Microstructures and mechanical properties of homogenization and isothermal aging Mg–Gd–Er–Zn–Zr alloy

The effects of homogenization and isothermal aging treatment on the mechanical properties of Mg–12Gd–2Er–1Zn–0.6Zr(wt%) alloy were investigated. The precipitated long-period stacking order(LPSO) structure and the aging precipitation sequence of the conditioned alloys were observed and analyzed, respectively. The results indicate that the 14H-LPSO structure occurs after the homogenization treatment and the b0 phase forms after the isothermal aging process. These two independent processes could be controlled by the precipitation temperature range. The significant increase in the elongation of the as-cast alloy after homogenization treatment is attributed to the disappearance of the coarse primary Mg5(Gd, Er, Zn) phase and the presence of the 14H-LPSO structure. The precipitation sequence of the investigated alloy is a-Mg(SSS)/b00(D019)/b0(cbco)/b.Furthermore, the yield tensile strength(YTS) and ultimate tensile strength(UTS) values of the isothermal aging alloy have a great improvement, which could be attributed to the high density of the precipitated b0 phase.     

Microstructural control and hardening response of Mg–6Zn–0.5Er–0.5Ca alloy

The effects of heat treatment on microstructures and hardening response of Mg–6Zn–0.5Er–0.5Ca(wt%) alloy were investigated by optical microscope(OM), scanning electron microscope(SEM), and transmission electron microscope(TEM) in this paper. The results show that the Mg–6Zn–0.5Er–0.5Ca alloy contains Mg_3Zn_6Er_1 quasicrystalline phase(Iphase) and Ca_2Mg_6Zn_3 phase under as-cast condition. Most of the Ca_2Mg_6Zn_3 phases and I-phases dissolve into matrix during heat treatment at 475 ℃ for 5 h. After the as-solution alloy was aged at 175 ℃ for 36 h, a large amount of MgZn_2 precipitate with several nanometers precipitate. It is suggested that the trace addition of Ca results in refining the size of the precipitate, and the presence of the nanoscale MgZn_2 phase is the main factor to improve the peak-aged hardness greatly to 87 HV, which increases about 40 % compared with that of as-cast alloy.     

Effect of Solution Treatment on Microstructure and Corrosion Properties of Mg–4Gd–1Y–1Zn–0.5Ca–1Zr Alloy

The as-cast multi-element Mg–4Gd–1Y–1Zn–0.5Ca–1Zr alloy with low rare earth additions was prepared, and the solution treatment was applied at different temperatures. The microstructural evolution of the alloy was characterized by optical microscopy and scanning electron microscopy, and corrosion properties of the alloy in 3.5% NaCl solution were evaluated by immersion and electrochemical tests. The results indicate that the as-cast alloy is composed of the a-Mg matrix,lamellar long-period stacking-ordered(LPSO) structure and eutectic phase. The LPSO structure exists with more volume fraction in the alloy solution-treated at 440 °C, but disappears with the increase in the solution temperature. For all the solution-treated alloys, the precipitated phases are detected. The corrosion rates of the alloys decrease first and then increase slightly with the increase in the solution temperature, and the corrosion resistance of the solution-treated alloys is more than four times as good as that of the as-cast alloy. In addition, the alloy solution-treated at 480 °C for 6 h shows the best corrosion property.     

Effects of Sb Addition on Microstructural Evolution and Mechanical Properties of Mg–9Al–5Sn Alloy

The Mg–9Al–5Sn-xSb(x=0.0,0.3,0.6,1.0,1.5 wt%) alloys were prepared by a simple alloying process followed by hot extrusion with an extrusion ratio of 28.2. The effects of Sb additions on the microstructure and mechanical properties of the Mg–9 Al–5 Sn alloys were investigated by optical microscopy, X-ray diffraction, transmission electron microscopy, scanning electron microscopy equipped with an energy-dispersive X-ray spectrometer. The results indicated that the phases α-Mg matrix, Mg_2_Sn, Mg_3Sb_2 and Mg_17 Al_12 exist in the as-cast Sb-containing alloys. Sb addition results in the precipitation of Mg_3Sb_2. The dendritic size of these alloys decreases with the addition of Sb. Both their ultimate tensile strength and yield strength of extruded alloys increase, and their elongation decreases gradually with increasing the content of Sb. The better mechanical properties of the as-extruded alloys were achieved due to the refined grains and the formation of dispersive second phases Mg_3Sb_2.     

