Thermal Stability of Ni3Al-Based Composite with Honeycomb Structure

Abstract

By powder metallurgy, materials in the form of a single-phase alloy based on Ni₃Al and the corresponding composite Ni₃Al + W with honeycomb structure are produced. The structural unit of the composite is a round granule (mean size 25 μm) of nickel alloy surrounded by a continuous tungsten coating (thickness ~0.4 μm) applied by chemical gas-phase deposition. Compressive tests at room temperature show that the yield point of the composite Ni₃Al + W at 20–1000°C exceeds that of the single-phase Ni₃Al-based alloy (by a factor of as much as 1.7). However, at higher temperatures, the yield points of the alloy and composite are comparable. The unit yield point (standardized at a density of 7.8 g/cm³ for the alloy and 9.5 g/cm³ for the composite) behaves analogously. At 1300°C, the single-phase Ni₃Al-based alloy exhibits solid–liquid behavior in compression. Creep tests with compression in vacuum at 1000–1200°C are conducted. By pair and parametric analysis of the creep processes according to Hollomon, regression equations for the creep rate, stress, and test temperature are obtained. The creep limit is calculated from the tolerances on the steady creep rate and its inverse. At all the test temperatures, the composite is characterized by lower creep rate (by a factor of seven) and higher creep limit (by a factor of 2.5) than for the nickel alloy on which it is based. The activation energy of creep is determined for the alloy and composite on the basis of an exponential relationship between the experimental quantities. The activation energy of creep for the nickel alloy is close to the activation energy of nickel self-diffusion in Ni₃Al and materials based on it (230–310 kJ/mol). For the composite, it is close to the activation energy of tungsten self-diffusion (503 kJ/mol).