Determination of Residual Stresses in Titanium Carbide‐Base Cermets by High‐Temperature X‐Ray Diffraction

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Abstract
The crystal structure and thermal expansion of titanium carbide, nickel, and two titanium carbide‐base cermets were determined between room temperature and 1100°C. (2012°F.). No structural changes were observed. An abnormal rate of expansion was observed for pure nickel near the Curie temperature, 353°C. (665°F.), and for the nickel and carbide phases in the cermets at about 816°C. (1500°F.). The expansion coefficient of pure nickel and the nickel phase in the cermets was found to be approximately twice that of pure titanium carbide and the carbide phase in the cermets. The brittleness and poor impact strength of the cermets was attributed to the large residual stresses present in these materials as a result of this difference in thermal expansion. The stress‐strain relations were interpreted on the basis of a mechanical interaction between the phases in the cermets. The carbide phase was found to be essentially under triaxial compression and the nickel phase under a triaxial tension of 158,000 lb. per sq. in. At elevated temperatures, increased solid solubility of carbide in the nickel phase and plastic deformation of this phase was believed to influence the stress‐strain relations and the thermal‐expansion behavior of the phases. It was concluded that replacement of the nickel phase in the cermets with a metal or alloy, such as a nickel‐chromium‐molybdenum alloy, which has a coefficient of thermal expansion similar to the carbide phase in the cermets, would improve the impact strength of these bodies. Equations were developed for the thermal expansion of titanium carbide and nickel. Values of the expansion coefficient were computed for each of the materials by differentiation of these equations.

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