Microstructural development in transient liquid-phase bonding
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INTRODUCTION
IN the transient liquid-phase (TLP) process, an insert is placed between two subsliates which forms a liquid phase at the bonding temperature. The liquid is then gradually removed by interdiffusion with the substrates. One of the most attractive features of TLP bonding is that it seemingly provides a method of joining materials, such as nonweldable gamma-prime or oxide dispersionstrengthened alloys, without the extensive precipitation of second phases. Hence, the process should allow the retention of parent metal properties in fabricated components. In order to understand the conditions required to allow the production of joints free from second phases, models of the TLP process have been proposed. Conventional models of the TLP bonding process tl,2,31 are based on binary alloy liquids. The binary alloy liquid can be produced either directly from a binary insert (for example, Ni-B) or from a single element insert that forms the binary liquid by reaction with the substrate. In these models, it is assumed that, after formation of the liquid, the solid and liquid phases initially equilibrate to their respective solidus and liquidus compositions. When equilibration is complete, solid-state diffusion of the melting point depressant (such as boron) in the substrate results in gradual isothermal solidification of the insert. According to the models, the process of isothermal solidification is completed without the precipitation of second phases during holding at the bonding temperature. The remaining melting point depressant can then be diluted in the substrate by a prolonged homogenization treatment. Hence, the precipitation of second phases on cooling (due to decreased solubility of the melting point depressant at lower temperatures) can be largely or entirely avoided. W.F. G A L E , Research Associate, and E.R. W A L L A C H , Lecturer, are with the Department of Materials Science and Metallurgy, University of Cambridge, Cambridge CB2 3QZ, United Kingdom. Manuscript submitted February 19, 1991. METALLURGICAL TRANSACTIONS A
In contrast to the binary alloy inserts considered in the conventional models, commercial TLP insert metals may consist of ternary alloys such as Ni-Si-B. In this article, it is proposed that with ternary Ni-Si-B inserts, deviations from the conventional models can occur. It is suggested that these deviations can result in the precipitation of boride phases at the bonding temperature during the initial stages of joint formation. II.
EXPERIMENTAL TECHNIQUES
Transient liquid-phase bonds were fabricated using a Ni-4.5 pet Si-3.2 pet B (weight percentages) insert metal (American Welding Society designation BNi-3) in the form of 51 /xm-thick melt-spun sheets and commercialpurity nickel substrates (99 wt pct Ni) in the form of discs of 35 mm in diameter and 2 mm in thickness. Highpurity (99.98 wt pct minimum) nickel substrates were also employed, although this difference in impurity level was not found to be of importance for these substrates. Bonds were fabricated under a nominally zero l
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