Homogenization heat treatment of 2099 Al–Li alloy

The microstructure evolution and composition distribution of as-cast and homogenized 2099 aluminum– lithium(Al–Li) alloy were studied by optical microscopy(OM), differential thermal analysis(DTA), scanning electron microscopy(SEM), energy dispersive spectrometry(EDS), area and line scanning, X-ray diffraction(XRD), and Vickers microhardness test methods. The results show that severe dendrite exists in the as-cast alloy. Cu, Zn, Mn, and Mg distribute unevenly from the grain boundary to inside. The low-melting point nonequilibrium eutectic phases dissolve into the matrix during the first-step homogenization, whereas the melting point of residual eutectic phases is elevated. After the second-step homogenization, most of the remaining eutectic phases dissolve into the matrix, except a small amount of Al–Cu–Fe phases. An optimized homogenization process of the 2099 Al–Li alloy is developed(515 °C 9 18 h ? 525 °C 9 16 h), which shows a good agreement with the homogenization kinetic analysis results.     

A High-Strength and Biodegradable Zn–Mg Alloy with Refined Ternary Eutectic Structure Processed by ECAP

In this study, the microstructure, mechanical properties and corrosion behaviors of a Zn–1.6 Mg(wt%) alloy during multipass rotary die equal channel angle pressing(RD-ECAP) processing at 150 °C were systematically investigated. The results indicated that a Zn + Mg_2 Zn_(11) + MgZn_2 ternary eutectic structure was formed in as-cast Zn–Mg alloy. After ECAP, the primary Zn matrix turned to fine dynamic recrystallization(DRX) grains, and the network-shaped eutectic structure was crushed into fine particles and blended with DRX grains. Owing to the refined microstructure, dispersed eutectic structure and dynamically precipitated precipitates, the 8 p-ECAP alloy possessed the optimal mechanical properties with ultimate tensile strength of 474 MPa and elongation of 7%. Moreover, the electrochemical results showed that the ECAP alloys exhibited similar corrosion rates with that of as-cast alloys in simulated body fluid, which suggests that a high-strength Zn–Mg alloy was successfully developed without sacrifice of the corrosion resistance.     

Research on microstructure in as-cast 7A55 aluminum alloy and its evolution during homogenization

The microstructure of the as-cast 7A55 aluminum alloy and its evolution during homogenization were investigated by means of optical microscopy (OM), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD), and differential scanning calorimetry (DSC) analysis. The results indicate that the microstructure of the as-cast 7A55 aluminum alloy mainly consists of the dendritic network of aluminum solid solution, Al/AlZnMgCu eutectic phases, and intermetallic compounds MgZn2, Al2CuMg, Al7Cu2Fe, and Al23CuFe4. After homogenization at 470°C for 48 h, Al/AlZnMgCu eutectic phases are dissolved into the matrix, and a small amount of high melting-point secondary phases were formed, which results in an increasing of the starting melting temperature of 7A55 aluminum alloy. The high melting-point secondary phases were eliminated mostly when the homogenization time achieved to 72 h. Therefore, the reasonable homogenization heat treatment process for 7A55 aluminum alloy ingots was chosen as 470°C/72 h.     

Microstructure,mechanical properties,and corrosion resistance of Mg–9Li–3Al–1.6Y alloy

Mg–9Li–3Al–1.6Y alloys were prepared through mixture method. The microstructure, mechanical properties, and corrosion resistance of the as-cast and asextruded alloys were studied by optical microscopy(OM),scanning electronic microscopy(SEM), X-ray diffraction(XRD), mechanical properties testing, and electrochemical measurement. The as-cast Mg–9Li–3Al–1.6Y alloy with the average grain size of 325 lm is composed of b-Li matrix, block a-Mg, and granule Al_2Y phases. After extrusion, the grain size of the as-cast alloy is obviously refined and reaches to 75 lm; the strength and elongation of the extruded alloy are enhanced by 17.20 % and49.45 %, respectively, owing to their fine microstructure and reduction of casting defects. The as-extruded alloy shows better corrosion resistance compared to the as-cast one, which may be related to the low stored energy and dislocation density in the extruded alloy, also the homogenization treatment before extrusion.     

