Influence of aging modes on microstructure and mechanical properties of AZ80 magnesium alloy

The microstructure and mechanical properties of AZ80 magnesium alloy after solid solution and aging treatments were studied by using optical microscope (OM), X-ray diffraction (XRD), scanning electron microscopy(SEM) as well as tensile testing. The results indicated that β-Mg17Al12 phase was getting to distribute discontinuously along the grain boundary after treated at 395℃ ageing for 12 h followed by water-cooling, but it did not dissolve into α-Mg completely. The residual β-Mg17Al12 phase distributed along the grain boundary and had block-like or island shapes. The size of α-Mg was getting to be coarsening but not significantly. The β-Mg17Al12 precipitates appeared in discontinuous and continuous patterns from supersaturated α-Mg solid solution after aged at 200℃. The precipitation patterns were associated with the aging time essentially. The tensile strength and elongation of the alloy increased significantly but the hardness and yield strength decreased after solid solution treatment. However, with the prolonging of aging time, the hardness and strength of alloy increased while the ductility decreased.     

Effect of Mg-Zn-Y quasicrystals on microstructure and mechanical properties ofAZ91 alloy

The influence of the additions of Mg-Zn-Y quasicrystals-containing master alloy on the microstructures and mechanical properties of AZ91 alloy under conventional casting condition was studied using XRD, SEM equipped with energy dispersive spectrometer （EDS）. The results show that the microstructure of Mg-Zn-Y quasicrystals reinforced AZ91 alloy consists of a-Mg supersaturating solid solution, β-Mgl7All2 phase and quasicrystals phase. Quasicrystals particles with excellent elevated temperature stability are dispersively distributed in the α-Mg matrix or at grain boundaries. After the addition of quasicrystals-containing master alloy, the matrix microstructure of AZ91 alloy is obviously grain-refined. The morphology of β-Mg17Al12 phase changes from continuous nets to discrete nets. At room and elevated temperatures mechanical properties of AZ91 alloy are also improved dramatically. The utility of Mg-Zn-Y quasicrystals as a reinforced phase provides new theoretical basis and technical support for the composite strengthening of magnesium alloys.     

Preparation ofAZ91 alloy by powder metallurgy method

Magnesium alloy AZ91 was prepared by powder metallurgy method, and the effect of sintering temperature on the densities and microstructures of the alloys was investigated. The results show that tight control of the sintering temperature within a narrow range around the liquidus level generates structures with high integrity and reduces some of the disadvantages caused by the higher temperatures. The matrix consists of α-Mg grains/cells surrounds by discontinuous precipitates of Mg17Al12 intermetallics which are normally located at the grain boundaries and triple junction.     

Effect of intermetallic compound β-Mg17Al12 on ductility of AZ91D magnesium alloy

The microstructures of the AZ91D die-cast magnesium alloy were investigated by scanning electron microscopy （SEM）, X-ray diffractometry （XRD） and energy dispersion spectroscopy （EDS）. Moreover, the microstructure change of AZ91D samples was observed during the process of heat treatment at 300 ℃ for different time. And the tensile testing was carried out for these samples and the fracture morphology of the tensile test samples was also examined. The results show that the microstructures of AZglD magnesium alloy are mainly composed of a-Mg phase and β-Mg17Al12 phase. The morphology of the fl phase alters from continuous distribution to discontinuous distribution during the process of heat-treatment. Meanwhile, the ductility of the materials reduces from 1.71% to 1.08% after a long time heat-treated at 300℃. Moreover, the quasi-cleavage fractures characters are also found in the fracture morphology. The granulation and discontinuous distribution of β-Mg17Al12 results in the deterioration of the ductility of AZ91D die cast magnesium alloy.     

Influence of annealing and spheroidizing treatment on microstructure and mechanical properties of AZ91 magnesium alloy

