合金熔体过热处理的研究进展

熔体过热处理主要研究的是金属或合金熔体的结构和物理化学性质随熔体过热处理温度、时间及其在冷却和凝固过程中的变化规律.通过适当的熔体过热处理可以改善合金的组织和提高综合力学性能.系统介绍了合金熔体过热处理工艺国内外的研究现状,探讨了今后的发展方向.     

熔体过热对Sb-Bi合金凝固组织的影响

以Sb-4.6%Bi合金为研究对象,在限定其它因素保持不变的情况下,考察了熔体过热温度对凝固过程的影响。,实验结果发现,随着熔体过热温度的提高,合金形过冷度增大,凝固组织显著细化,研究表明,熔体过冷倾向是熔体结构状态的一个必然反映,熔体结构状态随温度发生变化是导致合金结晶过冷度发生明显的变化的原因,随着过热温度的提高,Sb-4.6%Bi合金晶粒显著细化,其原因是经过热处理的合金熔体在较大的过冷度下凝固。     

综论偏晶合金的制备技术:外场下凝固、快速凝固及激光技术

当偏晶合金液过冷至液相分离温度(Tsep)以下时,进入亚稳态难混溶区间,由单一液相分离成两个液相:L1(主体液相,质量分数大于50%)与L2(次生液相,收缩成液滴)。微观组织演化呈现三个阶段:(1)相分离自发进行阶段;(2)主体相合金熔体进入结晶过程;(3)残余的次生相合金熔体进入凝固阶段。尤其是当次生相凝固后弥散分布于主体相基体内时,偏晶合金具有高强、高导以及高耐磨性能,其在航空航天和汽车等工业领域具有重要的应用前景,长期以来受到了研究者的广泛关注。偏晶合金组织结构特征有两种,即第二相弥散型和核/壳结构型。然而,常规凝固条件下,制备的偏晶合金极易形成严重偏析或分层组织,导致制备大块匀质偏晶合金变得困难。为了深入研究偏晶合金液相分离行为,以及微结构特征对偏晶合金性能的影响,研究者提出了许多制备偏晶合金的方法。早在1958年,液相分离现象就在Cu-Fe偏晶合金中被发现,当即引起学者们的广泛关注。近年来,为了制备组织均匀和性能优异的偏晶合金,开发了许多外场作用下的偏晶合金制备方法,旨在消除常规重力场下熔体对流造成的凝固组织偏析、位错、空洞等缺陷。例如,在微重力场条件下,对流作用减弱,可制备接近无偏析的凝固组织;在电磁场条件下,实现了对材料工艺过程的控制和材料组织与性能的改善;在直流磁场和电场交互作用下,熔体流动得到抑制,实现了电磁搅拌控制凝固;在交流磁场和电场交互作用下,实现了电磁搅拌和电磁悬浮,达到减小偏析和改善组织结构特征的目的;在超声场作用下,实现了材料无容器凝固。此外,快速凝固是一个典型的非平衡相变过程,可以消除合金的溶质偏析,获得常规凝固条件下无法获得的成分、相结构和显微组织,显著提高合金的强度、塑性、韧性、延展性和磁性等。为深入了解各类偏晶合金的制备方法,本文主要从外场下凝固、快速凝固、激光技术角度综述了偏晶合金的各种凝固制备工艺和研究方法。     

熔体过热对镍基高温合金凝固组织及性能的影响研究

基于对镍基高温合金熔体结构的认识,选择合适的熔体过热温度、过热时间和浇注温度,熔体过热处理可有效改善镍基高温合金的凝固组织并提高合金的力学性能.本文介绍了国内外熔体过热对镍基高温合金凝固组织及力学性能的研究进展.     

