Crystallization of Mg-based bulk metallic glass

1 Introduction Since INOUE et al[1,2] reported that amorphousalloy with the composition of Mg65Cu25Y10 could beproduced with thickness up to 4 mm by conventionamold casting technique, Mg-based bulk metallic glasses(BMGs) have been proposed as a new kind …     

Microstructures and properties of graphite and Al2O3 short fibers reinforced Mg-Al-Zn alloy hybrid composites

Graphite and Al2O3 short fibers reinforced Mg-Al-Zn alloy hybrid composites were fabricated by perform squeeze-infiltration route. The effects of the volume of graphite particles on the microstructure, mechanical properties and tr/bological behavior were investigated under the conditions of constant size of graphite particle and volume of Al2O3 short fiber. The results reveal that the uniform distribution of the reinforced graphite particles and Al2O3 short fiber can be obtained by this technique, and they have strong bonding with the metal matrix. Increasing graphite volume results in decrease in hardness, the ultimate tensile strength whereas the Al2O3 short fiber makes contribution to the increase in hardness of the composite. The composite exhibits good wear resistance, small wear mass loss and low coefficient of friction as compared with the metal matrix. The wear mechanisms transit from oxidation wear, abrasion wear into delamination wear as the applied load is increased, and a film of lubricant covering almost entire surface of specimen, is found to be formed, which separates the wear surfaces from metal to metal contact and thus improves the tribological properties.     

Phase and microstructure selection in directionally solidified peritectic alloys with convection

A boundary layer model was used to investigate the convection effects on phase and microstructure selection in directionally solidified peritectic alloy. Due to the convection effects, the steady-state compositions of one phase at interface corresponding to an initial composition reduce, which causes its steady-state point moves upward along its solidus line and the compositional range is not consistent with the band cycle in banding. A criterion of critical interface temperature was put forward to determine whether a phase entered steady-state growth or not. Furtherly by equivalent transformation, the equivalent solidus lines and subsequent equivalent phase diagram were derived for peritectic solidification with convection. Using this equivalent phase diagram, a phase and microstructure selection map is built for a peritectic alloy with convection effect, which shows that the compositional range for banding reduces, and moves to the hyperperitectic region, and also the coupled growth region of both solids comparing with purely-diffusive limit. The predicted map for directionally solidified Pb-Bi alloy agrees well with its experimental observations.     

Modified cellular automaton model for modeling of microstructure and microsegregation in solidification of ternary alloys

A modified cellular automaton （MCA） model has been extended to the ternary alloy system by coupling thermodynamic and phase equilibrium calculation engine PanEngine. In the present model the dendrite growth is driven by the difference between the local equilibrium liquidus temperature and local actual temperature, incorporating the effect of curvature. The local equilibrium liquidus temperature is calculated with PanEngine according to the local liquid concentrations of two solutes, which are determined by numerically solving the species transport equation in the domain. Model validation was carried out through the comparison of the simulated values to the prediction of the Scheil model for solute profiles in the primary dendrites. The simulated data with zero solid diffusivity and limited liquid diffusivity were increasingly close to the Scheil profiles as the solidification rate decreased. The simulated microstructure and microsegregation in an Al-Cu-Mg ternary alloy were compared with those obtained experimentally.     

Microstructure evolution and effect on mechanical property in AZ80 Mg alloy during thermal processing

The microstructure and mechanical properties of AZ80 alloy were investigated during thermal processing. The samples of 4 mm in thickness machined from cast ingot were compressed at 300℃ with a thickness reduction of 75% and cooled in the water to room temperature. Then ageing（T5） and solution＋ageing （T6） treatments were employed respectively. The results show that mechanical properties are significantly improved after thermal processing than those of as-cast AZ80 alloy due to grain refinement and discontinuous precipitates. The heat treatment has significant influence on microstructural evolution for sample formed at moderate temperature. Microstructural evaluation indicates that the β-phase increases because of sufficient solution and the alloy is strengthened evidently.     

Optimization of parameters and study of joint microstructure of resistance spot welding of magnesium alloy

Experimental investigations on the DC spot welding of Mg alloy AZ31B are presented. Experiments are carried out to study the influence of spot welding parameters （ electrode force, welding heat input and welding time） on the tensile shear load and the diameter of nugget, based on an orthogonal test and analysis method. The optimum parameters are as follows： electrode force is 2 000 N, welding heat input is 80% and welding time is 6 cycles. The microstrueture of spot weld is single fine equiaxed crystals in the nugget, of which the structure is β-Mg17Al12 precipitated on α-Mg boundaries induced by nonequilibrium freezing. And the surface condition of the workpiece has great influence on the joint quality.     

Solidification microstructural constituent and its crystallographic morphology of permanent-mould-cast Mg-Zn-Al alloys

1 Introduction Magnesium alloy exhibits light mass, high specific strength and stiffness. Therefore, extensive application of magnesium alloys to various automobile parts is expected to enhance fuel efficiency through mass reduction. However, the unsatisf…     

宝钢钛产业发展战略和目标

采用透射电子显微镜(TEM)研究了两相钛合金Ti-700在不同热处理制度下的显微组织和相结构.探讨了固溶温度,冷却方式以及时效处理对显微结构的影响.结果表明,在所选择的热处理制度下,合金为双态组织,除了基体(α β)外,还发现了α2相以及调质结构.这些析出物对Ti-700钛合金的性能有一定影响.     

热压烧结AlN陶瓷

以自蔓延高温合成(SHS)的AlN粉体为原料,以Y203-B20O-CaF2和YF3-B-CaF2系为烧结助剂,采用热压烧结工艺制备AIN陶瓷.结果表明,采用烧结助剂,在1750℃、压力为35 MPa、保温2 h的烧结条件下,可获得相对密度均98.8%、热导率为95W/(m·K)的AIN烧结体.通过对AlN试样断口的SEM分析可知AlN晶粒大多呈直接结合,晶界相较少,有少量气孔存在.对AlN陶瓷进行后续热处理可提高其热导率,这主要是由于后续热处理后AlN陶瓷的晶界比较干净、AlN晶粒间呈直接结合而晶界相呈孤岛状分布.     

Strength distribution at interface of rotary-friction-welded aluminum to nodular cast iron

The morphology, size and composition of intermetallic compound at the interface of AI 1050 and nodular cast iron were studied by electron microprobe analysis（EMPA） and scan electron microscopy （SEM）, respectively. The bond strength of the interface was measured by the tensile tests and the morphology of the fracture surface was observed by SEM. The observation of the interface reveals that there are two distinct morphologies： no interrnetallic compound exists in the central area at the interface; while numbers of intermetallic compounds （FexAly） are formed in the peripheral area due to the overfull heat input. The tensile tests indicate that the distribution of strength in radial direction at the interface is irdaomogeneous, and the central area of the interface performs greater bond strength than the peripheral area, which proves directly that the FexAly intermetallic compounds have a negative effect on the integration of interface. The morphology on the fracture surface shows that the facture in the central area at the interface has characteristic of the ductile micro-void facture. So it is important to restrain the form of the intermetallic compound to increase the bond strength of the AI 1050 and nodular cast iron by optimizing welding parameters and the geometry of components.