Phase analysis of sintered and heat treated alloy 718
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I.
INTRODUCTION
Table 1.
BESIDES cost savings, powder metallurgy techniques are used for the production of superalloy structural parts because of the much lower segregation compared to the cast or wrought material. This is due to the much smaller powder particle size relative to ingot size. However, undesirable phases, which can decrease or eliminate benefits of the powder metallurgy techniques, can form during the powder consolidation process. The main purpose of this work was to identify the phases which form in alloy 718, sintered and subsequently heat treated to produce suitable preforms for forging. Many studies were made on phases in cast and wrought alloy 718, but only the study by Merrick ~ was on alloy 718 made by PM techniques. The specimens analyzed by Merrick were, however, not sintered, but densified by hotpressing powders directly to the final form in a short time. In the present study, the specimens were submitted to a long sintering followed by heat treatments. The growth of some phases is then favored in the present specimens with respect to specimens hot-formed more rapidly.
II.
EXPERIMENTAL
PROCEDURE
Specimens investigated in this study were prepared by slip casting and vacuum sintering of alloy 718 powder. They were subsequently heat treated. The powder was fabricated by the vacuum atomization technique. 2 The chemical analysis of the powder was provided by the manufacturer* *Homogeneous Metals, Inc.
(Table 1). The C, O, N, and H contents were remeasured in our laboratories: they were 450, 470, 11, and 8 ppm, respectively. There is a discrepancy between the oxygen contents given by the manufacturer and our measurements. It should, however, be mentioned that the value of 470 ppm given above is an average of 37 oxygen measurements made
Chemical Analysis of Alloy 718 P o w d e r *
*Provided by the powder manufacturer.
on different size fractions of the powder and compared with standards. 3 None of these measurements was below 330 ppm. The high oxygen content of the powder was due to a few contaminated particles disseminated in the powder; they could easily be detected by optical microscopy becaase of their black color. 3 The particle size distribution is given in Table 2. The - 1 4 0 mesh size contained only spherical oi; elliptical particles, but the +140 mesh fraction contained platelets in proportion increasing with particle size. These platelets probably acquired their shape by solidifying on the atomizing vessel walls. To reduce possible contamination by these platelets, only the - 100 mesh fraction of the powder was used to produce specimens; it contained approximately one wt pct of plateletlike particles. The powder microstructure is illustrated in Figure 1. The large particles have Table 2.
K. HAJMRLE, formerly with the Department of Mining and Metallurgy, Laval University, Qurbec, Canada, is now Research Metallurgist at the Sherritt Research Centre, Sherritt Gordon Mines Ltd., Fort Saskatchewan, Alberta, Canada. R. ANGERS is Professor with the Department of Mining and Metallurg
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