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 of as-extruded 7136 aluminum alloy and its evolution during solution treatment

With the aid of scanning electron microscopy(SEM), energy-dispersive spectroscopy(EDS), X-ray diffraction(XRD), differential scanning calorimetry(DSC)analysis and electron backscatter diffraction(EBSD), the microstructure of the alloy in as-extruded state and various solution-treated states was investigated. The results indicate that second phase of the as-extruded 7136 aluminum alloy mainly consists of Mg(Zn, Cu, Al)_2and Fe-rich phases. The Mg(Zn, Cu, Al)_2phase directly dissolves into the matrix during solution treatment with various solution temperatures. After solution treated at 475 °C for 1 h,Mg(Zn, Cu, Al)_2phases are dissolved into the matrix,while Fe-rich phases still exist. Fe-rich phases could not dissolve into the matrix by prolonging solution time. The mechanical property test and EBSD observation show that two-stage solution treatment makes no significant improvement in mechanical properties and recrystallization of the alloy. The optimized solution treatment parameter is chosen as 475 °C/1 h.     

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.     

Horizontally Solidified Al–3 wt%Cu–(0.5 wt%Mg) Alloys: Tailoring Thermal Parameters,Microstructure, Microhardness,and Corrosion Behavior

In the present experimental investigation, Al–3 wt%Cu and Al–3 wt%Cu–0.5 wt%Mg alloys castings are produced by a horizontal solidification technique with a view to examining the interrelationship among growth rate(G_R), cooling rate(C_R), secondary dendrite arm spacing(λ_2), Vickers microhardness(HV), and corrosion behavior in a 0.5 M NaCl solution.The intermetallic phases of the as-solidified microstructures, that is, h-Al_2Cu, S–Al_2CuMg, and x-Al_7Cu_2 Fe, are subjected to a comprehensive characterization by using calculations provided by computational thermodynamics software, optical microscopy, and scanning electron microscopy/energy-dispersive spectroscopy. Moreover, electrochemical impedance spectroscopy and potentiodynamic polarization tests have been applied to analyze the corrosion performance of samples of both alloys castings. Hall–Petch-type equations are proposed to represent the HV dependence on λ_2. It is shown that the addition of Mg to the Al–Cu alloy has led to a considerable increase in HV; however, the Al–Cu binary alloy is shown to have lower corrosion current density(i_(corr)) as well as higher polarization resistance as compared to the corresponding results of the Al–Cu–Mg ternary alloy.     

MICROSTRUCTURES OF Mg-Zr,Mg-Zn AND Mg-Zn-Zr ALLOYS

Microstructural studies have been made on the Mg-0.54Zr,Mg-5.68Zn andMg-5.65Zn-0.50Zr alloys in the as-cast and homogenized states.The lenticular plates of{012}twins were found in three alloys.After homogenization,ZrH_2 phase formed in theMg-0.54Zr alloy.The Mg_7Zn_3 phase of cubic structure with a=1.417 nm distributes at thegrain boundaries of the Mg-5.68Zn alloy in the as-cast state and after homogenization,theMg_7Zn_3 phase dissolves and the MgZn_2 phase occurs.The as-cast Mg-5.65Zn-0.50Zr al-loy consists of MgZn_3 phase and Zn-Zr compounds.After homogenization,the dispersedacicular MgZn_2 phase may precipitate.     

Overheating temperature of 7B04 high strength aluminum alloy

The microstructure and overheating characteristics of the direct chill semicontinuous casting ingot of 7B04 high strength aluminum alloy, and those after industrial homogenization treatment and multi-stage homogenization treatments, were studied by differential scanning calorimetry(DSC), optical microscopy(OM) and scanning electron microscopy with energy dispersive X-ray spectroscopy(SEM-EDX). The results show that the microstructure of direct chill semicontinuous casting ingot of the 7B04 alloy contains a large number of constituents in the form of dendritic networks that consist of nonequilibrium eutectic and Fe-containing phases. The nonequilibrium eutectic contains Al, Zn, Mg and Cu, and the Fe-containing phases include two kinds of phases, one containing Al, Fe, Mn and Cu, and the other having Al, Fe, Mn, Cr, Si and Cu. The melting point of the nonequilibrium eutectic is 478 ℃ for the casting ingot of the 7B04 alloy which is usually considered as its overheating temperature. During industrial homogenization treatment processing at 470 ℃, the nonequilibrium eutectic dissolves into the matrix of this alloy partly, and the remainder transforms into Al2CuMg phase that cannot be dissolved into the matrix at that temperature completely. The melting point of the Al2CuMg phase which can dissolve into the matrix completely by slow heating is about 490 ℃. The overheating temperature of this high strength aluminum alloy can rise to 500-520 ℃. By means of special multi-stage homogenization, the temperature of the homogenization treatment of the ingot of the 7B04 high strength aluminum alloy can reach 500 ℃ without overheating.     

