Reaction Synthesis of Al/TiC Composites andthe Effect of Al Content

ＲｅａｃｔｉｏｎＳｙｎｔｈｅｓｉｓｏｆＡｌ／ＴｉＣＣｏｍｐｏｓｉｔｅｓａｎｄｔｈｅＥｆｆｅｃｔｏｆＡｌＣｏｎｔｅｎｔＺｈａｎｇＥｒｌｉｎ，ＺｅｎｇＳｏｎｇｙａｎ，ＺｅｎｇＸｉａｏｃｈｕｎａｎｄＬｉＱｉｎｇｃｈｕｎ（张二林）（曾松岩）（曾晓春）（李庆春...     

Effect of Al on Expansion Behavior of Mg–Al Alloys During Solidification

The cooling curves and the change of contraction/expansion during solidification and cooling were tested by using a selfmade device which could achieve the one-dimensional contraction instead of three-dimensional contraction of the casting.Then, the effects of Al content(0, 1.1, 3, 5, 10, 12.9, 15, 17, 19, 22, 24 and 30 wt%) on the thermal contraction/expansion of the binary Mg-Al as-cast alloys during solidification were obtained. The results showed that expanding instead of contraction was present in Mg-Al alloys with the addition of 0-30 wt% Al during solidification. The values of expansion significantly increased at first and then decreased with the increase in Al content. And the maximum expansion ratio of 0.44%(maximum expansion value: 0.841 mm) was present in the Mg-15 wt% Al alloy. Contraction instead of expansion occurred once the temperature drops to the temperature corresponding to the expansion value in total, indicating the occurrence of a continuous expansion during the solidification process in mushy zone for the Mg alloys with Al addition of 5-30 wt%. The expansion value in total consisted of two parts: the expansions occurring in the liquid-phase zone and mushy zone. The expansion in liquid zone was present in every Mg-Al alloy, and it contributed to the most proportion of the total expansion value when the Al content in Mg-Al alloy was lower than 10 wt% or higher than 22 wt%. However, the total expansion value was mainly determined by the solidification behavior in mushy zone when the Al content was among 10-22 wt% in Mg-Al alloys.     

Influence of Al content on cast microstructures of Ti–Al intermetallic compounds

Cast microstructures of Ti–(35–58) at% Al alloys have been studied using optical microscope, scanning electron microscope and transmission electron microscope. The critical compositions of transition from primary β to α phase and from α to γ phase were determined to be about 49.5 at% Al and 55.5 at% Al respectively. The peritectic compositions of α_p and γ_p were measured to be about 47 and 54 at% Al respectively. A minimum dendrite cell size was observed at 46.5 at% Al, which is believed to arise primarily from the variation behavior of the partition coefficient and liquidus slope with Al content. The mode of solid state transformation β→α during solidification at a given cooling rate varied from the massive transformation to the precipitation of Widmanstätten α plates and of α grains with Al content. The mode of transformation α→γ, on the other hand, varied from the precipitation to the massive transformation with Al content. This variation of transformation modes at a given cooling rate was explained in terms of the variation of the driving force and mobility with Al content.     

Effect of Al content on oxidation behaviour of ternary Fe–Al–C alloys

Iron aluminides produced by the electroslag refining technique, having the compositions: (1) Fe–16Al–1C, (2) Fe–10Al–1C, and (3) Fe–8Al–1C were used to investigate the effect of Al on the oxidation behaviour of the Fe–1C–Al system at 700 to 1000 °C. Prior to oxidation studies, phase and microstructure of alloys were analysed. The carbide phase, Fe₃AlC_0.69, was found to be distributed in the Fe₃Al matrix in alloy 1 and α (Fe–Al) matrix in alloys 2 and 3. The low Al content alloys displayed inversion in the oxidation kinetics below 800 °C, while, high Al content alloy displayed inversion phenomena at 1000 °C. The mechanism involving inversion in oxidation kinetics was found to be different in the two cases. In the former, it was attributed to the preferential oxidation of Al, while in the latter, to the phase transformation within the Al₂O₃. Carbides in the alloy having low Al content showed instability during oxidation.     

