A study of the effect of an intermediate phase on the dissolution and homogenization characteristics of binary alloys

The effect of an intermediate phase on the dissolution and homogenization characteristics of binary alloys was studied by mathematically modeling finite, three-phase, diffusion-controlled, binary systems for planar, cylindrical, and spherical interfaces with the mean composition in a terminal solid solution (α). Calculations were carried out using numerical methods and computer techniques. The major variables affecting the dissolution of the unstable terminal solid solution (γ) and the intermediate phase (β) were found as well as the variables affecting the homogenization process. Comparison of the results to existing solutions for one-phase (Heckel¹⁸) and two-phase (Tanzilli and Heckel¹⁶) binary systems led to predictions for the behavior ofn-phase binary systems. Calculations based on the spherical model were successfully compared to existing literature data for dissolution in a compacted blend of W?Re powders and for the formation of austenite from ferrite and cementite.     

The formation of multi-domain precipitates in a NiW alloy

The formation of twinned Ni₄W precipitates in a NiW alloy has been studied. On prolonged aging the dominant morphology found is one where two perpendicular twin related variants of Ni₄W form plate-shaped precipitates with a near (100)_Ni habit plane. The orientation relationship between the Ni₄W variants and the Ni matrix leads to a macroscopic shape change with the (100) habit plane interface nearly invariant. In addition each domain has a simple shear associated with it, which can be accomodated either elasticity or by arrays of screw dislocations. Elastic accommodation leads to a strain contrast effect which is similar to that observed at screw dislocations lying normal to the foil surface.     

Enthalpies of a binary alloy during solidification

The purpose of the paper is to present a method of calculating the enthalpy of a dendritic alloy during solidification. The enthalpies of the dendritic solid and interdendritic liquid of alloys of the Pb-Sn system are evaluated, but the method could be applied to other binaries, as well. The enthalpies are consistent with a recent evaluation of the thermodynamics of Pb-Sn alloys and with the redistribution of solute in the same during dendritic solidification. Because of the heat of mixing in Pb-Sn alloys, the interdendritic liquid of hypoeutectic alloys (Pb-rich) of less than 50 wt pct Sn has enthalpies that increase as temperature decreases during solidification. For some concentrations of Sn, the enthalpy of the dendritic solid at the solid-liquid interface also increases with decreasing temperature during solidification. Of particular concern, in formulating the energy equation, is the fact that the heat of fusion during solidification increases as much as 80 pct for hypoeutectic alloys and decreases as much as 25 pct for hypereutectic alloys. Thus the often applied assumptions of a constant specific heat and/or a constant heat of solidification could lead to errors in numerical modeling of temperature fields for dendritic solidification processes.     

Crystallography of grain boundary α precipitates in a β titanium alloy

The crystallography of α(hcp) precipitates formed on the β(bcc) matrix grain boundaries has been studied with transmission electron microscopy (TEM) in a Ti-15V-3Cr-3Sn-3Al alloy. The α precipitates have a near-Burgers orientation relationship with respect to at least one of the adjacent β grains. Among the possible 12 variants in this orientation relationship, the variant that [11•20]_α is parallel to the 〈111〉_β closest to the grain boundary plane tends to be preferred by the α precipitates. Additionally, further variant selections are made so as to minimize the deviation of orientation relationship with respect to the “opposite“ β grain from the Burgers one. Such rules in variant selection often result in the formation of precipitates with a single variant at a planar grain boundary. Prior small deformation of β matrix changes the variant of α precipitates at the deformed portion of grain boundary. It is considered that the stress field of dislocations in the slip bands intersecting with the boundary strongly affects the variants of α precipitates. Discussion of these results is based upon a classical nucleation theory. Formerly Graduate Student, Department of Materials Science and Engineering, Kyoto University Formerly Graduate Student, Department of Materials Science and Engineering, Kyoto University This article is based on a presentation made during TMS/ASM Materials Week in the symposium entitled “Atomistic Mechanisms of Nucleation and Growth in Solids,“ organized in honor of H.I. Aaronson’s 70th Anniversary and given October 3–5, 1994, in Rosemont, Illinois.     

On the effect of stress on nucleation and growth of precipitates in an Al-Cu-Mg-Ag alloy

A study has been made of the effect of an externally applied tensile stress on Ω and Θ′ precipitate nucleation and growth in an Al-Cu-Mg-Ag alloy and a binary Al-Cu alloy which was used as a model system. Both solutionized and solutionized and aged conditions were studied. The mechanical properties have been measured and the microstructures have been characterized by transmission electron microscopy (TEM). The volume fraction and number density, as well as the precipitate size, have been experimentally determined. It was found that for as-solutionized samples aged under stress, precipitation occurs preferentially parallel to the stress axis. A threshold stress has to be exceeded before this effect can be observed. The critical stress for influencing the precipitate habit plane is between 120 and 140 MPa for Ω and between 16 and 19 MPa for Θ′ for the aging temperature of 160 °C. The major effect of the applied stress is on the nucleation process. The results are discussed in terms of the role of the lattice misfit between the matrix and the precipitate nucleus.     

