Al-Fe-Mg-Mn-Si系合金凝固显微组织及其模型预测

对Al-Fe-Mg-Si和Al-Fe-Mn-Si2个四元系进行热力学优化评估,并对这2个四元系富Al角的零变量平衡反应温度和液相成分进行了计算。采用光学显微镜,扫描电镜和电子探针技术,系统研究了多组元Al基合金Al356.1定向凝固的显微组织及显微偏析。计算模拟了3个多组元Al合金(Al356.1,Al356.2,Al518.2)的平衡凝固和非平衡凝固的显微组织及显微偏析。模型计算结果与实测数据很吻合,证实了所建立的多组元体系热力学及动力学数据库的可靠性。     

急冷对Ti_(0.32)Cr_(0.345)V_(0.25)Fe_(0.03)Mn_(0.055)合金储氢性能的影响

为了研究急冷对储氢合金残余氢量的影响,利用真空电弧熔炼炉和铜模喷铸制备了Ti_(0.32)Cr_(0.345)V_(0.25)Fe_(0.03)Mn_(0.055)合金,采用XRD、PCT(压力-容量-温度)、TG/DTA等手段分析了急冷对储氢合金吸放氢性能的影响。结果表明,铸态合金和急冷合金均由BCC固溶体主相和Laves第二相组成;急冷对首次吸氢动力学行为影响较大,由铸态时的化学反应控制变为急冷时的新相晶核形成长大控制;急冷后,合金吸放氢平台压得到提高,且吸氢起始点左移,但吸放氢滞后性增大。TG/DTA曲线表明,急冷并没有改变合金的残余氢量,但氢化物放氢温度升高。     

Microstructure and mechanical properties of unidirectionally solidified Al-Si-Ni ternary eutectic

Al-10.98 pct Si-4.9 pct Ni ternary eutectic alloy was unidirectionally solidified at growth rates from 1.39μm/sec to 6.95μm/sec. Binary Al-Ni and Al-Si eutectics prepared from the same purity metals were also solidified under similar conditions to characterize the growth conditions under the conditions of present study. NiAl₃ phase appeared as fibers in the binary Al-Ni eutectic and silicon appeared as irregular plates in the binary Al-Si eutectic. However, in the ternary Al-Si-Ni eutectic alloy both NiAl₃ and silicon phases appeared as irregular plates dispersed in α-Al phase, without any regular repctitive arrangement. The size and spacing of NiAl₃ and Si platelets in cone shaped colonies decreased with an increase in the growth rate of the ternary eutectic. Examination of specimen quenched during unidirectional solidification indicated that the ternary eutectic grows with a non-planar interface with both Si and NiAl₃ phases protruding into the liquid. It is concluded that it will be difficult to grow regular ternary eutectic structures even if only one phase has a high entropy of melting. The tensile strength and modulus of unidirectionally solidified Al-Si-Ni eutectic was lower than the chill cast alloys of the same composition, and decreased with a decrease in growth rate. Tensile modulus and strength of ternary Al-Si-Ni eutectic alloys was greater than binary Al-Si eutectic alloy under similar growth conditions, both in the chill cast and in unidirectionally solidified conditions.     

Formation of silica and silicates during the solidification of Fe−Si−O alloys

Small (35 g) ingots of Fe?Si?O alloys have been solidified under controlled conditions and the form, distribution, and compositions of the oxides in the cast structures are reported. The composition of a melt was established by charging a preselected amount of silicon, 0.002 < pct Si < 0.7, and equilibrating the melt with the silica crucible at 1550° or 1650°C. The concentration of FeO and the morphology of the oxide particles in the cast structures indicate that during solidification the process of segregation causes the oxygen potential in the interdendritic liquid to increase over that in equilibrium with the silicon content of the liquid. It appears that nuclei for the formation of the oxide phases, which probably are in the liquid prior to the onset of solidification, become surrounded by solid iron and are made ineffective. It is possible at lower silicon levels that homogeneous nucleation may result by the melt becoming sufficiently supersaturated in oxygen to form FeO or an iron-silicate liquid phase.     

