Transient liquid-phase bonding in two-phase ternary systems

The process of isothermal solidification during transient liquid-phase (TLP) bonding in a ternary system is analyzed. In the most usual situation, and in contrast to the binary case, the composition of the liquid is required to change continuously as solidification proceeds. If the solubilities and/or diffusion coefficients of the two solutes are very different, the solidification stage is clearly divided into two parabolic regimes, the first dominated by the “faster” solute and the second by the slower of the two. In extreme cases, full solidification may not be realized in experimentally accessible times.     

Transient liquid-phase bonding in the NiAl/Cu/Ni system-A microstructural investigation

A transmission electron microscopy based investigation of microstructural development in NiAl-Ni transient liquid bonding, using commercial purity copper interlayers, is presented in this article. The article considers the mechanisms of isothermal solidification in NiAl/Cu/Ni joints and the influence of copper diffusion from the joint centerline on the microstructures of the adjacent NiAl and Ni substrates. Changes in the microstructure of the bond centerline due to entry of aluminum (from the NiAl substrate) and Ni (from both the NiAl and the Ni substrates) are discussed. Transfer of aluminum from the NiAl substrate to the Ni substrate is also examined. The precipitation of bothLl₂ typeγ′ andB ₂ typeβ phases at the joint centerline is investigated. Precipitation ofγ′ within both the NiAl and Ni substrates is considered. The formation ofAl typeγ phases in the NiAl substrate is also examined.     

Microstructural development in transient liquid-phase bonding

The applicability of conventional models of the transient liquid-phase (TLP) bonding process to the joining of nickel using ternary Ni-Si-B insert metals is considered in this article. It is suggested that diffusion of boron out of the liquid and into the solid substrate before the equilibration of the liquid and solid phases can result in the development of significant boron concentrations in the substrate. This, in turn, leads to the precipitation of boride phases in the substrate during holding at bonding temperatures below the binary nickel-boron eutectic temperature. The formation of boride phases during holding at the bonding temperature is of importance, because first, it is not predicted by the standard models of the TLP process, and second, the borides are not removed by prolonged holding at the bonding temperature and therefore may influence the in-service properties of the joint. In contrast, when bonding above the binary nickel-boron eutectic temperature, localized liquation of the substrate takes place. This liquid region resolidifies following prolonged holding and does not result in the formation of persistent boride phases. Experimental support is presented for the formation of borides during bonding, and characterization of the boride phases formed in the substrate is described.     

A study of transient liquid-phase bonding of Ag-Cu using differential scanning calorimetry

A novel approach using differential scanning calorimetry (DSC) to quantify interface kinetics in a solid/liquid diffusion couple is applied to characterize the isothermal solidification stage during transient liquid-phase (TLP) bonding of Ag and Cu using a Ag-Cu interlayer. When the DSC results are properly interpreted, the measured interface kinetics are more accurate than those obtained using traditional metallographic techniques. Experimental results are compared to predictions for isothermal solidification given by a selection of analytical models. The comparison yields close agreement with a solution that assumes a moving boundary; but accuracy of the predictions is very sensitive to selection of solute diffusivity. Metallographic inspection of the DSC samples and traditional TLP bonds validates the kinetics measured using this technique, and supports the prediction given by the analytical model. This study shows that the method of using DSC to quantify interface kinetics is valuable in the refinement of process parameters for TLP bonding. Furthermore, simple analytical solutions can be applied to predict the process kinetics of isothermal solidification in simple binary systems with considerable accuracy when the effects of grain boundaries can be neglected, thus reducing the need for complex numerical models when developing process parameters.     

Transient liquid phase bonding of ferritic oxide-dispersion-strengthened alloys

Joining of two ferritic oxide-dispersion-strengthened (ODS) alloys, MA956 and PM2000, using transient liquid phase (TLP) bonding is discussed. Thin-film boron coatings of different thicknesses were used as interlayers and different bond orientations with substrates cut along and normal to the direction of extrusion were studied, with postbond heat treatment and microscopic evaluation of the bonds. Microstructural continuity was achieved in bonds when joining fine grain substrates cut along the direction of extrusion.     