The low-strength and high-brittleness of AZ91 cast magnesium alloy mainly result from the coarse divorced eutectic phase.To solve these problems,the annealing treatment of AZ91 cast magnesium alloy was carried out at 415 ℃ and held for 24 h in this study and the alloy was then slowly cooled to room temperature in furnace.The microstructures of the alloy were observed using a metallographic microscope,a transmission electron microscopy and an emission scanning electron microscopy,respectively.The phase analysis was performed using the X-ray diffraction,and the tensile test of the specimen at ambient temperature was performed on a material test machine.The results indicate that the coarse divorced eutectic phase dissolves into the Mg matrix during the isothermal process,and the lamellar β-Mg17Al12 phase precipitates from the magnesium solid solution with a type of pearlite precipitation during furnace cooling.Consequently,the spheroidizing treatment was carried out at 320 ℃ for 20 h following the annealing process and the lamellar β-Mg17Al12 phase was spheroidized.Compared with the as-cast alloy,the strength and ductility of the AZ91 magnesium alloy are increased obviously after annealing treatment;the yield strength and tensile strength are increased to 137.8 MPa and 240.4 MPa from 102.9 MPa and 199.3 MPa,respectively;and the elongation is improved to 6.12% from 4.35%.After being spheroidized,the strength and hardness decrease a little,but the ductility is elevated to 7.23%.The nucleation,growth and spheroidizing mechanism of the lamellar β-Mg17Al12 phase were also discussed.     

Microstructures and mechanical properties of double hot-extruded AZ80＋xSr wrought alloys

The effects of Sr addition on microstructures and tensile properties of the as-cast and hot-extruded AZ80 alloys were studied by OM, SEM, EDS, XRD, DSC and Instron tester. The results show that the microstructures of as-cast alloys consist of a-Mg and β-Mg17Al12 phase. Sr gathers on the boundaries, and dissolves into β-Mg17Al12 phase or forms Mg17Sr2 phase. The grains of ascast alloys are refined and discontinuous net-shaped structure is formed. The compound phases on the boundaries become thicker with increasing Sr content. The ultimate tensile stress（UTS） and elongation are improved compared with the corresponding Sr-free alloy. After preliminary hot-extruding, the UTS is up to 308-320 MPa and elongation reaches 8.0%-13.5%. After double hot-extrusion, the dynamic recrystallization completes totally, and the UTS is up to 310-355 MPa, but the elongation does not change apparently. The alloy with 0.02%Sr （mass fraction） obtains the best comprehensive performance with the UTS of 355 MPa and elongation of 13.2%. The SEM morphology of fracture surface shows that the alloys with Sr present good ductility after double hot-extrusion.     

Microstructure and mechanical properties of AZ91 magnesium alloy with minor additions of Sm,Si and Ca elements

The effects of Sm, Si and Ca on the microstructure and mechanical property of AZ91 magnesium alloy were investigated by means of optical microscopy(OM), differential scanning calorimetry(DSC), scanning electronic microscopy(SEM), X-ray diffraction(XRD) and tensile testing. The results indicated that the addition of 1.5 wt.% Sm with or without 0.8 Si/Ca led to a decrease in the volume fraction of the β-Mg17 Al12 phase and the formation of the intermetallic compounds of Al-Sm, Mg2 Si, Mg Al Ca and Al2 Ca. The microstructure of AZ91 alloy was significantly refined and distribution became discrete with additions of Sm and Ca; the average grain size of the α-Mg matrix was reduced from 239.7 ± 16.9 μm to 66.34 ± 5.10 μm. The AZ91-Sm-Ca alloy exhibited a good combination of yield strength at 135 MPa, ultimate tensile strength at 199 MPa and elongation at 4.32%, which was ascribed to grain refinement strengthening. Furthermore, the T6 treated AZ91-Sm-Ca alloy possessed yield strength of 154 MPa and elongation of 7.1%, which was due to grain refinement strengthening and reduction in discontinuous precipitates.     

Microstructural characterization in 7 at% Al-containing NiTi-based alloys

The microstructural evolution of Ni–42Ti–7Al and Ni–41Ti–7Al alloys as a function of solution and aging heat treatment was investigated using transmission electron microscopy（TEM）, electron probe, and X-ray diffraction（XRD）. The results reveal that the volume fraction of Ti2 Ni phase as well as its composition does not change significantly after as-solution treated at 1200 °C and aged at 850 °C. At the early stage of the aging treatment at 850 °C for 1 h, the cuboidal β＇ precipitate keeps coherency with the matrix; further aging, β＇ precipitate coarsens, and the semicoherency between the β/β＇ two phases are observed.The shape of coarsened β＇ precipitates changes to the globule, and the interface dislocations are introduced accompanied by the occurrence of semicoherent precipitates. Under the same heat treatment, compared to the Ni–42Ti–7Al alloy, the lattice misfits of the Ni–41Ti–7Al alloy between the β and β＇ two phases are larger, so the β＇ precipitates in Ni–41Ti–7Al alloy are coarsened severely and easily lose coherency with the matrix. The thermal stability of Ni–41Ti–7Al alloy is much worse when aging at 850 °C.     

