, which contributed to a substantial increase in tensile yield strength. Adjusting the extrusion parameters further increased the tensile yield strength to 329 MPa, which was caused by the combined effects of further refined DRXed grains and an increased area fraction of un-DRXed grains. The damping performance of the Mg–0.03Cu–0.05Ca alloy sheet was superior to that of commercial AZ31(Mg–3Al–1Zn–0.3Mn in wt%) alloy sheet but lower than that of pure Mg, particularly at low frequencies. This revealed that the solute segregation at grain boundaries played a crucial role in the damping performance of Mg alloys.

Achieving enhanced mechanical properties of extruded Mg-Gd-Y-Zn-Zr alloy by regulating the initial LPSO phases

In this paper, “bimodal microstructure” and enhanced mechanical properties of extruded Mg-Gd-Y-Zn-Zr alloy is achieved by regulating the initial long-periodic stacking-ordered (LPSO) phases, the effect of blocky LPSO phase and lamellar LPSO phase on the dynamic recrystallization (DRX) behavior and the formation mechanism of bimodal microstructure are systematically investigated, the deformation modes, strengthening and toughening mechanism and fracture behavior of the “bimodal microstructure” are analyzed by slip traces. Blocky LPSO phase has significant particle stimulated nucleation (PSN) effect to promote DRX. The effect of lamellar LPSO phase on DRX depends on the size, fragmented lamellar LPSO with small size promotes numerous DRXed grains through the PSN effect, while continuous intragranular lamellar LPSO with large size inhibits DRX by hindering slip and grain boundary rotation, which is the key factor to form the bimodal microstructure. The almost fully DRXed H-510 sample has the lowest strength, and the H-525 sample with “bimodal microstructure” achieves enhanced comprehensive mechanical properties (ultimate tensile strength, yield strength and elongation of 383 MPa, 308 MPa and 9.2 %, respectively), which is related to the significant texture strengthening and dislocation strengthening of large-sized unDRXed grains, as well as strain coordination of higher DRXed grains fraction. The deformation mode of the DRXed regions is grain boundary sliding, while the unDRXed regions are dominated by prismatic slip, the strain incompatibility between the two is the decisive factor causing microcracks propagation and final fracture. This paper will provide theoretical guidance for the initial microstructure design of strength-plasticity synergistic Mg-Gd-Y-Zn-Zr alloys with bimodal microstructure.

Bimodal grain structure, phase transformation, and mechanical properties of a Mg-Gd-Y-Zn-Zr alloy during hot extrusion

In this study, we investigated the evolution of grain structures and second phases of a Mg-9.5Gd-4Y–2Zn-0.3Zr alloy during two-stage deformation that took place in the metal during hot extrusion, as well as their effects on its mechanical properties. We found that the alloy exhibited a bimodal grain structure consisting of equiaxed dynamic recrystallized (DRXed) grains and elongated unDRXed grains. As the extrusion process progressed, DRX occurred on a large scale, resulting in an increase in the proportion of DRXed grains from 17.8% to 93.2%, and a reduction in average grain size from 80.3 μm to 9.41 μm. The grain orientation also underwent a transition from a single orientation of an extrusion direction (ED) parallel to <0001> (i.e. ED//<0001>) to mixed orientations of ED//<01–10> and ED//<0001>. The primary mechanism of dynamic recrystallization (DRX) during extrusion involved particle-stimulated nucleation (PSN) accompanied by both continuous and discontinuous DRX. The Mg-9.5Gd-4Y–2Zn-0.3Zr alloy mainly contained intergranular 18R-long period stacking order (LPSO) phases and intragranular 14H-LPSO. As the extrusion process progressed, 18R-LPSO changed from blocky to lamellar to accommodate deformation, and β-Mg5(Gd,Y) phase particles dynamically precipitated along DRXed grain boundaries. The 14H-LPSO phase was found to be primarily distributed within coarse unDRXed grains. As deformation via extrusion progressed, the quantity of 14H-LPSO gradually decreased. Ultimately, with the gradual dissolution of Mg5(Gd,Y), 14H-LPSO reprecipitated within the recrystallized grain boundaries. As the extrusion process progressed, both the yield strength (YS) and elongation (EL) increased from 132.6 MPa to 287.6 MPa and from 10.8% to 15.5%, respectively. This was mainly attributed to the significant reduction in grain size and second phase size.

