The effect of thermal ageing on microstructure and mechanical properties of Al–Si–Fe/Mg alloys

In this paper the microstructure and ageing behavior of Al–Si–Fe/Mg alloys produced through sand-casting route is presented. 4.0–6.0 wt% Mg was added to Al–Si–Fe alloy. Standard mechanical properties test samples were prepared from the sand cast 25 mm diameter by 45 mm rods. Thermal ageing was done for 6 h at 200 °C. The ageing characteristics of these alloys were evaluated using tensile properties, hardness values, impact energy and microstructure as criteria. The thermal aged samples exhibited higher yield strength, tensile strength and hardness values as the weight percent of magnesium increased up to 5 wt% in the Al–Si–Fe/Mg alloys as compared to as-cast samples. The optimum values were obtained at 5 wt%Mg. Lower percent elongation, reduction in area and impact energy values were obtained for age-hardened Al–Si–Fe/Mg alloy samples as compared to as-cast samples. The increases in hardness values and strength during ageing are attributed to the formation of coherent and uniform precipitation in the metal lattice. It was found that the age-hardened showed acceleration in ageing compared to the as-cast alloy. However, the 5 wt%Mg addition to the alloy showed more acceleration to thermal ageing treatment. These results show that better mechanical properties are achievable by subjecting the as-cast Al–Si–Fe/Mg alloys to thermal ageing treatment.     

Solidification paths of multicomponent monotectic aluminum alloys

Solidification paths of three ternary monotectic alloy systems, Al–Bi–Zn, Al–Sn–Cu and Al–Bi–Cu, are studied using thermodynamic calculations, both for the pertinent phase diagrams and also for specific details concerning the solidification of selected alloy compositions. The coupled composition variation in two different liquids is quantitatively given. Various ternary monotectic four-phase reactions are encountered during solidification, as opposed to the simple binary monotectic, L′ → L′′ + solid. These intricacies are reflected in the solidification microstructures, as demonstrated for these three aluminum alloy systems, selected in view of their distinctive features. This examination of solidification paths and microstructure formation may be relevant for advanced solidification processing of multicomponent monotectic alloys.     

An Experimental Study of the Rapid Continuous Solidification of Al—Bi Immiscible Alloy

采用Al-（3．4—15）％Bi（质量分数）难混溶合金进行了快速连续凝固实验，研究了Bi含量和凝固速率对快速连续凝固组织的影响．结果表明，少量富Bi相粒子尺寸随着Bi含量的增加而增大，提高凝固速率有助于制备少量富Bi相粒子均匀分布的难混溶合金．通过分析富Bi粒子尺寸分布，发现沿试样轴向和径向方向粒子平均直径变化不大，试样表层处粒子尺寸较小．研究表明，快速连续凝固是制备均质难混溶合金的有效途径．     

Microstructure morphologies during the transient solidification of hypomonotectic and monotectic Al-Pb alloys

The solidification cooling rate (T), growth rate (v), temperature gradient (G), interphase spacing (λ) and diameter (d) of the Pb-rich phase have been experimentally determined for a hypomonotectic Al-0.9 wt%Pb and a monotectic Al-1.2 wt%Pb alloys directionally solidified under unsteady-state heat flow conditions. It is shown for both cases that, from the cooled bottom of the casting up to a certain position along the casting length, the microstructure was characterized by well-dispersed Pb-rich droplets in the aluminum-rich matrix, followed by a region of morphological transition (with the Pb-rich phase formed by droplets and fibers) and finally by a mixture of fibers and strings of pearls for positions closer to the top of the casting. It has been also observed that such microstructural transition was anticipated for the alloy with higher solute content. It is shown that the correlation between the morphology of the Pb-rich phase and the growth rate can be synthesized as follows: Al-0.9 wt%Pb alloy, droplets for v > 1.0 mm/s and fibers for v < 0.65 mm/s; Al-1.2 wt%Pb alloy, droplets for v > 1.1 mm/s and fibers for v < 0.87 mm/s. Experimental growth laws relating the interphase spacing to both G and v are proposed.     