7055铝合金多级均匀化工艺研究

采用中频炉熔炼7055铝合金,用光学显微镜、扫描电镜、能谱仪和万能拉伸机等方法研究多级均匀化工艺对7055铝合金显微组织及力学性能的影响。结果表明：未经均匀化处理的7055铝合金铸锭存在着严重的组织偏析,合金经470℃×30 h单极均匀化处理后,晶界上的非平衡相有所减少,已形成不连续分布,成分偏析得到改善;在单极均匀化基础上增加410℃×4 h的中间保温,均匀化效果更好,非平衡相的溶解数量增多,晶界更为平直,且形状规整;而经250℃×4 h＋410℃×4 h＋470℃×30 h三级均匀化处理后,消除了铸锭晶界上的非平衡凝固共晶组织,晶界呈线条状。经时效处理后,其力学性能较优。     

The fracture resistance of a binary TiAl alloy

The fracture resistance of a binary Ti-47Al (in at. pct) alloy has been investigated. The binary alloy was cast, forged, and heat treated to a fully lamellar microstructure with a colony size of either 640 or 1425 μm. Fracture toughness tests were performed in a scanning electron microscope (SEM) equipped with a loading stage. Direct observations of the fracture process indicated that crack extension commenced at a stress intensity level of 1.2 to 4 MPa√m. The crack path was primarily interlamellar and crack extension across an individual colony or across similarly oriented colonies was relatively easy. In contrast, crack arrest was prevalent when the crack encountered the boundaries of unfavorably oriented colonies. To extend into an unfavorably oriented neighboring colony, the K level of the approaching crack had to be increased significantly to renucleate a microcrack at a location away from the crack tip, resulting in the formation of an interconnecting ligament that must be fractured to further crack growth. This interaction between the crack and the microstructure led to a large variation in the slope of the K _R curves. Comparison of the K _R curves for the binary Ti-47Al alloy against published data for quinary Ti-47Al-xNb-yCr-zV alloys indicates that the initiation toughness of the quinary alloys is higher by a factor of 5 to 10, implying the existence of a significant beneficial effect of alloying additions on the initiation toughness.     

Elastic interaction energy of two spherical precipitates in an anisotropic matrix

Following the difference method of Eshelby, the elastic interaction energy between two spherical precipitates embedded in an infinite matrix of cubic anisotropy is studied as a function of their distance of separation and alignment direction. When the precipitates are positioned along the [100] direction of the matrix phase, the elastic interaction is found to be attractive and often to exhibit a maximum value at an intercenter distance of two to three radii. For the [110] and [111] alignments, the results depend on the sign of the anisotropic factor,H=2C₄₄+C₁₂−C₁₁, of the matrix phase. When it is positive as in Cu and Ni, the interaction is found to be repulsive. In the reverse case, the situation is substantially different; for the [111] alignment with a Mo matrix, the interaction is found to be of an attractive nature.     

Coarsening rate of Β precipitates in Al-11Mg alloy

The coarsening rates of the Β precipitates in an Al-11 wt pct Mg alloy at 250 ‡C, 316 ‡C, and 330 ‡C aging temperatures were investigated. As predicted by the modified Lifshitz-Slyozov-Wagner (MLSW) theory of diffusion controlled particle coarsening, a linear dependence ofr ³ ont in the long time aging region was observed. Interfacial energies of 8.1, 1.8, and 1.7 J/m² for 250 ‡C, 316 ‡C, and 330 ‡C, respectively, were computed from these data using the MLSW theory. The high values of interfacial energy indicate that the Β precipitates are incoherent with respect to the Al matrix. The especially high value of interfacial energy and low activation energy for the precipitate coarsening at 250 ‡C suggest that significant short circuit diffusion occurs at this aging temperature.     

均匀化处理对Cu-Cr-Zr合金显微组织的影响

采用OM、SEM和TEM分析手段对Cu-Cr-Zr合金的铸态和均匀化组织进行研究.结果表明,Cu-Cr-Zr合金的铸态组织呈典型的枝晶状组织,主要由网状的Cr枝晶、共晶组织和基体组成,其中共晶组织是由基体和层片状的Cu5Zr相组成.在Cu-Cr-Zr合金的均匀化退火过程中,发生共晶组织的溶解和Cr相的析出.随着均匀化退火温度和时间的升高和延长,共晶组织逐渐溶解,Cr相的析出体积分数逐渐减小.合理的均匀化退火制度为900℃×12 h.     

析出相对Monel K-500合金冲击韧性的影响

通过一系列实验研究了Monel K-500合金的冲击韧性。结果表明,合金经550~650℃时效后其冲击断口呈沿晶形貌;在750~850℃时效时,其冲击韧性与强度呈正相关关系。根据对合金在不同热处理状态的析出相形态及分布的研究,揭示了形成这些现象的原因。合金在550~650℃时效时,在晶内析出的γ’相呈球状均匀分布,而在部分晶界附近的γ’相呈条棒状或排成帘状垂直于晶界分布,由此导致时效态合金在断裂时,裂纹沿晶界扩展。在750~850℃时效会导致合金形成大量沿晶界网状分布的二次MC相,从而降低合金的冲击韧性。