300 MPa grade high-strength ductile biodegradable Zn-2Cu-xMg (x = 0.08, 0.15, 0.5, 1) alloys: The role of Mg in bimodal grain formation

300 MPa grade biodegradable Zn-2Cu-xMg (0.08, 0.15, 0.5, and 1 wt.%) alloys with different bimodal grain structures were obtained by casting and hot extrusion. The effects of the Mg element on the microstructure, mechanical properties, and dynamic recrystallization (DRX) behavior of the as-extruded Zn-2Cu-xMg alloys were investigated. The obtained results showed that CuZn4 butterfly particles and eutectic net structure (η-Zn + Mg2Zn11) are formed in the as-cast Zn-2Cu-xMg alloys. The as-extruded Zn-2Cu-0.08Mg and Zn-2Cu-0.15Mg alloys exhibited finer DRXed and coarser unDRXed grains with an average grain size of 8.5–8.8 μm, while Zn-2Cu-0.5Mg and Zn-2Cu-1Mg alloys were almost composed of completed DRXed grains with an average grain size of 4.2–6.5 μm. Nanoprecipitates ε-CuZn4 were uniformly precipitated in both DRXed regions and unDRXed regions. Continuous DRX (CDRX) and twinning-induced DRX (TDRX) were the main mechanisms at a low Mg content; Discontinuous DRX (DDRX) and particle-stimulated nucleation (PSN) were strengthened with the addition of Mg. The improved yield strengths in Zn-2Cu-xMg originate from grain boundary strengthening, Orowan strengthening, and hetero-deformation-induced (HDI) strengthening. The fracture elongations are mainly affected by the synergistic effect of bimodal grains, non-basal < c + a > dislocations, and the secondary phases.

Comparative Study on Mechanical Properties and Microstructure Evolution of Mg-3Zn-1Mn/Sn Alloy through Ca-La-Ce Addition.

This study systematically investigates the influence of the composite addition of Ce, La, and Ca elements on the microstructure evolution and mechanical properties of Mg-3Zn-1Mn/Sn (wt.%) alloys. It indicates that the strength of Mg-Zn-Mn series alloys is superior to that of Mg-Zn-Sn series alloys, due to the stronger restriction of nanosized Mn particles on the recrystallization process and grain growth compared with Mg2Sn phases. The addition of the Ca-La-Ce elements significantly enhances the strength of the Mg-3Zn-1Sn alloy (YS increased by approximately 92.5%, UTS increased by approximately 29.2%, and EL decreased by nearly 52.2%), while for the Mg-3Zn-1Mn alloy, a balanced effect on both the strength and performance can be achieved. This difference mainly lies in the more pronounced refined effect on the grain size and the formation of a bimodal grain structure with strip-like un-DRXed grains and surrounding fine DRXed grains for the Mg-3Zn-1Sn alloy. In contrast, the addition of the Ca-La-Ce elements has a less obvious hindrance on the recrystallization process in the Mg-Zn-Mn series alloy, while significantly weakening the extrusion texture while refining the grains. Through in-depth characterization and experimental analysis, it is found that Sn and Ca can promote the formation of brittle and fine secondary phases. A nanoscale Sn phase (Mg2Sn phase) is more likely to accumulate at the grain boundaries, and the size of the nanoscale Ca2Mg6Zn3 in Mg-Zn-Mn series alloys is finer and more dispersed than that in Mg-Zn-Sn series alloys, thus strongly hindering recrystallization and refining the recrystallized structure of the alloy.