磁场对偏晶合金定向凝固组织的影响

横向均匀静态磁场可以有效地抑制定向凝固过程中熔体中的对流;磁场的作用使偏晶系 Ａｌ６ ．５ Ｂｉ 合金纤维组织间距和纤维直径减小,而且组织分布更加均匀。     

Hydrolysis of ball milling Al–Bi–hydride and Al–Bi–salt mixture for hydrogen generation

In this paper, an effective method to produce hydrogen via the hydrolysis of the milled Al–Bi–hydride (or salts) in pure water at room temperature has been found. The result shows that the Al–Bi–hydrides (or salts) prepared by 5 h milling appear very effective to improve their hydrolysis reactivity. And the milled Al–Bi–hydrides (or salts) have high hydrogen yield in pure water, especially the Al–10 wt.% Bi–10 wt.% MgH₂ mixture or Al–10 wt.% Bi–10 wt.% MgCl₂ mixture all can produce 1050 ml/g within 5 min. The improvement mainly comes from three factors: (1) the additives (MgH_2, CaH₂, LiCl, MgCl₂, KCl, etc.) play an important role to decrease the mean size of the mixture particles; (2) the exothermic dissolution of the salt additive such as MgCl₂ can increase the temperature of aqueous solution, favoring the reaction between Al–Bi composite and water; (3) the hydrolysis of the additives can also offer conductive ions on the work of the micro-galvanic cell of Al–Bi composite. Furthermore, the high conductive ions around the Al–Bi composite are the uppermost effect for increasing the hydrogen yield and hydrogen generation rate.     

Al–Ti–C–Sr master alloy—A melt inoculant for simultaneous grain refinement and modification of hypoeutectic Al–Si alloys

Present article is focused on the microstructural features of Al–Ti–C–Sr master alloy, an inoculant for simultaneous grain refinement and modification of hypoeutectic Al–Si alloys. This master alloy is basically a metal matrix composite consisting of TiC and Al₄Sr phases formed in situ in the Al-matrix. TiC particles initiate the refinement of primary α-Al through heterogeneous nucleation in molten hypoeutectic Al–Si alloy, while Al₄Sr phase dissolves in molten Al–7Si alloy enriching the melt with Sr, which eventually leads to modification of eutectic silicon during solidification of the Al–7Si alloy casting. Thus present master alloy serves in both ways, as a grain refiner and a modifier for hypoeutectic Al–Si alloys.     

Formation, properties and microstructure of amorphous/crystalline composite Ag20Cu30Ti50 alloy using miscibility gap

The aim of the work was to produce the amorphous/crystalline composite with uniform distribution of fine crystalline soft phase. Silver–copper–titanium Ag₂₀Cu₃₀Ti₅₀ alloy was prepared using 99.95 wt% Ag, 99.95 wt% Cu, 99.95 wt% Ti that were arc-melted in argon atmosphere. Then the alloy was melt spun on a copper wheel with linear velocity of 33 m/s. Investigation of the microstructure for both arc-melt massive sample and melt-spun ribbons was performed with use of scanning electron microscope (SEM) with EDS, light microscope (LM) and X-ray diffraction. The thermal stability was evaluated by differential scanning calorimetry (DSC). The properties such as Young modulus and Vickers hardness number before and after crystallization of the amorphous matrix were measured with use of nanoindenter. The microstructure was investigated by transmission electron microscope (TEM). It was found, that the alloy has a tendency for separation within the liquid state due to the miscibility gap which resulted in segregation into Ti–Cu–Ag matrix and Ag-base spherical particles after arc-melting. During rapid cooling through the melt spinning the Ag₂₀Cu₃₀Ti₅₀ alloy formed an amorphous/crystalline composite of fcc silver-rich spherical particles within the amorphous Ti–Cu–Ag matrix.     

Microstructure and grain refining performance of a new Al–Ti–C–B master alloy

A kind of Al–Ti–C–B master alloy with a uniform microstructure is prepared using a melt reaction method. It is found that the average grain size of α-Al can be reduced from 3500 to 170 μm by the addition of 0.2 wt.% of the prepared Al–5Ti–0.3C–0.2B and the refining efficiency does not fade obviously within 60 min. It is considered that the TiC_xB_y and TiB_2−mC_n particles found at the grain center are the effective and stable nucleating substrates for α-Al during solidification, which accounts for the good grain refining performance.     

