强磁场下Cu-50%Ag合金定向凝固过程中的组织及固液界面形貌演变

目的 研究强磁场下Cu-50%(质量分数)Ag合金定向凝固过程中的组织演变、固液界面形貌变化及溶质迁移行为,分析强磁场对金属凝固过程的作用机制,为强磁场下的金属材料制备提供理论借鉴和指导。方法 在不同的凝固速率与磁场条件下进行定向凝固和淬火实验,对合金的定向凝固组织、糊状区与固液界面形貌以及溶质分布行为进行考察。结果 强磁场破坏了凝固组织的定向生长,使凝固组织转变为枝晶与等轴晶共存的形貌;强磁场诱发了熔体对流,减少了糊状区中溶质的含量;强磁场改变了固液界面处的溶质分布和固液界面形貌,破坏了固液界面的稳定性。结论 强磁场通过洛伦兹力和热电磁力的共同作用,诱发了糊状区内液相的纵向环流,改变了固液界面及糊状区中的组织形貌与元素分布。     

快速凝固过共晶铝硅合金的显微组织及摩擦学行为研究现状

过共晶铝硅合金由于具有高耐磨性、低热膨胀系数和高比强度而广泛应用于汽车和飞机制造业.该类合金在常规铸造下易产生粗大的脆硬初生硅相,降低合金力学性能及耐磨性.而利用快速凝固技术能够有效细化硅相,制备出高耐磨的过共晶铝硅合金.过共晶铝硅合金的性能可以通过改变共晶硅和初晶硅的形态及其分布、二次枝晶胞的尺寸或臂间距等方式加以改善.目前过共晶铝硅合金的研究大多是关于控制共晶和初晶硅的形态和分布,而针对常规铸造的细化晶粒工艺只对25％(质量分数)硅含量以下的过共晶铝硅合金有明显效果,因此研究人员聚焦于能对高硅含量的过共晶铝硅合金实现晶粒细化的快速凝固技术.快速凝固技术区别于常规铸造的特点是高冷却速度,研究发现冷却速度对过共晶铝硅合金的相平衡和微观结构有着显著的影响.随着冷却速度的增加,过共晶铝硅合金的微观结构细化、化学均匀性提高、固溶度增加,形成非晶及亚稳相,极大地改善了过共晶铝硅合金的性能.根据不同快速凝固技术制备的过共晶铝硅合金,其细化的显微组织及对应的摩擦学行为也有所不同.这些差异对于完善快速凝固过程中硅晶粒形核长大机制、形态演化机制及其对过共晶铝硅合金性能影响的理论体系能够起到有效的补充作用.本文综述了快速凝固过共晶铝硅合金四种主要制备方法:衬底急冷技术、快速凝固-粉末冶金技术、喷射沉积技术和选择性激光熔化技术,分析了相关快速凝固工艺的研究现状,对比了不同工艺制备的过共晶铝硅合金的显微组织及耐磨性能,并从理论体系、性能预测和技术工艺三方面对其未来的研究方向提出了一些可行建议.     

强磁场下Zn-2 wt.%Cu合金定向凝固的初步研究

本文进行了10T强磁场下Zn-2wt.%Cu合金的定向凝固的初步研究.结果发现下拉速度较低时,无磁场时晶体以平界面方式生长,而施加磁场则产生带状组织,并且随着磁场的增加带状组织越来越明显,带状组织间距越来越小;当定向凝固速度较高,晶体以枝晶方式生长时,磁场促进枝晶的分枝,并扰乱枝晶规则生长;随定向凝固速度提高,磁场的作用逐渐减弱.     

强磁场对Al-7%Si合金凝固过程中初生α-Al枝晶分布的影响

采用超导强磁场装置研究了磁场强度对有、无细化剂颗粒Al-7%Si(质量分数)合金凝固组织的作用效果。研究发现,施加强磁场使无细化剂颗粒合金中的初生α-Al枝晶转变为发达枝晶形貌,二次枝晶生长充分,三次枝晶分支明显,枝晶尖端清晰可见;晶粒细化,且枝晶主轴与磁场方向呈30°规则排列。施加强磁场后初生α-Al枝晶数量和Si在α-Al中的溶解度都有少量增加。强磁场抑制添加细化剂合金熔体中的对流,加剧了Ti的重力偏析,使初生α-Al相出现明显枝晶化趋势,方向性增强,枝晶臂粗化明显。     