熔体过热处理技术及其在镍基高温合金中的应用研究进展

镍基高温合金是先进航空发动机高温叶片不可或缺的关键核心材料,目前通过合金化来提高其承温能力已趋于极限。研究表明,材料熔体结构对合金凝固过程、凝固组织、性能以及成形质量具有重要的影响。熔体结构的变化能够直接导致熔体特性发生改变,进而对性能产生影响,然而在实际合金的制备过程中,熔体结构的作用通常被忽略。熔体过热处理技术通过利用合金熔体的遗传效应,将高温熔体的结构保留到低温熔体,从而大幅提高合金性能。系统介绍了熔体过热的原理、主要处理技术以及如何通过X射线衍射和物性参数测量来确定熔体过热处理参数,重点介绍了熔体过热处理技术在优化高温合金凝固组织和提升性能方面的应用,最后提出了熔体过热处理技术发展的方向和面临的挑战。     

高温合金定向凝固技术研究进展

首先回顾了定向凝固的发展历史,重点分析了液态金属冷却定向凝固的技术特点。总结了高温度梯度下制备的定向凝固法单晶高温合金在组织和性能方面的研究现状,结合作者在本领域的研究,着重分析了定向凝固温度梯度、凝固速率、晶体取向、熔体超温处理、熔体对流控制对组织和性能的作用规律和机制,认为高温度梯度定向凝固是细化组织、减少缺陷、提高合金性能的重要途径。最后展望了高温合金定向凝固的发展趋势。     

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。     

共晶Ni_(53)Si_(47)合金的液-固态结构相关性

利用高温液态X射线衍射仪研究了共晶Ni53Si47合金的液态结构。在衍射强度曲线的小角部分发现预峰,且预峰的强度随温度的降低而增强;研究表明,熔体中原子团簇结构稳定,有序度随温度降低而提高;通过纳米晶粒模型进一步研究了该合金的液-固态结构相关性,指出熔体衍射强度曲线上的预峰是化合物NiSi液态结构的一种体现,该合金液态结构与固态结构具有一定的相关性。     

Ni_3Al合金熔体结构的高温X射线衍射分析EI北大核心

采用高温X射线衍射技术对Ni3Al合金1703~1808K下的熔体结构进行了研究,获得了原始衍射强度、结构因子、平均最近邻原子间距离和配位数等结构信息。结果表明,Ni3Al合金在测定温度下的熔体结构基本没有发生改变,只是原子团簇尺寸上的增大或减小。熔体中除存在短程有序结构以外,还存在中程有序结构单元。通过固液X射线衍射曲线的对比分析,得到合金熔体中存在类γ-′Ni3Al和-βNiAl相两种有序原子团簇结构单元,固液结构之间存在密切的相关性。     

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。     

Effect of boron and zirconium on directional solidification behaviour and segregation of DS IN738 superalloy

The effect of boron and zirconium on the directional solidification behaviour and segregation of IN738 superalloy has been studied. It was found that additions of boron and zirconium enlarge the solidus-liquidus temperature interval and increase the amount of residual melt during solidification. Boron gives the alloy a tendency to form developed dendrites, while zirconium enhances a cellular solidified structure. Both boron and zirconium are shown to be rejected to the residual melt during solidification, and shown to change the segregation of alloying elements by interaction.     

Effects of Melt Thermal Treatment on A356 Alloy

To increase the casting quality of hypoeutectic Al-Si alloys, the effects of melt thermal treatment on the solidification structure of the A356 alloy were analyzed by a factorial experiment, in which the overheated melt was mixed with the low temperature melt. Experimental results show that the elongation ratio and strength of the treated samples increase remarkably compared with the control sample. The primary dendrite size reduces dramatically and the dendrite changes from columnar to equiaxed, with a little change of the secondary dendrite arm spacing (SDAS). Combined with the measurement of the nucleation undercooling, it is concluded that the solidification structure and refining effect are dependent primarily on the low temperature melt. The refining mechanism is believed as a result of the multiplication of the nuclei in the melt thermal treatment procedure.     

Influence of Liquid Structure on Solid Transformation of CuA1Ni Shape Memory Alloy

Molten Cu-13Al and Cu-13AI-4Ni (mass fraction) alloys have been investigated using X-ray diffraction method. A distinct pre-peak has been found in the structure factors. The pre-peak increases its intensity with decreasing temperature and addition of Ni. The structural unit size corresponding to the pre-peak equals to magnitude of (111) planar distance of β phase. The appearance of a pre-peak is due to existence of clusters with β-phase-like structure in melt. Quantity and size of clusters increase with decreasing temperature but their structural unit size remains constant. Cu-13AI-4Ni shape memory alloy ribbons can be fabricated by rapid solidification technique. Order degree of martensite and temperature of the reverse martensitic transformation increase with decreasing liquid quenching temperature.β phase particles develop from incorporating and growing of the clusters during solidification, thus result in the correlation between liquid structure and solid transformation,     

