A comparative study of the role of solute,potent particles and ultrasonic treatment during solidification of pure Mg,Mg-Zn and Mg-Zr alloys

Ultrasonic treatment(UST)applied during the solidification of pure Mg,eutectic(Mg-Zn)and peritectic(Mg-Zr)alloys was investigated in order to explore the grain refinement mechanisms.Temperature dependent grain refinement is observed in pure Mg where decreasing the superheat temperature(at which UST is applied from above the melting temperature,TM)from 100℃to 40℃produces significant refinement with a uniform grain structure.The presence of solute reduces the temperature dependence of the UST refinement and excellent grain refinement is obtained regardless of the superheat temperature(100℃or 40℃)and even with the use of preheated sonotrode in the Mg-6 wt.%Zn alloy.A further improvement in grain refinement is achieved when the alloy contains potent particles that introduce additional nucleation of grains in Mg-0.5 and 1.0 wt.%Zr alloys(producing an average grain size of≤100μm).At 40℃superheat,UST of Mg-Zn alloys produces excellent refinement(average grain size<200μm)with non-dendritic grains,which is normally achieved only with the addition of grain refining master alloy in the as-cast condition.The enhanced refinement observed in the eutectic alloy is explained through the undercooling imposed by a relatively cold sonotrode combined with high frequency vibrations and acoustic streaming.The advantages of using a cold sonotrode,a low superheat and solute are demonstrated for achieving significant refinement during solidification of Mg alloys under UST without or with a lower addition of grain refining master alloys.

Microstructural evolution of ultra-high strength Al-Zn-Cu-Mg-Zr alloy containing Sc during homogenization

The microstructural evolution and composition distribution of an Al-Zn-Cu-Mg-Sc-Zr alloy during homogenization were investigated by optical microscopy（OM）,scanning electron microscopy（SEM）,energy dispersive spectrometry（EDS）,X-ray diffraction（XRD） and differential scanning calorimetry（DSC）.The results show that severe dendritic segregation exists in Al-Zn-Cu-Mg-Sc-Zr alloy ingot.There are a lot of eutectic phases at grain boundary and the distribution of the main elements varies periodically along interdendritic region.The main eutectic phases at grain boundary are Al7Cu2Fe phase and T（Al2Mg3Zn3）.The residual phases are dissolved into the matrix gradually during homogenization with increasing temperature and prolonging holding time,which can be described by a constitutive equation in exponential function.The overburnt temperature of the alloy is 473.9 ℃.The optimum parameters of homogenization are 470 ℃ and 24 h,which is consistent with the result of homogenization kinetic analysis.

Microstructural evolution of Al-Zn-Mg-Zr alloy with trace amount of Sc during homogenization treatment

The microstructural evolution of Al-Zn-Mg-Zr alloy with trace amount of Sc during homogenization treatment was studied by means of metallographic analysis, scanning electron microscopy （SEM）, energy dispersive X-ray （EDX） and differential scanning calorimetry （DSC）. The results show that serious dendritic segregation exists in studied alloy ingot. There are many eutectic phases with low melting-point at grain boundary and the distribution of main elements along interdendritic region varies periodically. Elements Zn, Mg and Cu distribute unevenly from grain boundary to the inside of alloy. With increasing the homogenization temperature or prolonging the holding time, the residual phases are dissolved into matrix α（Al） gradually during homogenization treatment, all elements become more homogenized. The overburnt temperature of studied alloy is 476.7 °C. When homogenization temperature increases to 480 °C, some spherical phases and redissolved triangular constituents at grain boundaries can be easily observed. Combined with microstructural evolution and differential scanning calorimeter, the optimum homogenization parameter is at 470 °C for 24 h.

Effect of ball-milling parameter on mechanical and damping properties of sintered Mg-Zr alloy

Effects of ball-milling parameter on structures and properties of sintered Mg-l.5Zr （mass fraction, %） alloy were researched by metallographic analysis, mechanical properties tests and DMA technology. The results indicate that with 310 r/min rotation speed, the microstructure of the sintered alloy is greatly refined, and Zr-phase distributes uniformly. The micro-hardness, bending strength and damping capacities are the greatest under 310 r/min rotation speed. The damping peak of sintered Mg-l.5Zr alloy increases with increasing frequency under the testing conditions. The relaxation time meets the Arrhenius relationship, and shows the characteristics of relaxation damping.

