THE EFFECT OF RARE EARTH ELEMENTS ON THE SOLIDIFICATION BEHAVIOR OF Al-Mg ALLOYS

The effect of rare earth element on the solidification behavior of Al-Mg alloy was investigated in a directional solidification apparatus.It was found that during the solidification process.the rare earth element segregated in the liquid at sold-liquid interface,changed the solidification morphology and reduced the secondary arm spacing markedly.

Microstructure and crystal growth direction of Al-Mg alloy

The microstructures and crystal growth directions of permanent mould casting and directionally solidified Al-Mg alloys with different Mg contents have been investigated. The results indicate that the effect of Mg content on microstructure is basically same for the alloys prepared by these two methods. The primary grains change from cellular crystals to developed columnar dendrites, and then to equiaxed dendrites as the Mg content is increased. Simultaneously, both the cellular or columnar grain region and the primary trunk spacing decrease. All of these changes are mainly attributed to the constitutional supercooling resulting from Mg element. Comparatively, the cellular or columnar crystals of the directionally solidified alloys are straighter and more parallel than those of the permanent mould casting alloys. These have straight or wavy grain boundaries, one of the most important microstructure characteristics of feathery grains. However, the transverse microstructure and growth direction reveal that they do not belong to feathery grains. The Mg seemingly can affect the crystal growth direction, but does not result in the formation of feathery grains under the conditions employed in the study.

Microstructure, mechanical properties and deformation mechanisms of an Al-Mg alloy processed by the cyclical continuous expanded extrusion and drawing approach

Al-Mg alloys are an important class of non-heat treatable alloys in which Mg solute and grain size play essential role in their mechanical properties and plastic deformation behaviors.In this work,a cyclical continuous expanded extrusion and drawing(CCEED)process was proposed and implemented on an Al-3Mg alloy to introduce large plastic deformation.The results showed that the continuous expanded extrusion mainly improved the ductility,while the cold drawing enhanced the strength of the alloy.With the increased processing CCEED passes,the multi-pass cross shear deformation mechanism progressively improved the homogeneity of the hardness distributions and refined grain size.Continuous dynamic recrystallization played an important role in the grain refinement of the processed Al-3Mg alloy rods.Besides,the microstructural evolution was basically influenced by the special thermomechanical deformation conditions during the CCEED process.

Strengths and Fracture Mechanisms of Al_2O_3 Short Fiber Reinforced Al-Mg Alloy Matrix Composite at Elevated Temperatures

Using the experimental and theoretical methods, the tensile strengths and fracture mechanisms of AI2O3 short fiber reinforced AI-Mg alloy matrix composite at elevated temperatures were researched. The interfacial microstructural characteristics and the fracture surfaces of the composite at different temperatures were observed by transmission electron microscope (TEM) and by scanning electron microscope (SEM), respectively. Then, from the results of microscopic observation, the fracture mechanisms of the composite at different temperatures are discussed. Finally, the tensile strengths of the composite at elevated temperatures were predicted by statistical integration average (SIA) method with the consideration of various fracture mechanisms. It was shown that the strengths and fracture mechanisms of the composite at elevated temperature (300℃) were significantly different from those at room temperature due to the variations of interfacial bonding states. The tensile strengths predicted by the SIA method at elevated temperatures agreed well with the experimental results.

Effects of Mg content on pore structure and electrochemical corrosion behaviors of porous Al-Mg alloys

Porous Al-Mg alloys with different nominal compositions were successfully fabricated via elemental powder reactive synthesis, and the phase composition, pore structure, and corrosion resistance were characterized with X-ray diffractometer, scanning electron microscope and electrochemical analyzer. The volume expansion ratio, open porosity and corrosion resistance in 3.5%(mass fraction) Na Cl aqueous solution of the alloys increase at first and then decrease with the increase of Mg content. The maxima of volume expansion ratio and open porosity are 18.3% and 28.1% for the porous Al-56%Mg(mass fraction) alloy, while there is the best corrosion resistance for the porous Al-37.5% Mg(mass fraction) alloy. The pore formation mechanism can be explained by Kirkendall effect, and the corrosion resistance can be mainly affected by the phase composition for the porous Al-Mg alloys. They would be of the potential application for filtration in the chloride environment.

Effect of void size and Mg contents on plastic deformation behaviors of Al-Mg alloy with pre-existing void:Molecular dynamics study

The plastic deformation properties of cylindrical pre-void aluminum-magnesium(Al-Mg)alloy under uniaxial tension are explored using molecular dynamics simulations with embedded atom method(EAM)potential.The factors of Mg content,void size,and temperature are considered.The results show that the void fraction decreases with increasing Mg in the plastic deformation,and it is almost independent of Mg content when Mg is beyond 5%.Both Mg contents and stacking faults around the void affect the void growth.These phenomena are explained by the dislocation density of the sample and stacking faults distribution around the void.The variation trends of yield stress caused by void size are in good agreement with the Lubarda model.Moreover,temperature effects are explored,the yield stress and Young’s modulus obviously decrease with temperature.Our results may enrich and facilitate the understanding of the plastic mechanism of Al-Mg with defects or other alloys.