Influence of Liquid Structure on Solid Transformation of CuAlNi Shape Memory Alloy

Molten Cu-13Al and Cu-13Al-4Ni (mass fraction) alloys have been investigated using X-ray diffraction method.A distinct pre-peak has been found in the structure factors. The pre-peak increases its intensity with decreasing temperature and addition of Ni. The structural unit size corresponding to the pre-peak equals to magnitude of (111)planar distance ofβ phase. The appearance of a pre-peak is due to existence of clusters withβ-phase-like structure in melt. Quantity and size of clusters increase with decreasing temperature but their structural unit size remains constant. Cu-13Al-4Ni shape memory alloy ribbons can be fabricated by rapid solidification technique. Order degree of martensite and temperature of the reverse martensitic transformation increase with decreasing liquid quenching temperature. Β phase particles develop from incorporating and growing of the clusters during solidification, thus result in the correlation between liquid structure and solid transformation.     

Influence of Liquid Structure on Solid Transformation of CuAINi Shape Memory Alloy

Molten Cu-13AI and Cu-13AI-4Ni (mass fraction) alloys have been investigated using X-ray diffraction method. A distinct pre-peak has been found in the structure factors. The pre-peak increases its intensity with decreasing temperature and addition of Ni. The structural unit size corresponding to the pre-peak equals to magnitude of (111) planar distance of phase. The appearance of a pre-peak is due to existence of clusters with -phase-like structure in melt. Quantity and size of clusters increase with decreasing temperature but their structural unit size remains constant. Cu-13AI-4Ni shape memory alloy ribbons can be fabricated by rapid solidification technique. Order degree of martensite and temperature of the reverse martensitic transformation increase with decreasing liquid quenching temperature. phase particles develop from incorporating and growing of the clusters during solidification, thus result in the correlation between liquid structure and solid transformation.     

快速凝固法制备大尺寸MgB2超导体

简单化合物MgB2超导性的发现引起了世界科学家对其组织结构、超导原理、制备方法及应用前景的广泛兴趣.针对MgB2单晶制备过程中存在镁的气化温度很低,MgB2在镁的沸点以下溶解度很小等不利因素,在国际上率先提出了从过冷熔体中的相选择与控制入手,采用深过冷快速定向凝固技术来制备高品质、大尺寸MgB2单晶的新思路.     

Microstructure of Al-Si Alloys Rapidly Solidified from the Different Temperature Melts

1.IotroductionCoolingratehasasignificanteffectonsolidifyingmicrostructureofAl-Sialloys,ithaJsbeenshownthatthemorphologyofeutecticsiliconchangesfromplatestofiberswhenthecoolingrateisincreased[1'2].Rapidsolidificationprocessingprovidesawaytogreatlyre-finemicrostructure.Inmelt-spinningprocess,alargecircumferentialvelocityoftherotatingwheelisac-companiedbyalargecoolingandsolidificationrate.SoasmallsizeofAl-richandSi-richphaseswasgainedinthespunribbonsofAlSialloysl3].Obviously,themicrostructure…     

基于高温熔凝法Al_2O_3/ZrO_2/YAG共晶陶瓷显微组织演变规律

作为超高温结构材料,共晶氧化物陶瓷的力学性能和显微组织密切相关。采用高温熔凝法制备Al_2O_3/ZrO_2/YAG共晶陶瓷体,研究熔体温度和结晶种子对凝固组织影响规律,运用经典形核机制和Jackson-Hunt共晶生长模型探讨了凝固组织的演变机理。研究表明,随着熔体温度升高(1750~2000℃),凝固体物相组成从α-Al_2O_3,c-ZrO_2和YAG转变为α-Al_2O_3,c-ZrO_2和亚稳相YAP。凝固组织依次经历:非共晶Al_2O_3/ZrO_2/YAG、不规则共晶Al_2O_3/ZrO_2/YAG、纳米纤维状共晶Al_2O_3/ZrO_2/YAG和复杂粗大的亚稳复合陶瓷Al_2O_3/ZrO_2/YAP。分析表明,凝固组织的演变源于异质晶核点不断钝化导致形核过冷度和凝固路径改变,所以合理选择熔体温度和结晶种子是共晶组织调控的关键。