Incomplete Recrystallization Research Articles

Quasi-In-Situ Analysis of As-Rolled Microstructure of Magnesium Alloys during Annealing and Subsequent Plastic Deformation.

In this paper, quasi-in situ experiments were carried out on rolled AZ31 magnesium alloy sheets to track the recrystallization behavior of the rolled microstructure during the heat treatment process and the plastic deformation behavior during the stretching process. The as-rolled microstructures are classified into five characteristics and their plastic deformation behaviors are described. The research shows that annealing recrystallization leads to grain reorganization, resulting in the diversity of grain orientation, and it is easier to activate basal slip. Recrystallization preferentially nucleates in the regions with high stress, while it is difficult for recrystallization to occur in regions with low stress, which leads to the uneven distribution of the as-rolled structure of magnesium alloys. Slip can be better transmitted between small grains, while deformation between large and small grains is difficult to transmit, which can easily lead to the generation of ledges. Incomplete recrystallization is more likely to accumulate dislocations than complete recrystallization, and ledges are formed in the early stage of deformation. Microcracks are more likely to occur between strain-incompatible grains. It is of great significance to promote the application of rolled AZ31 magnesium alloys for the development of heat treatment and subsequent plastic working of rolled magnesium alloys.

Effect of Ca and Zr Additions on Microstructure and Mechanical Properties of As-Extruded Mg-3Sn Alloy.

In this paper, the effect of Ca and Zr additions on microstructure and mechanical properties at room temperature of Mg-Sn alloys was investigated by comparison of Mg-3Sn (wt.%) (T3), Mg-3Sn-1Ca (wt.%) (TX31), and Mg-3Sn-1Ca-1Zr (wt.%) (TXK311) alloys under extrusion. The results show that the main phases of as-extruded T3 alloy were α-Mg and Mg2Sn phases, while the CaMgSn phase was formed and the precipitation of Mg2Sn phase was inhibited in the TX31 and TXK311 alloys due to the addition of the Ca element. Zr did not form intermetallic compounds with other elements but dissolved in the grains of the matrix and became nucleating particles. Incomplete dynamic recrystallization (DRX) occurred in all alloys during hot extrusion. The coarse rod-like and fine block-like mixed CaMgSn phase was observed in α-Mg matrix of as-extruded samples of the TX31 alloy, and the dispersed granular CaMgSn phase was observed in the TXK311 alloy. Ca inhibited the dynamic recrystallization behavior of the alloys, while Zr promoted the dynamic recrystallization behavior. All the as-extruded alloys exhibit typical fiber texture of {0001} basal//ED. With the addition of Ca and Zr elements, the particle stimulated nucleation (PSN) effect excited by the second phase particles gradually weakened the texture. TXK311 alloy has good comprehensive mechanical properties at room temperature, with tensile strength, yield strength, and elongation of 261 MPa, 244 MPa, and 11%, respectively, and the average grain size was 1.8 μm. Grain refinement and second phase dispersion strengthening are considered to play critical roles in the strength optimization of the TXK311 alloy.

Microstructural and Mechanical Characteristics of Pure-Cu/brass Dissimilar Joints Welded by Friction Stir Welding Using Various Process Parameters

FSW (friction stir welding) is a solid-state joining method that attracts interest from all industries. The influence of various tool rotational speeds on the microstructure and mechanical characteristics of dissimilar pure Cu-brass joints has been investigated. The travel speed and vertical load were kept constant in the welding trials, at 40 mm/min and 10 kN, respectively, while the tool rotational speed varied from 1000 to 1400 rpm. The increase of the rotational speed to 1400 rpm resulted in degradation of the mechanical properties. The stir zone grain structure was refined; however, the grain size was irregular. Grain refining occurs due to a continuous and discontinuous dynamic recrystallization mechanism. In the grain interior, there were many large dislocations, identifying that incomplete recrystallization took part within the SZ. Instead, the lower rotational speed, i.e., 1000 rpm, led to more uniform grain refinement in the SZ. Moreover, in contrast to the welded zone where very fine grains exist, the base metal and thermomechanically heat-affected zone display coarser grains. Because of the microstructural modification, the stir zone’s mechanical characteristics were higher than the base materials, and the mechanical strength and plasticity were simultaneously upgraded. These results indicate that the size of the grains is independent of rotational speed. And mechanical properties like hardness and impact strength decreased as the rotational speed increased.