Microstructure and mechanical properties of a low-alloyed Mg–Zn–Al–Ca alloy: Effect of extrusion speed

A low-alloyed Mg-1.2Zn-0.6Al-0.1Ca (wt.%) alloy was extruded at 200 °C with different ram speeds (0.5–4.0 mm/s), and the microstructure and mechanical properties were studied systematically. Heterostructures with fine dynamic recrystallized (DRXed) grains and coarse unDRXed grains were achieved at lower ram speeds of 0.5 mm/s and 1.0 mm/s, and fully-DRXed microstructure was attained at 4.0 mm/s. Increasing the extrusion speed resulted in an increase in DRXed grain size from 0.9 μm to 3.8 μm, and a transformation of the DRXed texture component from <10 to 10>−<11–20> to a new orientation that deviated by approximately 14°. The sample extruded at 0.5 mm/s presented an excellent tensile yield strength (TYS) of 369 MPa along with a 7.8% elongation, which was mainly due to the high hetero-deformation induced (HDI) strengthening provided by its heterostructures. Increasing ram speed resulted in an improved elongation despite a decreased TYS. The reasons for the decreased strength with increasing extrusion speed were mainly associated with grain growth, reduced dislocation density and weakened HDI strengthening. The reasons for the improved ductility with increasing extrusion speed were largely due to the increased DRXed grains fraction with soft orientations.

Mechanical properties of Mg-Ce-Y-Zr-Zn-Ca microalloyed magnesium alloys composed of near-atomic solute

A new extruded Mg-0.43Ce-0.17Y-0.17Zn-0.12Zr-0.06Ca (wt%) alloy was prepared by adding five alloying elements in accordance with the principle of near-equivalent atomic weights, with the atomic number percentages of the various alloying elements added close to each other and with the sum of the mass fractions of all the added alloying elements being controlled to be less than 1 wt%. The alloy has a prominent extrusion weave after extrusion and possesses a bimodal organization consisting of fine dynamically recrystallized (DRXed) grains (0.91 μm) and coarse unDRXed grains with strong dislocation storage capacity, achieving a yield strength of 348 MPa and a plasticity of 10.3 %.

Effect of different extrusion parameters on the microstructure and mechanical properties of Mg-4Sm-3Gd-2Yb-0.5Zr alloy

The effect of different extrusion parameters on the microstructure and mechanical properties of Mg-4Sm-3Gd-2Yb-0.5Zr (SGY2) alloy was investigated. It was observed that under different extrusion parameters, unDRXed grains of SGY2 alloy exhibited a pronounced basal plane texture, specifically <011¯0>//ED, while the texture of DRXed grains was relatively dispersed. Under the condition of 420 °C and an extrusion ratio of 9.4 (420 °C-ER9.4), the basal plane texture strength of unDRXed grains in SGY2 alloy was the highest. Furthermore, SGY2 alloy at different extrusion parameters exhibited recrystallization mechanisms mainly characterized by continuous dynamic recrystallization (CDRX), with some DRXed grains and deformed grains experiencing discontinuous dynamic recrystallization (DDRX). Additionally, at the 420 °C-ER9.4, the second phase particles in the as-extruded SGY2 alloy were smaller in size and exhibited a dispersed distribution. Under this condition, a significant amount of dislocation accumulation, dislocation bypassing, and dislocation tangling phenomena were observed in the SGY2 alloy. The primary deformation mechanism of unDRXed grains in the SGY2 alloy at the 420 °C-ER9.4 may involve prismatic plane 〈a〉, pyramidal plane 〈a〉, and pyramidal plane 〈c + a〉 slip, thereby activating a significant amount of dislocations. Compared to other extrusion conditions, this condition is more prone to activate non-basal plane slip. The as-extruded SGY2 alloy exhibited superior mechanical properties, with ultimate tensile strength (UTS) and yield strength (YS) of 332 MPa and 278 MPa, respectively. This is mainly attributed to the extremely fine grains, with many DRXed grains having grain sizes smaller than 1 µm, and higher density grain boundaries produced under this condition. Additionally, the unDRXed grains contain a high density of dislocations with small Schmid factor (SF), thus effectively inhibiting basal plane slip and strengthening the alloy to some extent. Similarly, the increased presence of second phase particles will also contribute to strengthening the alloy matrix through precipitation hardening.