熔体深过冷过共晶铝硅合金的凝固组织研究

本工作采用熔体急冷装置对过共晶铝硅熔体进行深过冷处理,采用光学显微镜、扫描电子显微镜和X射线衍射仪等手段,研究了硅含量和熔炼工艺对熔体深过冷过共晶铝硅合金凝固组织的影响。研究结果表明,合金在800℃熔炼,保温时间为30 min时,熔体深过冷处理可抑制Al-(14～18) Si合金熔体在凝固过程中初晶硅的析出。当Al-18Si合金在800℃熔炼,保温时间超过30 min时,深过冷Al-18Si合金熔体在室温金属模型中凝固时可完全抑制初晶硅的析出,获得无初晶硅的凝固组织。     

强磁场下Al-Si过共晶合金组织细化原理研究

为了改善Al-Si合金性能,研究了强磁场对Al-Si过共晶合金组织的影响.依据热力学和晶粒形核理论,阐述了组织变化的原因.研究表明:当Al-Si过共晶合金在600℃施加强磁场并平行于磁场方向下凝固时,共晶组织被细化,但对初生硅相影响不大;磁场强度越大,细化效果越明显;强磁场降低了固态熵和磁自由能的影响,使共晶组织临界形核半径减小,而硅由于是逆磁质对其影响不大;强磁场使液态金属平行于磁场方向流动,进一步细化了组织.     

Si-Al-Sn合金熔体中块体硅定向生长行为研究

为从Si-Al-Sn合金中有效分离初晶硅, 本研究提出并实现了三元合金定向生长块体硅的技术。通过考察冷却速度、合金成分、温度梯度与晶体生长速度比值(G/R)等参数及其影响机制, 确定促进块体硅稳定生长的有利条件; 对比Si-Al、Si-Sn二元合金体系, 采用成分过冷理论分析金属Sn对三元合金中块体硅生长行为的影响; 采用电子探针显微分析仪考察块体硅微观组织形貌与杂质分布。研究结果表明: 定向凝固方法能有效分离块体硅, 同时抑制块体硅内金属夹杂物生成, 并将杂质含量控制在其固溶度范围内, 成为一种有效分离、回收高纯初晶硅的新途径。     

快速凝固和直流电处理对铝硅合金中硅形态影响

用氮气雾化法制取了Ａｌ－２１．５４％Ｓｉ合金微粒并进行７００℃重熔处理，对加Ｓｒ变质的Ａｌ－１３．２３％Ｓｉ合金熔体进行了直流电处理。用扫描和透射电研究了不同处理条件对Ａｌ－Ｓｉ合金中共晶硅生长形态的影响。结果表明，快速凝固法制取的合金微粒与重熔处理试样中，共晶硅呈球状和条状。加Ｓｉ变质的共晶硅为弯扭的纤维状，直流电熔体处理后共晶硅转变为圆状，其颗粒尺寸为５０－２００ｎｍ。     

电磁铸造对Al–Si12.6%共晶合金微观组织的影响

目的 了解外加耦合场作用下Al–Si合金的微观组织演变情况。方法 利用外磁场加强凝固过程中熔体的流动，从而获得合金中各相的形态、分布和取向等，利用扫描电子显微分析技术(SEM)和电子背散射衍射分析技术(EBSD)研究电磁场对Al–Si12.6%共晶合金微观组织的影响规律。结果 在磁场作用下，铸件的组织均匀性增加，初生铝和共晶硅的尺寸均减小，并且随着磁场强度的增大，共晶硅的含量和尺寸呈减小趋势，其形态趋于等轴状，共晶硅附近各铝枝晶之间的外延生长关系消失，相互独立形核和长大。结论 研究成果可以为电磁铸造对Al–Si系共晶合金微观组织研究提供一定的参考。     

强磁场对Al-18Si合金凝固组织的影响及其机理研究

研究了过共晶Al-18Si合金在稳恒强磁场中的凝固行为.实验表明,在稳恒强磁场作用下,过共晶Al-18Si合金中的初生Si发生显著偏聚.为解释初生Si的偏聚机理,建立了相应的量子理论模型.模型认为:初生Si在Al-18Si合金中的偏聚是由于部分Si原子在磁场中获得附加能后,沿磁场方向做旋进运动所致.Si原子在10T磁场中获得的附加能为1.854×10-22J.     

