Interdiffusion in CMSX-4 Related Ni-Base Alloy System at a Supersolvus Temperature

Interdiffusion in Ni-base superalloy CMSX-4 and alloys related to CMSX-4 was investigated at the temperature 1288 °C, which is 8 °C above the γ’-solvus temperature of this superalloy, 1280 °C. This temperature is of a special interest because it is a temperature of hot isostatic pressing applied to CMSX-4 and modeling of this process needs verified diffusion data for this specific temperature. Various diffusion couples were assembled from the investigated alloys, annealed at 1288 °C and studied by electron probe microanalysis. So far as the annealing temperature was higher than the γ’-solvus temperature of CMSX-4 and other investigated alloys have no strengthening γ’-phase, interdiffusion occurred in the fcc solid solutions of nickel. It was found that in the case when the γ’-forming and γ-stabilizing elements diffuse in the same direction (towards nickel) the diffusion rate accelerates, but when they diffuse in the opposite directions (counter diffusion) it slows down. Such an interdiffusion behavior is in agreement with the results predicted with diffusion simulation software Dictra.

On the Development Concept for a New 718-Type Superalloy with Improved Temperature Capability

The superalloy 718 stands out for its excellent manufacturability and strength at ambient temperature. However, its application temperature is limited to about 650 °C due to the instability of the γ’’ precipitates. Here, we provide an in-depth account of an alloy development concept, allowing for the design of superalloys with 718-type properties, yet with a significantly improved microstructural stability. The article begins with a detailed discussion on how the microstructural and chemical composition must be altered to achieve this objective. Then, model alloys were used to explore and validate the outlined strategy. Finally, it is shown how these considerations ultimately led to a new 718-type superalloy with far more improved microstructural stability— namely, VDM Alloy 780. The introduction of a large amount of Co as a substitute for Fe (and partially Ni) is the most important element of our alloy development concept in terms of chemical composition. The most important microstructural feature is the introduction of low solvus temperature, high misfit γ´-strengthening, replacing γ´´-hardening.