Phase stability and mechanical properties of carbide and boride strengthened chromium-base alloys
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FILIPPI
Phase stability and mechanical properties of five carbide and two boride strengthened chromium-base alloys are presented. Compositions examined were Cr-0.5 TaC (mole pet), Cr-0.5 TiC, Cr-0.5 Cb(Nb)C, Cr-0.5 HfC, Cr-0.5 ZrC, Cr-0.5 CbB, and Cr-0.5 TaB. A transition in stability from the carbide of the principal alloying metal to CrzsC6, complete at approximately 2800~ occurs in the Cr-0.5 TaC, Cr-0.5 TiC, and Cr-0.5 CbC alloys. Similarly, a change in phase stability from borides of columbium (niobium) and tantalum to CraB occurs at ~2800~ in the Cr-0.5 CbB and Cr-0.5 TaB compositions. The compounds HfC and ZrC, respectively, remained stable in the Cr-0.5 HIC and Cr-0.5 ZrC alloys at this temperature. Stress-rupture properties at 2100~ improved for several alloys when aged at this temperature to precipitate the carbide or boride of the principal alloying metal following higher temperature heat treatment to form the Cr23C6 or Cr4B phases. Rupture life of the Cr-0.5 TaC alloy, for example, was increased at 15 ksi and 2100~ from 4 hr for as-fabricated material, to 186 hr after heat treatment. Improvement of rupture life for similar material and test conditions from 24 hr to 382 hr was observed in the C r - 0 . 5 T a B c o m p o s i t i o n .
r K.~HROMIUM, principally because of its high melting point and elastic modulus, intermediate density, and fair oxidation resistance, has for some time been considered a possible substitute for nickel and cobalt-base superalloys in advanced design jet engine applications. Two undesirable characteristics of the element, however, have slowed its development. First, inherent low strength at elevated temperatures has demanded major improvement by alloying. Secondly, like many bcc metals, chromium undergoes a transition from ductileto-brittle behavior, occurring, for high purity wrought and recovered polycrystalline material, at approximately room temperature. I As a minimum, this undesirable c h a r a c t e r i s t i c should be m a i n t a i n e d unchanged in alloyed material. Unfortunately, alloying chromium to improve elevated temperature strength generally results in an increased ductile-brittle transition temperature, especially when the potent solid solution strengtheners tungsten or molybdenum are usedf -4 In a fairly recent study of chromium alloys, Ryan reported improvement in high temperature strength without sacrifice of low temperature ductility by dispersion hardening with the monocarbides of group IV B and V B metals.S The potential for carbide strengthening chromium was extended by this study. Alloys containing 0.5 at. pct Ta, Cb, Ti, Zr, or Hf plus an equal atomic amount of carbon to combine with and form 0.5 mole pct of the monocarbide (~1 vol pct) were examined. In addition, the potential for strengthening chromium by alloying to form 0.5 mole pct tantalum or columbium boride was also explored.
A. M. FILIPPI is Senior Engineer, Westinghouse Electric Corp., Astronuclear Laboratory, Pittsburgh, Pa. METALLURGICALTRANSACTIONS
EXPERIMENTAL PROCEDURES Melting an
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