The Aging Feature of Al-L-Cu-Mg-Zr Alloy Containing Sc

The aging behaviors of Al 1.42%Li 2.41%Cu 0.93%Mg 0.073%Zr 0.17%Sc(mass fraction, the same below) alloy at room temperature, 160 ℃ , and 160 ℃ after 8% pre deformation were studied respectively by hardness measurement. The microstructure of the alloy in various aging conditions was observed by TEM. The results show that the main precipitations of the alloy in quenching condition are the particles containing Sc and Zr which have certain coherent relation with the matrix. Addition of Sc in Al Li Cu Mg Zr alloy will be favorable to promoting precipitation. The particles can serve as preferred nucleation sites for δ′ phases which accelerate the aging hardening rate at initial aging. The main hardening phases of the alloy aged at 160 ℃ are δ′ and δ′/β′ composite precipitates. The size of the composite precipitates is very small (nanometer size). The composite precipitates will preclude efficiently the formation concentrative slip location and will improve the mechanical properties of the alloy. S′ phase will occur in the alloy aged at 160 ℃ after 8% pre deformation. It is found that 8% pre deformation has no obvious influence on the precipitation of the composite phase.

Effect of Scandium on Corrosion Resistance of Welded Joint ofAl-6Mg-Zr Alloy

The corrosion resistance of welded joints of Al-6Mg-Sc-Zr alloy was studied by neutral salt spray and exfoliation corrosion methods. The microstructure of welded joints was investigated by using optical microscope and transmission electron micrograph (TEM). It is demonstrated that the welded joints of Al-6Mg-Sc-Zr alloy are more corrosion resistance, comparing with Al-6Mg-Zr alloy. The addition of scandium in the alloy results in (Al_3Sc, Zr) particles, potently refined grains and restrained recrystallization process. The formation of homogeneous, discontinuous distribution of β-phase in welded joints improves the corrosion resistance of welded joints of Al-Mg-Zr alloy with high level content of magnesium.

Synthesis and spark plasma sintering of Al-Mg-Zr alloys

Although casting is commonly used to process aluminum alloys, powder metallurgy remains a promising technique to develop aluminum based materials for structural and functional applications. The possibility to synthesize Al-Mg-Zr alloys through mechanical alloying and spark plasma sintering techniques was explored. Al-10Mg-5Zr and Al-5Mg-1Zr alloyed powders were synthesized through wet ball milling the appropriate amount of elemental powders. The dried milled powders were spark plasma sintered through passing constant pulsed electric current with fixed pulse duration at a pressure of 35 MPa. The samples were vacuum sintered at 450, 500, 550, 600 and 620 ℃ for 10, 15 and 20 min. The Al-10Mg-5Zr alloy displays poor densification at lower sintering temperatures of 450, 500, 550 and 600 ℃. Its sinterability is improved at a temperature of 620 ℃ whereas sintering temperatures higher than 620 ℃ leads to partial melting of the alloy. It is possible to sinter the Al-5Mg-1Zr alloy at 450, 500 and 550 ℃. The increase of sintering temperature improves its densification and increases its hardness. The Al-5Mg-IZr alloy displays better densification and hardness compared to Al-10Mg-5Zr alloys.