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.     

Evolution of Microstructure,Residual Stress,and Tensile Properties of Mg–Zn–Y–La–Zr Magnesium Alloy Processed by Extrusion

The microstructure, texture, residual stress, and tensile properties of Mg–6 Zn–2 Y–1 La–0.5 Zr(wt%) magnesium alloy were investigated before and after extrusion process, which performed at 300 °C and 400 °C. The microstructural characterizations indicated that the as-cast alloy was comprised of α-Mg, Mg–Zn, Mg–Zn–La, and Mg–Zn–Y phases. During homogenization at 400 °C for 24 h, most of the secondary phases exhibited partial dissolution. Extrusion process led to a remarkable grain refi nement due to dynamic recrystallization(DRX). The degree of DRX and the DRXed grain size increased with increasing extrusion temperature. The homogenized alloy did not show a preferential crystallographic orientation, whereas the extruded alloys showed strong basal texture. The extrusion process led to a signifi cant improvement on the compressive residual stress and mechanical properties. The alloy extruded at 300 °C exhibited the highest basal texture intensity, the compressive residual stress and hardness, and yield and tensile strengths among the studied alloys.     

Effect of Partially Substituting Ca with Mischmetal on the Microstructure and Mechanical Properties of Extruded Mg-Al-Ca-Mn-Based Alloys

Mg-2Al-1.2Ca-0.2Mn(at%)-based alloys with Ce-rich mischmetal(MM) substitution of 0–0.6 at% for Ca were hot extruded at 400 °C. The effect of MM substitution on the microstructure and mechanical properties of the extruded alloys was investigated. The as-cast Mg-2Al-1.2Ca-0.2Mn alloy is mainly composed of a-Mg, Mg_2Ca and(Mg,Al)_2Ca phases and Al_8Mn_5 precipitates, whereas the substitution of MM brings about the formation of Al_(11)MM_3, Al_2MM phases and Al_(10)MM_2Mn_7 particles with the absence of (Mg,Al)_2Ca phase. The volume fraction of MM-containing phases increases with increasing MM contents. All of the extruded alloys exhibit bimodal microstructure comprising fine dynamically recrystallized grains with almost random orientation and coarse deformed grains with strong basal texture. Dense nanosized planar Al_2Ca and spherical Al–Mn phases precipitate inside the deformed grains. High tensile yield strengths of~ 350 MPa and moderate elongations to failure of 12% are obtained in all extruded alloys; the MM substitution induces negligible difference in the tensile properties at ambient temperature, while the highest MM substitution improves the strength at 180 °C due to the better thermal stability of the fragmented MM-containing phases.     

Microstructure and mechanical properties of ZA62 based magnesium alloys with calcium addition

As-cast microstructure and mechanical properties of Mg-6Zn-2Al-0.3Mn (ZA62) alloys with calcium addition were investigated.The as-cast microstructure of the base alloy ZA62 consists of the α-Mg matrix and eutectic phase Mg51Zn20.The Mg51Zn20 eutectic was gradually replaced by MgZn phase and Mg32(Al,Zn)49 phase when calcium is added into the base alloy.Further addition of calcium leads to the increase of grain boundary phases and formation of a new quaternary Mg-Zn-Al-Ca eutectic compound.In comparison with the base alloy,the increase of calcium addition to the base alloy results in the reduction of both strength and ductility at ambient temperature,but increase at elevated temperatures due to the thermal stability of Ca-containing phases.At elevated temperatures,the creep resistance of ZA62 based alloys containing calcium is significantly higher than that of AZ91 which is the most commonly used magnesium alloy.     

