Sintering atmosphere effects on tensile properties of heavy alloys
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INTRODUCTION
TUNGSTEN heavy alloys are a class of composites characterized by a high density. They are processed by classic liquid phase sintering from W-Ni-Fe or W-Ni-Cu elemental powder mixes, where the tungsten content ranges from 80 to 98 wt pct. The final sintered alloy consists of nearly pure tungsten grains dispersed in a ductile matrix which has an approximate composition of 53Ni-23Fe-24W. [~I The typical mean tungsten grain size varies from 20 to 60 micrometers, depending on the initial particle size, volume fraction of tungsten, sintering temperature, and sintering time. The combination of density, ductility, strength, conductivity, machinability, and corrosion resistance makes the heavy alloys unique in the field of materials. Due to this property combination, these alloys are used in many areas, [2'3'41 such as radiation shields, counter weights, kinetic energy penetrators, vibration damping devices, and heavy duty electrical contacts. Though the earliest work on heavy alloys dates back to the 1930's, tsj the effect of all the processing parameters on the properties of heavy alloys is not yet well understood. Considerable property variations have been reported for the same alloy composition apparently sintered under similar conditions. This is definitely due to subtle processing differences. Apart from the variation of properties brought about by a change in composition, many other factors are thought to affect the final properties of the heavy alloys. [6-36]Porosity greater than 0.5 pct can drastically reduce the mechanical properties, especially the ductility. EglIntermetallic precipitation is to be avoided; this is typically done by eliminating copper, using a 7Ni:3Fe ratio, and low volume fraction of matrix. [4Ar-t8,3~ Impurity segregation and embrittlement of interfaces can be controlled by using pure powders and post-sintering heat treatments such as quenching from high temperatures. 113'17'2~ Hydrogen embrittlement can be controlled by changing gases within the sintering cycle, tgl using vacuum anneal as a post-sintering heat treatment, I1~ or using vacuum as the sintering atmosphere.[36] A. BOSE, Postdoctoral Associate, and R.M. GERMAN, Professor, with the Department of Materials Engineering, Rensselaer Polytechnic Institute, Troy, NY 12180-3590. Manuscript submitted April 10, 1987.
In spite of all these factors, incomplete densification is still the primary concern. Incomplete densification may result from too short a sintering time, solidification shrinkage due to fast cooling through the solidus, trapped gases within the pores, or pores formed by a reaction product during sintering. The sintering atmosphere plays a vital role in controlling the residual porosity which in turn controls the final sintered properties. This paper addresses the roles of sintering atmospheres and alloy composition on pore elimination during sintering. The importance of hydrogen embrittlement and pore coarsening are discussed in light of the observations. Based on the theoretical concepts, processes have been
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