As-cast microstructure of Al-Zn-Mg and AI-Zn-Mg-Cu alloys added erbium

The effects of different contents of rare earth element, and erbium, on the as-cast microstructures of Al- 8Zn-2Mg and Al-6Zn-2Mg-1.8Cu alloys were studied by optical microscopy, scanning electron microscopy, X-ray diffractometry, transmission electron microscopy and EDS analysis. The results show that the netlike structure of as-cast alloys can be remarkably refined, and the distance of dendritic structure decreases, with Er addition. How- ever, the improvement results on Al-Zn-Mg-Cu are not better than that of Al-Zn-Mg. Er and Al can interact to form AlsEr phase, which is coherent with a（Al） matrix, with trace Er addition to the Al-Zn-Mg alloy. The refinement effect of Al-Zn-Mg alloys is familiar with the formation and precipitation of coherent AlsEr phases. The ternary compound AlCuEr, similar with AlCuSc phase, will form when Er is added to Al-Zn-Mg-Cu alloy, which suppresses the formation of Al3Er phase and doesn＇t solve in the following heat treatment.     

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 and properties of aging Cu–Cr–Zr alloy

The crystallography and morphology of precipitate particles in a Cu matrix were studied using an aged Cu–Cr–Zr alloy by transmission electron microscopy(TEM) and high-resolution transmission electron microscopy(HRTEM). The tensile strength and electrical conductivity of this alloy after various aging processes were tested. The results show that two kinds of crystallographic structure associated with chromium-rich phases, fcc and bcc structure, exist in the peak-aging of the alloy. The orientation relationship between bcc Cr precipitate and the matrix exhibits Nishiyama–Wasserman orientation relationship. Two kinds of Zr-rich phases(Cu4Zr and Cu5Zr)can be identified and the habit plane is parallel to {111}Cu plane during the aging. The increase in strength is ascribed to the precipitation of Cr- and Zr-rich phase.     

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 Different Scale Precipitates on Corrosion Behavior of Mg–10Gd–3Y–0.4Zr Alloy

A large amount of directional and willow-like β' phase was precipitated in Mg-10 Gd-3 Y-0.4 Zr(GW103 K) alloy after solution treatment and subsequently aged treatment(T6). In order to explore the effect of the precipitates on the corrosion behavior of the GW103 K alloy, the alloy was subjected to solution treatment(T4) at 773 K for 4 h at first, subsequently aged at 498 K for 193 h(T6). The microstructure evolution of the GW103 K alloy after this treatment was investigated by scanning electron microscopy and transmission electron microscopy. The high-angle annular detector dark-field scanning transmission electron microscopy was used to observe the typical corrosion morphologies of the nanoscale precipitation phases(β') in the T6-treated alloy. The corrosion rate was measured by potentiodynamic polarization test. Combining with the potential measurement results by scanning Kelvin probe force microscopy, the effects of the skeleton-like Mg_(24)(Gd,Y)_5 andf precipitates on the corrosion behavior of GW103 K alloy were explored. The results showed that the corrosion rate of the GW103 K alloy in different conditions was ranked as: as-cast alloy T4-treated alloy T6-treated alloy,attributing to the fact that the relative potential differences of skeleton-like Mg_(24)(Gd,Y)_5 were lower than those of the matrix, therefore Mg24(Gd, Y)5 phase formed micro-galvanic coupling with the matrix and corrosion dissolution occurred.The nanoscale β' precipitates in T6-treated alloy can retard the cathodic process.     

Phase structure of ZK60-1Er magnesium alloy compressed at 450℃

The phase structure of ZK60-1Er magnesium alloy thermally compressed at the temperature of 450℃ and the strain rate of 1×10 -4 s -1 was determined by transmission electron microscopy(TEM)and high-resolution electron microscopy(HREM).The results show that this magnesium alloy contains many new W phases(Mg3Zn3Er2,FCC structure)in the matrix.Those new W phases have two morphologies,either irregularly rectangular or rod morphology·Lattice constants of the two new W phases are slightly higher than those of W Phase(Mg3Zn3Y2)containing rare earth element of yttrium.     

Aging kinetics of 14H-LPSO precipitates in Mg-Zn-Y alloy

Alloys with long-period stacking ordered structures(LPSO)have good properties and are highly regarded.Mg-Zn-Y alloy containing LPSO phase was prepared by the traditional casting method,and the aging heat treatment was performed at different temperatures and times.The microstructure and phase constitutions of the alloy were observed by means of optical microscopy and scanning electron microscopy methods.Results show the microstructure of as-cast Mg95.5Zn1.5Y3 mainly consists ofα-Mg,W phases and LPSO phases.During the aging treatment,fine lamellar-shaped 14H-LPSO phase is formed at the grain boundaries and precipitates from the supersaturated magnesium matrix,and the volume fraction increases as the aging time increases.By controlling the aging time,Mg-Zn-Y alloys with different volume fractions of 14H-LPSO phase were prepared.The aging kinetics equation of the 14H-LPSO phase is summarized,that is f=1-exp(-0.2705 t 0.6368).The phase transformation mechanism of 14H-LPSO in Mg95.5Zn1.5Y3 alloy can be described as the change of dislocation energy.     

