The effect of thermal cycle on the microstructural development of a powder metallurgy superalloy braze material
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I.
INTRODUCTION
POWDER metallurgy (P/M) offers a number of unique attributes that are unavailable through conventional casting and forming processes. Metal powders, however, are used in a number of different applications that do not involve the primary synthesis route for parts. Brazing and weld repair are two such areas. Here, metal powder is deposited on the surface of a component. Sufficient heat is applied to melt one or more of the powder constituents. After cooling, the part is machined to final dimensions. For a braze powder to be effective in this function, it must meet several important criteria: (1) the liquid must form at a temperature less than the melting temperature of the base material; (2) the liquid should not degrade the properties of the base material; (3) the solid braze should have the same properties as the base material; and (4) the liquid should have a sufficiently low viscosity to fill the desired location without affecting the surrounding area. As the alloy chemistry becomes more complicated, it becomes increasingly more difficult to adhere to these four goals. Superalloy braze powders are a prime example. Liquid formation must occur at a low enough temperature so that the integrity of the base metal is uncompromised, yet after brazing, the material must have the same mechanical and thermal properties of the substrate. By allocating specific elements into different powders, the melting point of a braze alloy can be tailored such that it does not approach the solidus temperature for the base material. As the liquid diffuses between the particles, it alloys with the other elements present and disappears. The resultant alloy chemistry is consistent with the base material. The fundamentals of transient liquid-phase formation are well understoodtt,2~and have been applied to numerous materials systems,r34~ The thermal cycle used in superalloy brazing is quite
complicated. The time at the braze temperature is short compared to the total duration of the thermal cycle. The remainder of the time is for homogenizing the microstructure and dissolving undesirable phases. Because of the complexity of processing superalloy braze powders via P/M techniques, little is known about the effects of the thermal process on the final properties of the completed braze. Binary or even ternary diagrams can provide only limited information about the microstructural development in these alloys. Most of the data that are available in the literature have been obtained by experimental observations. The purpose of this investigation is to systematically examine the effects of braze and heat treatment on the microstructural development of a representative nickel-based superalloy braze alloy. Tools such as optical microscopy and the scanning electron microprobe were used to characterize the microstructure. II.
MATERIALS CHARACTERIZATION
Four different prealloyed component powders were mixed to produce the proper chemistry for the braze in this investigation. They were chosen from a slate of 12 possible commercially ava
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