Simulation of percolation structure of grain bonding in liquid-phase sintering by three-dimensional grain structure reconstruction

Computer simulations of the percolation structure often employ ordered lattice networks and assume all sites are monosized. However, in many situations, sites are located randomly in the space and have a continuous size distribution. In this article, we use a spatial microstructure reconstruction program to simulate the percolation structure of a classic liquid-phase sintering system, e.g., tungsten heavy alloy. The material was sintered under microgravity conditions to avoid settling. With this technique, the continuous three-dimensional (3-D) grain size distribution is created using the features obtained from a two-dimensional (2-D) metallographic section where grain and contacting neighbors are recorded. A recursive procedure is conducted to search for connected grain clusters. After all the grain clusters are taken into account, the dimension and the grain number of the largest cluster are recorded. These simulation results give improved understanding of percolation in liquid-phase sintering.     

The influence of solid-state and liquid-phase bonding on fatigue at Al/Al2O3 interfaces

Fracture and fatigue experiments have been conducted on liquid phase bonded (LPB) and solid-state bonded (SSB) aluminum-alumina interfaces. The LPB interfaces contain voids and dendritic FeAl₃ precipitates, whereas SSB interfaces are relatively defect-free. These precipitates result in local embrittlement, yet both interfaces are strong and tough. Upon cyclic loading, mode 1 cracks in both systems grow alternately along the interface and within the Al. The development of a tortuous crack path elevates the apparent fatigue threshold through crack closure. Under mixed mode loading, fatigue cracks approaching SSB interfaces propagate through the Al rather than along the interface. Conversely, for LPB interfaces, mixed mode cyclic crack growth along the interface occurs in preference to propagation in the Al. Correlation between the striation spacing and the crack tip opening displacement suggests a growth mechanism based on crack tip blunting. A.G. EVANS, Professor, formerly with the Division of Engineering and Applied Sciences, Harvard University, Cambridge, MA 02138     

The influence of solid-state and liquid-phase bonding on fatigue at Al/Al2O3 interfaces

Fracture and fatigue experiments have been conducted on liquid phase bonded (LPB) and solid-state bonded (SSB) aluminum-alumina interfaces. The LPB interfaces contain voids and dendritic FeAl₃ precipitates, whereas SSB interfaces are relatively defect-free. These precipitates result in local embrittlement, yet both interfaces are strong and tough. Upon cyclic loading, mode 1 cracks in both systems grow alternately along the interface and within the A1. The development of a tortuous crack path elevates the apparent fatigue threshold through crack closure. Under mixed mode loading, fatigue cracks approaching SSB interfaces propagate through the A1 rather than along the interface. Conversely, for LPB interfaces, mixed mode cyclic crack growth along the interface occurs in preference to propagation in the A1. Correlation between the striation spacing and the crack tip opening displacement suggests a growth mechanism based on crack tip blunting.     

Dilatometric investigations of the processes of liquid-phase sintering of materials of the Cu-Sn system

Conclusions The shrinkage of Cu-Sn powder compacts during liquid-phase sintering, which is due to dissolution of copper particles in molten metal and their regrouping, is preceded by growth caused by preferential diffusion of tin atoms from the liquid phase to the solid. The process of regrouping of solid-phase particles as a result of the destruction of the rigid skeleton of a compact at a high tin content at the instant of appearance of molten metal fails to manifest itself explicitly. The possibility cannot be ruled out that regrouping takes place at the instant of diffusional growth of compacts and, superimposing itself on the latter process, is responsible for the extent of relative growth being dependent on starting porosity.Translated from Poroshkovaya Metallurgiya, No. 9(261), pp. 27–31, September, 1984.     

Bulk-alloy microstructural analogues for transient liquid-phase bonds in the NiAl/Cu/Ni system

Transient liquid-phase (TLP) bonds between dissimilar materials can have complex microstructures that evolve both during holding at the bonding temperature and on cooling. In this article, an examination is made of the feasibility of producing bulk-alloy microstructural analogues for individual microstructural features of dissimilar material TLP bonds. The ultimate intent of this work is to enable the contribution of individual microstructural features to the overall properties of TLP bonds to be determined. Specifically, the article focuses on the production, characterization, and applications of microstructural analogues for TLP bonds in an NiAl/Cu/Ni model system. The article examines the use of five different cast Ni-Al-Cu alloys, together with heat treatment of selected materials, as bulk analogues for six distinct microstructural regions of the NiAl/Cu/Ni bonds. Each of these analogues is characterized in detail by transmission electron microscopy (TEM) and compared to the relevant target region of the bond. An initial examination is also made of the use of bulk alloys in aiding an understanding of phase transformations and structure-property relationships in these bonds.     

