Effects of isothermal process parameters on the microstructure of semisolid AZ91D alloy produced by SIMA

The effects of isothermal process parameters on the microstructure evolution of semisolid AZ91D alloy produced by strain-induced melt activation (SIMA) were investigated using the self-programmed analysis software based on quantitative metallography. The results showed that long isothermal time could make the semisolid particles more globular, but the size of the particles would grow larger; high semisolid isothermal temperature would reduce the solid volume fraction and accelerate the spherical evolution of the solid particles. It was found that the optimal process parameters should be 570 °C and 10–20 min of isothermal temperature and time respectively based on the conditions of this paper. The mechanism of the particles’ formation was also discussed during the isothermal treatment.     

自孕育法制备AZ31镁合金半固态流变成形组织(英文)

采用新型自孕育流变铸造技术对变形镁合金半固态组织进行控制。该工艺过程为将合金熔体与一定量的合金固体颗粒(自孕育剂)混合,然后将混合金属通过一个多流股导流器浇入铸型或收集器。结果表明:采用自孕育工艺,合金熔体处理温度690~710℃,孕育剂的加入量为3%~7%时能有效将AZ31镁合金传统铸造中的粗大枝晶组织转变为细小、近球状的非枝晶组织;当合金熔体处理温度较高时,增加孕育剂的加入量或减小导流器的倾斜角度有利于获得非枝晶组织。自孕育工艺制备的AZ31镁合金半固态浆料在620℃等温保温30s后能有效改善初生α-Mg颗粒的圆整度;延长保温时间有助于减小颗粒的圆整度,但同时颗粒发生粗化。利用Lifshitz-Slyozov-Wagner(LSW)理论对初生相颗粒在等温保温过程中的组织圆整、粗化过程进行了分析。     

Comparative study of the deformation behavior of hexagonal magnesium–lithium alloys and a conventional magnesium AZ31 alloy

Specimens of a conventional magnesium AZ31 alloy and a binary α-solid solution Mg4Li alloy with similar starting textures and microstructure were subjected to plane strain deformation under various deformation temperatures ranging from 298 K to 673 K. Lithium addition to magnesium exhibited remarkable room temperature ductility improvement owing to enhanced activity of non-basal slip, particularly, 〈c + a〉-slip mode. Furthermore, the addition of lithium to magnesium seemed to reduce the plastic anisotropy, typical for commercial magnesium alloys. This was evident in the flow curves and texture development obtained at 200 °C and 400 °C. At 400 °C prismatic slip gains strong influence in accommodating the imposed deformation. In terms of thermal stability against microstructure coarsening at elevated temperatures, the lithium containing alloy undergoes significant grain growth following recrystallization.     

Coarsening of γ (Ni–Al solid solution) precipitates in a γ′ (Ni3Al) matrix

The coarsening behavior of Ni–Al solid–solution precipitates in an Ni₃Al matrix was investigated in alloys containing 22.0–22.8 at.% Al aged at 650–800 °C for times exceeding 1800 h. The rate constant for coarsening increases with equilibrium volume fraction as predicted by the MLSW theory. The activation energy for coarsening, 314.1 ± 16.6 kJ mol⁻¹, agrees very well with results from conventional diffusion experiments. The particle size distributions are not in very good agreement with the predictions of any theory; possible reasons are discussed. The particles become more spherical with decreasing elastic self-energy. The results are consistent with the premise that a strong volume fraction effect is observed so long as diffusion in the matrix phase, and not through the precipitate–matrix interface, controls the kinetics.     