Control of solidified structures in aluminum–silicon alloys by high magnetic fields

In order to investigate the effects of high magnetic fields on the as-solidified structures of Al alloys, solidification experiments of hypoeutectic and hypereutectic Al–Si alloys under various high magnetic field conditions (up to 12 T) have been conducted. It was found that uniform magnetic fields and gradient magnetic fields affect the solidification process by Lorentz force and magnetization force, respectively. The primary silicon crystals of hypereutectic Al–Si alloys are distributed, relatively, homogeneously under uniform magnetic fields, whereas they congregate near the top surface or bottom of samples by the combined action of buoyancy and magnetization force under gradient magnetic fields. The results indicate that it is possible to control the behaviors of reinforced particles in the metal matrix and improve the material performances by using high magnetic fields in the solidification process of metal matrix composites. The experiments also showed that high magnetic fields decrease the interlamellar spacing of the eutectic structure, while there exists a certain optimum value of magnetic intensity corresponding to the minimum value of interlamellar spacing, and magnetic energy is capable of influencing thermodynamic equilibrium of solidifying system and makes the content of eutectic aluminum in eutectic structures increased.     

Effect of combined magnetic field on the eliminating inclusions from liquid aluminum alloy

The effect of electromagnetic field on the removal of inclusions in the aluminum alloy was investigated. Primary silicon particles precipitating from the solidification of Al-Si hypereutectic alloy were regarded as inclusions. An experimental apparatus applied with both rotating magnetic field (RMF) and traveling magnetic field (TMF) was employed to study the distribution of silicon particles in Al-Si alloy under magnetic field. The results showed that combined magnetic field (CMF) consisting of RMF and TMF eliminated the silicon particles from the molten alloy. Compared with TMF or RMF, CMF increased the separating effectivity substantially. It was proposed that CMF provided a highly effective approach for metal purification.     

Effects of high magnetic fields on solidified structures of Mn-90.4 wt% Sb hypoeutectic alloy

Mn-90.4 wt% Sb alloy specimens were solidified under both uniform magnetic field and magnetic field gradient conditions. The solidification behavior was examined to elucidate the effects of high magnetic fields on the solidified structure evolution of this hypoeutectic alloy. The macrostructures on the longitudinal section of the alloys were investigated by optical microscopy and x-ray diffraction (XRD). The volume fraction of primary MnSb phases and the interrod spacing of the eutectic were measured by metallographic analysis. It was found that the segregation of the primary MnSb particles at the certain regions of the specimens occurred under the influence of high magnetic field gradients. The MnSb phases obtained under magnetic fields were oriented with their (h0 l) planes along the direction of the magnetic field. Both the volume fraction of primary MnSb phases and the interrod spacing of the eutectic were decreased upon the application of the high magnetic fields.     

Effects of high magnetic fields on solidified structures of Mn-90.4 wt% Sb hypoeutectic alloy

Abstract

Mn-90.4 wt% Sb alloy specimens were solidified under both uniform magnetic field and magnetic field gradient conditions. The solidification behavior was examined to elucidate the effects of high magnetic fields on the solidified structure evolution of this hypoeutectic alloy. The macrostructures on the longitudinal section of the alloys were investigated by optical microscopy and x-ray diffraction (XRD). The volume fraction of primary MnSb phases and the interrod spacing of the eutectic were measured by metallographic analysis. It was found that the segregation of the primary MnSb particles at the certain regions of the specimens occurred under the influence of high magnetic field gradients. The MnSb phases obtained under magnetic fields were oriented with their (h0 l) planes along the direction of the magnetic field. Both the volume fraction of primary MnSb phases and the interrod spacing of the eutectic were decreased upon the application of the high magnetic fields.     

Computer simulation of solidification and microsegregation in presence of particles

A computer simulation was used to study the influence of particles on microsegregation during solidification. Particles in the melt can affect the solidification microstructure by changing the cooling curves, acting as barriers to solute diffusion, reducing the metal volume, affecting the coarsening process, etc. The computer simulation model used in the present work calculates the effect of particles of different thermal properties on the cooling curves and the consequent changes to microsegregation and dendrite arm spacing. The microsegregation calculations are valid for cases where the interparticle spacing is much smaller than the dendrite arm spacing. By using the simulation, it was possible to study various contributing factors in isolation as well as in combination so that the relative significance of each factor could be evaluated. It was observed that the reduction of liquid volume by the presence of the particles was the largest contributing factor to the influence of particles on the solidification microstructure. Thus, the changes in matrix microstructure of cast metal matrix composites depend more on the volume fraction of the reinforcing particles than on the properties of the particles themselves. Irrespective of the thermal properties of the particles, the dendrite arm spacing and microsegregation in the composite matrix was seen to be less than in the unreinforced alloy solidified under the same external conditions.

