The role of compound formation in reactive wetting: the Cu/SiC system

Wettability and reactivity in the liquid Cu/SiC couple are studied by the sessile drop technique in high vacuum or purified helium using monocrystalline or sintered α-SiC. This couple exhibits a very unusual wetting behaviour. Particularly, during spreading, an amazing drop shape is established and, for specific conditions, “hexagonal wetting” is observed. We show that these observations can be correctly explained using the concepts of reactive wetting developed recently.     

Quasi-static chip formation of intermetallic titanium aluminides

As a result of the development of new materials for high temperature applications the potential for mass reduction and increased process temperatures is constantly being expanded. Intermetallic γ-TiAl alloys can meet these demands to a large extent. The properties necessary for these applications have an adverse effect on the machinability however and render intermetallic titanium aluminides as difficult to machine materials. Cutting operations tend to produce damaged surfaces which are unsuitable for the intended applications. As the basis for a reliable and economic cutting technology, the chip formation of the intermetallic TiAl alloy TNBV5 has been examined in quasi-static cutting experiments. Observations showed that increased workpiece temperatures lead to a transition of the chip formation from segmented to continuous chips. By decreasing the undeformed chip thickness crack-free surfaces could be produced at low workpiece temperatures. In this case other mechanisms than the thermal activation of slip systems must be the reason for the observed large plastic deformations. The theory that hydrostatic pressure leads to this behavior is substantiated by the results of finite element simulations. This offers the possibility for damage free machining at lower cutting speeds, thus enabling the use of conventional tool materials at an acceptable tool life.     

Atomic morphology and chemical reactions of the reactive wetting front

Morphology at the reactive wetting front during spreading was first observed in situ at an atomic scale using a high resolution transmission electron microscope. Two typical nanomorphologies of an Ag–Cu–Ti molten alloy were observed on a SiC substrate depending on the nucleation position of the reaction product, TiC. In the case of the precursor atomic thin layer spreading ahead of the droplet of the molten alloy, the TiC nucleated in the thick region of the precursor, which was a few nm behind the tip of the precursor layer. In contrast, molten alloy spreading with TiC growing simultaneously at the tip was also observed. In this case, the precursor layer on SiC did not exist. Nucleation and growth rate of the reaction product were related to the tip morphology.     

Enthalpy of formation of Au-Sn intermetallic phases. Part II

The technique of solution calorimetry with liquid Sn has been used for the determination of enthalpy of formation of intermetallic compounds from the Au-Sn system. The intermetallic phases were prepared and homogenized in a glow-box operated in high purity argon. They were analyzed by an X-ray diffraction method before the calorimetric investigations to confirm their crystallographic structure. The obtained experimental values of enthalpy of formation were −5.8 ± 0.3 and −1.3 ± 0.4 kJ/mole of atoms for ζ′-Au₅Sn, and ζ-Au₅Sn of the composition Au₈₉Sn₁₁, respectively. Using the DTA technique the transition temperatures were measured for the following phases: η-AuSn₄, ?-AuSn₂, δ-AuSn, ζ′-Au₅Sn and ζ-Au₅Sn (Au₈₉Sn₁₁). It was found that the obtained data in the study agreed very well with the results previously presented in literature, except for the ζ-Au₅Sn (Au₈₉Sn₁₁) intermetallic phase, for which a difference of about 50° was observed for the liquidus temperature.     

Early stages of intermetallic compound formation and growth during lead-free soldering

We investigate the early stages of the morphological evolution of intermetallic compounds (IMCs) during lead-free soldering involving liquid Sn-based solder and Cu substrate considering the heterogeneous nucleation of the IMCs. The simulation is performed through the multi-phase-field approach [20], [25]. Initially, the liquid Sn-based solder and the Cu substrate are considered to be at metastable local equilibrium. Nucleation at the solder/substrate interface is modeled by considering it to be a Poisson process. The phase-field simulation accounts for variations in grain boundary diffusion in the η phase and interface energies between the η phase and liquid solder, and uses these variations to investigate the multiplicity of soldering reactions, and make comparisons with previous work [23]. The simulations address the kinetics of the IMC growth during lead-free soldering under the effect of nucleation at an early stage, illustrating the variation in Cu substrate thickness, IMC thickness and number of grains that nucleate or disappear due to grain growth-induced coalescence.     

Al-Li-Mg(Ti)合金形成焓的EAM研究

采用普适分析型的嵌入原子模型,通过拟合纯元素的晶格常数、空位形成能、结合能和体积弹性模量等,确定了Al、Li、Mg和Ti 4个元素的嵌入原子模型参数.不同元素之间的相互作用势的参数由拟合实验或第一原理计算结果确定.用确定的嵌入原子模型分别计算了由Al,Li、Mg和Ti元素构成的二元系以及三元系中的有序合金化合物的形成焓,计算结果与已有的实验结果和第一原理计算结果符合得比较好.     

Use of pre-oxidation to improve reactive wetting of high manganese alloyed steel during hot-dip galvanizing

The present study discusses hot-dip galvanizing of a Fe–23% Mn–0.6% C–0.3% Si steel using a Zn–0.22%Al bath. The paper concentrates on reactive Zn wetting on top a covering external oxide layer occurring after in-line annealing. Annealing was performed by soaking at 800 °C/60 s in 5% H₂–N₂ at different dewpoints. In-line pre-oxidation at 600 °C/10 s in 1.8% O₂–N₂ was further performed and the impact on selective oxidation as well as reactive Zn wetting was examined. After conventional annealing Zn wetting turns to increase if a roughly globular MnO layer appears on the external steel surface and Si is internally oxidized. Reactive wetting including the formation of Fe₂Al₅ crystals occurs on top of the MnO layer, because metallic-bond Fe exists incorporated within this MnO layer (→MnO·Fe_metall layer). The amount of metallic-bond Fe within the MnO·Fe_metall layer increases considerably if pre-oxidation is conducted. This results in an intensified Fe₂Al₅ formation on top of a MnO·Fe_metall, which improves liquid Zn wetting. Brittle Fe–Zn intermetallics were absent in all trials. These results offer a new way for hot-dip galvanizing (high) Mn alloyed steels. The absence of Fe–Zn intermetallics and (partial) MnO reduction implies that the currently discussed model of aluminothermic MnO reduction during hot-dip galvanizing Mn alloyed steel seems not to be dominating in the present case of reactive Zn wetting on top of a covering MnO·Fe_metall layer.     

