The Ductile-Brittle Transition in Tantalum Carbide

Abstract

Tantalum carbide was prepared in the composition range TaC0.73 to TaC1.26 and consolidated by hot pressing. Compositions at ends of the range were two phase. A modified cantilever‐beam test method was used to determine the ductile‐brittle transition temperature ( Tnormaldb ) at a given strain rate and for this geometry as a function of carbon content. The Tnormaldb range from TaC0.98 was 1825°–1925°C, and the median nominal bend stress in this range was 23,000 psi (1617 kg/cm2). At TaC0.90 the median nominal bend stress was at a minimum of 14,000 psi (984 kg/cm2) for the Tnormaldb range 1750°–1850°C. Tnormaldb increases approximately 150°C with increasing x across the single‐phase cubic region of TaCx .

Structure and properties of tantalum carbide crystals

“…Mechanical property measurements have been made on a range of substoichiometric polycrystalline tantalum carbides made by carburising tantalum [15][16][17][18] or by hot-pressing powders [19,20]. The hardness of tantalum carbide increases as the carbon content decreases from that of stoichiometric material [16,18,19].…”

“…Conversely, the bend and tensile strengths at room temperature and the tensile strength at elevated temperatures all pass through minimum values wtfich lie between TaCo.8 and TaC0. 9 [16,17].…”

In situ transmission electron microscopy observations of room-temperature plasticity in sub-micron-size TaC(100) and TaC(011) single crystals

“…The hardness of B1-structured TaC x was found to be maximum at an intermediate (x 0.8), rather than a higher/lower (x = 0.5 or 1), carbon vacancy concentration [2,5,8]. The hardness of TaC was found to decrease gradually, rather than abruptly, with increasing temperature [10].…”

“…They are thus attractive for applications in cutting tools, as wear-and oxidation-resistant coatings, as structural components (leading edges and nose-caps) in hypersonic vehicles, as diffusion barriers, as electrical conductors and as optical thin films [4][5][6][7]. Among the 30 group IV and V binary TMCs with a rocksalt (B1) structure, tantalum carbide (TaC) has one of the highest melting points (T m 4250 K) [1] and electrical conductivities (>5 Â 10 6 À1 m À1 at 300 K) [1,3].…”

“…Its mechanical properties are sensitive to carbon content [4][5][6] and comparable to those of other TMCs: TaC is stiff (elastic modulus $537 GPa) [1], moderately hard (10 s of GPa) [2,7] and, although localized plasticity is observed under microindents at room temperature [7][8][9], macroscopic ductility is more pronounced at temperatures 0.5T m [4,5]. temperature microindentation studies revealed that {1 11}h110i is the most commonly observed slip system in TaC [8,9].…”