Microstructure and phase transformation of as-cast and annealed Mg–4Zn–1Y alloy containing quasi-crystal phase

By means of optical microscope(OM), X-ray diffraction(XRD), scanning electron microscopy(SEM)and transmission electron microscopy(TEM) analyses, the microstructures of as-cast and heat-treated Mg–4Zn–1Y(wt%) alloy containing quasi-crystal phase were studied.The microstructure of the as-cast alloy consists of a-Mg solid solution grains, intermetallic particles and eutectic phases(W-phase and I-phase), and huge grains with serious dendritic segregation are clearly observed. After heat treatment, phase transformation and dissolution occur in the alloy and many phases remain. When the alloy was treated above 410 °C, the eutectic phases transform into spherical shape as the I-phase turns to W-phase. After heat treatment for long time, the alloy is over burnt and the W-phase decomposes to Mg–Y binary phase.     

Microstructure evolution during heat treatment of Mg–Gd–Y–Zn–Zr alloy and its low-cycle fatigue behavior at 573 K

In as-cast Mg–2.1Gd–1.1Y–0.82Zn–0.11Zr (mole fraction, %) alloy, lamellar microstructures that extend from grain boundaries to the interior of α-Mg grains are identified as clusters of γ′ using a scanning transmission electron microscope equipped with a high-angle annular dark-field detector. Under a total strain-controlled low-cyclic loading at 573 K, the mechanical response and failure mechanism of Mg–2.1Gd–1.1Y–0.82Zn–0.11Zr alloy (T6 peak-aging heat treatment) were investigated. Results show that the alloy exhibits cyclic softening response at diverse total strain amplitudes and 573 K. The experimental observations using scanning electron microscopy show that the micro-cracks initiate preferentially at the interface between long-period stacking order structures and α-Mg matrix and extend along the basal plane of α-Mg. The massive long-period stacking order structures distributed at grain boundaries impede the transgranular propagation of cracks.     

Corrosion fatigue behavior of epoxy-coated Mg–3Al–1Zn alloy in NaCl solution

The corrosion fatigue behavior of epoxy-coated Mg–3Al–1Zn alloy was investigated in air and 3.5 wt%NaCl solution. Epoxy coating as a new method was used to improve the corrosion fatigue property of the material.Results show that the fatigue limit(FL) of the coated specimens is higher than that of the uncoated specimens in3.5 wt% NaCl solution because of the strengthening and blocking functions of the epoxy coating. The FL of the coated specimens in 3.5 wt% NaCl solution is as high as that in air. It implies that the coated specimens are not as sensitive to the environment as the magnesium alloy. The low tensile strength and the short elongation of the pure epoxy coating lead to that the fatigue crack of the coated specimen is always initiated from the epoxy-coating film Pores and pinholes accelerate the fatigue crack initiation process. Pinholes are caused by the corrosion reactions between the epoxy coating and the NaCl solution.     

Hot deformation behavior and microstructure evolution of a Mg–Gd–Nd–Y–Zn alloy

The hot deformation behavior of homogenized Mg–6.5Gd–1.3Nd–0.7Y–0.3Zn alloy was investigated during compression at temperatures of 250–400 ℃ and at strain rates ranging from 0.001 to 0.100 s~(-1). Microstructure analyses show that the flow behaviors are associated with the deformation mechanisms. At the lower temperatures(250–300 ℃), deformation twinning is triggered due to the difficult activation of dislocation cross-slip. Dynamic recrystallization(DRX) accompanied by dynamic precipitation occurs at the temperature of 350 ℃ and influences the softening behavior of the flow.DRX that develops extensively at original grain boundaries is the main softening mechanism at the high temperature of 400 ℃ and eventually brings a more homogeneous microstructure than that in other deformation conditions. The volume fraction of the DRXed grains increases with temperature increasing and decreases with strain rate increasing.     

Plastic deformation behaviors of a Mg–Ce–Zn–Zr alloy

The stress–strain behaviors of a Mg–2.8%Ce–0.7%Zn–0.7%Zr (wt.%) alloy with various strain rates at different deformation temperatures were investigated. It is found that the alloy can be extruded at 623 K with σ_0.2=222.4 MPa, σ_b=257.8 MPa and δ=12.0%. The working hardening, the dynamic recovery and the dynamic recrystallization play important roles to affect the plastic deformation behaviors of the alloy at different temperature regions, respectively.     

Dual phases strengthening behavior of Mg–10Gd–1Er–1Zn–0.6Zr alloy

The microstructure evolution and strengthening mechanisms of Mg–10Gd–1Er–1Zn–0.6Zr (wt.%) alloy were focused in the view of the size parameters and volume fraction (f_p) of dual phases (long period stacking ordered (LPSO) structures and β′ precipitates). Results show that two types of LPSO phases with different morphologies formed, and the morphology and size of both LPSO phases varied with the solution conditions. However, the volume fraction decreased monotonously with increasing solution temperature, which in turn raised the volume fraction of β′ phase during aging. The alloy exhibited an ultimate tensile strength of 352 MPa, a yield strength of 271 MPa, and an elongation of 3.5% after solution treatment at 500 °C for 12 h and aging at 200 °C for 114 h. In contrast to the LPSO phase, the β′ phase seems to play a more important role in enhancing the yield strength, and consequently, a decreased f_LPSO/f_β′ ratio results in an increased yield strength.     

