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Better VPS Fabrication of Crucibles and Furnace Cartridges

The choice of alloy composition and processing parameters is important.

An experimental investigation has shown that by (1) vacuum plasma spraying (VPS) of suitable refractory metal alloys on graphite mandrels, and then (2) heat-treating the VPS alloy deposits under suitable conditions, it is possible to fabricate improved crucibles and furnace cartridges that could be used at maximum temperatures between 1,400 and 1,600 °C and that could withstand chemical attack by the materials to be heated in the crucibles and cartridges. Taken by itself, the basic concept of fabricating furnace cartridges by VPS of refractory materials onto graphite mandrels is not new; taken by itself, the basic concept of heat treatment of VPS deposits for use as other than furnace cartridges is also not new; however, prior to this investigation, experimental crucibles and furnace cartridges fabricated by VPS had not been heat treated and had been found to be relatively weak and brittle. Accordingly, the investigation was directed toward determining whether certain combinations of (1) refractory alloy compositions, (2) VPS parameters, and (3) heat-treatment parameters could result in VPS-fabricated components with increased ductility.

The table describes five refractory metal alloys that were considered in this investigation. In each case, during vacuum plasma spraying, the alloy powder or corresponding mixture of elemental metal powders was delivered to a plasma gun by a flow of argon. The plasma gun was located in a vacuum chamber that was evacuated and backfilled with argon at a low pressure. The plasma gun generated an argon/hydrogen plasma that melted the powder and projected it toward graphite mandrels, which were rotated so that the VPS deposits would form tubes. After plasma spraying, the tubes were removed from the mandrels.

Some of the VPS tubes were subjected to heat treatments based on current practice in the sintering and annealing of conventional powder-metallurgy components. Each tube was packed with high-purity alumina sand to prevent slumping of the thin-walled tubes during heating. Hydrogen was used during the heat treatment of three of the alloys (60Mo/40Re, 75W/25Re, and 95.5W/3.5Ni/1.0Fe) to aid in densification and in the reduction of oxides. Both a liquid-phase sinter (LPS) and a solid-state sinter (SSS) were used on the 95.5W/3.5Ni/1.0Fe alloy. Hydrogen was not used during heat treatment of the 90Ta/10W and 99Nb/1Zr alloys because these alloys are susceptible to the formation of brittle hydrides; instead, these alloys were annealed in vacuum.

Standard metallurgical polishing techniques were used to prepare specimens of the as-sprayed and heat-treated tubes of each alloy. These specimens were then examined in the as-polished and etched conditions, by use of an optical microscope. Quantitative microscopy was used to determine the densities of the specimens. Helium leak tests were performed on the as-sprayed and heat-treated specimens to determine whether any interconnected porosity was open to the surfaces. Some room-temperature compression tests were performed on heat-treated specimens to determine whether there were any improvements in mechanical properties.

The SSS and LPS heat treatments were found to effect significant increases in toughness and ductility of the 95.5W/3.5Ni/1.0Fe alloy, and to result in cartridge helium-leak rates of about 10-8 cm3/s — well below the maximum allowable rate of 10–6 cm3/s. For the other alloys and heat treatments investigated, there was a mix of favorable and unfavorable findings.

This work was done by Richard R. Holmes and Frank R. Zimmerman of Marshall Space Flight Center and J. Scott O'Dell and Timothy N. McKechnie of Plasma Processes Inc. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com/tsp under the Materials category. MFS-31301

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