Microstructure and grain boundary morphology evolution of a novel Co-9Al-9W-2Ta-0.02B alloy doped with yttrium

The microstructures and grain boundary morphologies of a novel Co-9 Al-9 W-2 Ta-0.02 B alloy doped with yttrium(Y)(0.01,0.05,0.10,and 0.20; at%) were investigated as functions of aging temperatures(900 and1000 ℃) and time(50 and 150 h). The aged alloys all exhibit a γ/γ'-Co_3(Al,W) coherent microstructure in grain interiors, whereas an intermetallic κ-Co_3(W) phase precipitates at grain boundaries. Y is found to fully segregate at grain boundaries and changes grain boundary precipitate morphologies. For 0.01 Y alloy, bright κ-Co_3(W) stripes precipitate along grain boundaries, where a needlelike κ-Co_3(W) phase grows from grain boundaries or κ-Co_3(W) stripes toward grain interior. As the nominal Y content increases, the stripe and needlelike κ-Co_3(W) precipitates at grain boundaries are strongly restrained and disappear in 0.20 Y alloy, leaving fine κ-Co_3(W) particles scattered at grain boundaries. It is noted that more Y segregation may increase the number of low-angle grain boundaries(LABS, with misorientations of 15°), whereas it eliminates O impurities from grain boundaries. Finally,the effect of Y segregation on tensile behavior of Co-AlW-Ta-B alloy was discussed from the viewpoints of grain boundary precipitate morphologies, grain boundary character distribution(GBCD), and impurity segregation.     

Influence of sub-rapid solidification on microstructure and mechanical properties of AZ61A magnesium alloy

The microstructure of sub-rapid solidification processed AZ61A magnesium alloy was presented and discussed. The results show that the grain size of the foil is significantly refined, and the grain morphology is cellular or globular. The eutectic transformation L→α-Mg+β-Mg17Al12 and microsegregation in conventionally solidified AZ61A alloy are suppressed to a great extent.The β-Mg17Al12 phases located in the α-Mg grain boundaries are largely decreased due to high solidification cooling rate. As a consequence, the alloying elements Al, Zn, Mn show much higher solid solubility and the sub-rapid solidification microstructure dominantly consists of supersaturated α-Mg solid solution. The mechanical properties and fractographic analysis reveal that the fracture mechanism and corresponding morphology of the rapture surface of tensile bars are linked to the microstructure obtained and depend on the sub-solidification processes.     

Effects of solution and aging treatments on microstructures and mechanical properties of AZ61 magnesium alloy welded joints

The effects of solution and aging treatments on the microstructures and mechanical properties of tungsten inert gas arc welded AZ61 magne-sium alloy joints were investigated by microstructural observations,microhardness tests and tensile tests.The results showed that the solution treatment led to the β-Mg17Al12 particles dissolved into the α-Mg grains.Hence,the microhardness of the fusion zone and the ultimate tensile strength of the welded joints were the lowest.With the increase of the aging temperature,the volume fraction of the β-Mg17Al12 particles in the fusion zone increased and this enhanced the microhardness of the fusion zone gradually.Also,the elongation of the welded joints was in-creased slightly with the increase of the volume fraction of the β-Mg17Al12 particles.However,the ultimate tensile strength of the welded joints increased at first and dropped at 190 ?C due to cracks formed at the boundaries of the β-Mg17Al12 particles.     

Aging Precipitation Behavior and Mechanical Properties of Inconel 617 Superalloy

Aging precipitation behavior and mechanical properties of Inconel 617 superalloy aged at 760 ℃ for up to 10000 h were investigated. The results showed that the precipitates of the aged alloy are M23C6 and M6C carbides and γ phase. The carbide particles precipitated both at the grain boundaries and within grains, and the γ phase particles were situated at intragranular sites in the process of aging. The carbide particles were discontinuously dispersed at grain boundaries after aging for 3000 h, while after aged for 5000 h the carbide particles are merged. The precipitates inside grains remained stable even after aging for 10000 h. The hardness was increased for the alloy aged for 300 h up to 3000 h, which was resulted primarily from the precipitation of carbides as discrete particles both at the grain boundaries and inside grains. Small quantity γ precipitates were formed inside grains, to some extent, which contributed to an enhanced hardness. However, a decrease of the hardness was observed after aging for 5000 h. A significant drop in toughness of the alloy aged for 300 h was attributed to the reduction of the bonding interface strength when carbides precipitated at grain boundaries. Thereafter, the toughness decreased slowly with the prolonged aging time. The high temperature tensile properties of the aged alloy are rather stable even aged for 300-3000 h.     