Quantitative mechanisms behind the high strength and electrical conductivity of Cu-Te alloy manufactured by continuous extrusion

The microstructure, mechanical performance, and electrical conductivity of Cu-Te alloy fabricated by continuous extrusion were quantitatively investigated. The results demonstrate that the grain size of the Cu-Te alloy is refined significantly by incomplete dynamic recrystallization. The Cu2Te phase stimulates recrystallization and inhibits subgrain growth. After extrusion, the tensile strength increases from 217.8 ± 4.8 MPa to 242.5 ± 3.7 MPa, the yield strength increases from 65.1 ± 3.5 MPa to 104.3 ± 3.8 MPa, and the yield to tensile strength ratio is improved from 0.293 ± 0.015 to 0.43 ±.0.091, while the electrical conductivity of room temperature decreases from 95.8 ± 0.38% International Annealed Cu Standard (IACS) to 94.0% ± 0.32% IACS. The quantitative analysis shows that the increment caused by dislocation strengthening and boundary strengthening account for 84.6% of the yield strength of the extruded Cu-Te alloy and the electrical resistivity induced by grain boundaries and dislocations accounts for 1.6% of the electrical resistivity of the extruded Cu-Te alloy. Dislocations and boundaries contribute greatly to the increase of yield strength, but less to the increase of electrical resistivity.

Effect of ultra-fast heating on microstructure and mechanical properties of cold-rolled low-carbon low-alloy Q&P steels with different austenitizing temperature

We performed ultra-fast heating (150 °C/s) quenching and partitioning (Q&P) processes to a cold-rolled low-carbon low-alloy steel. Adjusting the annealing temperature, we obtained two types of ferrite-containing Q&P steels with different matrix structure, i.e., martensitic matrix and ferritic matrix. The effect of ultra-fast heating on microstructure evolution and mechanical properties was analyzed, compared with the samples treated with conventional heating rate (10 °C/s). We also investigated the strain partition and fracture behaviors, basing on in-situ tensile tests under SEM. Results show that ultra-fast heating can significantly increase the A c1 and A c3 temperatures, and extend the temperature span of intercritical zone. It can also delay the complete recrystallization of deformed ferrite and promote the austenite nucleation, which leads to the structure refinement of the sample with martensitic matrix, improving both the strength and ductility. The increase of austenite nucleation rate and the incomplete recrystallization of deformed ferrite jointly promote the austenite transformation, which results in a distinct increase of martensite islands fraction in the sample with ferritic matrix, leading to its obvious increase of tensile strength and decrease of ductility. Strain partition is an important factor affecting the TRIP effect in ferrite-containing Q&P steels. For the sample with a martensitic matrix, retained austenite mainly possesses a film-like morphology and exists between martensite laths. The relatively high mechanical stability of retained austenite films and low local strain in martensitic regions result in the weak TRIP effect. The retained austenite in the sample with a ferritic matrix mainly distributes among ferrite grains and exhibits a blocky morphology. The relatively low mechanical stability of blocky retained austenite and high local strain in ferrite jointly enhance the TRIP effect. • Ultra-fast heating delays ferrite recrystallization. • Ultra-fast heating restrains martensite tempering. • Ultra-fast heating results in grain refinement of martensitic matrix. • Strain partition is a crucial factor controlling TRIP effect. • Main ductility origin is compatible deformation of matrix structures.

