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.     

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.     

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.     

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.     

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。     

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.     

A variational approach to grooving and wetting

Two bodies, e.g. grains with a certain surface contour, are assumed to be in contact at a plane interface, e.g. a common grain boundary with an arbitrary inclination relatively to the surface and with zero mobility and diffusivity. A groove appears due to surface diffusion along the triple line, i.e. the intersection line of the two surfaces and the grain boundary. The thermodynamic extremum principle is applied to derive the evolution equations for the surfaces of both bodies as well as the contact conditions at the triple line. Applications to grooving and wetting are demonstrated and compared with the results from the literature. The simulations indicate that the groove root angle can be significantly different from the value of the dihedral angle calculated from the equilibrium condition for the specific grain boundary and surface energies. Moreover, it is demonstrated that the groove angle is dependent on the kinetic parameters, e.g. surface diffusion coefficients of individual grains.     

Deformation behavior of TiAl intermetallic compounds and orientation control by reactive diffusion and high-temperature uniaxial compression deformation

To determine the optimum method of producing TiAl intermetallic compounds with an oriented lamellar microstructure the authors applied reactive diffusion and high-temperature deformation in a two-phase state on Ti-43 mol.%Al and Ti-46 mol.%Al. The reactive diffusion process produces an α single-phase state with orientations that reflect the texture of the starting titanium foils; the high-temperature deformation aims to rotate the lamellar interface by means of preferential activation of the crystal slip systems that are parallel to the interface. The reactive diffusion process can effectively control the texture in an α single-phase state, though many voids are formed at the initial stages of reactive diffusion. Furthermore, after the heating process for the construction of the lamellar microstructure, uniaxial compression at 1323 K is quite effective for ensuring that the lamellar interface is rotated parallel to the compression plane. Finally, the deformation causes about one half of all the lamellae to be arranged within 20 degrees of the compression plane.     

Annealing effects on the intermetallic compound formation of cold sprayed Ni,Al coatings

The annealing of Ni and Al coatings under various conditions on substrates fabricated by a cold gas dynamic spray process (CDSP) were investigated. The powder particles were accelerated through a standard De Laval-type nozzle with air used as the main carrying gas. The coatings were annealed at 450–550 °C in either argon or air atmospheres for 4 h. In the case of Ni coatings during annealing both in argon and air atmospheres, intermetallic compound layers such as Al₃Ni and Al₃Ni₂ were observed at the interfaces between the Ni coating and Al substrate. Also, the intermetallic layer formation of Al₃Ni and Al₃Ni₂ at the interfaces depended on the solid-state diffusion and the annealing temperature. The intermetallic compound AlNi was obtained at the interface of Al coating on a Ni substrate by low-temperature annealing under the melting temperature.     

Modelling intermetallic phase formation in dissimilar metal ultrasonic welding of aluminium and magnesium alloys

Abstract

Intermetallic compound (IMC) formation at the joint line usually has strongly detrimental effect on the performance of dissimilar metal welds. To understand the formation of IMC interlayers, and explore strategies to control their growth, a model has been developed and applied to the case of dissimilar joining of aluminium and magnesium alloys using ultrasonic welding. The model accounts for microbond formation during welding, diffusion across the joint line, as well as nucleation, spreading and thickening of the first IMC layer (Mg₁₇Al₁₂ phase) and the formation and simultaneous thickening of the second (Al₃Mg₂) layer. The model predictions match measurements reasonably well and the model has been used to predict the sensitivity of IMC layer thickness to weld temperature and time.     

Microstructural evolution and formation of nanocrystalline intermetallic compound during surface mechanical attrition treatment of cobalt

Nanocrystalline intermetallic Co₃Fe₇ was produced on the surface of cobalt via surface mechanical attrition (SMA). Deformation-induced diffusion entailed the formation of a series of solid solutions. Phase transitions occurred depending on the atomic fraction of Fe in the surface solid solutions: from hexagonal close-packed (<4% Fe) to face-centered cubic (fcc) (4–11% Fe), and from fcc to body-centered cubic (>11% Fe). Nanoscale compositional probing suggested significantly higher Fe contents at grain boundaries and triple junctions than grain interiors. Short-circuit diffusion along grain boundaries and triple junctions dominate in the nanocrystalline intermetallic compound. Stacking faults contribute significantly to diffusion. Diffusion enhancement due to high-rate deformation in SMA was analyzed by regarding dislocations as solute-pumping channels, and the creation of excess vacancies. Non-equilibrium, atomic level alloying can then be ascribed to deformation-induced intermixing of constituent species. The formation mechanism of nanocrystalline intermetallic grains on the SMA surface can be thought of as a consequence of numerous nucleation events and limited growth.     

Monitoring of intermetallic phase formation in jelly-rolled Nb/Al multifilamentary conductors by in-situ resistometry

In-situ resistance measurements were performed on jelly-rolled Nb/Al multifilamentary conductors during solid-state diffusion at temperatures less than 850 °C. During the heat treatment, formation of NbAl₃ and Nb₃Al phases can be clearly identified in the plot against temperature of the first derivative of resistivity with respect to temperature (dρ/dT). In addition, the two-stage formation of a single phase (NbAl₃) is also observed in the dρ/dT plot. The superconducting properties of the A15 Nb₃Al phase were measured after different heat treatments, and were found to be approximately independent of the extent of NbAl₃ formation at low temperatures.     

Refining alloy zinc-iron with intermetallic phases ZnnFem by formation phases AlnFem

Results on research work on hard zinc refining with aluminium on a laboratory and a commercial scale have been presented. The research was carried out according to a fractorial experiment design with the determination of four processes variables. Distribution of Al, Fe and Zn was examined by the X-ray micro-analyser and also intermetallic phases were identified. As a result of the formation of intermetallic phases which, lighter than zinc, were coming to the metal surface, a refined zinc containing 0.02 wt.% Fe and suitable for re-use in the processes of steel products coated with zinc, was obtained.