Transmission electron microscopy and thermodynamic studies of CaO-added AZ31 Mg alloys

We investigated the microstructural evolution of Mg–3Al–1Zn (AZ31) alloy systems, with Ca or CaO added, by carrying out microstructural characterizations in conjunction with thermodynamic calculations. A calculated phase diagram of the Mg–Ca–O ternary system showed that CaO can be dissolved in liquid Mg so as to have 12.6 wt.% Ca content in the liquid Mg at 700 °C. Therefore, for a 0.3 wt.% CaO-added AZ31 alloy, our thermodynamic calculation predicted a similar precipitation pathway to that of a 0.3 wt.% Ca-added AZ31 alloy during the solidification process. In fact, a thermodynamic analysis of the precipitation pathway assuming the Scheil model showed that the major precipitates in both alloys were Al₈Mn₅, CaMgSi, Laves C15 and Laves C36, in good agreement with our experimental observation. However, a microstructural characterization of the as-cast alloys using transmission electron microscopy revealed that the spatial distribution of the precipitates was significantly different in the two alloy systems; unlike in the Ca-added AZ31 alloy, the Ca-containing precipitates in the CaO-added AZ31 alloy exhibited strong agglomeration tendencies. Moreover, in an alloy solidified at a faster cooling rate, undissolved CaO particles were observed in the precipitate agglomerates that were connected to the other Ca-containing precipitates. These results suggest that an incomplete dissolution of CaO particles in the liquid results in the agglomeration of precipitates, as the undissolved CaO particles can act as local sources, supplying Ca to the liquid, and can thus act as preferential nucleation sites for the Ca-containing precipitates forming during the solidification of the alloy.     

Erbium and ytterbium solubilities and diffusivities in aluminum as determined by nanoscale characterization of precipitates

Binary aluminum alloys with 0.03–0.06 at.% RE (RE = Yb or Er) were aged to produce coherent, nanosize Al₃RE precipitates in an α-Al matrix. The temporal evolution of precipitate radii and matrix concentrations at 300 °C were measured by transmission electron microscopy and local-electrode atom-probe tomography, respectively. The temporal dependence of the matrix concentration of each RE was utilized to determine its solubility in Al. The solubility and the coarsening rate constants were used to determine the diffusivity of each RE in α-Al and the α-Al/Al₃RE interfacial free energies at 300 °C. When compared to Sc, both Yb and Er exhibited smaller solubilities but larger diffusivities in α-Al and larger α-Al/Al₃RE interfacial energies.     

Surface alloying of AZ91E alloy by Al–Zn packed powder diffusion coating

A new surface coating technique, namely packed powder diffusion coating (PPDC), for AZ91E magnesium alloy is reported. This new technique uses a powder mixture of aluminium and zinc as diffusion source and produces uniform and thick coatings at temperatures below 420 °C. Experimental results showed that zinc in the powder mixture significantly promotes the formation of intermetallic layers on the surface of the magnesium alloy at process temperatures between 350 °C and 413 °C, which is more than 50 °C lower than the previously reported processes. Depending on the temperature and the Zn-content in the powder, X-ray diffraction analysis identified three intermetallic phases and Mg(Al, Zn) solid solution that consist of the surface alloyed layer. The intermetallic compounds are τ-Mg₃₂(Al,Zn)₄₉, φ-Al₅Mg₁₁Zn₄ and β-Mg₁₇(Al,Zn)₁₂. The hardness of the over 500 μm thick surface alloyed layers is up to four times higher than the substrate. Both the β-Mg₁₇(Al,Zn)₁₂ phase and the τ-Mg₃₂(Al,Zn)₄₉ phase show one to two order magnitude higher corrosion resistance than the α-phase (solid solution) and the φ-Al₅Mg₁₁Zn₄ phase in 5% NaCl solution. A process parameter window for the layer thickness as well as a schematic model for the formation of the layer is proposed. The PPDC process is a promising technique that provides effective protection of AZ91E alloy from both wear and corrosion.     

Effects of boron on the microstructure and thermal stability of directionally solidified NiAl–Mo eutectic

Microalloying with 0.01 at.% B decreases the range of growth speeds over which a well-aligned fibrous eutectic microstructure can be obtained in directionally solidified NiAl–Mo. Compared to the undoped alloy, the size/spacing of the Mo fibers is larger, and the fiber density smaller, in the B-doped alloy. Annealing at 1400 °C coarsens the fibers by a mechanism involving fault migration and annihilation driven by diffusion along the fiber–matrix interface. The coarsening kinetics, given by the decrease in Mo fiber density with time, is exponential, and microalloying with B decreases the coarsening rate.     