A novel Al–Ti–B alloy for grain refining Al–Si foundry alloys

Al–Ti–B refiners with excess-Ti (Ti:B > 2.2) perform adequately for wrought aluminium alloys but they are not as efficient in the case of foundry alloys. Silicon, which is abundant in the latter, forms silicides with Ti and severely impairs the potency of TiB₂ and Al₃Ti particles. Hence, Al–Ti–B alloys with excess-B (Ti:B < 2.2) and binary Al–B alloys are favored to grain refine hypoeutectic Al–Si alloys. These grain refiners rely on the insoluble (Al,Ti)B₂ or AlB₂ particles for grain refinement, and thus do not enjoy the growth restriction provided by solute Ti. It would be very attractive to produce excess-B Al–Ti–B alloys which additionally contain Al₃Ti particles to maximize their grain refining efficiency for aluminium foundry alloys. A powder metallurgy process was employed to produce an experimental Al–3Ti–3B grain refiner which contains both the insoluble AlB₂ and the soluble Al₃Ti particles. Inoculation of a hypoeutectic Al–Si foundry alloy with this grain refiner has produced a fine equiaxed grain structure across the entire section of the test sample which was more or less retained for holding times up to 15 min.     

Retardation of grain growth in Mg–3Al–1Zn alloy processed by strip-casting method

Grain growth behaviors of the two AZ31 alloy sheets processed by slab- and strip-casting methods were examined and compared. Grain growth rate of the strip-casting processed AZ31 alloy was considerably lower than that of the slab-casting processed AZ31 alloy. The result could be ascribed to the presence of finer Al–Mn compound particles more uniformly and densely distributed in matrix of the strip-casting alloy. Low grain growth rate via effective Zener pinning of the Al–Mn particles notably improved tensile ductility of the AZ31 alloy at elevated temperatures.     

INVESTIGATION OF RAPID DIRECTIONAL SOLIDIFICATION OF AL-BASED IMMISCIBLE ALLOYS Ⅲ. EFFECT OF SOLID/LIQUID INTERFACE

分析了难混溶合金凝固过程中作用在凝固界面前沿液滴上的力,建立了液滴与凝固界面相互作用行为的动力学判据.结合实验,探讨了固/液界面与液滴间相互作用对Al-Bi基难混溶合金凝固组织演变的影响.结果表明,在界面排斥和Marangoni力共同作用下,较小尺寸的富Bi液滴被凝固界面排斥并富积在凝固界面前沿.当凝固界面为枝晶/胞状晶时,临界尺寸的富Bi液滴与界面发生部分捕获,凝固后形成"蝌蚪"型的富Bi粒子;尺寸较小的富Bi液滴被捕获在枝晶/胞状晶间,凝固后富Bi粒子分布于晶界和三叉交角处.     

The effects of lanthanum on microstructure and electrochemical properties of Al–Zn–In based sacrificial anode alloys

In order to improve the non-uniform corrosion of Al–0.5Zn–0.03In–1Mg–0.05Ti alloys, Al–5Zn–0.03In–1Mg–0.05Ti–xLa (x = 0.3, 0.5 and 0.7 wt.%) alloys were developed. Microstructures and electrochemical properties of the alloys were investigated. The results show that the optimal microstructures and electrochemical properties are obtained in Al–5Zn–0.03In–1Mg–0.05Ti–0.5La alloy. The main precipitate phase is Al₂LaZn₂ particles. The excellent electrochemical properties of Al–5Zn–0.03In–1Mg–0.05Ti–0.5La alloy is mainly attributed to fine grains and grain boundaries containing fine Al₂LaZn₂ precipitates. At the same time the fine grains can improve the non-uniform corrosion of Al–0.5Zn–0.03In–1Mg–0.05Ti alloy.     

Ag-Ni电接触材料快速/亚快速凝固过程研究

采用快速/亚快速凝固方法制备了富Ni相粒子弥散分布于Ag基体的Ag-Ni合金。建立了Ag-Ni合金凝固过程中组织演变的动力学模型,模拟计算了Ag-Ni合金凝固组织形成过程,分析讨论了合金成分对Ag-Ni合金凝固组织形成过程的影响。结果表明,合金的Ni含量越高,凝固组织中富Ni相粒子平均尺寸越大;Ag-Ni合金熔体冷却凝固时,富Ni相液滴/粒子的尺寸主要受形核和长大控制,Ostwald粗化作用很弱。     

A high-working-temperature CuAlMnZr shape memory alloy

The martensitic structure in the air-cooled Cu–11.91Al–2.48Mn–0.1Zr (wt%) alloy and its variation upon heating has been studied by X-ray diffraction and TEM. The forward and reverse thermoelastic transformation behavior has been studied by voltage measurement. The shape memory ratio of the alloy aged at 150 °C (in martensite state) for different times up to 100 h, or heated to different temperatures up to 620 °C followed by air cooling, has been measured. The air-cooled state of the alloy has a monoclinic martensitic structure M18R, which closely matches the N18R structure. This structure remains almost unchanged when the alloy is heated to 400 °C. When the alloy is heated to 620 °C, only a small amount of γ₂ phase precipitates and a shape memory ratio of 92% is achieved. When the alloy is aged at 150 °C for 100 h, a shape memory ratio of 97.2% is achieved.     

