Papers by Keyword: Rapid Solidification

Abstract: In the thermal spraying process, spray material is heated, melted, and accelerated by a high temperature flame. Thermal spraying can produce thick materials that rapidly solidify, because the alloy droplets accumulate successively on the substrate and solidify at a cooling rate in the range of 105-108Ks-1. Depending on the cooling conditions of the substrate and on the alloy composition, deposits are produced with metastable phases or extremely fine crystalline phases. Thermal spraying is an attractive method for the production of composite deposits with fine particles formed in-situ. In particular, iron based alloy with vanadium carbide, is useful in metal molds and also in pump parts due to its high wear resistance and high corrosion resistance. In the present work, low-pressure plasma spraying of Fe-C-V/Ni-Mg and Fe-C-V-Cr-Ni/Ni-Mg blend powders were iron based composite deposits with finely dispersed vanadium carbide particles. The as-sprayed deposit produced from Fe-C-V/Ni-Mg blend powder is composed of αFe and V8C7. The as-sprayed deposit produced from Fe-C-V-Cr-Ni/Ni-Mg blend powder is made up of γFe, αFe, V8C7 and Cr7C3. The fine precipitates of approximately 0.3μm in the as-sprayed deposit are carbide. With increasing the heat-treatment temperature up to 1273K, the carbide particles coarsen. The hardness of as-sprayed deposit produced from the Fe-C-V-Cr-Ni/Ni-Mg, which has many fine carbide precipitates, is the hardest of the deposits.

Abstract: The paper outlines some research work that has been conducted in Harbin Institute of Technology, on the fabrication of TiAl alloys. The review is presented with special emphasis on some different manufacturing routes of TiAl alloys, including investment casting, canned forging and sheet rolling, mechanical milling and rapid solidification. Investment casting has been developed to manufacture near-net shape TiAl blades. Also included are current development of canned forging and rolling for TiAl sheets. Then, TiAl nanocrystalline powders gained through low temperature and two steps mechanical milling were researched. And considerably refined TiAl alloys with Y additions were produced by rapid solidification and the microstructure evolution with Y addition was studied. Details of the processing route and microstructure related to different processing method will be presented.

Abstract: A high Zn content Al-Zn-Mg-Cu alloy was prepared by spray forming process and the precipitate behavior and microstructure of the extruded alloy were also investigated. The precipitate sequence of the spray-formed alloy could be described as “α-solid solution → GPI zone → GPII zone (also called Metastable ′ )→ Stable  (MgZn2)” during artificial ageing treatment. In the early stage of artificial ageing treatment, the GPI zone was the main strengthening phase and kept coherent relationship with the matrix. With the increasing of ageing time, ′ phase dominate strengthening phase and kept semi-coherent relationship with the matrix. With the further increasing of ageing time,  phase took the place of ′ phase, and dominated the strengthening phase in the alloy. The grain size of the spray deposit is finer than that of cast alloys. The ultimate tensile strength of the alloy is over 810MPa in peak ageing condition.

Abstract: Hydrogen (H) behaviour in materials was investigated in rapidly solidified (RS) foils of pure aluminium (Al), Al-0.4 Cr and Al-0.25 Zr alloys (at %) by means of thermal desorption spectroscopy (TDS). In addition, Al-0.25; 0.3 Zr alloys were examined with respect to microstructure and its instability during the thermal process using SEM and microhardness measurements. The effect of dopes and heating rate on H desorption was summarized. The lowest energy desorption is attributed with significant thermal desorption peak which temperature was found is correlated with sample composition.

Abstract: With an aim of clarifying the strength of rapidly solidified P/M materials strengthened by solid solution of Mg and dispersion of transition metal compounds at elevated temperature, Al-2mass%Mn, Al-4mass%Mn and Al-6mass%Mn alloys with varied Mg additions of 0, 1 and 3 mass% were prepared by rapid solidification techniques. Rapidly solidified (RS) flakes were produced by remelting alloy ingots in a graphite crucible, atomizing the alloy melt and subsequent splat-quenching on a rotating water-cooled copper roll under argon atmosphere.　The RS flakes were consolidated to the P/M materials by hot extrusion after vacuum degassing. Cast ingots of these alloys were also hot-extruded under the same conditions to the I/M as reference materials. Metallographic structures and constituent phases were studied for the P/M and I/M materials by optical microscope and X-ray diffraction. Mechanical properties of as-extruded and annealed P/M materials and as-extruded I/M materials were examined by tensile test at room and elevated temperatures under various strain rates. Uniform dispersion of fine intermetallic compounds (Al6Mn) was observed in all the as-extruded P/M materials. Added Mg was present as the solute in I/M and P/M materials alloy even after annealing. The P/M materials containing Mg exhibited higher hardness and strength at room temperature, than those without Mg. It was considered that both solid solution of Mg and dispersion of intermetallic compounds were contributing the hardness and strength increase in the rapidly solidified Al-Mn-Mg alloys. Tensile strength increases with increasing amount of Mg in I/M materials at all testing temperatures. However, strength of as-extruded P/M materials decreases with addition of Mg at 573K and 673K. Thus the positive effects of Mg additions on tensile strength of as-extruded P/M materials disappeared at higher testing temperature. Tensile strength of annealed P/M materials in which dislocation density decreased and compound particle coarsened increased with addition of Mg at elevated temperatures.

