Sintering Atmosphere Effects on the Ductility of W- Ni- Fe Heavy Metals
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INTRODUCTION
HEAVYmetals are W-Ni-Fe or W-Ni-Cu alloys fabricated by liquid phase sintering mixed elemental powders. They are two phase composites with dispersed tungsten grains (typically 20 to 50/xm in diameter) existing in an alloy matrix phase. An attraction to the heavy metals is the combined high density, strength, ductility, and corrosion resistance. Many applications have been documented for this unique property combination.~-6 To optimize the density, it is desirable to maintain a high tungsten content. However, as the tungsten content increases the ductility decreases because of greater tungsten contiguity. As a consequence of the alloy complexity and the apparent elusive cause of variable ductility, considerable research has gone into the mechanical properties. 3-28 Besides the systematic property variations with composition, there are property variations attributed to impurity segregation, 7'13'17-19'27 hydrogen embrittlement, 5'7'n'14'~6'17 intermetallic phase formation,4' ~5,~9,2~.nresidual porosity, 8.23,28ductile-brittle transition temperature effects, 9m tungsten saturation in the matrix, 11,24-26and incomplete oxide reduction. 28 Analysis of this information with respect to the W-Ni-Fe compositions provides some guidelines for high ductility. One of the primary detriments to ductility is incomplete densification. 8'1~ Pores can result from too short a sintering time, entrapped gas, or solidification shrinkage of the matrix. The material exhibits a high sensitivity to residual porosity, especially at levels greater than approximately 0.5 pct. Both the strength and ductility are negatively affected with ductility having the greater sensitivity. With full densification, there are several additional possible causes of poor ductility including hydrogen embrittlement, residual oxygen, intermetallic phase formation, and impurity segregation. The residual oxygen is easily controlled by proper selection of powders, sintering schedule, and reducing atmosphere. ~~ Impurity segregation to tungstenmatrix interfaces is controlled by input powder purity and post-sintering heat treatments. 13'~7-~9'27 Likewise the R. M. GERMAN is Associate Professor, Materials Engineering Department, Rensselaer Polytechnic Institute, Troy, NY 12181. K.S. CHURN, formerly Postdoctoral Fellow at Rensselaer Polytechnic Institute under a fellowship from the Korea Science and Engineering Foundation, is Systems Manager at Daejeon Machinery Depot, P.O. Box 35, Daejeon, Korea. Manuscript submitted August 1, 1983. METALLURGICALTRANSACTIONS A
hydrogen embrittlement effect can be controlled by postsintering plastic deformation u or sample degassing anneals. 5'1~ Intermetallic formation can be avoided by using alloys with no copper, high Ni: Fe compositions, with low volume fractions of matrix phase. 3,4'1~ Additional small improvements in the properties can be attained by control of the mean tungsten grain size and the mean free path between tungsten grains. Typically, ductility is the more sensitive parameter with respect to improper
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