Carbide composition change during liquid phase sintering of a wear resistant alloy
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I.
INTRODUCTION
LIQUID phase sintering is a process by which complete or nearly complete densification of powder compacts can be achieved at temperatures high enough to generate a liquid phase in multicomponent systems. Most of the research reported in the literature has examined the liquid phase sintering of relatively simple two component systems in order to isolate various phenomena such as rearrangement or dissolution and reprecipitation during sintering. ~-9 In these systems (e.g., W-Ni, Fe-Cu) the liquid phase evolves in a straight forward manner by the melting of the low melting point element or solid solution. However, no research seems to have been reported on the role of compositional changes in one of the constituent phases in triggering the formation of a liquid phase in more complicated systems. Such an occurrence has been observed during the sintering of STELLITE* Alloy No. 6 PM (Co-Cr-W-C-B) *STELLITE is the registered trademark of Cabot Corporation.
which is reported here. STELLITE Alloy No. 6 has been characterized as being a hypoeutectic Co-Cr-W-C alloy which forms a dendritic Co-rich solid solution fcc phase and an interdendritic CoM7C3 type carbide eutectic phase during coolingJ ~ It has been shown that the solidification range of this alloy is approximately 100 ~ Such a narrow solidification range means that the liquid phase sintering range is even smaller since shape stability must be maintained during sintering. The alloy investigated here, STELLITE Alloy No. 6 PM, has a nominal composition similar to that of STELLITE Alloy No. 6 (Co-29 Cr-4.5 W-1.2 C- 1.0 Si) with an addition of less than 1 pct boron. ~2This alloy can be sintered to near full density over a much broader temperature range than its P.J. McGINN, Graduate Research Assistant, and A.E. MILLER, Professor, are with the Department of Metallurgical Engineering and Materials Science, University of Notre Dame, Notre Dame, IN 46556. P. KUMAR is Powder Metallurgy Group Leader, Cabot Corporation Technology Department, Kokomo, IN 46901. A.J. HICKL is Director of Technology, Cabot-Berylco, Reading, PA 19603. This paper is based on a presentation delivered at the symposium "Activated and Liquid Phase Sintering of Refractory Metals and Their Compounds" held at the annual meeting of the AIME in Atlanta, Georgia on March 9, 1983, under the sponsorship of the TMS Refractory Metals Committee of AIME.
METALLURGICALTRANSACTIONS A
counterpart without boron, thereby facilitating commercial processing. In previous research 13 the microstructural development during the sintering of STELLITE Alloy No. 6 PM was investigated. The following observations were made: 1. The initial dendritic structure present after atomization spheroidizes upon heating to temperatures near the solidus. A eutectic structure appears after sintering at temperatures above the solidus. 2. The eutectic develops throughout the particles simultaneously, but large pools of eutectic first appear in the interparticle neck region. It was postulated that capillary tensile stresses pre
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