Institute of Metals Division - High-Temperature Solid Solution-Strengthened Columbium Alloys

The mechanical properties of solid-solution-strengthened columbium alloys have been assessed as a function of alloying additions. Studies included the effects of tungsten, tantalum, molybdenum, and vanadium. Elevated temperature fabrication was required at alloying levels above about 15 pct W or 10 pct Mo. Tungsten, molybdenum, and vanadium additions increase tensile strength at room temperature and 2200°F, but tantalum additions exert little strengthening. Similarly, tungsten, molybdenum, and probably vanadium additions increase the ductile-to-brittle transition temperature, and tantalum additions decrease this parameter. The 2200°F stress-rupture strength is increased by tungsten and molybdenum additions; with 20 to 25 at. pct additions, 100-hr rupture strengths of 30,000 psi are obtainable. Small vanadium additions exhibit mild stress-rupture strengthening, and tantalum additions are innocuous. At 2500°F, tungsten additions retain stress-rutpure strengthening capability, but molybdenum additions are much less effective. Tantalum additions augment the stress-rupture strengthening of tungsten plus molybdenum additions at 2500°F. Of the refractory metals, columbium possesses specific potential for structural use in the temperature range from 2000" to about 2500°F. Desirable features of columbium are its good low-temperature ductility, ease of fabrication, oxidation mechanics (lack of a volatile oxide at moderate temperatures), good availability and reserves, and relatively low density. However, alloying is required to provide attractive strength at elevated temperatures. Published information concerning alloying be- havior of columbium relative to mechanical properties is sketchy, particularly in consideration for long-time applications at temperatures in excess of 2000°F. Only a few stress-rupture data at temperatures from 2000" to 2500°F have been reported;'-' these are too meager to allow rationalization of the effects of various alloying additions on creep or stress-rupture properties. Additional published data do, however, give some insight toward understanding of systematic alloying behavior for columbium relative to low-temperature strength4-' and ductility,' and elevated temperature tensile behavior.4,6 This paper is intended to augment previous knowledge, and to promote understanding of the alloying behavior of columbium for long-time structural service at elevated temperature. BASIC CONSIDERATIONS Solid-solution strengthening is recognized to be of prime importance in improving strength in columbium-base alloys. Other strengthening mechanisms may be (and are, in current alloys) used to augment solid-solution strengthening. As strengthening results primarily from lattice distortion, the strengthening effectiveness per atom depends upon atomic size mismatch between solvent and solute. Another factor in this regard is conformance to ideal solution behavior in the solid solution. Thus, for example, despite appreciable differences in the degree of mismatch between tungsten (5 pct), molybdenum (5 1/2 pct), and vanadium (7 1/2 pct) in solution in columbium, all are about equally effective in straining the columbium lattice at room temperature.'-' Similarly, tantalum additions do not strain the columbium lattice, and little strengthening is expected. Size effects being equal, maximum solution strengthening at high temperatures is anticipated to result from the addition of elements that increase the melting temperature of columbium. This expectation is based on an increase in the electronic bonding forces exhibited in the transition metal series as electronic structures approach those of the Group Vl metals. Of the metals that exhibit ex-