Papers by Keyword: Rapid Solidification

Abstract: High-silicon aluminum alloys as light-weight structural materials are attractive because of their high wear resistance, low density and low thermal expansion. However, the Si phase in the high-silicon aluminum alloys always has big size, which deteriorates the mechanical properties and machinability of alloys. In this paper, alloy ribbon of high-silicon aluminum alloy with the composition of Al₇₀Si₃₀ was prepared by melt-spinning method. Then alloy ribbon was broken to powder by ball milling. The alloy powers were consolidated by spark plasma sintering (SPS) at different sintering parameters including time and temperature under vacuum atmosphere. The density, hardness and compression property of the compacts were investigated. The sintered samples were characterized by scanning electron microscopy (SEM) and X-ray diffractometry (XRD), and compared with their alloy power and original alloy ribbons. The results showed that the density of Al₇₀Si₃₀ alloy, prepared under the condition of 320 °C and 500 MPa, was above 98%, the grain size was refined to sub-micron, and the wear resistance was good, micro-hardness was 311.70 Hv. It is believed that the existence of local high temperature and discharge plasma can break down and disrupt oxidation film on powder surface, and therefore improving the efficiency of sintering. In the case of large current, short time and low temperature, sintering grain growth was not obvious, there was less supersaturated solid solution precipitation and the rapid solidification microstructure , and the properties can be preferably reserved.

Abstract: Rapid solidification of Co-Si alloys was investigated by using vacuum suction casting in this study. Different microstructures and intermediate phases were obtained. Eutectic εCo phase and eutectoid εCo + αCo₂Si structures were obtained in the first eutectic Co_76.9Si_23.1 alloy. The microstructures of hypereutectic Co₇₀Si₃₀ alloys were composed of primary αCo₂Si phase and interdendritic lamellar eutectoid εCo + αCo₂Si. While for hypoeutectic Co₆₃Si₃₇ alloy at the second eutectic point, CoSi dendrites were the primary phase, and αCo₂Si+CoSi eutectoid structures can be seen at the interdendritic region. Especially, a metastable CoSi₂ phase was found in Co₆₃Si₃₇ alloy. This indicates that eutectoid decomposition βCo₂Si→ CoSi+αCo₂Si is restrained in eutectic Co_60.3Si_39.7 alloy. For rapid solidified Co₅₅Si₄₅ and Co₅₂Si₄₈ alloys, αCo₂Si+CoSi eutectoid structures were not observed, while metastable CoSi₂ were obtained. The higher hardness achieved in Co-Si alloys at the second eutectic point, for the reason of the higher volume fractions of compound phases.

Abstract: The NiAl/Cr (Mo,Dy) hypoeutectic alloy was fabricated by rapid solidification. The microstructure and mechanical properties as well as tribological properties for the alloy at different temperatures were investigated. The results revealed that the rapidly solidified NiAl/Cr (Mo,Dy) hypoeutectic alloy was composed of primary NiAl, fine NiAl/Cr (Mo) eutectic lamella, Ni₅Dy phase and Cr₇Ni₃ precipitate. The compression test showed that the rapid solidification improved the mechanical properties of the NiAl/Cr (Mo,Dy) hypoeutectic alloy obviously. The dry sliding test results showed that alloy had excellent tribological properties at about 1073 K, which obtained wear rate of 4.9 10^-14m³/m·N and friction coefficient of 0.16 μ. The excellent tribological properties at high temperature may be attributed to the continuous and intact protecting lubricant film which was composed of amorphous, Cr₂O₃ and Al₂O₃ nanoparticles. Between 700 K to 900 K, the alloy demonstrated bad tribological properties, especially the high wear rate, which may be ascribed to the softening of NiAl and Cr (Mo) phases.

Abstract: Development of liquid-phase separated bulk metallic glasses is retarded due to difficulties in finding of immiscible systems with high glass-forming ability (GFA) of coexistent liquids. Zr-Ce alloy is a typical liquid immiscible system characterized by a liquid miscibility gap. We added Co and Cu into the Zr-Ce immiscible system and optimized the composition of the designed Zr-Ce-Co-Cu immiscible alloys. The solidification experiments were carried out for the quaternary alloys. The result indicates that the melt separated into ZrCo-rich and CeCu-rich liquids upon cooling through the miscibility gap. By optimizing the relative atomic ratio of Co:Cu, the coexistent ZrCo-rich and CeCu-rich liquids automatically assembled eutectic compositions during the liquid-liquid phase separation (LLPS). Under the condition of fast quenching, the two liquids subsequently undergo liquid-to-glass transition, resulting in the formation of composite structure with two glasses in the samples. We successfully developed phased-separated metallic glasses based on the Zr-Ce-Co-Cu immiscible alloys. This work not only strengthens the understanding in the LLPS but also provides a new strategy on the design of the dual glassy composites.