Improvement of the Mechanical Characteristics, Hydrogen Crack Resistance and Durability of Turbine Rotor Steels Welded Joints

This article is devoted to the following issues: calculating the values of temperatures obtained by simulating welding heating and the subsequent implementation of the welding process at the given mode parameters made it possible to obtain a welded joint of the rotor with an improved initial structure and increased mechanical properties, hydrogen resistance and durability by up to 10–15%; simulating welding heating in the areas of fusion, the overheating and normalization of the HAZ and the formation of austenite grains; specified welding heating creates the conditions for the formation of new products of austenite decomposition in the form of sorbitol in the area of the incomplete recrystallization of the HAZ. In air and gaseous hydrogen, the destruction of the combined joints took place on the weld metal, as well as on the fusion areas, the overheating and the incomplete recrystallization of the HAZ of 20H3NMFA steel as the base metal. Structural materials have a relatively low strength and high fracture toughness in air. This is manifested in a significant reduction in the elongation (δ), the area (ψ) and critical stress intensity factor (KIc) of welded joints and the endurance limit of cylindrical smooth rotor steel specimens, which were cut from transverse templates. Welded joints in the whole range of load amplitudes are sensitive to the action of hydrogen.

Study of the structural changes in oxide ceramics using polarization microscopy

The mechanical and thermal properties of heat-resistant ceramic materials based on refractory oxides largely depend on the defect-free nature of their crystal structure, the presence of extraneous phases, the size and orientation of the grains. We present the results of studying the structure of ceramic fibers of mullite-corundum composition during high-temperature heating using optical polarization microscopy. The structural features of a continuous fiber of Al2O3 – SiO2 system heat-treated in a free state and under tension, as well as the structure of short fibers of a similar composition obtained by injecting a solution with subsequent heat treatment of the fiber mass at different temperatures were studied. It is shown that the structure of a continuous fiber after heat treatment in the free state has a multidirectional orientation of the grains, whereas heat treatment at the same temperatures under tension leads to the formation of an oriented uniaxial structure. Thus, the fibers obtained from the same solution and heat-treated at the same temperatures are characterized by different strength values. Moreover, the strength of a fiber with an oriented uniaxial structure is one and a half times higher than that of a fiber with a multidirectional grain orientation, which greatly facilitates its further processing. It is also revealed that structural defects like incomplete crystallization, collective recrystallization, and inhomogeneous inclusions of the second phase, characteristic of short fibers obtained by nozzle spraying, negatively affect the thermal properties of thermal insulation made of such fibers. The results obtained can be used to improve the methods of analyzing the microstructure of aluminum oxide fibers using the method of optical polarization microscopy.

Microstructure evolution and process optimization of molybdenum rods during loose tooling forging

Defects such as abnormal grain growth and cracking are easy to occur in the process of loose tooling forging of large-size molybdenum rods prepared by powder metallurgy. In this work, the forging process of molybdenum rods was simulated, which were heated to 1350 °C, and one end of them was subjected to loose tooling forging with deformation of 20%, 30% and 40%, and then they were subjected to secondary heating at 1200 °C, 1250 °C, 1300 °C and 1350 °C for 60 min. The microstructure of the molybdenum rods was characterized by optical microscope (OM), and the equivalent stress, equivalent strain and damage distribution characteristics of during the forging process were calculated by Deform-3D finite element software. The microstructure evolution law and crack formation mechanism during deformation were analyzed. The results show that when the two ends of the molybdenum rods were separately forged and heated, incomplete static recrystallization was easy to occur in the middle gradient deformation zone due to the uneven distribution of stress and strain, resulting in the abnormal growth of local grains. In the subsequent forging process, the surface of the molybdenum rods was cracked due to the excessive stress at the grain boundary of the coarse grains in the transition zone. The initial temperature and softening temperature of static recrystallization in tempering process were different with the deformation of molybdenum rods. Finally, the process optimization of molybdenum rods forging was put forward.