Achieving High Strength-Ductility Synergy in Low-Alloyed Mg-Li-Er Extrusion Alloys via Tailoring Bimodal-Grained Structure.

Low-alloyed Mg-Li-Er alloys were developed in this study and a bimodal-grained structure was obtained by varying the trace Er content and extrusion temperature. The alloys displayed a good strength-ductility synergy, i.e., a tensile yield strength (TYS) of 270 MPa and an elongation (EL) of 19.1%. Microstructural characterization revealed that the formation of numerous submicron Mg24Er5 particles favored a high density of low-angle grain boundaries (LAGBs) inside the deformed grains and inhibited dynamic recrystallization (DRX). The resultant coarse unDRXed grains with a strong basal texture and considerable LAGBs, together with the fine DRXed grains, contributed to the high strength-ductility synergy.

Role of Ca content on microstructure, mechanical properties and strain evolution of as-rolled Mg-Al-Ca-Zn-Mn alloy

The role of Ca content (0.5, 1.0, 2.0 wt.%) on microstructure, mechanical properties and strain evolution of as-rolled Mg-Al-Ca-Zn-Mn alloy was thoroughly investigated in this work. The results indicate that the primary second phase transformed from the Mg17Al12 phase to the Al2Ca phase after homogenization, and the amount of Al2Ca phase increased significantly with increasing Ca content. After hot rolling, the alloys exhibited the typical bimodal microstructure composed of fine dynamic recrystallized (DRXed) grains and coarse elongated un-DRXed grains, the area fraction of the DRXed regions increased with increasing Ca content. Besides, a large number of submicron-sized as well as nano-scaled spherical Mg17Al12 phases dynamically precipitated along the DRXed grain boundaries in all alloys, which promoted the DRX and restricted the grain growth. During rolling deformation, DRX preferentially occurred near the primary second phases and shear bands by the particle stimulated nucleation (PSN) and shear band induced nucleation (SBIN) mechanism in the alloys. The ultimate tensile strength (UTS), yield strength (YS), and elongation to failure (EF) along the rolling direction (RD) of the Mg-8.0Al-1.0Ca-1.0Zn-0.4Mn (wt.%) sheet were 393 MPa, 334 MPa and 8.7 %, respectively. Such high strength was mainly attributed to fine DRXed grains, high number density of dynamically precipitated Mg17Al12 phases and strongly textured un-DRXed grains with numerous sub-structures. The reasonable DRX ratio moderated strain localization and thus stabilized tensile deformation, leading to moderate plasticity of the alloy.

Evolution of bimodal-structure and achieving ultra-high yield strength in the as-extruded ZK70 alloy via Gd addition

Achieving ultra-high tensile yield strength over 400 MPa in RE modified Mg–Zn-based alloys using conventional thermomechanical processing is a challenging task. In this study, we successfully produced a Mg–7Zn–2Gd–0.5Zr (wt. %) alloy with exceptional strength through traditional hot-extrusion techniques. This alloy exhibits a tensile yield strength of 440 MPa and an acceptable ductility of 4.5%, surpassing a majority of previously reported Mg–Zn-RE-based alloys. Our findings demonstrate that the addition of Gd in Mg–Zn-based alloys effectively retards dynamic recrystallization (DRX) during extrusion, leading to the formation of a distinctive bimodal structure comprising approximately 13.8% coarse un-recrystallized (unDRXed) grains and about 86.2% fine DRXed grains. Furthermore, the alloying effect of Gd enhances particle density, including micron-sized particles and dynamic nano-precipitates. These dynamic precipitates play a crucial role in impeding dislocation movement during extrusion, thereby contributing to the formation of bimodal structure. Additionally, both nano-precipitates and segregation of Zn and Gd atoms towards grain boundaries facilitate the formation of fine DRXed grains through pinning effect. Consequently, the significantly increased tensile yield strength observed in our Gd-modified alloy can be attributed to multiple strengthening mechanisms: fine DRXed grains, strong texture exhibited by unDRXed grains, numerous dynamic precipitates, high-density residual dislocations, and co-segregation of solutes. However, the formation of bimodal structure worsens the yield asymmetry of the Gd-modified alloy due to its strong basal texture. These results provide a valuable insight for further development efforts aimed at achieving ultra-high-strength Mg–Zn-RE-based alloys.