Fatigue Behavior of Dissimilar Al–Mg–Si/Al–Zn–Mg Aluminum Alloys Friction Stir Welding Joints

In this study, fatigue properties and fracture mechanism of dissimilar Al–Mg–Si/Al–Zn–Mg aluminum alloys friction stir welding(FSW) joints were investigated and the effect of the sheet configuration on the fatigue behavior of the FSW joints was also discussed. Results showed that the joints owned better fatigue properties when the Al–Zn–Mg aluminum alloy was placed at the advancing side(AS). At 10~7 cycles, the fatigue strengths of Al–Zn–Mg–AS and Al–Mg–Si–AS joints were, respectively, 105.6 and 90.1 MPa. All joints fractured at the heat-affected zone at the Al–Mg–Si alloy side. Transmission electron microscopy results showed that better fatigue property of the Al–Zn–Mg–AS joint was associated with the bridging effect of the bigger secondary phase particles.     

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.     

Precipitation behavior of 14H-LPSO structure in Mg–12Gd–2Er–1Zn–0.6Zr Alloy

The microstructures of as-cast and as-solution Mg–12Gd–2Er–1Zn–0.6Zr alloys were investigated by optical microscopy(OM), scanning electron microscopy(SEM), transmission electron microscopy(TEM), highresolution transmission electron microscopy(HRTEM)X-ray diffraction(XRD) and selected-area electron diffraction(SAED) in the present investigation. The results show that the primary eutectic phase Mg_5(Gd, Er, Zn) and some flocculent features are found in the as-cast alloy; the SAED pattern indicates that these flocculent features are the dense areas of stacking faults. The 14H-LPSO structure precipitates in the temperature range of 673–793 K, and the volume fraction of 14H-LPSO structure increases with the extension of heating time; however, there is no precipitation of 14H-LPSO structure when the temperature reaches up to 803 K. In addition, the Mg_5(Gd, Er, Zn) phase dissolves gradually along with the precipitation of 14H-LPSO structure.     

Effects of Zinc and Calcium Concentration on the Microstructure and Mechanical Properties of Hot-Rolled Mg–Zn–Ca Sheets

Four kinds of Mg alloys with different Zn and Ca concentration were selected to analyze the effect of Zn and Ca concentration on the microstructure and the mechanical properties of Mg–Zn–Ca alloys. It was found that Zn and Ca concentration has a great influence on the volume fraction, the morphology and the size of second phase. The Mg–1.95Zn–0.75Ca(wt%) alloy with the highest volume fraction, continuous network and largest size of Ca2Mg6Zn3 phase showed the lowest elongation to failure of about 7%, while the Mg–0.73Zn–0.12Ca(wt%) alloy with the lowest volume fraction and smallest size of Ca2Mg6Zn3 phase showed the highest elongation to failure of about 37%. It was suggested that uniform elongations of the Mg–Zn–Ca alloys were sensitive to the volume fraction of the Ca2Mg6Zn3 phases, especially the network Ca2Mg6Zn3phases; post-uniform elongations were dependent on the size of the Ca2Mg6Zn3 phase, especially the size of network Ca2Mg6Zn3 phase. Reduction in Zn and Ca concentration was an effective way to improve the roomtemperature ductility of weak textured Mg–Zn–Ca alloys.     

Microstructural evolution of direct chill cast Al-15.5Si-4Cu-1Mg-1Ni-0.5Cr alloy during solution treatment

Heat treatment has important influence on the microstructure and mechanical properties of Al-Si alloys. The most common used heat treatment method for these alloys is solution treatment followed by age-hardening. This paper investigates the microstructural evolution of a direct chill (DC) cast Al-15.5Si-4Cu-1Mg-1Ni-0.5Cr alloy after solution treated at 500, 510, 520 and 530℃, respectively for different times. The major phases observed in the as-cast alloy are α-aluminum dendrite, primary Si particle, eutectic Si, Al7Cu4Ni, Al5Cu2Mg8Si6, Al15(Cr, Fe, Ni, Cu)4Si2 and Al2Cu. The Al2Cu phase dissolves completely after being solution treated for 2 h at 500℃, while the eutectic Si, Al5Cu2Mg8Si6 and Al15(Cr, Fe, Ni, Cu)4Si2 phases are insoluble. In addition, the Al7Cu4Ni phase is substituted by the Al3CuNi phase. The α-aluminum dendrite network disappears when the solution temperature is increased to 530℃. Incipient melting of the Al2Cu-rich eutectic mixture occurrs at 520℃, and melting of the Al5Cu2Mg8Si6 and Al3CuNi phases is observed at a solution temperature of 530℃. The void formation of the structure and deterioration of the mechanical properties are found in samples solution treated at 530℃.