Nanocomposites of Aluminum Alloys by Rapid Solidification Processing

Aluminium alloys reinforced with transition metal aluminide (Al₃Ti, Al₃Fe, Al₃Ni, etc.) particles possess high specific strength both at ambient and elevated temperature. The improved strength of these alloys are the results of slower coarsening rate of the intermetallic particles due to low diffusivity of the transition metals in aluminium. However, the strength can be enhanced further by refining the microstructure of the alloys to nanometer range. The authors have successfully attempted two important non-equilibrium processing techniques i.e. rapid solidification processing (RSP) and mechanical alloying for the refinement of the microstructure in various aluminium alloys. In this report, authors present a short review of their work on RSP of Al?CTi and Al?CFe alloys to produce nanocomposites.     

快速凝固技术在铝基合金材料中的应用

介绍了快速凝固技术的特点以及快凝铝基合金材料的发展现状,综述了快速凝固技术在铝基合金材料中的应用以及铝基晶态合金、铝基准晶合金、铝基非晶合金的发展现状。最后指出,快速凝固技术需要进一步完善工艺,降低成本,实现量产;需要优化合金成分,加强多元系合金快凝过程的理论研究以及计算机模拟在铝基合金材料快凝过程中的模拟应用。     

Nb-Based Nb-Al-Fe Alloys: Solidification Behavior and High-Temperature Phase Equilibria

High-melting Nb-based alloys hold significant promise for the development of novel high-temperature materials for structural applications. In order to understand the effect of alloying elements Al and Fe, the Nb-rich part of the ternary Nb-Al-Fe system was investigated. A series of Nb-rich ternary alloys were synthesized from high-purity Nb, Al, and Fe metals by arc melting. Solidification paths were identified and the liquidus surface of the Nb corner of the ternary system was established by analysis of the as-melted microstructures and thermal analysis. Complementary analysis of heat-treated samples yielded isothermal sections at 1723 K and 1873 K (1450 °C and 1600 °C).     

快速冷凝(RS)Al-Li合金的韧化途径

通过对快速冷凝(RS)A1—Li合金低塑韧性原因的分析，介绍了A1—Li合金制备、成材、热处理等过程中一些主要韧化途径，并简单讨论了获得较好综合性能的A1—Li合金制品的工艺方法及大致工艺参数范围。     

Nonequilibrium Solidification Behavior of Co-Si Alloys Near the First Eutectic Point

Adopting a fluxing purification and cyclic superheating technique, Co-10 wt pct Si and Co-15 wt pct Si alloys had been undercooled to realize rapid solidification in this work. It was investigated that the solidification modes and microstructures of Co-Si alloys were deeply influenced by the undercooling of the melts. Both alloys solidified with a near-equilibrium mode in a low undercooling range; the peritectic reaction occurred between the primary phase and the remnant liquids, and it was followed by the eutectic reaction and eutectoid transformation. With the increase of undercooling, both alloys solidified with a nonequilibrium mode, and the peritectic reaction was restrained. As was analyzed, a metastable Co₃Si phase was found in Co-10 wt pct Si alloy when a critical undercooling was achieved.     

The Effect of Rapid Solidification Velocity on the Microstructure of Ag-Cu Alloys

Electron beam solidification passes have been performed on a series of Ag-Cu alloys between 1 wt pct Cu and the eutectic composition (28.1 wt pct Cu) at speeds between 1.5 and 400 cm per second. At low growth rates conventional dendritic or eutectic structures are obtained. The maximum growth rate of eutectic structure is 2.5 cm per second. At high growth rates microsegregation-free single phase structures are obtained for all compositions. The velocity required to produce this structure increases with composition for dilute alloys and agrees with the theory of absolute stability of a planar liquid-solid interface with equilibrium partitioning. For alloys between 15 and 28 wt pct Cu, the velocity required to produce the microsegregation-free extended solid solution decreases with composition and is related to nonequilibrium trapping of solute at the liquid solid interface. At intermediate growth rates for alloys with 9 wt pct Cu or greater, a structure consisting of alternating bands of cellular and cell-free material is obtained. The bands form approximately parallel to the local interface. On leave at the Center for Materials Research, The Johns Hopkins University, Baltimore, MD 21218.     