MST/3405     

Fracture features of a silicon-dispersed aluminium alloy extruded from rapidly solidified powder

The tensile fractography of an AI-20Si-3Cu-1 Mg alloy consolidated from rapidly solidified powder by extrusion has been investigated using optical and electron microscopy, and related to the processing conditions as well as the tensile behaviour of the alloy at room and elevated temperatures. The alloy studied shows distinct fracture features owing to the presence of dispersed silicon crystal particles with a bimodal distribution in size and of prior powder particle boundaries in the extrudate. It has been found that at room temperature cracks initiate by cracking the primary silicon crystal particles. Crack propagation occurs along the interfaces between the eutectic silicon crystal particles and the matrix and also between the prior powder particles, where microvoids are formed by the interfacial decohesion. At 300 °C, the fracture of the alloy involves microvoid nucleation, growth and coalescence at the interfaces between the silicon crystal particles and the aluminium matrix and between the prior powder particles. It has also been observed that the fractographic features of the alloy correspond well to the processing conditions including extrusion temperatures and subsequent heat treatment. The importance of minimizing the coarsening of the silicon crystals in processing in order to use the full strength potential of the alloy investigated is emphasized.     

Microstructural modification of Sn–Bi and Sn–Bi–Al immiscible alloys by shearing

Sn–20?wt-%Bi and immiscible Sn–20?wt-%Bi–1?wt-%Al alloys were used to understand the effect of high-intensity shearing on microstructural refinement. Novel ACME (Axial Centrifugal Metal Expeller) shearing device, based on axial compressor and rotor–stator mechanism to generate high shear rate and intense turbulence, was used to condition the melts prior to solidification. Microstructure in the Sn–Bi alloy deviated from dendritic grains with coarse eutectic pockets under conventional solidification to compact grains with well-dispersed eutectic under semisolid-state shearing. Decreasing the shearing temperature and increasing shearing time increased the globularity of grains. Following shearing, remnant liquid solidified into fine grain structure. In the immiscible Sn–Bi–Al alloy, shearing produced uniform dispersion of refined Al-rich particles in Sn-rich matrix as opposed to severe segregation under conventional solidification. The primary effect of shearing appears to originate from the thermo-solutal homogenisation of the melt and its effect on interface stability during solidification.     

Characterization of rapidly solidified aluminium-silicon alloy

A metallographic investigation of as-cast LM-13 aluminium-silicon alloy, solidified at different cooling rates (using permanent moulds or a single-roll melt spinner), is presented with special reference to the modification of eutectic silicon. and the refinement of primary aluminium. The refinement of microstructure with the increase in cooling rate is mainly attributed to the limited growth kinetics of the nucleated phase during solidification. The rapidly solidified ribbon was heat-treated in order to determine the microstructural stability at high temperature. The tensile strength, percentage elongation and hardness values of as-solidified and heat-treated samples correlate well with the changes in the microstructure observed.     

Solidification of Lead Tin and Lead Telluride Eutectic Alloys in Microgravity

The Brazilian Microgravity Program is mainly based on experiments carried out on sounding rockets. A solidification furnace, capable of producing temperatures up to 900 °C, was developed to process metal and semiconductor alloys in microgravity environment. This paper describes a solidification experiment made in this furnace during a parabolic flight, with two eutectic alloys. The behavior of the eutectic alloys PbSn with 26.1 Pb at. % composition, and PbTe with 10.9 Pb at. % composition were presented and compared with laboratory solidifications carried out in the same furnace and thermal cycle. It was concluded that the formation of dendritic structures in PbSn alloy is related to the presence of sedimentation and convective flow during solidification, and the size of these structures is connected to the solidification time. Thus, in the microgravity alloy, there was no formation of dendritic structures and the profile of solute distribution remained constant throughout the sample. For the PbTe eutectic alloy the microgravity conditions have not caused significant changes compared to the earth solidified sample.