Rearrangement densification in liquid-phase sintering

Grain rearrangement has a significant role in sintering densification when a liquid phase forms. Although grains may change shape, bringing the grain centers closer to each other and producing overall densification, this contribution is slow and localized to the grain contacts regions. On the other hand, the movement of individual grains through capillary force effects can provide a burst of rapid densification. To describe the spatial grain arrangement quantitatively, a new parameter is proposed, termed the grain-arrangement factor. An increase in the grain-arrangement factor indicates the microstructure is packing to a higher density. Analysis of the relations between this factor and other microstructure parameters provides an indirect method to measure the state of grain arrangement in liquid-phase sintering. Experiments performed on a tungsten heavy alloy show that the grain-arrangement factor mainly depends on the solid volume fraction and the dihedral angle.     

Special features of formation of the ring-shaped structure in solid- and liquid-phase sintering alloys of the TiC-Ni-Mo system

Translated from Poroshkovaya Metallurgiya, No. 6(354), pp. 75–79, June, 1992.     

Freeze-off limits in transient liquid-phase infiltration

Transient liquid-phase infiltration (TLI) involves a powder-metal skeleton and an infiltrant with similar composition containing a melting-point depressant (MPD). Upon infiltration, the MPD diffuses into the skeleton, causing isothermal solidification and allowing a homogeneous final-part composition. Diffusional solidification of the infiltrant can restrict liquid flow and result in premature freeze-off if the liquid solidifies before filling the entire part. A capillary-driven fluid-flow model was developed to predict the infiltration rate and freeze-off limit using a variable skeleton permeability. Diffusional solidification was measured via quenching experiments, compared to theory, and used to define the change in permeability of the skeleton. The infiltration rate was measured via mass increase and compared to the flow model for various skeletons with powder sizes ranging from 60 to 300 μm. The predicted horizontal infiltration freeze-off limits were proportional to the square root of d ³ γ/μDβ ², where d is the average powder diameter, γ and μ are the infiltrant surface tension and viscosity, respectively, D is the solid diffusivity, and β is a function of the solidus and liquidus concentrations. These relations can be used for selection of processing parameters and for evaluation of new material systems.     

Macrosegregation and sedimentation in liquid-phase sintering

Macrosegregation has been observed in a series of Pb-Sn alloys isothermally treated in an α + L (or β + L) two-phase region. Depending on the alloy composition, the Pb-rich α (or the Sn-rich β) solid phase quickly settles (or floats) to the bottom (or top) of the crucible as the alloy is heated into the solid-plus-liquid two-phase region. This settling or floating persists until such time as a skeletal solid structure is formed at the crucible bottom or top. The skeleton (the “mushy” zone) resembles liquid-phase-sintered structures; that is, it consists of interconnected solid and liquid phases. The initial settling can give rise to macrosegregation. Elimination of macrosegregation requires substantial time and is accompanied by a reduction in length of the mushy zone, giving rise to apparent sedimentation of this zone. We argue that this sedimentation results from the compositional readjustments accompanying macrosegregation elimination. On this basis, it appears that many observations of sedimentation in liquid-phase-sintered structures are due to these readjustments.     

Densification and shape distortion in liquid-phase sintering

Densification and dimensional control are important aspects of liquid-phase sintering. The capillary force and the solid bonding affect both densification and shape preservation. Capillarity, which is orientated isotropically, causes uniform shrinkage and holds grains together to preserve the component shape in the early stage of sintering. On the other hand, solid bonding resists viscous flow and inhibits densification and shape distortion. The capillary force decreases with densification and approaches zero as pores are eliminated. Thus, shape retention eventually requires solid-grain bonding. The solid-grain bonding provides compact rigidity, which is represented by compact strength. Shape distortion occurs when the compact loses its strength. For every situation, there is a critical compact strength above which no shape distortion occurs. Distortion in liquid-phase sintering indicates that the compact strength passed below a critical level.