Effects of Trace Zn on the Deformation Mechanisms of Mg–Gd–Y–Zr Alloy Investigated by In Situ Synchrotron Diffraction

Addition of trace Zn can impose notable effect on the deformation behavior of Mg–Gd–Y–Zr alloy, while the underlying mechanisms have not been revealed. In the present work, Mg–8.5Gd–2.5Y–0.3Zr (wt%) (0Zn) and Mg–8.5Gd–2.5Y–0.5Zn–0.3Zr (wt%) (0.5Zn) alloys are fabricated by casting and extrusion, to obtain a bimodal microstructure containing fine dynamic recrystallized (DRXed) grains and coarse unDRXed grains. The as‐extruded 0Zn and 0.5Zn alloys are studied by in situ synchrotron diffraction under uniaxial tension and compression to clarify the effect of trace Zn on the deformation mechanisms. It is found that both alloys exhibit stress softening in the basal slip‐favored grains under both tension and compression, and adding trace Zn highly enhances the stress softening. Furthermore, the addition of Zn stimulates the occurrence of discontinuous yielding which is in connection with the stress softening, resulting from the interaction between basal dislocations and solute atmospheres. The tension–compression‐yielding symmetry of the alloy is improved by adding Zn, which is mainly attributed to the suppression effect of γ′ phases and Gd/Y–Zn dimers on tensile twinning. Finally, the addition of Zn can promote the tensile and compressive strength, owing to the strength enhancement of the relevant deformation modes and the increase of the fraction of unDRXed regions.

Unveiling the microstructure evolution and deformation mechanisms of micro-alloying Mg-1.5Zn-0.5Zr-0.5Sr alloy via extrusion-shearing

Obtaining high strength without sacrificing ductility in Mg alloys is of great importance. However, the hexagonal close-packed crystal structure critically limits the deformation of Mg alloys. In this work, novel Mg-1.5Zn-0.5Zr-0.5Sr (wt.%) alloys were fabricated through homogenization, direct extrusion (DE), and extrusion-shearing (ES) processes. The effects of three different processes on microstructures, texture, and mechanical properties were thoroughly investigated. The results show that the average grain size of the alloy was greatly refined after hot extrusion. The alloy after the DE process exhibited a typical bimodal microstructure with dynamic recrystallized (DRXed) grains showing a strong <101‾0 > fiber texture parallel to extrusion direction (ED), while the small-sized unDRXed grains showed relatively weak basal texture. In addition, the strength was significantly improved, with the value of yield strength (YS) reaching ∼229 MPa and ultimate tensile strength (UTS) reaching ∼262 MPa. This is mainly attributed to grain boundary strengthening and dislocation strengthening. Compared to the DE-processed alloy, the basal texture intensity was significantly decreased and non-basal slip was activated after the ES process, which indicates that the shearing deformation during ES could improve the ductility, with the fracture elongation (FE) increased from ∼13.9 % to ∼21.4 %. This work aims to unveil the deformation mechanisms in Mg-1.5Zn-0.5Zr-0.5Sr alloy and provides an efficient way for preparing micro-alloying Mg alloys with low cost and significant improvement of comprehensive properties.

Preparation of ultrafine-grained Mg–Mn–Ce alloy with high ductility and energy absorption by coupling extrusion and forging

An ultrafine-grained (UFGed) Mg–0.9Mn–0.5Ce (wt.%) alloy with high compressive ductility and energy absorption was developed using a low-temperature deformation method of coupling extrusion and forging. Specifically, the alloy was extruded at 150 °C with an extrusion ratio of 12:1, followed by forging at 200 °C to a true strain of 1.2 along the extrusion direction. The microstructure and plastic mechanism were uncovered using optical microscopy, scanning electron microscopy, electron backscatter diffraction and transmission electron microscopy. The microstructure was found to consist of ultrafine dynamically recrystallized (DRXed) grains and fragmented unDRXed grains, exhibiting an average grain size of 1.9 μm after low-temperature extrusion. After forging, new UFGs were gradually generated, a uniform UFGed microstructure with an average grain size of 2.7 μm was obtained, and the texture intensity was decreased as well. The homogenous UFGs and basal slip collaboratively contributed to better comprehensive performance of the UFGed alloy, such as a high compressive ductility of 45.0% and a high energy absorption of 122.72 MJ/m3.

