Mechanical properties of Ti-Cr-Nb alloys and prospects for high-temperature applications
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I.
INTRODUCTION
I N T E R M E T A L L I C compounds that are usefully strong at high temperature are sought for aerospace and engine applications. The recent renewed interest in these compounds IL2'3] is based on the prospects of high specific strength and specific modulus at elevated temperatures and the fact that there are hundreds of, as yet, mechanically untested binary compounds. Iq In particular, titanium-base materials have received intensive attention because of their low specific gravities, fSl Screening of a set of 36 titanium-containing intermetallics and intermetallic-metal two-phase alloys showed several promising systems based on hardness, toughness, and elastic moduli. ~6JOf these systems, Ti-Cr-Nb, which contains the C15 (cF24) Laves phase Ti40Cr6o, had the best properties. This work reports the results on such alloys of more extensive mechanical tests than were used in the screening experiments.
II.
SELECTION OF COMPOSITIONS
In the previous work, [61 microhardness was used as a measure of strength; crack formation at indentations was used to define a brittle-to-ductile transition temperature, T~, and resistance to fracture caused by a chisel and hammer to define a relative toughness scale. Figure 1 shows these data for a group of Ti-Cr, Ti-Cr-Zr, and TiCr-Nb alloys. Increasing Cr produces improved hightemperature strength but lowers the room-temperature toughness and raises Tbd- Zirconium lowers the toughness, whereas Nb strengthens the alloys without degrading toughness and also enhances the moduli, tTl These trends suggest that Ti-Cr-Nb alloys should be examined in more detail, applying some of the more conventional tests to evaluate yield stress, try, ultimate tensile strength, trtrrs, maximum extension, er~Ax, and stress intensity
factor, K. The four compositions selected are shown in Table I.
III.
EXPERIMENTAL
A. Preparation of Samples Samples were arc melted into disk-shaped ingots using Cr, Nb, and Ti of 99.99 pct purity or better. They were then cut to fit into steel retainer rings, capped with Ta sheet, encapsulated with argon in a stainless steel jacket, and (in order to test the effect of forging temperature) press forged at 1100 ~ (411 and 412) or 1000 ~ (D8 and D9) to true strains in the range of 0.76 to 0.81, as noted in Table I. Subsequent anneals are also listed in the table. Bend, compression, and notched samples were cut by electrical discharge machining (EDM) and then ground or polished to shape. The surface finish for bend samples was 0.20 ~ m root-mean-square (rms) on tension and compression faces and 0 . 3 8 / x m on the sides. The notched bars have 1.6/~m rms relief. The compression samples were polished on one face, as they were electrical discharged machined on the other three sides. Tests at elevated temperatures were done in argon.
B. Mechanical Tests Conventional compression tests were run on samples of dimensions 2.8 x 2.8 • 8.4 mm at a strain rate of 10-4/s. Three-point bending tests were conducted at a crosshead rate of 2.1 - 10 -3 c m / s on a span (maximum
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