Effects of stirring parameters on rheocast structure of Al–7.1wt.%Si alloy

In this study, effects of stirring time and stirring speed on the microstructure of semisolid rheocast (SSR) Al–7.1 wt.%Si were examined. The results demonstrated that the non-dendritic structure could be formed by a short stirring period below liquidus temperature and further stirring had little impact on the final morphology of the primary particles. Stirring was shown, however, to affect the average particles size mainly during the initial stages of solidification. Although the average shape factor of primary particles was relatively insensitive to large variations in the stirring speed, higher stirring speeds made the shape and size of the primary particles more uniform. Higher stirring speeds also rendered smaller and more rounded agglomerates of primary particles. The results of two stability models employed suggest that, in general, the primary particles generated in the initial stages of solidification can attain growth stability before pouring and maintain this stability during the secondary cooling stage.     

STRUCTUREAND PROPERTY OF DIRECTIONALLY SOLIDIFIED EUTECTIC Bi/MnBi ALLOY

Eutectic Bi/MnBi magnetic alloy was prepared by directional solidification.When thegrowth rate,R>2 cm/h,the MnBi fibre spacing,λ,distributed homogeneously inBi matrix,follows λ~2R=constant.The thermal gradient.G_L,does not influenceλ.The magnetic property,B_r of Bi/MnBi alloy decreases with the increase of R andincreases with the increase of G_L.     

Effect of manganese on the microstructure of Mg–3Al alloy

The effect of manganese on the microstructure of Mg–3Al alloy, especially the nucleation efficiency of Al–Mn particles on primary Mg, has been investigated in this paper. Mg–0.72Mn was used to fabricate Mg–3Al–xMn (x = 0, 0.1, 0.2, 0.3, 0.4 and 0.5) alloys, and the grain sizes of these alloys fluctuate at 390 μm indicating addition of manganese does not evidently influence the grain size of Mg–3Al alloy. Through XRD, FESEM and TEM detection, it is found that Al_0.89Mn_1.11 compound is the dominant Al–Mn phase in Mg–3Al–0.3Mn, Mg–3Al–0.4Mn and Mg–3Al–0.5Mn, and distributes in primary Mg matrix and interdendritic regions with an angular blocky morphology. The number of Al_0.89Mn_1.11 increases gradually with increasing manganese content while the grain sizes of primary Mg are nearly the same in Mg–3Al, Mg–3Al–0.3Mn, Mg–3Al–0.4Mn and Mg–3Al–0.5Mn, indicating Al_0.89Mn_1.11 has low nucleation efficiency on primary Mg.     

Crystallography and phase transformation mechanisms in TiAl-based alloys – A synthesis

The variation in cooling rates of Ti–46.8Al–1.7Cr–1.8Nb (at. %) alloy from the high-temperature α domain produces lamellar, Widmanstätten, feathery-like (α₂ + γ) structures, as well as γ-massive phase, often coexisting together. Earlier reported crystallographic and morphological details of each of these structures are compiled together and a combined view on the generation of their crystallographic related possible variants and their solid phase transformation mechanisms is proposed. Sympathetic nucleation is suggested as a common mechanism for the lamellar structure at slow cooling rate, the Widmanstätten structure and the feathery-like structure.     

In situ XRD studies on Al–Ni and Al–Ni–Sr alloys prepared by rapid solidification

In this paper the structure and stability of Al–17 wt.%Ni(Al–17Ni) and Al–17 wt.%Ni–2 wt.%Sr alloys prepared by rapid solidification was investigated by means of XRD techniques. Our work demonstrates that both alloys are crystalline and composed of fcc (Al–Ni) solid solution and orthorhombic Al₃Ni phases. The ternary alloy shows in addition the presence of small amount of tetragonal Al₄Sr phase. In situ XRD experiment demonstrates the stability of the solute solution up to 650 °C, Al₃Ni above 750 °C while Al₄Sr overcomes melting of the major phases at 800 °C. High-temperature structure analysis proved strong bindings between Al and Ni atoms in Al₃Ni phase, corroborating its covalent nature, linear and faster increase of the fcc volume with annealing temperature. The linear correlation between constituting atoms decreases with increase of the temperature.The work also documents the applicability of pair distribution function (PDF) analysis to the study of multiphase crystalline systems.