非平衡凝固时包晶反应滞后现象的研究

利用差示量热扫描分析仪(DSC)测量了Al-Ni系、Sn-Sb系二元合金的包晶反应温度.同样的凝固速度时,Al-Ni合金包晶反应严重滞后,Sn-Sb合金包晶反应则滞后较小.研究表明,先析出相和包晶相晶格结构的异同是造成包晶反应严重滞后的原因,两者结构不同时,包晶反应严重滞后;两者结构相同时,包晶反应滞后较小.     

基于CA方法的Ti-45Al合金定向凝固过程微观组织模拟

采用CA方法建立了Ti-45Al(at.%)合金定向凝固过程中的组织演变数值计算模型,考虑了二元合金包晶反应过程,对液态金属冷却定向凝固过程中Ti-45Al合金的组织演变进行了模拟。模拟结果表明,定向凝固组织分为初生等轴晶组织、稳定生长的柱状晶以及中间过渡区域3部分,柱状晶一次臂间距随抽拉速度增大而减小。Ti-45Al合金在0.1 mm/min抽拉速度条件下,得到了以初生β相为主的凝固组织,在枝晶间溶质富集处有少量的包晶相析出,与相关试验结果基本一致。     

考虑包晶相的Ti-Al合金定向凝固枝晶生长数值模拟

考虑二元合金包晶反应和包晶转变过程,采用改进的CA方法建立了Ti-Al合金定向凝固过程中的组织演变数值计算模型,对液态金属冷却定向凝固过程中Ti-47.8Al(原子分数,%)合金包晶相的形成和具有不同优先生长方向的柱状晶的竞争生长进行了模拟。模拟了40 K/cm和80 K/cm两种温度梯度下合金的组织演化,并与相应的Ti-47Al-2Cr-2Nb合金定向凝固试验结果进行对比。结果表明,较之40 K/cm,温度梯度为80 K/cm时组织更快地进入稳定生长区,过渡区相对较短,而且柱状晶区晶粒的连续性更好,一次枝晶臂间距减小。     

稀土对Al-Cu合金凝固过程力学行为及固液共存区的影响

本文用自行设计的实验装置研究了富Ce混合稀土,La,Y和Ce对Al-Cu合金凝固过程力学行为和固液共存区的影响.研究表明,加入稀土后,都不同程度地使得准固相线下移,缩小了准固态区,扩大了准液态区,稀土能提高台金在固相线附近的强度,明显地增加合金在准固态的强度增长率.同时,稀土使合金的热裂纹形成温度降低,增加了热裂纹形成应力,降低了合金的热裂倾向性.通过对合金的微观组织断口分析发现,稀土在α-Al中的溶解度很低,使枝晶细化,并在凝固后期使枝晶间的联生增强。     

INFLUENCE OF RARE EARTHS ON MECHANICAL BEHAVIOR AND SOLID-LIQUID COEXISTING ZONE OF Al-Cu ALLOYS DURING SOLIDIFICATION

本文用自行设计的实验装置研究了富Ce混合稀土,La,Y和Ce对Al-Cu合金凝固过程力学行为和固液共存区的影响.研究表明,加入稀土后,都不同程度地使得准固相线下移,缩小了准固态区,扩大了准液态区,稀土能提高台金在固相线附近的强度,明显地增加合金在准固态的强度增长率.同时,稀土使合金的热裂纹形成温度降低,增加了热裂纹形成应力,降低了合金的热裂倾向性.通过对合金的微观组织断口分析发现,稀土在α-Al中的溶解度很低,使枝晶细化,并在凝固后期使枝晶间的联生增强。     

ZA22合金急冷凝固组织的研究

采用极小试样快速液淬方法研究了ZA22合金从不同温度快速冷却所获得的凝固组织的差异。在研究的温度范围内，液淬前液态合金温度越高获得的凝固组织越细密，且ZA22合金凝固组织中富铝相的晶内成分偏民随着液态合金温度的升高而减轻。结合液态合金的微观结构，对造成凝固组织差异的原因进行了初步分析。     