Study of strain softening behavior of Al–Al3Sc multilayers using microcompression testing

Multilayer thin films with bilayer thicknesses in the nanometer range have been reported to have very high strengths. A previous study has shown that Al–Al₃Sc multilayers, with bilayer thicknesses as small as 6 nm, have hardnesses as high as ～3 GPa as measured by sharp tip nanoindentation. In the present study, we have avoided some of the complications associated with sharp tip nanoindentation by directly measuring the yield strengths and strain hardening/softening properties of Al–Al₃Sc multilayers using microcompression testing methods with a nanoindenter. The results show the expected trend of increasing yield strength with decreasing bilayer thickness, and compare favorably with estimates of the yield strengths based on sharp tip nanoindentation. During deformation, the Al–Al₃Sc multilayer pillars with smaller bilayer spacings experience considerable strain softening, resulting in a “flat-top mushroom” shape after deformation. We have developed a numerical model to account for this inhomogeneous deformation behavior and to calculate stress–strain relationships during strain softening. A new transmission electron microscopy study of a deformed pillar shows that the softening is a result of destruction of the layered structure due to shearing and rotation.     

Crystallographic structure of Al3Nb in laser-processed Al–Nb alloys

The aim of the present work is to study the influence of the solidification rate of Al₃Nb on the crystallographic structure and on the order parameter of Al₃Nb in Al–Nb coatings produced by laser-alloying, using a CO₂ laser. The laser treatments were performed using a laser power density of 1.7×10³ W/mm² and scanning speeds of 5, 10, 20 and 40 mm/s, corresponding to interaction times in the range 0.03–0.24 s. The microstructure of all the surface coatings consists only of the phases predicted by the phase diagram, i.e. α-Al and Al₃Nb. X-ray diffraction on polished bulk samples revealed a strong (001) texture, which increases with increasing scanning speed. After filing, the particles of Al₃Nb become randomly oriented, leading to a better agreement between the relative intensities the experimental and calculated diffraction patterns. The long-range order parameter of Al₃Nb varies in the range 0.8–0.9 and does not depend on the solidification rate, within the limits of experimental error.     

Pseudo-semi-solid thixoforging of cup shell with Al/Al2O3

1 Introduction The composite has high strength and high hardness when some ductile metal is added into the ceramics, at the same time the fracture toughness can be improved availably[1?4]. We also know that the increase of toughness is because of the bri…     

Twinning-like Deformation Behaviour of Ti_3Al

Ｔｗｉｎｎｉｎｇ－ｌｉｋｅＤｅｆｏｒｍａｔｉｏｎＢｅｈａｖｉｏｕｒｏｆＴｉ＿３ＡｌＭａｏＷｅｉｍｉｎ，ＹｕＹｏｎｇｎｉｎｇａｎｄＹａｎｇＢｉｋｅ（毛卫民）（余永宁）（杨萍科）（ＤｅｐａｒｔｍｅｎｔｏｆＭａｔｅｒｉａｌｓＳｃｉｅｎｃｅａｎｄＥｎｇｉｎｅ...     

Solidification of Al alloys under electromagnetic field

1　INTRODUCTIONCastingisakeystepinAlalloyprocessing .From productionweknowthatalmostallqualityproblemsarerelatedtothedefectsincasting ,mostofwhichcannotbeeliminatedinfollowingformingandheattreatments ,socastingisnotonlyaformingpro cessbutalsoaprocessforstructuralcontrol.Toapplyoutfieldisanefficientandeconomicalmethodtocon trolstructures .Theelectromagneticfieldiswidelyusedinindustry ,becauseofitshighefficiency ,econ omy ,cleannessandnocontact[15] ,forexample ,electromagneticstirring ,elec…     