A comprehensive model of ordered porosity formation

The model presented in this paper considers heat transfer, gas diffusion and simultaneous growth of solid and gas fractions in an ordered porosity ingot. Special attention is paid to the gas pore nucleation. The diffusion boundary layer ahead of the solid/liquid interface is analysed with respect to the diffusion process and structure formation, both controlled by external gas pressure, solidification velocity and gas fluxes between solid, liquid and pores. The relationships between processing parameters and structure characteristics are described. How the porosity structure is affected by the size of gas nuclei and the number of nucleation sites on the solid/melt interface is analysed.     

反应合成Ni-Al金属间化合物的组织及形成机理(英文)

以Ni粉和Al粉为原料,通过热爆反应合成制备Ni-Al系金属间化合物,研究了反应物摩尔比对生成物物相、组织结构和显微硬度的影响。结果表明:当Ni、Al摩尔比为1:1时,反应生成物为单一的NiAl相;当Ni、Al摩尔比为2:1时,生成物相组成为NiAl+Ni3Al相,因为非平衡冷却,NiAl含量多,呈不规则块状形态分布,Ni3Al相析出量少,主要沿初生NiAl晶界分布,当进一步提高Ni含量,且Ni、Al摩尔比为3:1时,生成物组织形态复杂,除枝晶状的β-NiAl、粗大块状和网状的γ-Ni3Al外,还有板条状和针片状的M-NiAl和不规则的块状或者尖条状的γ′-Ni3Al,这主要是由非平衡冷却和β-NiAl内的成分差异而引起的。     

Polarity effect of electromigration on intermetallic compound formation in SnPb solder joints

The polarity effect of electromigration on the interfacial reactions of micro ball grid array (μBGA) solder joints was studied in terms of microstructural evolution. A dummy μBGA package with the same pair of solder joints was used to obtain reproducible results for a practical application. The μBGA package with Ni(P)/SnPb/Ni(P) structure showed a polarity effect on intermetallic compound (IMC) formation after stress by a current density of 3.0 × 10³ A cm^?2 at 120 °C, compared to the no-current case. Electric current enhanced the growth of Ni₃Sn₄ at the anode and retarded it at the cathode because of the change in Sn diffusion induced by electromigration. The IMC growth at the anode was about one order of magnitude faster than that at cathode. The growth of IMC at the anode had a parabolic dependence on time since the square of thickness of IMC increases linearly with time. The real μBGA package has unique and more complicated geometry compared to ordinary diffusion couples. So not only the polarity growth of IMC but also the fast dissolution of Cu and Ni at specific positions was found with the μBGA package. The unique geometry of μBGA solder joints caused heat generation at the upside interfaces due to Joule heating. Because of the heat generation and the high interfacial energy at the triple point of upside interfaces, the electroless Ni(P) finishes and Cu trace are consumed rapidly in the region where the current is crowded. Thus electromigration induced the rapid dissolution of Ni and Cu into solder and then formed (Cu,Ni)₆Sn₅ at the triple point of the current crowding region at high temperature. This is a rapid failure process because the localized fast dissolution of Ni and Cu accelerated the solid-state reaction between solder and electroless Ni(P) or Cu trace.     

The role of a preceding chemical reaction and reactive clusters in phase transitions of intermetallic compounds

By means of chemical and X-ray phase analysis with the use of a rotating disc electrode, it is found that the dissolution of Mg-Cu and In-Bi intermetallic compounds proceeds via magnesium ionization and subsequent $Mg_2 Cu\xrightarrow[{ - Mg^{2 + } }]{}MgCu_2 $ and $In_2 Bi\xrightarrow[{ - In^{3 + } }]{}InBi$ InBi phase regrouping. It is postulated that the process is accompanied by the formation of a defective crystal lattice and the appearance of reactive clusters, which interact to produce, at first, nuclei of a new phase and, then, the crystal lattice of a new intermetallic compound that is enriched in the electrochemically less active component.     

Effect of ultrasonic treatment on formation of iron-containing intermetallic compounds in Al-Si alloys

Iron is generally regarded as an unavoidable impurity in Al-Si casting alloys.The acicular AI_3Fe andβ-AI_5FeSi(or Al_9Si_2Fe_2) are common iron-containing intermetallic compounds(IMCs) in conventional structure which have a detrimental impact on the mechanical properties.In this paper,ultrasonic field(USF) was applied to modify acicular iron phases in AI-12%Si-2%Fe and AI-2%Fe alloys.The results show that the USF applied to Al-Fe alloys caused the morphological transformation of both primary and eutectic AI_3Fe from acicular to blocky and granular without changes in their composition.In the case of Al-Si-Fe alloys,ultrasonic treatment led to both morphological and compositional conversion of the ternary iron IMCs.When the USF was applied,the acicular β-AI_9Si_2Fe_2 was substituted by star-like α-AI_(12)Si_2Fe_3.The modification rate of both binary and ternary iron IMCs relates to the USF treatment duration.The undercooling induced by the ultrasonic vibration contributes to the nucleation of intermetallics and can explain the transformation effect.