合金凝固过程再辉现象的数值模拟

本文采用二元合金的等温相场模型,在假定温度在计算区域空间上保持一致的情况下耦合计算热平衡方程,以Al4.5%Cu合金为例模拟了在不同冷却速率下二元合金凝固过程的枝晶生长过程及其再辉现象.结果表明:利用相场模型可以逼真地模拟二元合金过冷熔体中的枝晶生长过程,与试验得到的枝晶形貌吻合较好;凝固潜热和冷却速率显著影响枝晶的生长行为,再现其凝固过程的再辉现象,模拟的结果与经典枝晶生长理论相吻合.     

过冷液态金属的研究进展

过冷是液态金属凝固过程中的一种现象.由于过冷对凝固后的组织结构和性能有重要影响,因此,研究液态-过冷液态-固态的结构、性能及其演变,对于控制液态金属的结构及其凝固过程,开发新型金属材料具有重要意义.本文综述了实现过冷的方法以及过冷液态金属的研究现状,同时介绍了通过测量电阻率研究过冷液态结构变化的新方法,并对深过冷技术的应用进行了展望.     

不同冷速Ag-5Sc合金的凝固组织

采用感应熔炼钢模凝固、慢速平衡凝固、液态快速凝固制备Ag-5Sc二元合金。借助差示扫描量热仪(DSC)、金相显微镜、电子探针(EMPA)和X射线衍射仪(XRD)研究合金在不同冷速条件下的组织结构、钪的分布和形成的化合物相。结果表明,钢模凝固形成环状Ag4Sc相与共晶组织交替的环形层状组织结构,其它组织为致密细小的共晶组织。慢冷凝固是粗大的胞状共晶组织,胞状间存在块状Ag4Sc相。液态快冷凝固为呈细小蜂窝状结构,组织为等轴晶粒。钢模凝固环形和慢冷凝固块状Ag4Sc相中钪元素分布密度比共晶中的Ag4Sc相高,液态快冷合金中钪元素分布均匀。     

液态金属喷淋冷却定向凝固设备的研制

高温度梯度是控制高温合金定向凝固组织的重要参数。为了获得高的温度梯度，基于液态金属喷淋冷却定向凝固技术，设计并建造了一套通过喷淋液态锡使高温合金定向凝固的设备。热流分析发现，液态金属喷淋冷却定向凝固技术的热流密度比传统的高速凝固技术提高了47%。采用DZ22高温合金进行了喷淋液态锡使之定向凝固的试验。结果表明：在液态锡喷淋冷却条件下定向凝固的?15 mm DZ22高温合金试样枝晶组织细小，三次枝晶发达，这是液态锡喷淋冷却提高了温度梯度所致。     

高温金属间化合物的定向凝固特性

高温金属间化合物是一类重要的高温结构材料，其中Ti-Al及Ni-Al等许多合金系在凝固时都存在连续的包晶反应。本文就包晶合金凝固的特性从多方面加以总结和分析。凝固过程中的包晶相变对组成相的生长与取向有不同的影响。除此之外，定向凝固包晶反应过程中，相的竞争与选择及界面形态的演化也将同时对金属间化合物的凝固组织产生重要的影响。在考虑了界面温度对生长速率的响应后，可以发现生长速度超过某一值后亚稳相将析出，而根据凝固条件不同可以以不同的凝固方式进行长大，形成如带状、浅胞状和枝／胞双相共生等形式的组织，其中共生组织更有利于提高材料的单向力学性能。虽然定性的分析已获得某些规律，但作为工程应用的具有包晶反应特点的高温金属间化合物的定向凝固研究还极不充分，需进行大量的、基础的理论与实验研究。     