Microstructure and mechanical properties of unidirectionally solidified Al-Si-Ni ternary eutectic

Al-10.98 pct Si-4.9 pct Ni ternary eutectic alloy was unidirectionally solidified at growth rates from 1.39μm/sec to 6.95μm/sec. Binary Al-Ni and Al-Si eutectics prepared from the same purity metals were also solidified under similar conditions to characterize the growth conditions under the conditions of present study. NiAl₃ phase appeared as fibers in the binary Al-Ni eutectic and silicon appeared as irregular plates in the binary Al-Si eutectic. However, in the ternary Al-Si-Ni eutectic alloy both NiAl₃ and silicon phases appeared as irregular plates dispersed in α-Al phase, without any regular repctitive arrangement. The size and spacing of NiAl₃ and Si platelets in cone shaped colonies decreased with an increase in the growth rate of the ternary eutectic. Examination of specimen quenched during unidirectional solidification indicated that the ternary eutectic grows with a non-planar interface with both Si and NiAl₃ phases protruding into the liquid. It is concluded that it will be difficult to grow regular ternary eutectic structures even if only one phase has a high entropy of melting. The tensile strength and modulus of unidirectionally solidified Al-Si-Ni eutectic was lower than the chill cast alloys of the same composition, and decreased with a decrease in growth rate. Tensile modulus and strength of ternary Al-Si-Ni eutectic alloys was greater than binary Al-Si eutectic alloy under similar growth conditions, both in the chill cast and in unidirectionally solidified conditions.     

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.     

Review of Microstructure Evolution in Hypereutectic Al–Si Alloys and its Effect on Wear Properties

Al–Si alloys with silicon content more than 13 % are termed as hypereutectic alloys. In recent years, these alloys have drawn the attention of researchers due to their ability to replace cast iron parts in the transportation industry. The properties of the hypereutectic alloy are greatly dependent on the morphology, size and distribution of primary silicon crystals in the alloy. Mechanical properties of the hypereutectic Al–Si alloy can be improved by the simultaneous refinement and modification of the primary and eutectic silicon and by controlling the solidification parameters. In this paper, the effect of solidification rate and melt treatment on the evolution of microstructure in hypereutectic Al–Si alloys are reviewed. Different types of primary silicon morphology and the conditions for its nucleation and growth are explained. The paper discusses the effect of refinement/modification treatments on the microstructure and properties of the hypereutectic Al-Si alloy. The importance and effect of processing variables and phosphorus refinement on the silicon morphology and wear properties of the alloy is highlighted.     

Microstructural variations induced by gravity level during directional solidification of near-eutectic iron-carbon type alloys

The effects of gravity on the microstructure of directionally solidified near-eutectic cast irons are studied, using a Bridgman-type automatic directional solidification furnace aboard a NASA KC-135 aircraft which flies parabolic arcs and generates alternating periods of low-g (0.01 to 0.001 g, 30 seconds long) and high-g (1.8 g, 1.5 minutes long). Results show a refinement of the interlamellar spacing of the eutectic during low-g processing of metastable Fe-C eutectic alloys. Low-g processing of stable Fe-C-Si eutectic alloys (lamellar or spheroidal graphite) results in a coarsening of the eutectic grain structure. Secondary dendrite arm spacing of austenite increases in low-g and decreases in high-g. The effectiveness of low-gravity in the removal of buoyancy-driven graphite phase segregation is demonstrated.     

枝晶凝固过程中的微观偏析半解析数学模型

基于典型微观凝固单元体内的溶质质量守恒,结合前人的研究工作,建立了一个适合于枝晶凝固方式的二元合金微观偏析半解析模型.本模型同时考虑了反向扩散和粗化对微观偏析的影响,并对枝晶臂间距的粗化直接进行计算,因此更为精确.若只考虑反向扩散的影响,本模型可以简化为BF模型形式;而如果只考虑粗化的影响,本模型可简化为Mortensen模型.本模型完整地统一了以BF模型为代表的反向扩散类模型和以Mortensen模型为代表的粗化模型.利用本模型同样可以对多元合金的微观偏析进行很好预测.以Fe-C-X（Si,Mn,P,S）合金体系为例对本模型的求解过程进行了详细的阐述.本模型可以很好地预测Al-4.9%Cu二元合金的共晶分数以及Fe-C-X（Si,Mn,P,S）多元合金体系的零强度温度和零塑性温度,并与实测值吻合良好.      

Steady-state cellular solidification of Al-Cu Reinforced with alumina fibers

The steady-state directional solidification of aluminum-4.5 wt pct copper and aluminum-1.0 wt pct copper alloys reinforced with parallel,continuous, closely spaced alumina fibers is investigated under growth conditions that produce a plane front or cells in corresponding unreinforced alloys. Specimens were designed to have a central reinforced region surrounded by unreinforced metal of the composite matrix composition. Each was produced by pressure infiltration, subsequently remelted, directionally solidified, and quenched to reveal the liquid/solid metal interface. Both unreinforced and composite sections were characterized to determine solidification front morphology and degree of microsegregation. In the unreinforced portion of the samples, the transition from plane-front to cellular solidification was observed to correspond to a coefficient of diffusion of copper in liquid aluminum of 5 − 10⁻⁹ m² − s⁻¹, in agreement with published values. Cell lengths, analyzed using a finite-difference model of microsegregation, are in agreement with the Bower-Brody-Flemings (BBF) model for cell tip undercooling. In the composite portion of the samples, the alloys solidify free of lateral microsegregation for all solidification conditions investigated, in agreement with theory. The shape of the liquid/solid metal interface near the fibers indicates a much lower fiber/liquid metal interfacial energy than fiber/solid metal interfacial energy. In the composite, plane front solidification is therefore not observed even when plane front solidification obtains in the unreinforced alloy. It is shown that geometrical constraint imposed on deep cells by the fibers causes significant increases in cell tip undercoolings, in agreement with current analyses of deep cell solidification.     