Hot compressive deformation behavior and constitutive relationship of Al-Zn-Mg-Zr alloy with trace amounts of Sc

Abstract： The hot deformation behaviors of AI-Zn-Mg-Sc-Zr alloy were investigated in a temperature range of 340-500℃ and a strain rate range of 0.001-10 s 1 using uniaxial compression test on Gleeble-1500 thermal simulation machine. The results show that the flow stress increases with increasing strain and tends to be constant after a peak value. The flow stress increases with increasing strain rate and decreases with increasing deformation temperature. The phenomenon of dynamic recovery and dynamic recrystallization can be observed by microstructural evolutions. Based on the hyperbolic Arrhenius-type equation, the true stress-true strain data from the tests were employed to establish the constitutive equation considering the effect of the true strain on material constants （α, β, Q, n and A）, which reveals the dependence of the flow stress on strain, strain rate and deformation temperature. The predicted stress-strain curves are in good agreement with experimental results, which confirms that the developed constitutive equations are suitable to research the hot deformation behaviors of Al-Zn-Mg-Sc-Zr alloy.

EFFECT OF POWDER SIZE ON MICROSTRUCTURE AND PROPERTIES OF RAPIDLY SOLIDIFIED Al-Li-Mg-Zr ALLOY

The microstructure and properties of the rapidly solidified Al-Li-Mg-Zr alloy,in relation to the particle size of supersonic atomizing powder,have been investigated.The finer the size and the structure of powder,the higher the strength of the alloy.While the overfine powder may worsen plasticity of the alloy.The proper powder seems to be sized 40—100um.

MICROSTRUCTURE AND MECHANICAL BEHAVIOUR OF Al-Li-Zr AND Al-Li-Cu-Mg-Zr ALLOYS

The aging response,tensile and impact properties of two kinds of Al-Li based alloys have been studied.The microstructure,deformation as well as fracture behaviour in the alloys were observed with SEM and TEM.It was found that the mechanisms of deformation and fracture for different heat-treated alloys with the same chemical composition are quite different.The causes leading to the drop of ductility,toughness as well as anisotropy in peak-aged alloys have been analysed.Finally,possible methods to improve the ductility and toughness of the al- loys have been discussed.

MICROSTRUCTURE AND MECHANICAL PROPERTIES OF AN Al-Li-Cu-Mg-Zr ALLOY CONTAINING MINOR LANTHANUM ADDITIONS

The effect of the addition of minor amounts of La on the microstructure and mechanicalproperties of an Al-Li-Cu-Mg-Zr alloy was studied.The results showed that in Al-Li alloyssome impurities,such as Fe,Si and Na segregated on the grain boundaries,and the addition ofminor amounts of La decreased the segregation of these impurities.In addition,La could giverise to grain refinement,and retard the growth of precipitates.The tensile properties andtoughness of the Al-Li alloy containing minor amounts of La were improved compared with theLa-free Al-Li alloy.

EFFECT OF TENSION CONDITIONS ON SUPERPLASTIC BEHAVIOUR OF AN Al-Li-Cu-Mg-Zr ALLOY

A much higher elongation of a warm rolled superplastic Al-Li-Cu-Mg-Zr alloy was made under two-stage strain rate tests comparing with the single ones.During initial stage of deformation a deformation-induced continuous recrystallization which converted a subgrain structure into a recrystallized grain structure by a continuous increase in boundary misorientations had occurred.The higher the strain rate,the faster the continuous recrystallization and the finer the recrystallized grains.The fine recrystallized grain structure formed during the first stage deformation is the essential condition for the material to have high strain rate hardening and strain hardening during the low second stage superplastic deformation.The combination of strain rate hardening and strain hardening is the reason why the higher elongation may be obtained during two-stage superplastic deformation of the alloy.

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

The hot deformation behavior of a new Al-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.

Effects of minor Sc on microstructure and mechanical properties of Al-Zn-Mg-Zr alloy welded joint