Microstructure and mechanical properties on aging behavior of Zn containing Mg–2Sn–0.4Mn alloy

The effects of Zn addition were examined by observing the microstructure and measuring the mechanical properties of Mg–xZn–2Sn–0.4Mn with different Zn contents. The addition of Zn to the Mg–2Sn–0.4Mn alloy caused the precipitation of secondary phases and an improvement in the mechanical properties. The aged alloys showed improved elongation at break, which led to a slight decrease in yield strength and ultimate strength. The results suggest that the formation of precipitates containing Zn affects the mechanical properties of the alloys.     

Mechanical properties and energy absorption of extruded Mg–2.0Zn–0.3Zr alloy with Y addition

The effects of the rare earth element Y addition on mechanical properties and energy absorption of a low Zn content Mg–Zn–Zr system alloy and the deformation temperature of optimized alloy were investigated by room tensile test, optical microscopy(OM), X-ray diffraction(XRD), scanning electron microscopy(SEM), and transmission electron microscope(TEM). The results show that,after homogenization at 420 °C for 12 h for the as-cast alloys, Mg Zn phase forms, which decreases the strength of Mg–2.0Zn–0.3Zr alloy with Y content of 0.9 wt%. The tensile strength and elongation of the alloy with a Y addition of 5.8 wt% reach the max value(281 ± 2) MPa and(30.1 ± 0.7) %, respectively; the strength and elongation of Mg–2.0Zn–0.3Zr–0.9Y alloy at the optimized extrusion temperature of 330 °C reach(321 ± 1) MPa and(21.9 ± 0.7) %, respectively. The energy absorption increases with the increase of Y content, the max value reached 0.79 MJ m-3with Y content of 5.8 wt%, and the energy absorption of Mg–2.0Zn–0.3Zr–0.9Y alloy at the optimized extrusion temperature of 330 °C reaches0.75 MJ m-3.     

Effect of chemical milling on low-cycle fatigue behavior of an Al–Mg–Si alloy

The effect of alkaline chemical milling used for dimensionally reducing aluminum-alloy structures is assessed in terms of total fatigue life and crack-initiation mechanisms. Chemically milled Al–Mg–Si specimens exhibited a 50% reduction in average fatigue lives compared to electropolished Al–Mg–Si specimens at comparable peak-applied loads above macroscopic yield. The fatigue-life reduction of the chemically milled specimens is likely associated with early onset of crack initiation due to pit-induced-stress concentrations. Fractographic analyses suggest a transition in the crack-initiation mechanisms from predominantly {1 1 1}-slip plane cracking to partly or predominantly pit-induced-stress driven depending on the depth of surface pits.     

Quasicrystal dissolution and performance of isothermally heat-treated Mg–Zn–Y alloy

Conventional casting method was employed to prepare Mg–Zn–Y alloy only with a-Mg+I-phase; however, the grain size of quasicrystal is quite large in the ascast state.Therefore, isothermal treatment was applied to refine the quasicrystal phase.The result shows that after the Mg–Zn–Y alloy was isothermally treated at 500 °C for several hours, the coarse quasicrystal can be gradually dissolved and thus refined.Generally, the dissolving processes of quasicrystal are slow first and then accelerate;after isothermally treated with 8 h at 500 °C, the quasicrystal is almost completely dissolved into the matrix only with 1–5 lm tiny quasicrystals remained.Refinement of quasicrystal can markedly reduce the wear resistance, but increase the corrosion resistance.     

Hot deformation behavior of Al-Cu-Li-Mg-Zr alloy containing Zn and Mn

The hot deformation behavior of a new AI-Cu-Li-Mg-Zr alloy was studied, and its microstructure and true stress were characterized as function of the deformation temperature and the strain rate using Gleeble-1500 thermal mechanical simulator. The results show that, with the increase of the strain rate from 0.001 s^-1 to 10 s^-1, the peak value of true stress is elevated at the same deformation temperature, and at the same strain rate the peak value of the true stress decreases with the increase of the deformation temperature from 360 ℃ to 520℃. Dynamic recrystallization easily occurs in the new Al-Cu-Li-Mg-Zr alloy under the lower strain rate and the higher deformation temperature, and dynamic recovery can usually be seen in this alloy under the higher strain rate of 10 s^-1 and the lower deformation temperature.     