Study on Improving Solidification Structure of Rare Earth Al-Si Alloys with Electropulse Acting on Liquid Alloy

Electropulse modification (EPM) process, a new physical field method for improving the solidification structure of metals was introduced.Different from other research, EPM is only acting pulse current on melt under liquid state.The solidification structure of Al-Si alloys, A1-Cu alloys,cast iron and steel can be modified obviously with this method: the solidification structure of ZL101 alloy presented the Na and Sr modification and the mechanical properties were enhanced; a large number of primary silicon appeared in the microstructure of ZL109 alloy; the equiaxed grain zone was expanded and the grains were fined in Al-5.0wt% Cu alloy; the graphitization took place in solidification process of molten cast iron; the grain sizes of solidification structure of T8 steel were reduced significantly and the shape of steel pearlites also changed; the equiaxed grain zone increased to 88% from original untreated 19%, the equiaxed grains were fined and the intercrystalline crack was avoided in concasting billet by continuously treating liquid electrical sheet steel in tundish.Effects of rare earths on casting Al-Si alloys were also summarized.The method of modifying the solidification structure of rare earth Al-Si alloys with EPM in producing the alloys was proposed.     

快速凝固铝硅合金的制备、组织特征及断裂行为

快速凝固Al-Si合金是一种高性能结构材料,目前主要通过快速凝固/粉末冶金(RS/PM)和喷射沉积(SF)两种方法进行制备;此类合金具有超细化显微组织、成分均匀分布、非平衡亚稳相和过饱和固溶体及高密度位错等组织特征;其断裂方式主要呈脆性断裂。本文主要对上述内容进行综合评述     

Effect of chilling and cerium addition on microstructure and cooling curve parameters of Al–14%Si alloy

Abstract

Al–14%Si alloys, with and without cerium, were cast at varying cooling rates by solidifying them in a crucible and against chills. The effect of melt treatment and chilling on microstructure and cooling curve parameters of the alloy was assessed. Ce treated alloys solidified in clay graphite crucible at a slow cooling rate showed refinement of primary silicon and the formation of Al–Si–Ce ternary intermetallic compound. The addition of Ce to the alloy solidified against chills resulted in simultaneous refinement and modification of primary and eutectic silicon. Nucleation temperatures of both primary and eutectic silicon decreased on addition of cerium. The formation of the intermetallic compound decreased with increase in cooling rate, leading to the modification of the eutectic silicon. The increase in the degree of modification of the eutectic Si was associated with the decrease in the volume fraction of the intermetallic compound formed.     

Effects of Solute Elements on Primary and Secondary Dendrite Arm Spacings in Fe‐based Alloys

There have been many experiments and theoretical studies of primary dendrites, and most of which have focused on the effects of extrinsic factors such as solidification rate and temperature gradient. However, intrinsic factors such as the kind of solute element and initial solute concentration are also considered to be important for controlling the spacing of primary dendrites. In this study, the effects of intrinsic factors on primary and secondary arm spacings of dendrites were investigated, and changes in a microsegregation index with changes in primary arm spacings and equilibrium distribution coefficient of each solute element were also examined. Fe‐Cr‐C, Fe‐C‐Cr, Fe‐C‐Cr‐Mo, Fe‐C‐Cr‐Mo‐V and Fe‐C‐Cr‐Mo‐V‐W alloys were solidified unidirectionally and water‐quenched. The primary arm spacing increased with increase in initial solute concentration in all alloys because the range of solidification temperature increased with increase in solute concentration. On the other hand, the secondary arm spacing in all alloys decreased with increase in initial solute concentration, and it was proportional to about the one‐third root of local solidification time. The microsegregation index increased with increasing primary arm spacing and with decreasing equilibrium distribution coefficient.     

