A Comparative Study on Precipitation Strengthened Co-based Superalloys for High Temperature Applications

Sammanfattning

Cobalt-based superalloys have the potential to replace conventional Ni-based superalloys thanks to their higher melting point and superior oxidation behaviour. Nevertheless, Co-9.2Al-9W alloy, the first Co-based superalloy with γ′-precipitations, has high density and is limited by a low γ′ solvus temperature compared to Ni-based superalloys. Therefore, there is a need for lighter and thermally more resistant Co-based superalloys. CALPHAD-based high-throughput computational screening (HTSC) facilitates the alloy development and phase characterisation process in multicomponent systems.Alloying elements such as Ti, Ni and V make it possible to increase the solvus temperature while decreasing the density of the alloy by substituting for W. However, uncontrolled usage of alloying elements may also result in TCP phasesand inhibit the precipitation strengthening mechanism. This study aims to realise comprehensive research on modelling Co-based superalloys and experimental investigations. With the help of HTCS using Thermo-Calc TCNI12 database, over 100,000 compositions were generated, and evaluated using a multifunctional desirability approach. Three alloys, Co-9Al-10Ni-8Ti-1V-1W, Co-9Al-10Ni-6Ti-5W and Co-10Al-10Ni-1Ti-8W were designed and produced with Co-9.2Al-9W as a reference alloy. Heat treatment of the designed alloys was carried out in two steps. In the first step, designed alloys were solutionised at 1250 °C for 2 hours and aged at 950 °C for 2 hours, followed by air cooling. After solutionizing in the first step, precipitates and secondary phases were detected in some alloys. Therefore, as a second step of heat treatment, these alloys were solutionised again at 1200 °C for 2 hours to dissolve these phases that formed during solutionizing in the first step and aged at 950 °C for 2 hours, followed by water-quenching. The reference alloy was heat-treated in a sequence of solutionizing at 1250 °C for 2 hours with air cooling andageing at 800 °C for 24 hours, followed by water-quenching. Microstructural analysis was conducted using scanning electron microscopy (SEM) with energy dispersive spectrometry (EDS) and wavelength dispersive spectrometry (WDS). Differential scanning calorimetry (DSC) was performed to identify the phase transformation temperatures, and oxidation resistance of the alloys was tested by thermogravimetry analysis (TGA). The density of the alloys was measured by Archimedes' method, and the hardness of the alloys was tested using HV0.5. Analysis showed that the compositions with high Ti concentration elevated the solvus temperature up to 1100 °C, reducing the density by approximately 12% below the ternary reference. Solutionizing unexpectedly resulted in cubic-like precipitates and Al-rich secondary phases. Alloy Co-9Al-10Ni-6Ti-5W exhibited the best properties with the hardness of 374 HV, mass gain of 0.39% as an oxidation resistance, and a density of only 8.51 g/cm3. In contrast, the alloy with a relatively low W/V content did not precipitate γ′, with the lowest hardness value and obtained the highest oxidation rate.