Al-11.80Cu-24.22Mg三元包共晶合金的凝固机制

采用液淬方法对Al-11.80Cu-24.22Mg(质量分数,%)三元包共晶合金在不同温度下淬火,而后,对其相组成和凝固组织进行了研究,对凝固组织的演化规律以及凝固机制进行了分析.实验结果表明,该合金凝固过程中确实发生了三元包共晶反应,初生相为S相(Al_2CuMg),包共晶组织为α-Al相和T相(Al_6CuMg_4)组成,最终凝固组织由残余初生S相、包共晶组织(α-Al+T)、两相共晶(α-Al+T)及三相共晶(α-Al+T+β)组成;虽然包共晶组织中和两相共晶中含有相同的相,即α-(Al)相和T相,但二者的组织形态不同,包共晶组织呈条带状,而两相共晶呈规则的蛛网状和球状.此外,三元包共晶反应时,包共晶组织依附于初生S相的周围生长,随着冷却速率的增加,三元包共晶反应变慢甚至被抑制,致使初生S相剩余而滞留在基体中.     

Effect of solidification path and contraction on the cracking susceptibility of carbon peritectic steels

The solidification path and contraction generated during the solidification of three carbon steels were studied. Two of the carbon steels were of hypo-peritectic chemical composition, with 0.11 % C and 0.15 % C, while one of the carbon steels were hyper-peritectic with 0.16 % C. The steels with 0.11 % C and 0.16 % C solidified as expected due to their chemical composition. In contrast, the chemically hypo-peritectic steel with 0.15 % C solidified as hyper-peritectic steel, which was associated with the microsegregation of Mn. For the steel exhibiting a hypo-peritectic solidification path, peritectic transformation occurred at solid fraction values higher than 0.9, where it was assumed that the contraction generated in the mushy shell cannot be fed by the liquid. However, for steels exhibiting a hyper-peritectic solidification path, peritectic transformation began at solid fraction values lower than 0.9, where the contraction generated by the peritectic transformation was partly fed by the liquid. Hence, the highest cracking susceptibility was associated with the hypo-peritectic solidification mode.     

Competition of the primary and peritectic phases in hypoperitectic Cu–Sn alloys solidified at low speed in a diffusive regime

Directional solidification experiments on hypoperitectic Cu–Sn alloys have been performed at very low velocity in a high thermal gradient to ensure planar front growth of both phases. The diameter of the sample has been reduced to 500 μm to strongly reduce convection. Lamellar and fibrous peritectic cooperative growth of the primary α- and peritectic β-phases has been observed on length spanning several millimeters. For the first time in a high solidification interval peritectic alloy, a quenched interface of both phases in contact with the liquid has been obtained. An unexpectedly high volume fraction of the primary phase, which furthermore fluctuates over time, has been observed. This is attributed to the transient state of the (α + β) growth front to a steady state and the associated evolution of the large diffusion layer ahead of the solid–liquid interface.     

Ti-Al包晶合金定向凝固及组织选择

考察了包晶合金分别在定向凝固初始过渡及稳态生长阶段的凝固特性;分析了凝固过程中液-固界面前沿两相形核过冷与成分过冷的条件及温度梯度(G)和生长速度(v)对两相竞争生长的影响和带状结构形成与共生生长的条件,确定了出现带状结构所必需的成分和G/v条件.计算了TiAl合金在初始过渡阶段初生相及包晶相的竞争生长和向稳态发展的过渡机制,并应用界面响应函数建立了不同成分TiAl合金在稳态生长过程中相与组织随生长速度演化的竞争选择模型及相应的凝固组织图,预测了TiAl合金在不同凝固条件下可能形成的相与组织形态.在所选的合金成分范围内定向实验所得的凝固组织与计算预测的组织选择图有较好的一致性.     

Growth morphologies in peritectic solidification of Fe–C: A phase-field study

We use a thermodynamically consistent multi-phase, multi-component phase-field model, where the evolution equations for the different fields are derived from an entropy functional, for simulating peritectic growth structures in two and three dimensions. Different solidification morphologies are obtained in the computations and the characteristic properties of the growth forms are discussed. The phase-field method allows for a prediction of the surface energies in the three-phase system δ-ferrite, γ-austenite and liquid based on comparison between experimentally observed and simulated structures. Additionally an investigation of possible nucleation sites in evolving domains is presented and its dependence on the solid–solid surface energy is examined.     