Glass-Fibre-Reinforced Aluminium Alloys Formed by Hot Extrusion: Part II: Heat-Treatment

Abstract

The microstructures of rapidly solidified 7050 alloy powders have been observed by optical microscopy, scanning electron microscopy, and transmission electron microscopy and compared with those observed in a cast and homogenized ingot of the same material. The powder alloy was then consolidated by cold compaction followed by extrusion, the resulting structures being compared with those of extruded cast alloys. Finally, the structure and properties of the two materials were compared after heat treatment. It is shown that metastable phases not previously reported are present in the powder particles and that localized melting occurs in the powder alloy at a lower temperature than in the cast alloy; this is the primary cause of blistering, but appropriate modification to the heat treatment will prevent it occurring. Properties comparable to or better than the cast material may be produced by designing a more suitable heat treatment to avoid liquation of this metastable phase. The evidence presented suggests that the degassing procedure employed historically to counteract blistering is probably unnecessary. PM/0322     

Variations of Microsegregation and Second Phase Fraction of Binary Mg-Al Alloys with Solidification Parameters

A systematic experimental investigation on microsegregation and second phase fraction of Mg-Al binary alloys (3, 6, and 9 wt pct Al) has been carried out over a wide range of cooling rates (0.05 to 700 K/s) by employing various casting techniques. In order to explain the experimental results, a solidification model that takes into account dendrite tip undercooling, eutectic undercooling, solute back diffusion, and secondary dendrite arm coarsening was also developed in dynamic linkage with an accurate thermodynamic database. From the experimental data and solidification model, it was found that the second phase fraction in the solidified microstructure is not determined only by cooling rate but varied independently with thermal gradient and solidification velocity. Lastly, the second phase fraction maps for Mg-Al alloys were calculated from the solidification model.     

Dendrite coarsening and microsegregation in Al-Cu alloys

Solute redistribution and the amount of eutectic are predicted in the solidification structure of a dendritic Al-Cu alloy. The effect of dendrite coarsening on microsegregation during solidification is formulated. Local solidification time and alloy composition are found to change the type of operating dendrite coarsening. While one type of dendrite coarsening operates for a shorter local solidification time and/or lower alloy composition, at longer solidification times and/or higher alloy composition a different type of dendrite coarsening prevails. Local solidification time and alloy composition are the parameters which determine the homogenization and solution treatment, and the structural heterogeneity of the solidified ingot. METIN BASARAN, formerly Associate Professor, Department of Mechanical Engineering, Technical University of Istanbul, Turkey     

激光诱导击穿光谱原位分析仪和电子探针在CrMo耐磨铸钢元素偏析分析中的联合应用

CrMo耐磨铸钢是重要的耐磨钢铁材料,凝固过程中的溶质元素偏析是影响CrMo耐磨铸钢组织和性能的重要因素,了解凝固过程中的溶质元素偏析对于CrMo耐磨铸钢的工业化生产具有重要的借鉴意义。宏观偏析和微观偏析是衡量材料偏析程度的两个指标,准确的测量其偏析状况是研究溶质元素偏析的基础。实验以CrMo耐磨铸钢为研究对象,采用激光诱导击穿光谱原位分析仪(LIBSOPA)和电子探针(EPMA)分析钢锭不同部位的宏观偏析和凝固组织中的微观偏析,结果发现,Cr、V和Mn元素在CrMo耐磨铸钢铸锭中宏观偏析程度较小,偏析比接近1,而Mo元素宏观偏析程度较大,其最大宏观偏析比超过1.20;Cr、Mo、V和Mn元素在CrMo钢凝固组织中均存微观偏析,且随着冷却速度的增加,Cr、Mo、V和Mn微观偏析程度也随之增加,其最大微观偏析比分别为1.39、2.63、3.47和1.83。LIBSOPA与EPMA在CrMo耐磨铸钢元素偏析分析中的联合应用,对全面了解CrMo钢铸锭元素偏析,优化铸造以及后续的热加工工艺具有重要借鉴意义。     

Formation of Nondendritic Primary Aluminum Phase in Hypoeutectic Alloys in Controlled Diffusion Solidification (CDS): A Hypothesis

Controlled diffusion solidification (CDS) is a novel process wherein specific Al alloys can be cast by mixing two precursor alloys of specific compositions and temperature and subsequently casting the resultant mixture. This process enables a nondendritic morphology of the primary Al phase in the cast samples, which is beneficial in mitigating hot tearing tendencies and enabling castability of dilute Al (wrought) alloys to obtain castings with superior mechanical and performance properties. In this study, a hypothesis is proposed to describe the mechanism of the CDS process, specifically the processes of mixing two precursor alloys and a subsequent solidification process. Al – 4.5 wt pct Cu was used as an example alloy system to propose a hypothesis and to verify the various features in the mechanism of mixing two alloys. Experimental results show that the mixing process naturally causes copious nucleation of one of the alloys mixed and that the turbulence energy during mixing distributes these nuclei uniformly to enable a favorable solidification condition for a nondendritic cast microstructure. It is critical that the alloy with the higher thermal mass (mass and temperature) is mixed into the alloy with lower thermal mass to obtain a valid CDS process and that the reverse will not yield a favorable homogeneous cast sample. Certain critical parameters during the CDS process have also been identified and quantified for a favorable microstructure of the casting.