Alloys are being developed for nozzles of hypersonic wind tunnels to be used in testing components of future hypersonic missiles, aircraft, and space transportation systems. The nozzle components made from these alloys will be required to retain sufficient strength to withstand stresses of as much as 600 MPa at throat surface gas temperatures as high as 1,700 K while resisting erosion and oxidation by impinging hypersonic flows of air and possibly other gases. In some applications, back-side cooling or film/transpiration cooling may be used to reduce the temperature rises in nozzles. Alternatively, in some applications, nozzles may be used, without active cooling, in either of two heat-sink modes. In one mode, exposure time would be limited in order to limit the maximum temperature rise. In the other mode, denoted the self-limiting heat-sink mode, a nozzle throat would be exposed long enough to come into thermal equilibrium with the gas, and, hence, the nozzle throat material must be chosen to withstand the maximum surface gas temperature (e.g., 1,700 K) for an indefinite time.

The alloys investigated thus far are of three types: one based on Ir; one based on various proportions of Mo, Si, and B; and one comprising a baseline Mo-Re alloy modified by incorporation of small amounts of other metals:

  • Prior Ir-based alloys have proven high resistance to oxidation but limited strength at temperatures above 1,200 ºC. Therefore, the approach taken in the present development of Ir-based alloys has involved increasing strength by means of both solid-solution hardening and particle strengthening through alloy additions, while ensuring that any increase in strength is not accompanied by loss of oxidation resistance or reduction in melting temperature.
  • Prior Mo-Si-B alloys have exhibited potential for retention of high strength at elevated temperatures as long as thermal shock is minimized. They do not resist oxidation as well as do prior Ir-based alloys. The Mo-Si-B portion of the present development has been oriented toward choosing process conditions and proportions of Mo, Si, and B to increase fracture toughness (to increase resistance to thermal shock).
  • Prior Mo-Re alloys are known to be more ductile (and, hence, less vulnerable to thermal shock) than are prior Mo-Si-alloys. The Mo-Re portion of the present development has been oriented toward strengthening through addition of other alloying elements.

This Nozzle Was Fabricated by electrical-discharge machining of an ingot of the alloy described in the text.
The most promising alloy identified thus far is composed of Ir with the following proportions of other elements in atomic percentages: 4.5Zr, 0.3W, 0.31C, and 0.005Th. The alloy appears to have good oxidation resistance, and the basic strength requirements at high temperature (600 MPa at 1,700 K) are satisfied. However, a more detailed analysis that includes pressure and thermal stresses indicates that some surface damage is to be expected, and, hence, the operational lifetime of the nozzle is expected to be limited. At the time of reporting the information for this article, a nozzle had been made from this alloy (see figure) and it was planned to perform survivability tests of the nozzle.

This work was done by E. J. Felderman and D. T. Akers of Aerospace Testing Alliance and C. T. Liu and J. Schneibel of Oak Ridge National Laboratories for the Air Force Research Laboratory.


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Alloys for Nozzles of Hypersonic Wind Tunnels

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This article first appeared in the October, 2007 issue of Defense Tech Briefs Magazine.

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