Retarded grain boundary mobility in activated sintered molybdenum
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INTRODUCTION
THE significance
of grain boundaries to densification during sintering has been well recognized ever since the classic experiments by Alexander and Balluffi. 1 There are a few examples where sintering enhancement has been attained by controlling grain growth during sintering, the most notable being alumina doped with MgO. 2'3 Other examples include alumina additions to iron4 and molybdenum additions to alumina. 5 In such cases, the dispersoids inhibit grain growth during sintering, providing an opportunity for better sintered properties. The unique mechanical properties of refractory metals at high temperatures make them quite difficult to fabricate. In this sense, powder metallurgy has played a vital role in the production of molybdenum components. The addition of appropriate transition metals as co-mixed phases eases fabrication by lowering the sintering temperature. 6-14 Unfortunately, while a transition metal such as nickel can enhance the sintering of molybdenum at low temperatures, extensive microstructural coarsening is observed. 13-16In tungsten activated with nickel or palladium, it has been found that sintering grain growth decreased both the sintered strength and density. 17This study examines the effect of grain growth on densification and experimentally demonstrates that retarding grain growth is a means to enhance densification in molybdenum activated with nickel. Since grain growth and densification are competitive processes, it is advantageous to sintering kinetics and mechanical properties to retard grain growth. Consequently, experiments are conducted to reduce the grain boundary mobility by dispersing silica at the interparticle grain boundaries. II.
BACKGROUND
Activated sintering of refractory metals with certain transition metal additives occurs because the additive lowers the P.E. ZOVAS, formerly Research Assistant in the Rensselaer Polytechnic Institute, Materials Engineering Department, is now on the Engineering Staff, Sikorsky Aircraft, Stratford, CT. R.M. GERMAN is Associate Professor, Materials Engineering Department, Rensselaer Polytechnic Institute, Troy, NY 12181. 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
energy barrier for refractory atom transport. 18 The lower activation energy results in increased diffusion of the refractory metal through the additive. The additive remains segregated at the particle-particle contact points because of the unipolar solubility relationship. That is, the refractory metal is soluble in the additive, but the additive is relatively insoluble in the refractory metal. The segregated lower melting additive provides a rapid short-circuit mass transport path throughout the sintering process. It is well known that grain boundaries act as effective vacancy sinks when dens
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