Al-Y包晶合金非平衡凝固过程及组织特征

包晶凝固过程中小平面-小平面两相复相生长方式是凝固领域研究的一个热点。以初生相和包晶相都是严格计量比金属间化合物的Al-Y包晶合金作为研究对象,利用DSC热分析技术,严格控制冷却速度,获得不同凝固条件的非平衡凝固试样,研究了两相为小平面相的Al-Y包晶合金的凝固行为。发现小平面-小平面包晶系合金包晶凝固过程中,非平衡凝固特性及宏观偏析特点比非小平面包晶系更加明显。凝固特征温度与平衡相图偏差明显,包晶反应温度和包晶相直接凝固温度都远高于平衡相图给定的值,相对于相图是在“过热”条件下发生的,而固溶体型包晶合金一般是在“过冷”条件下发生的。包晶转变过程非常微弱,致使初生相残留量远高于平衡相图。即使对于过包晶成分的合金,其凝固组织中仍存在大量的共晶凝固组织,最终得到的凝固组织与平衡相图存在显著差异。     

Two-phase growth in peritectic Fe–Ni alloys

Bridgman crystal growth experiments were carried out to investigate the solidification behavior of Fe–Ni alloys containing nominally between 4 and 4.5 at.% Ni. Due to macrosegregation, a radial concentration gradient was established across the cylindrical specimens. Due to this gradient, a series of solid/liquid interface morphologies was observed. Oriented two-phase microstructures, which formed either lamellar or fibrous δ-ferrite in an austenite (γ) matrix, were found in the central region of specimens with a composition of some 4.2 at.% Ni and a G/V ratio close to the critical ratio for solid/liquid interface breakdown. At slightly smaller concentrations, oscillatory two-phase structures formed which were similar to the 2-λ instabilities of off-eutectic alloys. The observations confirm that at low solidification rates the stable growth morphology in peritectic alloys cannot be selected by the highest growth temperature criterion. A recently developed nucleation and constitutional undercooling criterion (NCU) was applied to establish a solidification microstructure selection map. Reasonable agreement was obtained between calculated and experimental results. Based on eutectic growth theory the possibility of simultaneous two-phase growth in peritectic alloys is discussed.     

Solidification microstructures and solid-state parallels: Recent developments,future directions

Rapid advances in atomistic and phase-field modeling techniques as well as new experiments have led to major progress in solidification science during the first years of this century. Here we review the most important findings in this technologically important area that impact our quantitative understanding of: (i) key anisotropic properties of the solid–liquid interface that govern solidification pattern evolution, including the solid–liquid interface free energy and the kinetic coefficient; (ii) dendritic solidification at small and large growth rates, with particular emphasis on orientation selection; (iii) regular and irregular eutectic and peritectic microstructures; (iv) effects of convection on microstructure formation; (v) solidification at a high volume fraction of solid and the related formation of pores and hot cracks; and (vi) solid-state transformations as far as they relate to solidification models and techniques. In light of this progress, critical issues that point to directions for future research in both solidification and solid-state transformations are identified.     

包晶合金定向凝固中的共生生长

对包晶合金定向生长特性进行分析，并与共晶系经典共生生长模型（Jackson-Hunt模型）所描述的共生生长特性进行对比，提出包晶稳态过热共生生长模型，同时以Fe-Ni合金系为对象，对其亚包晶和过包晶成分在不同温度梯度G和生长速度1，下进行定向凝固实验及组织分析。研究结果表明：在定向凝固条件下，包晶系两相严格的共生生长只有在进入稳态且温度高于包晶相变温度时才可能进行；包晶合金两相的共生生长尤其在G／v值高的平／胞界面形态，确实发生在Tp温度以上，与提出的模型所预测的一致。