Title

IRON-RICH LOW-COST SUPERALLOYS (CARBIDES, EUTECTIC, WEAR)

Date of Completion

January 1985

Keywords

Engineering, Metallurgy

Degree

Ph.D.

Abstract

An iron-rich low-cost superalloy has been developed in conjunction with United Technologies Research Center under the NASA program, Conservation of Strategic Aerospace Materials. The alloy, when processed by conventional chill casting, has physical and mechanical properties that compare favorably with existing nickel and cobalt-based superalloys while containing significantly lower amounts of strategic elements. The composition of the alloy is Cr(15)-Mn(15)-Mo(2)-C(1.5)-Si(1.0)-Nb(1.0)-Fe(bal.), and it can be produced with chromite ore deposits located within the United States.^ Studies were also made on the properties of Cr(20)-Mn(10)-C(3.4)-Fe(bal.), a eutectic alloy processed by chill casting and directional solidification (D.S.) which produced an aligned microstructure consisting of M(,7)C(,3) fibers in an (gamma)-Fe matrix. This good alignment vanishes when molybdenum or aluminum is added in higher concentrations. Thermal expansion of the M(,7)C(,3) (M=Fe, Cr, Mn) carbide lattice was measured up to 800(DEGREES)C and found to be highly anisotropic, with the a-axis being the predominant mode of expansion.^ Repetitive impact-sliding wear experiments performed with the Fe-rich eutectic alloy showed that the directionally solidified microstructure greatly improved the alloy's wear resistance as compared to the chill-cast microstructure and conventional nickel-base superalloys.^ Studies on the molybdenum cementite phase have proven that the crystal structure of the (xi) phase is not orthorhombic. The molybdenum cementite has a monoclinic cell (space group C2/m) a = 10.870, b = 7.761, c = 6.563 (ANGSTROM) and (beta) = 120.1(DEGREES) with a volume approximately one-fourth the size of the previously proposed cell. The crystal structure of the (xi) phase is made up of octahedra building elements consisting of four Mo and two Fe-atoms and trigonal prisms consisting of four Fe and two Mo-atoms. The voids are occupied by carbon atoms. The previous chemical formula for the molybdenum cementite "MoFe(,2)C" is now clearly seen to be Mo(,12)Fe(,22)C(,10). ^