Two kinds of Al-6.0Zn-2.0Mg-0.12Zr and Al-6.0Zn-2.0Mg-0.2Sc-0.12Zr alloy plates were prepared by ingot-metallurgy. The alloy plates with 3 mm thickness were welded by argon shield welding method,and the mechanical properties and microstructures of the two welded joints filled with Al-Mg-Sc welding wire were studied comparatively. The results show that firstly,minor Sc can raise the mechanical properties of the Al-Zn-Mg-Zr base alloy greatly. The reason for the increment is the fine grain strengthening,precipitation strengthening and the substructure strengthening caused by Al3(Sc,Zr). Secondly,η′ phase(MgZn2) and grain size in the heat-affected zone of the alloy without Sc become coarse obviously,the η′ phase(MgZn2) in the heat-affected zone of the alloy with Sc becomes coarse also,but the grain size has no visible change. Al3(Sc,Zr) particles are rather stable and can inhibit the movement of dislocation and sub-grain boundaries,overaging softening is not serious. Thirdly,adding minor Sc can raise the strength of welded joint remarkably,the tensile strength of alloy with Sc increases from 395 MPa to 447 MPa and the welding coefficient increases from 0.7 to 0.8 as well. The reason for the high strength of welded joint with Sc addition is the fine grain strengthening,precipitation strengthening and the increasing of resistance to thermal cycling softening caused by Al3(Sc,Zr).

Preparation of FeCoNi medium entropy alloy from Fe^(3+)-Co^(2+)-Ni^(2+)solution system

In recent years,medium entropy alloys have become a research hotspot due to their excellent physical and chemical performances.By controlling reasonable elemental composition and processing parameters,the medium entropy alloys can exhibit similar properties to high entropy alloys and have lower costs.In this paper,a FeCoNi medium entropy alloy precursor was prepared via sol-gel and coprecipitation methods,respectively,and FeCoNi medium entropy alloys were prepared by carbothermal and hydrogen reduction.The phases and magnetic properties of FeCoNi medium entropy alloy were investigated.Results showed that FeCoNi medium entropy alloy was produced by carbothermal and hydrogen reduction at 1500℃.Some carbon was detected in the FeCoNi medium entropy alloy prepared by carbothermal reduction.The alloy prepared by hydrogen reduction was uniform and showed a relatively high purity.Moreover,the hydrogen reduction product exhibited better saturation magnetization and lower coercivity.

Influence of introducing Zr,Ti,Nb and Ce elements on externally solidified crystals and mechanical properties of high-pressure die-casting Al–Si alloy

High pressure die casting(HPDC)AlSi10Mn Mg alloy castings are widely used in the automobile industry.Mg can optimize the mechanical properties of castings through heat treatment,while the release of thermal stress arouses the deformation of large integrated die-castings.Herein,the development of non-heat treatment Al alloys is becoming the hot topic.In addition,HPDC contains externally solidified crystals(ESCs),which are detrimental to the mechanical properties of castings.To achieve high strength and toughness of non-heat treatment die-casting Al-Si alloy,we used AlSi9Mn alloy as matrix with the introduction of Zr,Ti,Nb,and Ce.Their influences on ESCs and mechanical properties were systematically investigated through three-dimensional reconstruction and thermodynamic simulation.Our results reveal that the addition of Ti increased ESCs'size and porosity,while the introduction of Nb refined ESCs and decreased porosity.Meanwhile,large-sized Al_3(Zr,Ti)phases formed and degraded the mechanical properties.Subsequent introduction of Ce resulted in the poisoning effect and reduced mechanical properties.

Effects of the extrusion parameters on microstructure,texture and room temperature mechanical properties of extruded Mg-2.49Nd-1.82Gd-0.2Zn-0.2Zr alloy

Microstructure,texture,and mechanical properties of the extruded Mg-2.49Nd-1.82Gd-0.2Zn-0.2Zr alloy were investigated at different extrusion temperatures(260 and 320℃),extrusion ratios(10:1,15:1,and 30:1),and extrusion speeds(3 and 6 mm/s).The experimental results exhibited that the grain sizes after extrusion were much finer than that of the homogenized alloy,and the second phase showed streamline distribution along the extrusion direction(ED).With extrusion temperature increased from 260 to 320℃,the microstructure,texture,and mechanical properties of alloys changed slightly.The dynamic recrystallization(DRX)degree and grain sizes enhanced as the extrusion ratio increased from 10:1 to 30:1,and the strength gradually decreased but elongation(EL)increased.With the extrusion speed increased from 3 to 6 mm/s,the grain sizes and DRX degree increased significantly,and the samples presented the typical<2111>-<1123>rare-earth(RE)textures.The alloy extruded at 260℃ with extrusion ratio of 10:1 and extrusion speed of 3 mm/s showed the tensile yield strength(TYS)of 213 MPa and EL of 30.6%.After quantitatively analyzing the contribution of strengthening mechanisms,it was found that the grain boundary strengthening and dislocation strengthening played major roles among strengthening contributions.These results provide some guidelines for enlarging the industrial application of extruded Mg-RE alloy.