High-Temperature Oxidation of Al–Mg Alloys

The effect of the atmosphere on the oxidation rates of aluminum-can alloyswas studied using thermogravimetric methods. The atmospheres included: air,Ar+1%O₂, Ar+5%O₂, and CO₂. Temperaturesranged from 450 to 800°C. The oxidation rate was influenced by thesurface condition and by the time elapsed after specimen preparation. Increasingtemperature increased the oxidation rate of both AA 3004 and 5182. Parabolickinetics were observed for AA 3004 and linear kinetics were observed forAA 5182 at 450 and 500°C. From 550 to 800°C, parabolic behavior wasobserved for AA 5182. The reduction of free oxygen in the atmosphere reducedthe rate of oxidation. The reactivity of the atmospheres decreased in thefollowing sequence: air, Ar+5%O₂, Ar+1%O₂, and CO₂.     

Effect of silicon on oxidation of Ni-15Al alloy

1　INTRODUCTIONChromium, aluminum, and silicon can formsatisfactory protective scales on Ni based alloys.Chromium is expensive and not suitable for use attemperatures above 1 000℃ due to the evaporationof CrO3. It has also been well established that theincorporation of Si in many alloy systems has abeneficial effect on their oxidation resistance[1, 2].In addition, silicon is abundant and cheap. More over, Si has one of the largest solubility in Ni3Alwhere it …     

Influence of ultrasonic treatment on formation of primary Al3Zr in Al–0.4Zr alloy

The effect of ultrasonic treatment on the formation of primary Al₃Zr was investigated by applying ultrasound to an Al–0.4Zr alloy. Three temperature ranges were selected, i.e., 830 to 790 °C (above liquidus), 790 to 750 °C (cross liquidus) and 750 to 710 °C (below liquidus) for ultrasonication. Using the scanning electron microscopy, both the size and morphology of the primary Al₃Zr particles were examined. It was found that the size was significantly reduced and the morphology changed from large throwing-star shape to small compact tablet shape. The mechanisms for refinement of primary Al₃Zr were discussed. It is suggested that sonocrystallization theory via activation of aluminium oxide particles is responsible for the refinement of primary Al₃Zr when ultrasonic melt treatment (UST) is applied within the fully liquid state. The refinement of primary Al₃Zr particles when UST is applied in the slurry (growth stage) is due to the sonofragmentation.     

Effect of Mg addition on mechanical and thermoelectrical properties of Al–Al2O3 nanocomposite

The effects of Mg addition on mechanical thermo-electrical properties of Al–Mg/5%Al₂O₃ nanocomposite with different Mg contents (0, 5%, 10% and 20%) produced by mechanical alloying were studied. Scanning electron microscopy analysis (SEM), X-ray diffraction analysis (XRD) and transmission electron microscopy (TEM) were used to characterize the produced powder. The results show that addition of Mg forms a predominant phase (Al–Mg solid solution). By increasing the mass fraction of Mg, the crystallite size decreases and the lattice strain increases which results from the atomic penetration of Mg atoms into the substitutional sites of Al lattice. The microhardness of the composite increases with the increase of the Mg content. The thermal and electrical conductivities increase linearly with the temperature increase in the inspected temperature range. Moreover, the thermal conductivity increases with the increase of Mg content.     

Three-Dimensional Analysis of Fragmentation Process of Al3Ti Platelet Particles in Al–Al3Ti Multiphase Alloy Deformed by Asymmetric Rolling

Asymmetric rolling (ASR) can introduce larger shear strains than symmetric rolling (SR). Therefore, application of ASR to metal-based multiphase alloy containing platelet particles will yield different particle distributions compared with SR. In this study, differences in the fragmentation behavior of Al₃Ti platelet particles in Al–Al₃Ti multiphase alloy after SR or ASR were investigated in three dimensions. The size of the Al₃Ti platelet particles in the ASR-treated multiphase alloy was smaller than that in the SR-treated multiphase alloy. The platelet shape of the Al₃Ti particles in the multiphase alloy deformed by rolling was retained because deformation by rolling occurs via plane strain. The results show that both SR and ASR can be used to modify the distribution of Al₃Ti platelet particles in Al–Al₃Ti multiphase alloy, but that ASR can change it more severely.