Structural evolution and bandgap modulation of layered β-GeSe2 single crystal under high pressure

Germanium diselenide (GeSe2) is a promising candidate for electronic devices because of its unique crystal structure and optoelectronic properties. However, the evolution of lattice and electronic structure of β-GeSe2 at high pressure is still uncertain. Here we prepared high-quality β-GeSe2 single crystals by chemical vapor transfer (CVT) technique and performed systematic experimental studies on the evolution of lattice structure and bandgap of β-GeSe2 under pressure. High-precision high-pressure ultra low frequency (ULF) Raman scattering and synchrotron angle-dispersive x-ray diffraction (ADXRD) measurements support that no structural phase transition exists under high pressure up to 13.80 GPa, but the structure of β-GeSe2 turns into a disordered state near 6.91 GPa and gradually becomes amorphous forming an irreversibly amorphous crystal at 13.80 GPa. Two Raman modes keep softening abnormally upon pressure. The bandgap of β-GeSe2 reduced linearly from 2.59 eV to 1.65 eV under pressure with a detectable narrowing of 36.5%, and the sample under pressure performs the piezochromism phenomenon. The bandgap after decompression is smaller than that in the atmospheric pressure environment, which is caused by incomplete recrystallization. These results enrich the insight into the structural and optical properties of β-GeSe2 and demonstrate the potential of pressure in modulating the material properties of two-dimensional (2D) Ge-based binary material.

Insights into the effect of different thermomechanical processing on the microstructure, phases, texture and tensile properties in Mg-0.9Al-0.6Mn-0.2Si-0.1Ca alloy

In this study, a Mg-0.9Al-0.6Mn-0.2Si-0.1Ca alloy (in as-cast form) has been subjected to homogenization treatment followed by different thermomechanical processing, namely hard plate hot forging (HPHF) and hot rolling (HR), at a fixed working temperature (723 K). The HPHF specimen has developed a bimodal grain size distribution comprising of fine recrystallized grains and coarse deformed (non-recrystallized) grains due to incomplete dynamic recrystallization. Most of these grains have basal orientations along the forging plane, leading to a strong basal fiber texture with low Schmid factor and low Taylor factor. The HR specimen, on the contrary, has developed a uniform grain size distribution consisting of mostly fine grains, with twins in some of the relatively coarser grains. Although a large number of grains in the HR specimen have basal orientations, some of the very fine newly recrystallized grains and the twinned regions attain non-basal orientations, leading to an overall weaker basal fiber texture. The Mg17Al12 phase from the as cast specimen has dissolved during both the thermomechanical treatments owing to its low melting point. Contrarily, the thermally stable Al8Mn5 precipitates are present in all the specimens. The finer grain size and higher dislocation density, along with the finer nano-sized Al8Mn5 precipitates, have resulted in greater strengthening in the HR specimen, as compared to the HPHF specimen. Furthermore, the weaker basal fiber texture, higher Schmid factor and higher Taylor factor have led to much better ductility as well as work hardening response in the HR specimen, than in the HPHF condition.

Improved mechanical performance of double-pass extruded Mg-Gd-Er-Zr alloys with various rare earth contents

The present work focuses on the effects of rare earth (RE = Gd and Er) contents on the microstructural evolution and mechanical performance of Mg-Gd-Er-Zr alloys. Three different alloys, i.e., Mg-12Gd-2Er-0.4Zr, Mg-14Gd-1Er-0.4Zr, and Mg-14Gd-2Er-0.4Zr, are fabricated and subjected to double-pass extrusion. Incomplete dynamic recrystallization (DRX) resulted in a non-homogeneous grain structure with coarse and refined equiaxed grains during single-pass extrusion. In contrast, the grain size decreased dramatically after double-pass extrusion, and a high number of fine Mg5RE particles precipitated in the Mg-14Gd-2Er-0.4Zr alloy than the other two extruded alloys. Moreover, the Mg5RE particles larger than 1 μm contributed to increase the DRX fraction, whereas the finer ones exerted a strong pinning effect on grain boundaries. The volume fraction of β′ precipitates increased with increasing RE content in the matrix. A large volume fraction of β′ precipitates in the alloy will primarily result in higher hardness and strength in the peak-aged condition. After peak-aging treatment, the double-pass extruded Mg-14Gd-2Er-0.4Zr alloy exhibited a good yield strength of 481 ± 3.7 MPa and an adequate elongation of 3.2 ± 0.6%. The influence mechanisms of grain refinement, fine particles, and the strengthening of β′ precipitates are discussed.