Phase Stability of the Laves Phase Cr2Nb in a Two-Phase Cr-Cr2Nb Alloy

The phase stability of the Laves phase Cr2Nb in a two-phase Cr-Cr2Nb alloy produced by arc-melting was investigated using X-ray diffraction, transmission electron microscopy and selected area electron diffraction. The experimental results indicated that the as-cast ingot was consisted of C15-Cr2Nb and Cr phases; the intermediate C36 and high-temperature C14 modifications of Cr2Nb, reported in literatures, were not detected in this study. These results, combined with a detailed analysis of the actual solidification conditions, revealed that the C14-Cr2Nb was a metastable rather than a stable high-temperature modification. Moreover, a kind of extremely fine lamellar structure was found to be randomly distributed in the eutectic cells, which may be formed by decomposition of the supersaturated C15-Cr2Nb via a discontinuous precipitation reaction.     

Microstructure characterization and tensile properties of a Ni-containing TiAl-based alloy with heat treatment

The effects of Ni addition on solidification micro structure and tensile properties of Ti-48Al-2Cr-2Nb alloy were investigated using differential scanning calorimetry(DSC),X-ray diffraction(XRD),scanning electron microscopy(SEM) equipped with energy-dispersive spectroscope(EDS) and transmission electron microscopy(TEM).Results show that with 3 at%Ni addition,the as-cast micro structure is mainly composed of fine lamellar colonies(~50 μm),γ grains and Ni-ridied τ_3 phase.After heat treatment at 1380℃,the Ni-containing alloy is characterized by fine fully lamellar micro structure(~90 μm).The heat-treated Ni-containing specimen exhibits superior room temperature tensile properties than other specimens.The tensile properties are discussed in light of the microstructure evolution and role of Ni-riched τ_3 phase.     

Microstructure of phases in a Cu–Zr alloy

The morphology and crystallography of phases in the Cu-0.12% Zr alloy were investigated by scanning electron microscope(SEM), transmission electron microscope(TEM), and high-resolution transmission electron microscope(HRTEM). The results show that the as-cast microstructure of Cu–Zr alloy is mainly Cu matrix and eutectic structure which consist of Cu and Cu5Zr phases with a fine lamellar structure. The disk-shaped and plateliked Cu5Zr phases with fcc structure are found in the matrix, in which habit plane is parallel to {111}a plane of the matrix.Between the copper matrix and Cu5Zr phase,there exists an orientation relationship of [112]a|| [011]Cu5Zr;(111)a||(111)Cu5Zr. The space structure model of Cu5Zr phase can be established.     

Solidification characteristics of high Nb-containing γ-TiAl-based alloys with different aluminum contents

The effect of Al content on the microstructure and solidification characteristics of Ti–Al–Nb–V–Cr alloys in as-cast and isothermally treated states was investigated using X-ray diffraction(XRD), scanning electron microscopy(SEM) equipped with energy dispersive spectroscope(EDS), and transmission electron microscopy(TEM). The typical solidification characteristics are due to the joint influence of both the crystal temperature range and the solidification path. The wide crystallization temperature range contributes to obtaining coarse dendrites in the as-cast Ti47Al7Nb2.5V1.0Cr(at%) alloy solidifying through the peritectic reaction. The b-solidifying Ti46Al7Nb2.5V1.0Cr(at%) alloy with the narrow crystallization temperature range is attributed to the formation of a homogeneous finegrained microstructure. However, the crystallization temperature range of Ti48Al7Nb2.5V1.0Cr(at%) alloy is equivalent to that of Ti46Al7Nb2.5V1.0Cr alloy, but it is solidified by peritectic reaction, leading to the formation of finer dendrites.     

Obtaining Ultra-High Strength and Ductility in a Mg–Gd–Er–Zn–Zr Alloy via Extrusion,Pre-deformation and Two-Stage Aging

The Mg–12Gd–1Er–1Zn–0.9 Zr(wt%) alloy with ultra-high strength and ductility was developed via hot extrusion combined with pre-deformation and two-stage aging treatment.The age-hardening behavior and microstructure evolution were investigated.Pre-deformation introduced a large number of dislocations,resulting in strain hardening and higher precipitation strengthening in the subsequent two-stage aging.As a result,the alloy showed a superior strength–ductility balance with a yield strength of 506 MPa,an ultimate tensile strength of 549 MPa and an elongation of 8.2% at room temperature.The finer and denser β' precipitates significantly enhanced the strength,and the bimodal structure,small β-Mg_5RE phase as well as dense γ' precipitates ensured the good ductility of the alloy.It is suggested that the combination of pre-deformation and two-stage aging treatment is an eff ective method to further improve the mechanical properties of wrought Mg alloys.     

