Directional eutectic casting of high temperature alloys

Under directional solidification, fibers. or laminar reinforcing phase grows simultaneously with the matrix from the alloy melt and is regularly arranged in rows after solidification. Since the reinforcing phase and the matrix phase are formed simultaneously in the same matrix, they are also called primary composites.

Directional eutectic cast high-temperature alloys were developed in the late 1960s as materials for aviation gas turbine blades with higher service temperatures. The matrix phase is a nickel-based or cobalt-based alloy solid solution, while the strengthening phases are tantalum carbide, niobium carbide, chromium carbide, molybdenum and Ni3Nb intermetallic compounds. The use of temperature than other high temperature alloys to improve 40 ~ 110 ℃, including niobium carbide or tantalum carbide reinforced nickel-based directional eutectic alloy has a better overall performance. The U.S. General Electric Company has the grade NiTaC14B of this type of alloy made of hollow - level turbine blades and installed successfully. The main problem of this alloy is the low production efficiency, the directional solidification rate of ≈ 0.6cm/h; in addition, like all composite materials, there are still problems in matching the thermal expansion coefficient of the reinforced phase and the matrix phase, as well as the mutual diffusion dissolution and chemical interaction at high temperatures.

The strength σc of oriented eutectic cast high temperature alloys is often related to the organization as follows: σc = σi + kλ-1/2, where σi and k are constants, related to the alloy system; λ is the spacing of the reinforcing phases. The strength of the alloy is oriented, and the more the stress direction deviates from the direction of the reinforcing phase arrangement, the lower the strength is.