Metadynamic recrystallization in A5083 aluminum alloy with homogenized and as-extruded initial microstructures

The static softening behavior of A5083 aluminum alloy with different initial microstructures was studied using various characterization techniques. To calculate the recrystallized fraction, in the compression test, a new method based on the offset yield stress method and the Hall–Petch relationship was proposed, which is more accurate than the conventional methods and applicable even when the initial microstructure is complex. And in the stress relaxation test, a method proposed in our previous study was applied, indicating that this method could be employed in materials with incomplete recrystallization or a complex initial microstructure. Although the as-extruded initial microstructure led to a more developed metadynamic recrystallization, the recrystallization became heterogeneous, compared with the homogenized initial microstructure. When the anvil was not unloaded during the interpass time, the resulting external stress accelerated static recovery but impeded metadynamic recrystallization. New insights were discussed that challenge the conventional view in metadynamic recrystallization. First, metadynamic recrystallization always required an incubation time, unless the experimental conditions were sufficiently favorable for recrystallization. Second, when recovery was active, not only the dynamic recrystallized grains but also the recovery-enhanced subgrains were the potential nuclei for the subsequent metadynamic recrystallization.

Effect of grain size on the thermal stability of electrodeposited nanocrystalline nickel: X-ray diffraction studies

Nanocrystalline (NC) Ni coatings have widespread engineering applications, many of which are in high-temperature environments. Therefore, it is important to study the thermal stability of NC Ni. The present study compares the thermal stability of electrodeposited (ED) NC Ni of varying grain sizes: 20 nm, 50 nm, and 100 nm. In-situ high-temperature X-ray diffraction (XRD) investigations have been employed to follow the grain growth behavior of 〈111〉 and 〈200〉 grains under isochronal annealing up to 550 °C. Based on the calculated activation energies, the anisotropic growth mechanisms of 〈111〉 and 〈200〉 grains are evaluated. The studies revealed that the onset of grain growth occurred at progressively higher temperatures with increasing initial grain sizes. While near uniform grain size distribution is observed for the 20 nm grain size variant, bimodal distributions are observed for its counterparts. Electron Backscatter Diffraction studies of NC Ni samples in the final-processing temperature of XRD showed that <100>//ED texture is preserved during the annealing process. While high fractions of Σ3 coincidence site lattice (CSL) boundaries are observed in the 20 nm sample, reduced fractions with no particular trend is observed for the 50 nm and 100 nm grain-sized samples. It is shown here that the incomplete recrystallization arising from inhomogenous strain distribution is the reason for the observed bimodal distribution and deviation from an expected trend in the formation of Σ3 CSL as a function of initial grain size.

Further understanding of metadynamic recrystallization through thermomechanical tests and EBSD characterization

To further understand metadynamic recrystallization, various measurement techniques were performed with two materials: SUS316 austenitic steel and A5083 aluminum alloy, which were selected to represent materials with low to medium and high stacking fault energies, respectively. Microstructure characterization using electron backscatter diffraction with the grain orientation spread approach was utilized as a direct measurement technique to study metadynamic recrystallization. Additionally, indirect measurement techniques, including double-pass compression, stress relaxation, and microhardness tests, were conducted. A new approach, including its simplified version, is proposed to determine the recrystallized fraction from the stress relaxation test, which is effective even in the case of incomplete recrystallization. A comparison between various approaches for determining the recrystallized fraction shows their advantages and disadvantages. A further understanding of the metadynamic recrystallization is achieved. First, with careful setup of water cooling, unlike the conventional understanding that metadynamic recrystallization does not require an incubation time, the incubation time is approximately 1 s in both materials and can be reduced to 0 s at high temperatures in SUS316. Second, the retardation effect of external stress on recrystallization in both materials is different because on the one hand recovery competes with recrystallization, while on the other hand recovery promotes the nucleation of recrystallization.