Aging precipitation behavior and properties of Al–Zn–Mg–Cu–Zr–Er alloy at different quenching rates

The effect of quenching rate on the aging precipitation behavior and properties of Al–Zn–Mg–Cu–Zr–Er alloy was investigated. The scanning electron microscopy, transmission electron microscopy, and atom probe tomography were used to study the characteristics of clusters and precipitates in the alloy. The quench-induced η phase and a large number of clusters are formed in the air-cooled alloy with the slowest cooling rate, which contributes to an increment of hardness by 24% (HV 26) compared with that of the water-quenched one. However, the aging hardening response speed and peak-aged hardness of the alloy increase with the increase of quenching rate. Meanwhile, the water-quenched alloy after peak aging also has the highest strength, elongation, and corrosion resistance, which is due to the high driving force and increased number density of aging precipitates, and the narrowed precipitate free zones.     

Effect of microstructure on tensile and fatigue properties of cast Mg–10Gd–2Y–0·5Zr alloy

Abstract

The microstructure and its effect on tensile properties and fatigue properties of a Mg–10Gd–2Y–0·5Zr (wt-%) cast alloy have been studied. The microstructures of as-cast, solution treated and T6 treated specimens were examined by optical and scanning electron microscopy (SEM). Tensile properties and fatigue properties of the specimens were determined and fractography was carried out. The SEM examination showed that the precipitates after T6 treatment were mainly distributed at grain boundaries, which accounts for the intergranular brittle fracture observed. The average grain size of the specimens measured after solution treatment varied from 87 to 128 μm. The mechanical tests showed that the tensile strength and low cycle fatigue strength increase with decreasing average grain size, whereas high fatigue strength is less sensitive to grain size. The fractography indicated that ductile and brittle fracture patterns coexist.     

Superplastic behavior of friction-stir welded Al–Mg–Sc–Zr alloy in ultrafine-grained condition

The present work was undertaken to improve superplastic ductility of friction-stir welded joints of ultrafine-grained (UFG) Al–Mg–Sc–Zr alloy. In order to suppress the undesirable abnormal grain growth, which typically occurs in the heavily deformed base material, the UFG material was produced at elevated temperature. It was suggested that the new processing route could reduce dislocation density in the UFG structure and thus enhance its thermal stability. It was found, however, that the new approach resulted in a relatively high fraction of low-angle boundaries which, in turn, retarded grain-boundary sliding during subsequent superplastic tests. Therefore, despite the successful inhibition of the abnormal grain growth in the base-material zone, the superplastic deformation was still preferentially concentrated in the fully-recrystallized stir zone of the material. As a result, the maximal elongation-to-failure did not exceed 700%.     

Effects of fabrication methods on deformability and microstructure of Mg–Zn–Zr wrought alloy

The effects of fabrication processing methods on the workability of Mg–Zn–Zr wrought magnesium alloy (ZK60A) were investigated based on the microstructure and inherent internal defects. Three different billets, semi-continuously cast, semi-continuously cast and subsequently extruded, and die-cast, were fabricated and uniaxially compressed at elevated temperatures and two different strain rates to determine the deformation capabilities. The grain structure of the billets was investigated using electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM). The internal defects were inspected by an X-ray scanner. The enhanced deformability was observed in semi-continuously cast billet compared to the one fabricated by conventional die-casting, and the subsequent extrusion further improved the deformability.     

Deformation behavior of Mg–Zn–Y alloys reinforced by icosahedral quasicrystalline particles

New Mg-rich Mg–Zn–Y alloys, reinforced by quasicrystalline particles, have been developed by thermomechanical processes. The deformation behavior of these alloys at room and elevated temperatures has been investigated. Yield strength of these alloys, which increases with an increase in the volume fraction of quasicrystalline particles, is relatively high due to their strengthening effect. The variation of the flow stress in the alloys is characterized by linking the microstructural evolution during deformation at high temperatures. The flow softening is related to dynamic recrystallization developed under the dislocation climb controlled creep; the flow hardening is related to grain growth that occurs under the grain boundary diffusion controlled creep. Quasicrystalline particles in the Mg–Zn–Y alloys resist coarsening due to their low interfacial energy, thereby forming of stable quasicrystalline particle/matrix interface and also prohibit against microstructural evolution of the α-Mg matrix during deformation at temperatures up to near the eutectic temperature. Stability of both quasicrystalline particles and matrix microstructure in the Mg–Zn–Y alloys provides large elongation to failure with no void formation at the quasicrystalline particle/matrix interface.     

Microstructure and phase compositions of as-cast Mg–3.9Zn–0.6RE(Gd,Y) alloy with different Gd/Y ratios

Mg–Zn–RE(Gd, Y) alloys with different Gd/Y atomic ratios were prepared by conventional casting, and the microstructure of the alloys was studied by multiple means. Icosahedral quasicrystal phases are observed in all alloys. The different Gd/Y atomic ratios affect the microstructures of the alloys irregularly. The alloy with more Gd has large dendritic structure and more complicated phase composition which are composed of I-phase lamellar eutectic, W-phase divorced eutectic, Mg–RE cuboid particles and Mg–Zn binary phases. Other two alloys show similar microstructures and phase compositions with very thin lamellar eutectics which distribute along the interdendritic region, and the lamellar eutectics are formed by I-phase and Mg. The element contents of the I-phases and Mg–RE phases are partially controlled by the Gd/Y atomic ratio.