Effects of Er on the microstructure and properties of AZ31 magnesium alloy prepared via the EMS process

The effects of small amounts of the rare-earth element erbium on the microstructure and the mechanical properties of AZ31 magnesium alloy via the electromagnetic stirring(EMS) process have been studied.It has been shown that AZ31-Er alloys are mainly composed of α-Mg solid solution and β-Mg17Al12 phases.When the Er content reaches 0.12 wt.%,the characteristic peaks of Al2Er can be observed.The micro-structure is obviously refined and the tensile strength of the AZ31-based alloy at ambient temperature is significantly improved by contents of 0.03 wt.% Er,especially the elongation(δ=19%).More addition of Er obviously decreases the tensile strength and elongation of the AZ31-based alloy because of the grain coarsening and the reduction of β-Mg17Al12 phases.     

Effects of Ce addition on microstructure,mechanical properties and corrosion resistance of as-cast AZ80 magnesium alloy

In this study, Ce was introduced into the AZ80 alloy and the effects of Ce addition on the microstructure, mechanical properties and corrosion resistance of the as-cast AZ80 magnesium alloy were investigated. The results show that the addition of Ce into the AZ80 alloy can not only refine the microstructure, but also result in the formation of the needle-like Al4Ce phase. These tiny Al4Ce phases are homogeneously distributed at grain boundaries and within grains. An appropriate Ce addition can also change the β-Mg17Al12 phase at the grain boundaries from continuous network to small island-like. At the same time, with the increase of Ce content from 0 to 2.0wt.%, the macro-hardness of the as-cast alloy is enhanced linearly, while impact toughness, tensile strength and elongation all firstly increase and then decrease. The AZ80 alloy containing 1.0wt.% Ce exhibits the optimal properties. Its macro-hardness, impact toughness, tensile strength and elongation are 61.90 HB, 15.50 J·cm^-2, 171.80 MPa and 3.35%, increase by 9.95%, 63%, 13.3% and 36.7%, respectively compared with the base alloy. In addition, Ce can enhance the corrosion resistance of the AZ80 magnesium alloy.     

Effects of in-situ ZrB2 particle on grain refinement of ZrB2/AZ91D magnesium matrix composite

In-situ ZrB2/AZ91D magnesium matrix composite was successfully synthesized with Al/K2ZrF6+NH4BF4 by means of Direct Melt Reaction.The fabricated ZrB2/AZ91D magnesium matrix composite through direct melt mixing method was investigated.Results from X-ray diffraction (XRD) and energy dispersive X-ray spectroscopy (EDS) confirmed the existence of ZrB2 particles in the AZ91D alloy,and most ZrB2 particles were in the size range of just several microns,some even to 100 nm.The cast specimens were studied through corrosion testing and heat treatment.The average grain size of AZ91D decreased markedly from about 250μm to 50μm.In addition,the shape and size of the β-Mg17Al12 phase as well as the morphologies of primary α-Mg in the magnesium matrix composite were greatly changed.The network structure of the β-Mg17Al12 phase was broken into small blocks and the size of α-Mg decreased significantly.     

Microstructure and mechanical properties of magnesium alloy prepared by lost foam casting

The microstructure and mechanical properties of AZ91 alloy prepared by lost foam casting(LFC) and various heat treatments have been investigated. The microstructure of the AZ91 alloy via LFC consists of dominant α-Mg and β-Mg17Al12 as well as a new phase Al32 Mn25 with size of about 5-50μm, which has not been detected in AZ91 alloy prepared by other casting processes. The tests demonstrate that the as-cast mechanical properties are higher than those of sand gravity casting because of chilling and cushioning effect of foam pattern during the mould filling. The solution kinetics and the aging processes at different temperatures were also investigated by hardness and electrical resistivity measurements. The kinetics of aging are faster at the high temperature due to enhanced diffusion of atoms in the matrix, so the hardness peak at 380℃ occurs after 10 h; while at the lower aging temperature(150℃), the peak is not reached in the time(24 h) considered.     