     

Analysis on Internal Oxidation Technology of Al in Cu—Al Pre--Alloyed Powders

通过在铁素体和奥氏体不锈钢中添加适量Cu，经过特殊的抗菌处理，使不锈钢具有了优良的抗菌特性．探索了含Cu不锈钢经过抗菌处理后抗菌析出相的分布和形貌，抗菌检测结果表明，含有一定量Cu的铁素体和奥氏体不锈钢显示出了很强的广谱抗菌性能．在生物电镜下观察到抗菌钢表面细菌形态的改变与组织液溢出现象．     

High-Temperature Oxidation of Fe3Al and Fe3Al–Zr Intermetallics

The oxidation behavior of Fe₃Al and Fe₃Al–Zr intermetallic compounds was tested in synthetic air in the temperature range 900–1200 °C. The addition of Zr showed a significant effect on the high-temperature oxidation behavior. The total weight gain after 100 h oxidation of Fe₃Al at 1200 °C was around three times more than that for Fe₃Al–Zr materials. Zr-containing intermetallics exhibited abnormal kinetics between 900 and 1100 °C, due to the presence and transformation of transient alumina into stable α-Al₂O₃. Zr-doped Fe₃Al oxidation behavior under cyclic tests at 1100 °C was improved by delaying the breakaway oxidation to 80 cycles, in comparison to 5 cycles on the undoped Fe₃Al alloys. The oxidation improvements could be related to the segregation of Zr at alumina grain boundaries and to the presence of Zr oxide second-phase particles at the metal–oxide interface and in the external part of the alumina scale. The change of oxidation mechanisms, observed using oxygen–isotope experiments followed by secondary-ion mass spectrometry, was ascribed to Zr segregation at alumina grain boundaries.     

Effect of cold-rolling on tensile strength of SiCw/Al composite

SiCw/Al composite was fabricated through a squeeze cast route and cold rolled to about 30%, 50% and 70% re-duction in thickness, respectively. The length of whiskers in the composite before and after rolling was examined using SEM. Some of the rolled composites were recrystallization annealed to remove the work hardening of matrix alloy. The tensile strength of the rolled and annealed SiCw/Al composites was examined and then associated with the change of the whisker length and the work hardening of matrix alloy. It was found that the tensile strength is a function of the degree of cold rolling. For the cold rolled composites, with the increase in the degree of cold rolling, the tensile strength increases at first, and decreases when the degree of cold rolling exceeds 50%. For the annealed ones, however, the tensile strength de-creases monotonously with the increase in rolling degree. The different changes in tensile strength between the rolled and annealed composites could be attributed to the result of     

Coupled macro-micro modeling for prediction of grain structure of Al alloy

A 3D stochastic modeling was presented to simulate the dendritic grains during solidification process of aluminum alloy. Shape functions were proposed in 2D and 3D to describe equiaxed dendritic shape. A growth model was presented to describe the growth of a nucleated grain and the capturing of the neighboring cells. On growing,each grain continues to capture the nearest neighboring cells to form the final grain shape. If a neighboring cell has been captured by other grains, the growth along this direction stops, which can reflect the grains impingement phenomenon occurring in solidification process. 2D and 3D calculations were performed to simulate the evolution of equiaxed dendritic grains. In order to verify the modeling results, step-shaped sample castings were cast in sand mold. The microstructure in various positions of the sample was observed. In addition the quantitative metallographic analysis also has been done to evaluate the grain size. Experimental and numerical results agree well.