The ignition temperature of Mg alloys WE43, AZ31 and AZ91

A simple furnace test explored ignition of three Mg alloys. WE43 had the highest ignition temperature (644 °C) compared with 628 °C for AZ31 and 600 °C for AZ91. Tests to measure the ignition temperature appear to be highly controlled. However, this appearance is deceptive because the ignition temperature depends on test details. Flame tests appear to be able to allow direct study of the behaviour in a flame and so appear to be a good approach to study the behaviour Mg alloys in an aircraft fire accident.     

Effect of heat treatment on the microstructure and mechanical properties of the consolidated Mg alloy AZ91D machined chips

Influence of heat treatment on the properties of the consolidated AZ91D Mg alloy chips was performed in this study. The chips were pressed into a die to form a compact with a green density of 1.6 g/cm³. The 50-mm diameter green compact was then extruded into a 20-mm rod at 350 °C. The extruded rod was solution treated at a temperature of 415 °C for 24 h; the solution treated specimens were then aged at two temperatures: 170 and 215 °C. Heat treatments were conducted to explore the microstructure evolution and mechanical properties of the extruded rod. Heat treatments revealed that the age hardening effect was related to the transformation of the microstructure. Over aging during age heat treatment was believed to be caused by the formation of a lamellar structure composed of alternating layers of Mg₁₇Al₁₂ phase and magnesium matrix.     

Structure stability and mechanical properties of Mg–Al-based alloy modified with Y-rich and Ce-rich misch metals

Mg–3.4 wt.% Al-based alloy modified with 2.4 wt.% Ce-rich and 0.3 wt.% Y-rich misch metals was prepared by high-pressure die-casting. The microstructure, thermal stability of intermetallic phases and mechanical properties were investigated. The cross-section of test bar is divided into the fine skin region and the wide core region by a narrow band. Two intermetallic phases Al₁₁RE₃ and Al₂RE, with the former being the dominant one, are mainly distributed at the interdendritic regions. The Al₁₁RE₃ intermetallics possess high volume fraction, fine acicular/lamellar morphology and layered arrangement. It is suggests that each rare earth element has the individual preference for the above two Al–RE intermetallics. The thermal stability of Al₁₁RE₃ is conditioned. It is basically stable at temperature up to 200 °C within 800 h, while almost all Al₁₁RE₃ intermetallics transform to Al₂RE at higher temperature of 450 °C for 800 h. The alloy exhibits remarkably improved tensile and compressive yield strengths at room temperature and 200 °C and they are the results of the reinforcement of dendrite boundaries with Al₁₁RE₃ intermetallics, the fine dendritic arm spacing effect as well as the solid solution strengthening with various rare earth elements.     

Preparation of non-dendritic microstructure of AM60 alloy for rheoforming using self-inoculation method

Abstract

A rheocasting process, i.e. self-inoculation method (SIM), has been developed for preparing sound semisolid slurry of AM60 alloy. This process involves mixing between alloy melt and particles of solid alloy (self-inoculants in SIM) and subsequent pouring of the mixed melt with a multistream fluid director. Results show that the combined action of self-inoculants and fluid director can create high nucleation and survive rate of the primary α-Mg phase even with high superheat (85°C) in the AM60 alloy. The melt treatment temperature and addition amounts of self-inoculants are the main factors influencing the slurry microstructure. In order to prepare high quality semisolid slurries, proper melt treatment temperature range of 680–700°C and self-inoculants addition range of 5–7% are suggested. The grain multiplication derives from the heterogeneous nucleation induced by self-inoculants and free crystals and dendrite fragments enhanced by the fluid director; globular grain growth and coarsening of particles should be contributed to the microstructure formation.     