Microstructural origin and control mechanism of the mixed grain structure in Ni-based superalloys

Mixed grain structure, as an incomplete recrystallization, should be homogenized and refined by subsequent processing to improve the mechanical properties of the aeroengine components. The microstructural origin and control mechanism is quite necessary for the microstructure homogenizing and grain refining through thermo-mechanical processing. In present research, microstructural origin and control mechanism of the mixed grain structure have been conducted deeply. With the consideration of temperature drop in practical industrial forging, the optimum processing parameters can be determined as 1110–1140 °C/1.0 s−1 to obtain homogeneous equiaxed grains. The microstructure evolution corresponding to the identified optimal parameters, viz., deformation temperature and strain rate, were systematically investigated. Higher dynamic recrystallization (DRX) fraction and coarser DRX grains can be observed at super-solvus temperature deformation (1140 °C). In addition, sub-solvus deformation (1110 °C) shows random orientated distribution while near< 001 > compression texture can be detected in super-solvus deformation. Particular attention was paid to the DRX mechanisms during hot deformation. Discontinuous dynamic recrystallization (DDRX) and continuous dynamic recrystallization (CDRX) occur simultaneously, and the synergistic effect of them contributes to microstructure control. Moreover, DDRX characterized by grain boundary bulging dominants the microstructure control. Progressive subgrain rotation labeled as CDRX can be regarded as an important assistant mechanism. The findings would provide theoretical support for microstructure control of Ni-based superalloys with mixed grain structure, which is of vital importance for components to obtain excellent performance.

Hetero-deformation-induced (HDI) plasticity induces simultaneous increase in both yield strength and ductility in a Fe50Mn30Co10Cr10 high-entropy alloy

Low yield strength is the bottleneck of the face-centered cubic-structured high-entropy alloys (HEAs). Here, the strategy of hetero-deformation-induced (HDI) plasticity is applied by hetero-structuring to induce strengthening and strain hardening for a simultaneous increase in both yield strength and ductility in a Fe50Mn30Co10Cr10 HEA. The coarse-grain (CG) microstructure is a dual phase consisting of face-centered cubic (γ) and hexagonal close-packed (ε) phases, along with phase transformation from γ to ε to happen during tensile deformation. The hetero-structure (HS) was designed, besides recrystallized γ and ε, specifically to reserve a part of deformed γ after cold rolling followed by incomplete recrystallization. Yield strength increases from 200 MPa in CG to 760 MPa in HS, while uniform elongation (i.e., ductility) increases from 35% to 38%. The tensile load-unload-reload testing showed the ceaselessly presence of hysteresis loop during each unload-reload cycle. Both the residual plastic strain and HDI stress were measured with tensile strains in both HS and CG, providing solid evidence of the effect of HDI plasticity. To be specific, the HDI stress is found to account for a large proportion of global flow stress in HS as compared to that in CG. It turns out that the HDI plasticity facilitates both HDI strengthening and HDI strain hardening, which play the crucial role in enhancing strength and ductility. The microstructural origin of HDI plasticity in HS was ascribed to plastic incompatibility at hetero-interfaces of among varying grains as evidenced by the evolution of Schmid factor and KAM values as well.

Study on grain refinement of copper-based liner by vacuum gradient heat treatment process using response surface methodology

Copper and copper alloys are some of the most studied and widely used liner materials. In this study, we investigated different heat treatment parameters on the tensile strength and grain size of copper alloys, and our experimental methods utilized a combination of central composite design (CCD) and response surface methodology (RSM). The material was subjected to vacuum gradient heat treatment under the optimized heat treatment parameters, and the material microstructure after the heat treatment was assessed using electron backscatter diffraction (EBSD) analysis. Studies have shown that an increase in heating temperature and holding time results in grain coarsening and lower tensile strength values. As a result, the optimized heat treatment process allowed the material to undergo incomplete recrystallization. Furthermore, additional grain refinement and a reduction in texture strength and dislocation density improved the performance of the liner.