The Solidification and Corrosion Behavior Determination of the Ti/B Added Zn-Al-Mg Alloys

The solidification and corrosion behavior of the Ti/B added Zn-Al-Mg alloys were experimentally investigated by means of microstructure characterization and electrochemical test.The basic calculations were carried out to predict the characteristics of the Ti-added Zn-Al-Mg alloys.The Zn-Al-Mg ingots with minor doping of Ti/B were prepared and solidified under different cooling rate,including air cooling,water quenching and furnace cooling.The scanning electron microscopy(SEM)and the X-ray diffraction method(XRD)were used to determine the microstructures and phase types of the alloy samples.It could be discovered that trace TiAl3 particles were dispersed in the Ti/B added alloy samples which provide the heterogeneous nucleation sites to refine the size of the dendrites and the eutectic microstructures.More fined microstructures with the addition of both Ti and B were obtained compared with those with the merely addition of Ti,and the water cooled alloys presented the finest microstructures due to the fastest cooling rate.It could also be noticed that with the increasing solidification rate,the percentage of the MgZn_(2) phase turned out to be higher because of the Mg_(2)Zn_(11)↔MgZn_(2) transition,which is in consistent with the results in the actual hot-dip galvanizing process.Electrochemical experiments in the previous work included methods the of the Tafel polarization test and the electrochemical impedance spectroscopy test(EIS).Results show that the quenched Zn-Al-Mg alloy with the addition of both Ti and B takes on best corrosion resistance.Consequently,the addition of certain amount of Ti/B elements and the appropriate elevation of the cooling rate will be the practicable approaches to optimize the microstructure and the corrosion resistance of the Zn-Al-Mg coatings in the actual galvanizing process.

Influence of precipitation on the Portevin-Le Chatelier effect in Al-Mg alloys

In the alloy with solute content higher than the limiting solubility,the solute atoms that have failed to dissolve will precipitate from the solid solution and form precipitations.In this study, the Portevin-Le Chatelier(PLC) effects in annealed 5456 and 5052 aluminum alloys with different precipitation contents have been investigated under different applied strain rates.The results suggest that precipitations have significant effect on the PLC effect and the more the precipitations are, the greater the influence is.Furthermore,the solute diffusion is pipe diffusion in 5052 alloy with lower precipitation content.However,for 5456 alloy with higher precipitation content,the diffusion is no longer the case but more complex.

Microstructure of the Hypoeutectic Al-Mg Alloy Solidified at 4 GPa High Pressure

High pressure solidification rules of Al-Mg alloy needs to be discussed further for its wide range of application. Microstructures and phases of Al-25wt% Mg alloy solidified at 4 GPa were studied by optical microscope,X-ray diffractmeter,energy dispersive X-ray spectroscopy and transmission electron microscopy( TEM). The microstructure evolution mechanism of Al-25Mg alloy under high pressure was analyzed. The result shows that the alloy consists of α-Al phase and Al 3 Mg 2 phase under normal pressure. However,only Al 12 Mg 17 phase forms without Al 3 Mg 2 phase at 4 GPa. In addition,Mg concentration in α-Al phase increases and that of the lattice constant also increases. The α-Al dendrite presents the broken arms under normal pressure, after high pressure solidification,the morphology of the dendrite tends to integrate and the size of the dendrite arms

Mechanism of Secondary Electron Emission of Cu-Al-Mg Alloy

The slices of Cu-Al-Mg alloys with thickness of 0.1mm were prepared. Some samples were activated at the temperature of 590°C and 620°C respectively. The secondary electron emission (SEE) factor (σ) was determined. The composition of surface and the electron binding energy were studied by means of XPS and AES. It is shown that the oxide mainly consisting of Al2O3 and MgO can be formed spontaneously at room temperature. This oxide layer exhibits lower electron binding energy, high σ and satisfactory stability.