Effect of La and Nd on microstructures and mechanical properties of AZ61 wrought magnesium alloy

Effects of La and Nd addition on the microstructure and mechanical properties of the AZ61 alloy have been investigated. The results show that when La and Nd are added into the AZ61 alloy respectively, the β(Mg17-Al12) phase is refined and granulated, and new phases are formed in the form of small rod-like shape, which are verified as La3 Al12 and Nd3Al11 phase by X-ray diffraction and TEM observation. Microstructure observations show that the effective efficiency of La addition is higher than that of Nd addition, thus the sizes of β(Mg17 Al12) and La3Al11 phase are relatively smaller than those of β(Mg17 Al12 ) and Nd3 Al11 phases in both AZ61 alloy and Nd-containing alloy. The increase of the tensile strength and elongation of AZ61 alloy refers to the existence of small rod-like La3Al11 and Nd3Al11 phases, and fine granulated β(Mg17 Al12 ) phase.     

Effect of Pr addition on microstructure and mechanical properties of AZ61 magnesium alloy

To improve the strength, hardness and heat resistance of Mg-6Al-1Zn(AZ61) alloy, the effects of Pr addition on the as-cast microstructure and mechanical properties of AZ61 alloy were investigated at room and elevated temperatures by means of Brinell hardness measurement, optical microscope(OM), scanning electron microscope(SEM), energy dispersive spectroscopy(EDS), X-ray diffractometer(XRD) and DNS100 electronic universal testing machine. The results show that the microstructures of Pr-containing AZ61 alloys were refined, with primary β-Mg17Al12 phase distributed homogeneously. When the addition of Pr is up to 1.2wt.%, the β phase becomes fi ner, and new needle-like or short-rod shaped Al11Pr3 phase and blocky AlPr phase appear. As a result, optimal tensile properties are obtained. However, greater than 1.2wt.% Pr addition leads to poorer mechanical properties due to the aggregation of the needle-like phase and large size of grains. The present research fi ndings provide a new way for strengthening of magnesium alloys at room and elevated temperatures, and a method of producing thermally-stable AZ61 magnesium alloy.     

Effects of La2(CO3)3 on microstructure and mechanical properties of ZM-5 magnesium alloy

The optical microscope (OM), scanning electron microscope (SEM) with energy dispersive spectrometer(EDS) and X-ray diffractometer (XRD) are employed to analyze the effect of La2(CO3)3 on microstructure and mechanical properties of ZM-5 magnesium alloy, and effect of holding time on mechanical properties of the alloy.Experimental results show that the as-cast microstructure of ZM-5 alloy with the addition of La2(CO3)3 is composed of α-Mg, β-Mg17Al12, Al11La3 and Al8LaMn4 phases and the microstructure can be refined significantly by adding La2(CO3)3.As a result, the morphology of coarse β-Mg17Al12 phase can be alternated from the semi-continuous net shape to a discontinuous, even to a granular shape, and to be diminished in their sizes and dispersed in their distributions.     

Computer simulation of γ＇ and θ phase precipitation of Ni-Al-V alloy using microscopic phase-field method

The precipitation processes of γ‘ and θ phases in Ni75Al6.5 V18.5 alloy were simulated at different temperatures and the precipitation sequence of two phases and morphological evolution were investigated. The simulation demonstrates that the two phases precipitate simultaneously at high temperature and γ‘ phase precipitates earlier than θ phase at 1000 K and 1120 K. With the temperature decreasing, the velocity of precipitation quickens, the quantity of θ phase increases and the size reduces; but the volume fraction increases, the quantity of phase increases and the size reduces as well. The two phases nucleate and grow independently at high temperature and the θ phase precipitates from the boundaries of γ‘ phase at 1000 K and 1120 K. We also find that there are many kinds of domain boundaries between the same and different phases. The results of average deviation of composition and average absolute long range order parameter show that the ordering and compositional clustering of γ‘ phase happen simultaneously at high temperature, the congruent ordering occurs prior to spinodal decomposition at 1 000 K and 1 120 K and the ordering advances and quickens as the temperature decreases. Ordering and compositional clustering of θ phase occur simultaneously at each temperature and are quickened with temperature decreasing.