Effects of Cr element on the crystal structure, microstructure, and mechanical properties of FeCrAl alloys

In the present work, on the foundation of Fe–13Cr-4.5Al–2Mo–1Nb-0.2Si alloy, FeCrAl alloys with low Cr content (7, 8, 9 and 10 wt%) and other minor alloying elements (Ta, Zr and Ni) were prepared. The crystal structure, microstructure, tensile properties and oxidation resistance to high-temperature steam of lean-Cr FeCrAl alloys were investigated. As with the decrease of Cr content, the matrix phase of FeCrAl alloys still exhibited the α-ferrite phase, but the lattice parameter of the matrix phase gradually increased. After annealing, the 13Cr alloy occurred complete recrystallization with an average grain size of 33.5 μm. Meanwhile, the amount of Fe2Nb-type Laves phase in FeCrAl alloys gradually decreased with the decrease of Cr content below 9 wt%. Owing to the weakest pinning effect from the small amount of Laves phase particles, many coarse equiaxed grains were observed in 7Cr alloy. 10Cr alloy did not occur full recrystallization due to relatively slow diffusion of elements resulting from the high content of alloying elements and recovered microstructure. Reducing the Cr content did not damage tensile properties of FeCrAl alloys at both room temperature and 800 °C. Due to the incomplete recrystallization, the 10Cr alloy retained higher strength and relatively poor ductility than other low Cr content alloys at room temperature. Decreasing the content of Cr made the oxidation resistance of lean-Cr alloys worse, which was still better than the traditional Zr alloy. The present work showed that small changes in Cr will not have a remarkable impact on tensile properties and oxidation resistance to high-temperature steam, and low-Cr FeCrAl alloys can be used as an accident-tolerant fuel cladding in light water reactors.

Microstructure evolution and mechanical properties in an ultralight Mg-2.76Li–3Al-2.6Zn-0.39Y alloy

A novel Mg-2.76Li–3Al-2.6Zn-0.39Y alloy was prepared by multidirectional forging (MDF) and rolling (R) to improve strength and ductility, and its microstructural evolution and mechanical properties were investigated by an optical microscope, X-ray diffraction (XRD), transmission electron microscope (TEM), and tensile tester. After MDFR, the ultimate tensile strength was 275 ± 8 MPa, the yield strength was 207 ± 7 MPa, and the elongation was 29% at a grain size of 6.8 ± 1.1 μm. The specific modulus of this alloy was 30.04 MN m kg−1, and the specific strength was 183.58 kN m kg−1. Microstructural examination of decreased-temperature MDF revealed the presence of a bimodal microstructure, which is favorable for the enhancement of both strength and ductility. The deformation mechanisms of MDF are intense mechanically shearing fragmentation and incomplete dynamic recrystallization. XRD analysis revealed that the present alloy consists of α(Mg) phase, and Al2Y, Mg17(Al,Zn)12, Mg2Y, and AlLi intermetallic compounds. TEM analysis confirmed the existence of Mg17(Al,Zn)12 compound and dislocation wall and pile-up. Portevin–Le Chatelier (PLC) effect or serrated flow was discovered in this alloy. The relationship between the break-away stress and the experimental stress was proposed as a criterion for judging the occurrence of the PLC effect. The estimated stress is consistent with the experimental stress. The contributions of different strengthening mechanisms to tensile yield strength were estimated. The estimated tensile yield strength is in good agreement with the experimental tensile yield strength.

Microstructure and mechanical properties of pure magnesium prepared by CEE-AEC at different temperatures

Cyclic expansion extrusion with an asymmetrical extrusion cavity (CEE-AEC) was carried out on pure magnesium up to 3 passes at different deformation temperatures of 250 °C and 350 °C. The microstructure and texture evolution of its edge and center regions are studied respectively, and their mechanical properties are correlated. The results show that there is an incomplete dynamic recrystallization (DRX) region in pure magnesium deformed at 250 °C. Therefore, pure magnesium processing at 250 °C has a larger grain size and higher texture strength than that processing at 350 °C. According to the tensile test, the ultimate tensile strength (UTS) of different positions of different temperatures is very close because the DRXed grains grow at 350 °C. But the value of tensile yield strength (TYS) is nearly doubled, the main reasons are that the effect of (0001) basal slip and texture softening is greater than that of grain refinement.