Effect of scandium on superplasticity of Al-Mg alloys

The superplastic behavior of adding 0.22%Sc into the Al 6Mg alloy was studied by simple superplastic pretreatment process—warm rolling and cold rolling. The optimum superplastic temperature and strain rate of the alloys were defined and satisfactory results were obtained during the superplastic deformation at 811 K and initial strain rate 0=1.67×10 -3 s -1 . The average elongation of Al 6Mg 0.22Sc alloys reaches to 1 125%, and the maximum elongation is 1 200%, maximum m value (strain rate sensitive index) is 0.879. But under the same condition the elongation and maximum m value for Al 6Mg alloy were only 377% and 0.595, respectively. The superplastic deforming mechanism for Al 6Mg 0.22Sc alloys was also discussed. [

APPLICATION OF GENETIC NEURAL NETWORK IN Zn-27 % Al-Mg ALLOY

A genetic neural net work m odel about design of Mg content in the alloy , based on tested databetw een Mg and tensile intensity or elongation in Zn 27 % Al alloy , has been established . Theresult has sho w n that the genetic neural netw ork is a better an d m ore applied method for m a terials design than the regress analysis .

Effects of minor Zn content on microstructure and corrosion properties of Al-Mg alloy

The effects of different Zn contents in Al-Mg alloy on the microstructure characterizations were observed by advanced electron microscopy and the corrosion properties were investigated by the inter-granular corrosion tests,the exfoliation corrosion tests,and the Potentiodynamic polarizaion tests.The τ phase(Mg_(32)(Al,Zn)_(49)) forms on the pre-existing Mn-rich particles and at the grain boundaries.According to the theory of binding energy,the formation of τ phase is much easier than that of β phase(Al_3Mg_2),somehow replacing β phase and reducing the possibility of β phase precipitation.This change dramatically decreases the susceptibility of corrosion.The Zn addition increases the corrosion resistance of Al-Mg alloy with an optimal value of 0.31%.When the Zn addition is increased to 0.78%,however,the corrosion resistance of alloy decreases once again but it is still better than that of the alloy without Zn addition.

Special nanostructures in Al-Mg alloys subjected to high pressure torsion

Deformation twins and stacking faults were observed in nanostructure Al-Mg alloys subjected to high pressure torsion.These observations are surprising because deformation twinnings have never been observed in their coarse-grained counterparts under normal conditions.Experimental evidences are introduced on non-equilibrium grain boundaries,deformation twinnings and partial dislocation emissions from grain boundaries.Some of these features can be explained by the results reported from molecular-dynamics simulations of pure FCC metals.Special emphasis is laid on the recent observations of high density hexagonal and rhombic shaped nanostructures with an average size of 3 nm in the Al-Mg alloys processed by high pressure torsion.A possible formation process of these nanostructures is proposed based on molecular-dynamics simulations.

Study on Dynamic Mechanical Behavior of Al-Mg-Si Alloy

The dynamic mechanical behavior of Al-Mg-Si alloy was investigated under different strain rates by mechanical property and microstructure characterization,constitutive behavior analysis and numerical simulation in the present study.As the strain rate increases,the yield strength,ultimate tensile strength and elongation increase first,then remain almost constant,and finally increase.The alloy always exhibits a typical ductile fracture mode,not depending on the strain rate.However,as the strain rate increases,the number of dimples gradually increases.Tensile deformation can refine grains,however,the grain structure is slightly affected by the strain rate.An optimized Johnson-Cook constitutive equation was used to describe the mechanical behavior and obtained by fitting the true stress-strain curves.The parameter C was described by a function related to the strain rate.The fitting true stress-strain curves by the JC model agree very well with the experimental true stress-strain curves.The true stress-strain curves calculated by the finite element numerical simulation agree well with the experimental true stress-strain curves.

Preheating-assisted solid-state friction stir repair of Al-Mg-Si alloy plate at different rotational speeds

Additive friction stir deposition(AFSD)is a novel structural repair and manufacturing technology has become a research hotspot at home and abroad in the past five years.In this work,the microstructural evolution and mechanical performance of the Al-Mg-Si alloy plate repaired by the preheating-assisted AFSD process were investigated.To evaluate the tool rotation speed and substrate preheating for repair quality,the AFSD technique was used to additively repair 5 mm depth blind holes on 6061 aluminum alloy substrates.The results showed that preheat-assisted AFSD repair significantly improved joint bonding and joint strength compared to the control non-preheat substrate condition.Moreover,increasing rotation speed was also beneficial to improve the metallurgical bonding of the interface and avoid volume defects.Under preheating conditions,the UTS and elongation were positively correlated with rotation speed.Under the process parameters of preheated substrate and tool rotation speed of 1000 r/min,defect-free specimens could be obtained accompanied by tensile fracture occurring in the substrate rather than the repaired zone.The UTS and elongation reached the maximum values of 164.2MPa and 13.4%,which are equivalent to 99.4%and 140%of the heated substrate,respectively.