快速凝固粉末铝合金的研究现状和发展趋势

本文综述了快速凝固粉末铝合金的研究现状,讨论了合金经快速凝固而产生的晶志组织结构特点。并对研究中的一些问题作了评述。从合金设计的角度探讨了末来快速凝固粉末铝合金的研究方向及发展趋势。     

Solidification Reaction Sequence of Co-Rich Nb-Al-Co Alloys

The freezing reaction sequence of Co-rich Nb-Al-Co ternary alloys with emphasis on the formation of Laves and Heusler phases has been examined. For Co-rich alloys, the solidification reaction sequence is observed as primary freezing of α-Co and CoAl phases, subsequent [Co + C36] and [CoAl + C36] eutectics, and the final ternary eutectic reaction [L → α-Co + C36 + CoAl]. The compositions of solidified α-Co and C36 phases agree with the corresponding vertices of the tie-triangle at the solidus temperatures. When the Nb concentration is over 20 at. pct in Co-rich alloys, the quasi-peritectic reaction [L + Co₂AlNb → C36 + CoAl] does not occur as equilibrium prediction. The formation of C36 and CoAl phases occurs through solid precipitation and must be distinguished from a solidification reaction.     

Rapid Solidification Microstructure and Carbide Precipitation Behavior in Electron Beam Melted High-Speed Steel

Metallurgical and Materials Transactions A - The solidified microstructure and carbide precipitation behavior in an S390 high-speed steel processed by electron beam melting (EBM) have been fully...     

快速凝固过程分析

从快速凝固的基本原理着手,详细总结了快速凝固技术的冷却方式判定方法,从物质的导热原理出发讨论了单辊熔体急冷法、双辊熔体急冷法的冷却方式,分别概括了它们的界面导热系数的计算,分析了冷却速度的计算方法。对快速凝固的过程分析具有参考价值。     

Microstructure,Mechanical Properties,and Crack Propagation Behavior in High-Nb TiAl Alloys by Directional Solidification

Titanium aluminide (Ti-47Al-6Nb-0.1C) alloys were prepared using a cold crucible directional solidification technique with an input power range of 35 to 55 kW under a withdrawing velocity of 0.4 mm/min. The macro/microstructure was characterized, and the mechanical properties were evaluated. The results show that the directional solidification (DS) ingots exhibit β-solidification characteristics at an input power range of 35 to 55 kW, and a well-developed DS microstructure was acquired at an input power range of 40 and 45 kW. With the increasing input power, the lamellar spacing decreases, resulting in the increasing tendency in the average room-temperature yield strength. The steady-state creep rate strongly depends on the lamellar spacing, and the creep life depends on the DS microstructure. The well-developed DS alloy can significantly improve the creep properties but has little influence on the room-temperature tensile properties. Moreover, after testing the fracture toughness, the crack propagation showed interlamellar cracks, translamellar cracks, and intercolony boundary cracks that primarily propagated along the colony boundary after creep testing.

     

Simulation of Microsegregation in Multicomponent Alloys During Solidification

A phase‐field model is applied to the simulation of microsegregation and microstructure formation during the solidification of multicomponent alloys. The results of the one‐dimensional numerical simulations show good agreement with those from the Clyne–Kurz equation. Phase‐field simulations of non‐isothermal dendrite growth are examined. Two‐dimensional computation results exhibit different dendrites in multicomponent alloys for different solute concentrations. Changes in carbon concentration appear to affect dendrite morphology. This is due to a larger concentration and a lower equilibrium partition coefficient for carbon. On the other hand, changes in phosphorus concentration affect the dendrites and interface velocity in multicomponent alloys during solidification when phosphorus content is increased from 10^?3 mol% P. With additional manganese, the solidification kinetics slow down; dendrite morphology, however, is not affected. The potential of the phase‐field model for applications pertaining to solidification has been demonstrated through the simulations herein.     

Rapid Solidification of Sm-Co Permanent Magnets

In this article, we report the microstructural evolution of rapidly solidified Sm-Co alloys from 4 to 16 at. pct Sm. We have observed a wide variety of phase formation and microstructures, ranging from primary Co dendrite formation to eutectic structures to the formation of the metastable SmCo₇ compound. In particular, we observed nonequilibrium formation of Co along with SmCo₇, whose presence caused a decrease in coercivity from ∼10 kOe to 500 Oe. Alloying elements reduced the scale of the microstructure, effectively offsetting the detrimental effects of the Co phase formation and leading to a recovery of the coercivity. The eutectic structure with Co rods surrounded by SmCo₇ provides a natural path to nanoscale hard/soft magnetic nanocomposites, where control of scale and phase content is critical. This article is based on a presentation made in the symposium entitled “Phase Transformations in Magnetic Materials”, which occurred during the TMS Annual Meeting, March 12–16, 2006, in San Antonio, Texas, under the auspices of the Joint TMS/MPMD and ASMI-MSCTS Phase Transformations Committee.