Study on the hydrogen absorption properties of a YGdTbDyHo rare-earth high-entropy alloy

This study investigated the microstructure and hydrogen absorption properties of a rare-earth high-entropy alloy(HEA),YGdTbDyHo.Results indicated that the YGdTbDyHo alloy had a microstructure of equiaxed grains,with the alloy elements distributed homogeneously.Upon hydrogen absorption,the phase structure of the HEA changed from a solid solution with an hexagonal-close-packed(HCP)structure to a high-entropy hydride with an faced-centered-cubic(FCC)structure without any secondary phase precipitated.The alloy demonstrated a maximum hydrogen storage capacity of 2.33 H/M(hydrogen atom/metal atom)at 723 K,with an enthalpy change(ΔH)of-141.09 kJ·mol^(-1)and an entropy change(ΔS)of-119.14 J·mol^(-1)·K^(-1).The kinetic mechanism of hydrogen absorption was hydride nucleation and growth,with an apparent activation energy(E_(a))of 20.90 kJ·mol^(-1).Without any activation,the YGdTbDyHo alloy could absorb hydrogen quickly(180 s at 923 K)with nearly no incubation period observed.The reason for the obtained value of 2.33 H/M was that the hydrogen atoms occupied both tetrahedral and octahedral interstices.These results demonstrate the potential application of HEAs as a high-capacity hydrogen storage material with a large H/M ratio,which can be used in the deuterium storage field.

Dynamic softening and microstructural evolution during hot deformation of Al-Cu-Mg-Zr alloys with different homogenization cooling rates

An Al-Cu-Mg-Zr alloy,which obtained different homogenization cooling rates by changing the heattreated sample size,was compressed to various strains at the deformation temperature of 300℃ and strain rate of 0.01 s^(-1).The results showed that the homogenization cooling rate had strong effects on the hot deformation behavior of the alloy.The flow stress and relative dynamic softening rate of the alloy were significantly higher under a high cooling rate(HCR) than those under a low cooling rate(LCR).Furthermore,based on X-ray diffraction,scanning electron microscopy,transmission electron microscopy,and thermodynamic equilibrium phase calculation,the substructure evolution in the grain interior,morphology,and spatial distribution of the precipitates were studied to determine the differences in the flow softening mechanism.The main softening mechanism could be summarized as dynamic recovery and precipitation coarsening for the LCR alloy and dynamic precipitation for the HCR alloy.

Improvement in grain refinement efficiency of Mg-Zr master alloy for magnesium alloy by friction stir processing

Previous studies have proved that the zirconium(Zr)alloying and grain refining performance of a Mg-Zr master alloy on Mg alloy is closely related to the distribution of Zr particle size,and a Mg-Zr master alloy with more Zr particles in size range of 1-5μm exhibits a better refining efficiency.In this paper,friction stir processing(FSP)was used to modify the Zr particles size distribution of a commercially available Mg-30 wt.%Zr master alloy,and the subsequent grain refinement ability was studied by trials on a typical Mg-3Nd-0.2Zn-0.6Zr(wt.%,NZ30K)alloy.It is found that plenty of large Zr particles in the as-received Mg-30%Zr master alloy are broken by FSP.Grain refinement tests reveal that the refining efficiency of Mg-30%Zr alloy is significantly improved by FSP,which is attributed to the better distribution of Zr particles.The refinement effect by adding 0.6%FSP-ed Mg-30%Zr is approximately equivalent to that by adding 1.0%as-received Mg-30%Zr.Due to the easy and convenient operation of FSP,this study provides a new method to develop a more efficient Mg-Zr refiner.