Electrochemical Performance of Nanocrystalline and Amorphous Mg–Nd–Ni–Cu-Based Mg_2Ni-type Alloy Electrodes Used in Ni-MH Batteries

Nanocrystalline and amorphous Mg2Ni-type(Mg24Ni10Cu2)100–xNdx(x = 0, 5, 10, 15, 20) alloys were prepared by melt-spinning technology. The structures of as-cast and spun alloys were characterised by X-ray diffraction,scanning electron microscopy and transmission electron microscopy. Electrochemical performance of the alloy electrodes was measured using an automatic galvanostatic system. The electrochemical impedance spectra and Tafel polarisation curves of the alloy electrodes were plotted using an electrochemical work station. The hydrogen diffusion coefficients were calculated using the potential step method. Results indicate that all the as-cast alloys present a multiphase structure with Mg2 Ni type as the major phase with Mg6 Ni, Nd5Mg41 and Nd Ni as secondary phases. The secondary phases increased with the increasing Nd content. The as-spun Nd-free alloy exhibited nanocrystalline structure, whereas the as-spun Nd-doped alloys exhibited nanocrystalline and amorphous structures. These results suggest that adding Nd facilitates glass formation of Mg2Ni-type alloys. Melt spinning and Nd addition improved alloy electrochemical performance, which includes discharge potential characteristics, discharge capacity, electrochemical cycle stability and high-rate discharge ability.     

Phases and microstructures of high Zn-containing Al–Zn–Mg–Cu alloys

Phases and microstructures of three high Zncontaining Al–Zn–Mg–Cu alloys were investigated by means of thermodynamic calculation method, optica microscopy(OM), scanning electron microscopy(SEM)energy dispersive spectroscopy(EDS), X-ray diffraction(XRD), and differential scanning calorimetry(DSC) analysis. The results indicate that similar dendritic network morphologies are found in these three Al–Zn–Mg–Cu alloys. The as-cast 7056 aluminum alloy consists of aluminum solid solution, coarse Al/Mg(Cu, Zn, Al)_2 eutectic phases, and fine intermetallic compounds g(MgZn_2). Both of as-cast 7095 and 7136 aluminum alloys involve a(Al)eutectic Al/Mg(Cu, Zn, Al)_2, intermetallic g(MgZn_2), and h(Al_2Cu). During homogenization at 450 °C, fine g(MgZn_2) can dissolve into matrix absolutely. After homogenization at 450 °C for 24 h, Mg(Cu, Zn, Al)_2 phase in 7136 alloy transforms into S(Al_2Cu Mg) while no change is found in 7056 and 7095 alloys. The thermodynamic calculation can be used to predict the phases in high Zncontaining Al–Zn–Mg–Cu alloys.     

Double-Shelled L1_2 Nano-structures in Quaternary Al–Er–Sc–Zr Alloys: Origin and Critical Significance

The formation of highly coherent double-shelled L1_2 nano-precipitates in dilute Al–Er–Sc–Zr alloys was investigated with the combined use of Cs-corrected transmission electron microscopy characterization and first-principles energetics calculations. The double-shelled nano-precipitates are primarily featured with an Er-rich core surrounded by a Sc-rich inner shell and a Zr-rich outer shell. First-principles energetics analyses based on the classic homogenous nucleation theory suggested that once forms, this double-shell structure can be thermally stable. The predominant formation of this double-shell structure has thus both profound kinetic and thermodynamic origins. Its formation and stability preference to all other possible L1_2 nano-structures would become more pronounced as its size increases, no matter what the solute ratio and aging temperature of interest.     

Effect of erbium on microstructure and thermal compressing flow behavior of Mg-6Zn-0.5Zr alloy

A series of thermal compressing tests of Mg-6Zn-0.5Zr and Mg-6Zn-0.5Zr-1Er alloys were performed on a Gleeble-1500D thermal simulator. The microstructures of thermal compressed Mg-6Zn-0.5Zr and Mg-6Zn-0.5Zr-1Er alloys were determined by optical microscopy, transmission electron microscopy and X-ray diffractometry. The results show that Mg-6Zn-0.5Zr alloy mainly consists of α-Mg and MgZn2 phase, while Mg-6Zn-0.5Zr-1Er alloy comprises α-Mg phase, coarse Mg3ZnnEr2 eutectic, rod-liked Mg3Zn4Er2 precipitated phase, fine I phase particle （Mg3Zn6Er, icosahedral quasicrystal structure）. The peak flow stress becomes larger with increasing strain rate and erbium addition at the same temperature, and gets smaller with increasing deformation temperature at the same strain rate. The deformation activation energy increases with increasing temperature, strain rate and erbium addition. In addition, it is observed that the growth of dynamic recrystallization （DRX） grains of Mg-6Zn-0.5Zr-lEr alloy was markedly suppressed due to the pinning effect of fine I phase and Mg3Zn4Er2 phase during thermal compression.