Influence of bimodal microstructure on strength and ductility of as-extruded Mg-Gd-Y-Zr alloy

This paper studied the role of layered bimodal structure in the strength-ductility balance of Mg-8Gd-4Y-0.4Zr (wt%) alloys under as-extruded conditions. The various bimodal microstructure, consisting of fine dynamically recrystallized grains and elongated un-dynamically recrystallized grains along extrusion direction, were controlled by different extrusion ratios. The mechanical properties were then discussed in detail. The local strain evolution was investigated in the alloys based on in situ testing and analyzed with electron backscattered diffraction and digital image correlation. The fracture behavior of the alloys was characterized by 3-Dimensional X-Ray Microscopy. The alloys with an extrusion ratio of 7.5 exhibited high tensile yield strength of 338 MPa and ductility of 13.2%, resulting from grain boundary strengthening of fine DRXed grains, and dislocation strengthening of elongated un-DRXed grains and Mg5RE particles. The strain evolution analysis showed that the slips and rotations of DRXed grains contributed to relieving strain localization at the interface. Additionally, the layered structure assisted local strain dispersion and uniformed strain distribution, improving the strength and ductility of alloys. The microcracks primarily nucleated and propagated at the un-DRXed regions. The DRXed regions can refine microcracks and enhance deformation compatibility.

Development of low rare-earth containing magnesium alloy with high strength-ductility synergy by engineering nano-spaced lamellae

Conventional Mg-RE-based alloys containing a high content of RE solutes can generate high strength, but there is often low ductility. Herein, we reported a low RE alloying Mg–5Er-1Nd-1Zn-0.2Zr extrusion alloy with only 6 wt% RE addition, showing a high strength-ductility synergy, such as a yield strength of ∼380 MPa and an elongation of ∼12.5 %), which is superior to the majority of traditional Mg-RE-based wrought alloys with at least 12 wt% RE addition. Nano-spaced lamellae was formed in the α-Mg matrix during extrusion, such as stacking faults (SFs) evolved in dynamically recrystallized (DRXed) grains and the 14H long-period stacking-ordered (LPSO) phase developed in unDRXed grains as a result of dynamical solute partitioning during extrusion. And this alloy characterizes a high proportion (∼70 %) of elongated unDRXed grains with a strong basal texture. Thus, high strength is mostly related to SFs/LPSO lamellae strengthening and texture hardening, instead of the commonly accepted refinement strengthening and precipitation hardening in those of high RE containing Mg alloys. While high ductility is heavily dependent on SFs/LPSO lamellae to trigger strain hardening and <c + a> dislocations glide, independent of fine grains as in traditional Mg alloys with high elongation. The findings give an insight into the development of high-performance Mg wrought alloys with a high strength-ductility synergy.

Optimization in strength-ductility of Mg-Gd-Y-Zn-Zr alloy processed by upsetting-extrusion combined with intermediate thermo-mechanical treatment (ITMT)