Abstract: Microstructure evolution and mechanical properties of 7A09 aluminum alloy ribbon prepared by rapid cooling solidification are studied. Single roller is applied to produce rapid solidification ribbon of 7A09 aluminum alloys. Microstructure characteristics and mechanical properties of the rapid solidified ribbon of 7A09 aluminum alloys are studied by means of X-ray diffraction (XRD), scan electron microscopy (SEM) and hardness measurement. The results show that the rotate speed of the roller is the key factor affecting the ability of the forming ribbon. At a roller rotate speed of 1500rpm, the ribbon with a good quality can be obtained. Microstructure features of the rapid solidification ribbon are refined with the increase of the cooling rate, all of the crystals translate into nanocrystalline. All the hardness of the rapid solidification ribbon of 7A09 aluminum alloys is higher than that of the original alloys and increases with the rotate speed of the rotor.

Abstract: In the present work, rapidly solidified hypereutectic Al-Si-Cu-Mg alloys strips was prepared by single roller melt-spinning method. The microstructures, phase and morphology characteristics of the resultant strips were characterized by means of SEM, TEM and XRD technique. The results show that the grains are refined after rapid solidification processing, and the micro-nanocrystals are formed. Compared with equilibrium solidification, the microstructures are changed obviously. The nucleation and growth of primary silicon are suppressed and primary silicon can not deposited, meanwhile, α-Al phase is nucleated which prior to eutectic. Therefore, the microstructures become into the metastable state. The microstructures of the strips are composed of primary micro-nanostructure α phase and feather-needles-like (α+Si) eutectic which set in the α phase. The mechanism of the formation for microstructures of melt-spinning Al–Si alloy have also been discussed.

Abstract: The microstructure of FeAl40 intermetallic alloy produced by spray atomization and deposition, with boron additions and reinforced with Al2O3 particles was studied. This technique allowed the co-deposition of particulate reinforcement and the addition of boron in order to obtain a boron concentration of 0.4 at. %. The additions of alumina particles produce a grain refinement. High resolution transmission electron microscopy observations shows a precipitation of boron in the FeAl matrix in despite of the rapid solidification process. Fe3B precipitates were found which is a metastable phase formed during the rapid solidification process. In addition, FeB precipitates were observed.

Abstract: A series of multielementary AB-type shape memory alloys, quaternary (Ti,Hf)50(Ni,Cu)50 and quinary (Ti,Zr,Hf)50(Ni,Cu)50, have been produced by means of the melt-spinning (ribbons), twin-roll casting (strips) and injection casting (bars) techniques. The glass forming ability (GFA) has been investigated by means of calorimetric studies (DSC, DTA), microscopic observations (TEM/SAD, SEM, XRD, optical microscopy) and deformation tests (bending, tension). The glass transition (Tg) and crystallization (Tx) temperatures with the corresponding supercooled liquid window ΔT = Tx – Tg have been determined for the mentioned rapid solidification techniques. GFA is found to be strongly dependent on specific ratio between the constituent alloying elements, with effective element A containing mixture of the early transition elements A = (Ti,Zr,Hf) and effective element B containing mixture of the late transition elements B = (Ni,Cu), respectively. We report a choice of the alloy composition, which is amorphous after fast cooling while recovers a reversible B2 ↔ B19 ↔ B19' martensitic transformation sequence and demonstrates shape memory and superelastic properties (up to MS ~ 400 K, AF ~ 450 K in the bulk material versus MS ~ 250 K, AF ~ 350 K in the ribbon and strip states) after an adequate thermal treatment.

Abstract: Microstructural evolution of the spray atomized and powder thixoformed hyper-eutectic A390 aluminum alloy was investigated. The spray atomized powder revealed homogeneous and very fine silicon particles distribution, due to the rapid solidification of the alloy. The semi-solid powders were extruded into a closed die cavity through a hole for the plastic deformation of the powder particles. A drop forge of 45kg weight at different heights was used in this investigation. Remarkable rearrangement and growth of the silicon rich phase was revealed in the final stage.