INCOLOY 908, a low coefficient of expansion alloy for high-Strength cryogenic applications: Part I. Physical metallurgy
- PDF / 5,260,098 Bytes
- 16 Pages / 597 x 774 pts Page_size
- 43 Downloads / 219 Views
I.
INTRODUCTION
Low coefficient of thermal expansion (COE) ironnickel-cobalt based superalloys find applications in gas turbine engines and are used for components that require minimal thermal distortion over a wide operating temperature range. Their mean COEs are approximately one half those of conventional superalloys. Values of the mean COE for these alloys range from 7 x 10 -6 ~ to 7.5 • 10 -6 ~ over a temperature range from 25 ~ to 540 ~ The low COEs of these alloys are made possible by a nickel-iron-cobalt matrix whose composition, after precipitation hardening, is similar to that of an Invartype alloy (iron-35 pct nickel), t4'51 Other than the matrix balance of Ni, Fe, and Co, the primary compositional change that distinguishes these alloys from conventional superalloys is their low chromium content. This is a result of the extreme effect that chromium has on the Curie temperature. Achieving a low COE thereby requires that the chromium content of the matrix be maintained at close to 0 w t pct. I6'71
More recently, a new application for low expansion superalloys has emerged. The high magnetic fields present in advanced superconducting magnets, such as those used in magnetic fusion systems, can subject structural materials to very large forces. To withstand the stresses generated by these forces requires a material with high strength and toughness at cryogenic temperatures. In some applications, the structural material is used to reinforce the brittle and relatively weak superconductor. The socalled "cable-in-conduit-conductor" or CICC magnet is an example of an application where, in addition to acting M.M. MORRA, Research Assistant, Department of Materials Science and Engineering, R.G. BALLINGER, Associate Professor, Departments of Nuclear Engineering and Materials Science and Engineering, and I.S. HWANG, Research Scientist, Department of Materials Science and Engineering, are with the Massachusetts Institute of Technology, Cambridge~ MA 02139. Manuscript submitted September 16, 1991. METALLURGICAL TRANSACTIONS A
as a conduit, the CICC magnet design utilizes the highstrength conduit for structural support, t8-~2] This design consists of a superconducting cable enclosed in a conduit that is wound into a magnet coil. The cable-in-conduit assembly is fabricated with enough free volume to allow pumping of liquid helium through the conduit for cooling. t8,9,~~ The low expansion characteristics of these alloys are used to advantage in this case to reduce the amount of strain imposed on the superconductor due to contraction of the conduit material during cooldown to operating temperature, t~2-~51 The superconductor used in most advanced, high magnetic field designs is Nb3Sn, which is incorporated into a composite wire form, termed Nb3Sn conductor. While Nb3Sn exhibits excellent superconducting properties, the material exhibits essentially zero fracture ductility. For this reason, magnet designs using this material are fabricated to final shape with the Nb3Sn in its precursor form, usually a wire composite consis
Data Loading...