High temperature oxidation of AZ31 + 0.3 wt.%Ca and AZ31 + 0.3 wt.%CaO magnesium alloys

Magnesium alloys of AZ31 + 0.3 wt.%Ca and AZ31 + 0.3 wt.%CaO were cast and oxidized between 450 and 650 °C in atmospheric air. The initially added Ca and CaO enabled to cast the alloys in air without using environmentally hazardous SF₆ gas, by forming a thin CaO-rich barrier layer at the surface during casting. A thin CaO-rich barrier layer was also formed at the surface during oxidation in air, thereby increasing the oxidation resistance of the AZ31 alloy considerably. The initially added Ca and CaO reacted with Al to become Al₂Ca along the grain boundaries of the AZ31 alloy during casting.     

The effect of annealing on the microstructure and tensile properties of a β/γ′ Ni–Al–Fe alloy

The evolution of the microstructure of a (β/γ ′) Ni–32 at.% Al–5 at.% Fe alloy during annealing has been studied by electron microscopy and X-ray diffraction. Annealing at 800°C and 1100°C causes a reverse martensitic transformation, L1₀→B2 (β), and a B2→L1₂ (γ ′) phase transformation. The lower annealing temperature leads to a higher volume fraction of the γ ′-phase but a smaller size of the γ ′-particles. The kinetic laws of the coarsening and of the increase in the volume fraction of the γ ′-phase are discussed. The orientation relationships between the β and γ ′ phases appeared to be mainly of Nishiyama–Wassermann and Bain types after 800°C annealing, while Kurdjumov–Sachs and Bain orientation relationships were predominant in the alloys annealed at 1100°C. A strong correlation between the volume fraction of the γ ′-phase and the tensile characteristics of the alloy has been established.     

Effect of hot-dip aluminizing on the oxidation resistance of Ti–6Al–4V alloy at high temperatures

Hot-dip aluminizing and interdiffusion treatment were used to develop a TiAl₃-rich coating on Ti–6Al–4V alloy. Interrupted oxidation at temperatures from 600 to 900 °C and isothermal oxidation at 700 and 800 °C of the coating were conducted. The coating markedly decreases the oxidation rate in comparison with the alloy at temperatures below 800 °C during the interrupted oxidation. The oxidation kinetics follows parabolic relations at 700 and 800 °C during the isothermal oxidation. A layered structure of Al₂O₃/TiAl₃/TiAl₂/TiAl/alloy from the outside to the inside forms after oxidation at 700 °C without changing the main body of the coating.     

Investigation of the age hardening behaviour of 6063 aluminium alloys refined with Ti,RE and B

The purpose of this study is to obtain a better understanding on the effect of ageing on the hardness of 6063 aluminium alloys refined with Ti, cerium-rich mixtures of rare earth (RE) and B with the aids of SEM, EDS, TEM, DSC, etc. The following conclusions have been obtained: the 6063 alloy with the joint additions of Ti, B and RE (10 w(Ti)/w(B) mass ratio) has the finest grain, compared to alloys with Ti or Ti and RE additions. Artificial ageing must be performed more than 7 d after extrusion. The 6063 alloys refined with Ti, Ti + RE or Ti + RE + B all have better ageing behaviour. It takes shorter ageing time to obtain a hardness near peak ageing hardness and there is no obvious decrease of ageing hardness of the alloys until ageing for 6 h at 200 °C. The addition of RE forms Al–Si–Mg–RE intermetallic constituents, resulting in lower strength of alloys than that with Ti addition.     

Creep- and coarsening properties of Al–0.06 at.% Sc–0.06 at.% Ti at 300–450 °C

Upon aging at 300–450 °C, nanosize, coherent Al₃(Sc_1−xTi_x) precipitates are formed in pure aluminum micro-alloyed with 0.06 at.% Sc and 0.06 at.% Ti. The outstanding coarsening resistance of these precipitates at these elevated temperatures (61–77% of the melting temperature of aluminum) is explained by the significantly smaller diffusivity of Ti in Al when compared to that of Sc in Al. Furthermore, this coarse-grained alloy exhibits good compressive creep resistance for a castable, heat-treatable aluminum alloy: the creep threshold stress varies from 17 MPa at 300 °C to 7 MPa at 425 °C, as expected if the climb bypass by dislocations of the mismatching precipitates is hindered by their elastic stress fields.