Abstract: Rapid solidification (RS) combined with plastic consolidation by hot extrusion was used to produce Al alloys with additions of varied concentration of Mn. RS flakes were manufactured using an inert gas atomizing of the molten alloy and the spray deposition on the water-cooled cooper roll. Rods of 7mm in diameter were received using cold pressing of RS-flakes, vacuum degassing and hot extrusion procedures. Mechanical properties of as extruded materials were tested in hot compression at temperature range 293K - 773K. It was found that the flow stress was reduced monotonically with deformation temperature for all tested materials. RS alloys exhibit higher mechanical properties than those produced by conventional metallurgy methods. Higher mechanical properties of RS materials are ascribed to beneficial particles morphology obtained due to the rapid solidification. Development of fine Al₆Mn particles was observed in all tested RS-materials.

Abstract: This paper describes direct cladding of magnesium (Mg) and aluminum (Al) alloys using a tandem horizontal twin roll caster that has three pairs of upper and lower rolls. Manufacturing conditions that are appropriate for fabricating Al/Mg and Al/Mg/Al cladded material were investigated. The surface condition of the cladded cast strip was examined. An electron probe micro analyzer was used to observe the interface between Al alloy and Mg alloy. The thickness of the mixed layer of Al and Mg alloy was 15μm, and how the materials were connected was clarified. Microscopic observation and backscattered electron analysis were used to investigate the cladding mechanisms of the Al and Mg alloy layers. Average hardness was determined using the Vickers hardness test at the Al layer and at the diffused layer between Mg and Al alloys. Cladding of Al/Mg alloy and A/Mg/Al alloy was possible using a tandem twin-roll caster. In addition, Al₃Mg₂ and Al₁₂Mg₁₇ phase precipitation at the interface of the Al and Mg alloys was confirmed during direct cladding from molten metals.

Abstract: Series of experiments on a series of Al-Fe-Mg alloys were performed to determine the effect of rapid solidification (RS) on the material strengthening, which result from the refining of thegrain size and intermetallic compound. Additionally, an enhancement of the material strengthening due to magnesium addition was also observed. Manufacture of RS Al-Fe-Mg alloys combined a spraydeposition of the molten alloy on the rotating water-cooled copper roll and plastic consolidation bymeans of powders pressing and hot extrusion methods. The results suggest that the rapid solidification provides an effective method of microstructure refinement and, in combination with solid solutionhardening due to Mg, leads to significant improvement of mechanical properties of Al-Fe-Mg based alloys.

Abstract: Ultra-fine grained metallic materials are characterized by higher mechanical properties comparing with their conventional equivalents. However increase in strength under static load is not always accompanied by improved fatigue behaviour. Previous investigations on submicrocrystalline RS442 aluminium alloy produced by plastic consolidation of rapidly solidified flakes in the extrusion process revealed increase in its high cycle fatigue bending strength caused by annealing at 450°C. The aim of present studies was to evaluate the influence of heat treatment – also precipitation hardening – on static mechanical properties (hardness, tensile and yield strength) and fatigue strength of the alloy determined in high cycle stress controlled bending tests. Correlation between microstructure, static mechanical properties and fatigue behaviour was analyzed too.

Abstract: Experiments on Al-1Fe-1Ni-5Mg alloy were performed to determine the effect of rapid solidification (RS) on the material strengthening, which result from the refining of the grain size and intermetallic compounds. Additionally, an enhancement of the material strengthening due to magnesium addition was also observed. RS procedure was performed using spray deposition of the molten alloy on the rotating water-cooled copper roll. As a result, highly refined structure of rapidly solidified flakes was obtained. Using common powder metallurgy (PM) techniques, i.e. cold pressing, vacuum degassing and hot extrusion, as received RS-flakes were consolidated to the bulk PM materials. For comparison purposes, the conventionally cast and hot extruded Al-1Fe-1Ni-5Mg alloy was studied as well. RS process combined with hot pressing and extrusion procedure was found to be very effective method for the manufacture of fine grained material and effective refinement of intermetallic compounds. However some inhomogenity of particles distribution was observed, which was ascribed to varied cooling rate dependent on the particular spray-drop size. Mechanical properties of as-extruded material were examined using compression test at 293K – 873K. High strength and ductility of as-extruded RS material with respect to conventionally produced alloy were observed. However, the effect of enhanced mechanical properties of RS material is observed only at low deformation temperatures. It was found that increasing deformation temperature above 400K results in negligible hardening of RS samples if compared to conventionally produced material.

Abstract: Since the growth velocity can be comparable with or even larger than the solute diffusion velocity in the bulk phases, modeling of rapid solidification with non-equilibrium solute diffusion becomes quite an important topic. In this paper, an effective mobility approach was proposed to derive the current phase field model (PFM). In contrast with the previous PFMs that were derived by the so-called kinetic energy approach, diffusionless solidification happens not only in the bulk phases but also inside the interface when the growth velocity is equal to the solute diffusion velocity in liquid. A good agreement between the model predictions and experimental results is obtained for rapid solidification of Si-9at.%As alloy.