Intermetallic compound layer growth at the interface of solid refractory metals molybdenum and niobium with molten alumi
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INTRODUCTION
T H E formation and growth of one or more intermetallic compound layers during dissimilar metal contact at high temperatures is a commonly occurring phenomenon. Such behavior by solid-state interdiffusion often is observed in fiber-matrix reactions in composites, solid-state joining of dissimilar metals by diffusion, and the formation of diffusional coatings, u,2,31 and by solid-liquid metal interaction in hot-dip coating, liquid metal corrosion, soldering, brazing, and dissimilar metal wetding.~4-7~ In many instances, the formation of brittle intermetallic layered phases between the dissimilar metals can lead to a reduction in desirable mechanical and physical properties of a joint. Often, the kinetics of layer growth of a phase at a given temperature is also of importance in defining the processing conditions necessary for optimum performance or in predicting the duration of successful usage. In general, in binary systems, the number of possible intermetallic compounds present at a given temperature is dictated by the alloy constitution diagram if local equilibrium is attained at the phase boundaries. However, according to experimental observations, often not all o f the possible phases will form since this depends upon the nucleation and growth properties of each phase, the wettability of the contact surface, chemical reactions between the interacting phases, and the rates of diffusion of the reacting species through the interlayer. [1-3,81 N. TUNCA, Research Associate, and R.W. SMITH, Head of Department of Metallurgical Engineering, are with Queen's University, Kingston, ON, Canada K7L 3N6. Manuscript submitted November 20, 1987. METALLURGICAL TRANSACTIONS A
The purpose of the present study was to examine the interfaces between the refractory metals Mo and Nb and molten aluminum in order to determine the types of intermetallic layers formed, their rates and growth and probable growth mechanisms, and their mechanical properties, in both saturated and unsaturated melts. These experiments can provide fundamental information on the growth and diffusion kinetics of the intermetallic compound phases of single and multiphase diffusion couples.
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EXPERIMENTAL PROCEDURE
Two sets of experiments were carried out to investigate the growth of layers of intermetallic phases in saturated and unsaturated molten aluminum. To study layer growth in a saturated melt, a small quantity of aluminum (approximately 50 g) was melted in air under a commercial brazing flux. After reaching the required temperature, a clean (Mo and Nb disc) specimen was dipped into the crucible, initially into the flux layer for approximately one minute to heat up the sample and to break the oxide film on the disc surface, and then into the aluminum melt beneath the flux. The sample was removed after one minute and quenched in water. The lateral surfaces of the specimens were protected by alumina tubes. The resulting disc was covered by a thick aluminum layer. Each disc was divided into several pieces to give small pieces with a layer of
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