Advanced Materials Research Vols. 399-401

Abstract: Observation of fratcure morphology and energy spectrum analysis of fracture toughness specimens of high strength aluminum alloy was carried out on scanning electron microscope (SEM), and the fracture behaviour of this alloy has also been studied. It is revealed that the fracture surface can be divided into 3 regions: precrack region, propagation region and rapid fracture region, the appearance of which is similar to that of cleavage fracture, mixed mode fracture and static fracture, respectively. The fracture of high strength aluminum alloy is primarily influenced by coarsed Fe-rich particles which is relatively brittle, and the major fracture mode of this kind of alloy is toughness fracture casued by coarsed second phase particles.

Abstract: The PbO₂ film electrodes have been prepared by different electrodeposition process onto aluminum substrate.The electrochemical activity of the electrode for oxygen evolution reaction in 50g•L^-1Zn²⁺+150g•L^-1H₂SO₄ solutions have been studied. The results show that the best activity of the electrodes obtained A1/conductive coating was electrodepositing PbO₂ in alkaline lead solutions firstly, then plating PbO₂ in acidic lead nitrate solutions. This can be attributed to the conductivity of the substrate, the crystal phase structure and property of PbO₂. SEM-EDX analysis shows the beta-form, deposited from acidic solutions of Pb(II), has a distorted rutite structure, and the alpha-form, deposited from alkaline solutions, has the spherical cell. The high non- stoichiometry of PbO2 was obtained in alkaline lead solutions.

Abstract: Imposing ultrasound on D.C.casting of the 7050 aluminum alloy could refine grains significantly. Among the physical effects accompanying the propagation of high power ultrasonic oscillations in melt, cavitation, or the formation of cavities filled mainly with gases dissolved in the molten melt. In this research, we have analyzed solidified structures which influenced by different power of ultrasound via using cavitation theory and experiment. The results indicated that after treating by ultrasound, the solidified structures of the aluminum alloy could be refined considerably and the degree of the structure refining rose along with the increase of the ultrasound power. Software MATLAB was chosen to simulate the relationship between acoustic pressure and relative radii of cavitation bubbles, ultrasonic frequency and relative radii and initial equilibrium radii and relative radii respectively. After that, the mechanism of grain refining have been discussed under the condition of stable cavitation and transient cavitation, providing some related references for ultrasound applying in the field of foundry industry.

Abstract: Alumina–spinel castables were prepared using tabular alumina and coarse spinel as aggregates, calcium aluminate cement, fused alumina fines, spinel fines and ultrafine alumina as matrixes. Effect of microsilica and dispersant on the properties of alumina–spinel castables were investigated in this paper. The results show that with the same content of microsilica addition, the values of permanent linear change and apparent porosity of samples with different additives fired at 1500°C and 1600°C are noticeably increased. The maximum expansion value is 2.01%, 1.47% and 2.59%, respectively, for Sample B (additive: FS 10), Sample E (additive: sodium tripolyphosphate) and Sample F (additive: sodium tripolyphosphate +H₃BO₃). There is a decrease in the final expansion value with the soaking time increases, which is associated to the sintering shrinkage of the castables in this temperature range under a compression load of 0.2 MPa, Expansion behavior of alumina–spinel castables is much different with different additive addition due to abrupt CA6 crystal growth.

Abstract: By introducing ultrasonic field during the solidification process of aluminum alloy melt, the effects of ultrasonic treatment (UST) on macroscopic and microscopic segregation were investigated. The results show that under 170W of UST, the macrosegregation degree of Zn is diminished to the minimum, the segregation index reaches 0.0583. At a higher ultrasonic intensity, the UST greatly contributes to the positive centerline segregation in the ingot. Compared with conventional casting (without UST), the solid solubility of Zn, Cu and Mg inside crystals of ultrasonic casting can be strengthened so that the microsegregation tends to be minimized. Overall, the ultrasonic cavitation and acoustic stream promote the redistribution of solute elements in the ingots, and a reasonable ultrasonic power is beneficial to weaken the segregation.

Abstract: The coarsening kinetics of main strengthening phase γ′ and the growth behavior of grain boundary carbides have been investigated on NiCr20TiAl alloy aged at 550~750°C for 200~10,000h. The precipitates of NiCr20TiAl alloy at standard heat treatment condition are γ′, M₇C₃, M₂₃C₆ and MC. The coarsening of γ′ precipitates proceeds by Ostwald ripening controlled by volume diffusion in the alloy. Grain boundary carbides M23C6 and M7C3 increase with ageing times and temperatures. The morphologies of precipitates after long-time ageing almost remain the same as that at standard heat treatment condition except 750°C.

Abstract: Mcrostructure and properties of Mg-Zn-Nd-Cd-Zr alloy have been investigated on different solution treatments and artificial aging conditions by optical microscope, SEM (scanning electron microscope) and TEM(transmission electron microscope). The results indicate that the grain boundarys become thinner and the second phases distributed on the grain boundarys become finer by double-stage homogenization at 400°C for 2h plus 460°C for 12h than that of homogenization at 420°C for 24h. Double-stage homogenization brings a greater peak-ageing hardness and longer time to reach peak hardness 20h about., and the ultimate tensile strength is 345MPa at 140°C for 34h, higher 15% than that of as-extruded

Abstract: Phase transformation of the Zr-1.0Sn-0.39Nb-0.31Fe-0.05Cr alloy was investigated by high temperature X-ray diffraction (XRD). The XRD results revealed that the alloy contained two precipitates at room temperature, namely β-Nb and hexagonal Zr(Nb,Fe,Cr,)₂. β-Nb was suggested to dissolve into the α-Zr matrix at the 580oC. Thin oxide film formed at the alloy’s surface was identified as mixture of the monoclinic Zr0.93O₂ and tetragonal ZrO₂, when the temperature reached to 750oC and 850 oC. The thermal expansion coefficients of αZr in this alloy was of αa = 8.39×10^-6/°C, αc = 2.48×10^-6/°C.

Abstract: Hydrogen is widely used to prepare tungsten from tungsten oxides. Such a gas is combustible and strict measures must be taken. However, assisted by Mg or Li₃N, tungsten can be prepared through the ball milling at room temperature. Al powder is supposed to be one of the proper reducers. Simulations on the loss of Gibbs energy of these reactions have been investigated. Studies reveal that the reduction of Na₂WO₄ is feasible when reducers such as Al, Mg and Li₃N are used; and the intermediate tungsten oxides such as WO₂, WO_2.72 and WO_2.9 can be seldom generated. In addition, the calculation favors that NH3 does not react with Na₂WO₄ or WO₃ theoretically.

Abstract: The effect of cooling rate and iron content on the solidification of copper-iron alloys during the wedge-mold casting had been investigated in this paper. The experimental results show that the morphology of as-cast structure is mainly determined by iron content and the grain size is affected by both the iron content and cooling rate. The as-cast structure is obviously refined and its morphology changes gradually from columnar grains to equiaxed ones with the increase in iron content. When iron content is greater than 3% (wt%) the as-cast structure is composed of equiaxed grains. With the increase in cooling rate the as-cast structure is gradually refined and primary γ-Fe phase is gradually scattered uniformly throughout the matrix.