In this work, the effects of intermediate annealing and intermediate annealing subsequent aging treatment on the second phase precipitation, dynamic recrystallization (DRX) behavior, and mechanical properties of Mg-9Gd-4Y–2Zn-0.5Zr (wt.%) alloy were comparatively studied. The results showed that the one-pass upsetting-extrusion (UE) deformation intermediate annealing treatment sample (UAE) exhibited the highest dynamic recrystallized (DRXed) grains number fraction and the largest average DRXed grain size, which was mainly attributed to the precipitation of a large number of β-Mg5RE phases (>1 μm) at the grain boundaries (GBs) and within unDRXed grains after annealing treatment. These β-Mg5RE phases promoted DRX through particle-stimulated nucleation (PSN) mechanism during subsequent extrusion process. However, the lack of the pinning effect of the fine second phase precipitations led to the growth of DRXed grains. The DRXed grains of the deformation intermediate annealing subsequent peak-aging treatment sample (UAAE) were significantly smaller than the UAE sample. This is mainly due to the fact that the densely distributed prismatic β′ phases precipitated by the peak-aging treatment were rapidly transformed into nano-scale β-Mg5RE phases and pinned at the DRXed grain boundaries to inhibit the growth of the DRXed grains during the subsequent extrusion process. The ultimate tensile strength (UTS) and failure elongation (EL) of the UAAE sample were 274 MPa and 14.3%, respectively, 4.2% and 85.7% higher than those of the UE sample, showing excellent strength-ductility synergy. The randomly oriented fine DRXed grains, coarse unDRXed grains with high-density dislocations and substructures, and nano-scale β-Mg5RE particles were co-contribute to the strength-ductility synergy.

Effect of bimodal microstructure on the tensile properties and fracture toughness of extruded Mg-9Gd-4Y-0.5Zr alloy

The effects of bimodal microstructure on the tensile properties and fracture toughness of Mg-9Gd-4Y-0.5Zr alloys were investigated. The Mg-9Gd-4Y-0.5Zr alloy was extruded at different temperatures to produce samples with bimodal and fully dynamic recrystallized (DRXed) grain structures. The results show that the bimodal grain structure exhibits a better combination of yield strength (YS of 277 MPa) and plane strain fracture toughness (KIc = 21.5 MPa m1/2), compared to the fully DRXed microstructure (YS of 207 MPa, KIc = 16.3 MPa m1/2). The fracture mechanisms for the bimodal and fully DRXed microstructures are the ductile fracture mode and quasi-cleavage fracture mode, respectively. Both hetero-deformation induced (HDI) strengthening effect and strong basal texture can contribute to high tensile strength in the bimodal grain structured sample. As compared to the sample with fully DRXed microstructure, the stronger strain hardening capacity and larger hardened region around the crack tip in the bimodal grain structured sample can bring about the improvement in the fracture toughness. The tortuous crack propagation path and the hindrance of propagating crack by the un-DRXed grains result in larger fracture resistance and more energy dissipation. The increase of geometrically necessary dislocation (GND) density and the activation of non-basal dislocations also have positive effects on the fracture toughness of the samples with bimodal grain structure. The fracture toughness is effectively improved via intrinsic and extrinsic toughening mechanisms associated with the bimodal microstructure.

Deformation behaviors and the related high-temperature mechanical properties of Mg–11Gd–5Y–2Zn–0.7Zr via regulating extrusion temperatures

The effects of extrusion temperatures (430, 450, 470 and 490 °C) on the microstructures and mechanical properties of Mg–11Gd–5Y–2Zn–0.7Zr (GW) alloys at room temperature (RT), 250, 300, 350 °C were systematically investigated. The dynamic precipitation of blocky and lamellar long period stacking ordered (LPSO) phases was taken place during hot extrusion. The dominant texture changed from <10 1¯0 >//ED to <0001>//ED with increasing extrusion temperature. Tensile results indicated that the GW alloy extruded at 430 °C (GW430) exhibited the highest mechanical properties at both RT and elevated temperatures, which was ascribed to the fine dynamic recrystallized (DRXed) grains, un-DRXed grains with an orientation of <10 1¯0 >//ED, and the LPSO phase strengthening. When tensile tested below 300 °C, slips on the blocky LPSO phase and kinks of lamellar LPSO phase were the primary deformation modes, followed by grain boundary crack. At 350 °C, grain boundaries became the vulnerable sites to be activated. For GW alloys extruded at 450, 470 and 490 °C, an abnormal increase of mechanical properties was observed at 300 °C. This was due to the occurrence of multiple-slip, which showed the intersection and entanglement of dislocations and thus improving the strength of Mg matrix.