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板坯凝固过程夹杂物运动行为

 本文使用离散相模型,利用数值模拟的方法对结晶器中的钢液流动、传热、凝固以及夹杂物的运动进行了耦合计算。通过追踪夹杂物的运动轨迹,并在钢渣界面处对夹杂物进行采样分析,最终计算出夹杂物在结晶器中的上浮率。研究表明,夹杂物在结晶器中的上浮率与其尺寸及拉速的大小均有关系,但受夹杂物密度的影响很小。夹杂物越大、拉速越小,越有利于夹杂物上浮至自由液面。小颗粒夹杂在结晶器中并不能被有效去除。对于粒径为50μm的夹杂物,当拉速为1m/min时其上浮率仅为46%,有37%的夹杂物被凝固坯壳捕捉,主要分布在铸坯表皮下10~25mm处。夹杂物被宽面坯壳捕捉的位置多集中在宽面靠近窄面处,在水口下方被捕捉的夹杂物较少。以往的研究认为只要夹杂物上浮至钢渣界面就能够被保护渣吸收,J.Strandh等的研究表明,夹杂物能否被吸收还取决于保护渣的粘度和润湿性等因素。因此,对于粒径较小的夹杂物,必须在精炼后的软吹氩过程中适当增大钢液的静置时间,尽量减少钢液中小颗粒夹杂的数量。另外,结晶器保护渣的选用对钢液中夹杂物的去除也很重要,不仅要满足其对钢液的保温润滑作用,还要考虑其对夹杂物吸附的影响。     

Solidification of Aluminium Alloys Under Ultrasonication: An Overview

An overview of our investigations on solidification microstructure formation under ultrasonication in various Al alloys and comparison against unrefined or chemically modified microstructures under identical cooling conditions is presented. Primary α-Al grains show significant refinement under ultrasonication, even better than established chemical inoculation, in the small ingots investigated. Increased solute content appears to promote grain refining efficiency under ultrasonication. Regular lamellar eutectic in Al–33 wt%Cu was observed to degenerate into rounded particle morphology and the irregular eutectic of long Si plates in Al–11 wt%Si were spheroidised into compact form near the ultrasound radiator. Grain refinement under ultrasonication appears to originate from enhanced heterogeneous nucleation under cavitation showing distinct reduction in nucleation undercooling. Eutectic modification, on the other hand, appears to originate from coarsening as the strong fluid flow created under cavitation disturbs the thin diffusion boundary layer ahead of the eutectic growth front.     

High Strain-Rate Behavior and Transformation-Induced Plasticity of a High-Strength FeCrMoVWC Alloy Manufactured by Rapid Solidification Technique

By employing the rapid solidification technique and special manufacturing conditions, e.g., using only pure elements, an alloy was produced that exhibits major differences to conventionally produced high-speed steels. Within this study, the strain rate dependent material behavior of the examined alloy was characterized under compressive loading for a strain rate range from 10^?3?seconds^?1 to 10³?seconds^?1. The aim was to understand the inherent mechanisms when low and high strain rates are applied. Initially, microstructural observations of the base material, which revealed ????-martensite, retained austenite, and complex carbides, were conducted. The material exhibits extraordinarily high ultimate compression strength of up to 4800?MPa, a high work-hardening behavior, and a good deformability of 15?pct. Moderate strain rate sensitivity was detected. Furthermore, a strain-induced transformation (transformation induced plasticity [TRIP]-effect) from retained austenite to ????-martensite occurred. Interrupted compression tests at different strains and strain rates were carried out to understand the microstructural evolution. The examinations showed that adiabatic heating decreases the transformation rate of retained austenite to ????-martensite and counteracts the work-hardening behavior. For higher strain rates higher ????-martensite contents in the initial deformation region as well as a pronounced saturation behavior of ????-martensite was detected. A hypothesis is given for the strong work-hardening behavior of the alloy.