碳纳米管增韧Al-Mg合金复合材料的制备与性能研究

选择碳纳米管为增强相,通过搅拌铸造法制备了碳纳米管增韧Al-Mg合金复合材料,研究了碳纳米管的添加量对复合材料力学性能、微观组织以及韧性的影响,并探究了碳纳米管对Al-Mg合金的增韧机理。结果表明,掺入适量的碳纳米管后细化了Al-Mg合金复合材料的晶粒,晶粒尺寸约为80μm,当碳纳米管的添加量为0.8%(质量分数)时,复合材料的断口形貌最均匀。随着碳纳米管添加量的增大,复合材料的抗拉强度、断裂伸长率、硬度均表现出先增大后降低的趋势,当碳纳米管的添加量为0.8%(质量分数)时,抗拉强度、断裂伸长率和硬度均达到了最大值,分别为208.6 MPa、15.8%和53.1 HB。适量碳纳米管的添加显著改善了Al-Mg合金的韧性,使断裂延伸率逐渐增大,屈服阶段延长,韧性得到提高,这是因为适量的碳纳米管添加后能够与合金发生较好的结合,并贯穿于合金的裂纹中,发挥出了桥联作用,阻碍了裂纹的萌生和扩展,从而改善了Al-Mg合金复合材料的韧性,延缓了合金的失效过程,延长了合金的屈服阶段。综上分析可知,碳纳米管的最佳添加量为0.8%(质量分数)。

Al-Mg系合金中合金化元素作用及其对力学性能的影响

铝镁合金是轻量化材料应用领域中一种重要的金属材料,属于中高强度铝合金,具有较高的塑性、良好的耐蚀性以及优良的焊接性等优势,目前在航空航天、交通运输和军工制造等领域具有广阔的应用前景。笔者综述了铝镁合金力学性能特点以及用途,介绍了Al-Mg系合金中的强化机制,重点阐述了Al-Mg系合金中主合金化元素Mg及其含量对合金微观组织和力学性能的影响规律及机理,详细论述了Mn、Zr、Ti、Sc、Er、Y等微合金化元素的作用以及对Al-Mg系合金微观组织和力学性能的影响规律。最后,结合Al-Mg系合金当前研究现状,提出了今后值得研究的方向。

Analysis on phase selection and microstructure evolution in directionally solidified Zn-Al-Mg-Ce alloy

In the process of hot-dip Zn-Al-Mg alloy coating,the plating solution dissipates heat in the direction perpendicular to the steel plate,which is considered to be a process of directional solidification.To understand the relationship between microstructure and cooling rate of Zn-Al-Mg alloys,both the phase constitution and microstructure characteristic length scales of Zn-9.5Al-3Mg-0.01Ce(wt.%)alloy were investigated by the directional solidification experiments at different growth velocities(V=40,80,160,250μm·s^(-1)).The experimental results show that the microstructure of directionally solidified Zn-9.5Al-3Mg-0.01Ce alloy is composed of primary Al dendrites and(Zn-Al-Mg2Zn11)ternary eutectics at the growth velocities ranging from 40 to 250μm·s^(-1).The primary Al dendrites are aligned regularly along the growth direction,accompanied with obvious secondary dendrites.The relationship between the microstructure length scale and the thermal parameters of solidification is obtained:λ1=374.66V-0.383,andλ2=167.5V-0.563(λ1is the primary dendrite arm spacing,andλ2 is the secondary dendrit arm spacing).In addition,through the interface response function(IRF)and the nucleation and constitutional undercooling(NCU),the phase selection of Zn-9.5Al-3Mg-0.01Ce is obtained:(Zn+Al+Mg2Zn11)ternary eutectics in the Zn-9.5Al-3Mg-0.01Ce alloy will be replaced by ternary eutectics(Zn+Al+MgZn2)when the growth rate is lower than 7.53μm·s^(-1).

Microstructure evolution and precipitation behavior of Al-Mg-Si alloy during initial aging

The microstructure evolution and precipitation behavior of Al-Mg-Si alloy during initial aging were studied using hardness testing, conductivity testing, differential scanning calorimetry(DSC), and high resolution transmission electron microscopy(HRTEM). The results show that the precipitation sequence of the Al-Mg-Si alloy during initial aging can be represented as: supersaturated solid solution → spherical Mg/Si clusters → needle-like Guinier Preston(GP) zone → β″. Clusters are completely coherent with the Al matrix. The GP zone with relatively complete independent lattice parameters that differ slightly from the Al matrix parameters, is oriented along the direction of <111>Aland lying on {111}Alplane. The strength of the Al-Mg-Si alloy is greatly enhanced by the threedimensional strain field that exists between the β″ phase and the two {200}Alplanes. After aging at 170 ℃ for 6 h, the hardness reaches the peak of 127 HV and remains for a long time. At this stage, the electrical conductivity keeps relatively stable due to the formation of coherent precipitates(Mg/Si clusters/GP zones) and the reduction in solute atom concentration in the Al matrix. The severe coarsening and decreased number density of the β″ phase during the over-aging stage result in a significant decrease in the hardness.