Effects of Nickel Content on Microstructural Evolution and γ′ Dissolution of Co-8.8Al-9.8W Base Alloys

Abstract:

To study the effects of nickel content on microstructural evolution and γ′ dissolution of Co-Al-W base alloys, the microstructure,γ′ precipitates transformation temperatures, γ′ precipitates microstructural evolution and microhardness of Co-Al-W base alloys with aging treatment were analyzed by using scanning-electron microscope(SEM) and X-ray diffraction(XRD) etc. The results showed that with the increase of Ni,γ ′ solvus temperature increased at different degree and the volume fraction of γ′ precipitates increased gradually. The solidus temperatures andγ′ precipitates morphology of four Co-Al-W base alloys are nearly unchanged. When the nickel content in Co-8.8Al-9.8W alloy reaches 25%γ′ precipitates transformation temperatures increased to 1100℃. The typical of γ/γ′ two-phase micro-structure is generated in four alloys after the heat treatment at 900 ℃ for 50 h. When prolonged aging at 900 ℃ for 100 h , the γ′ precipitates volume fraction decreases to different degree and there is an obvious phenomenon of the γ′ precipitates become coarsen as a function of Ni concentration . The microhardness results of the four alloys after heat treatment at 900 ℃ for 50 h and 100 h indicate that the microhardness is risen from 5Ni to 15Ni alloy, lowered in alloys with higher Ni content. With the aging treatment time prolonged from 50h to 100h, the microhardness of alloys becomes small.

Key words:Nickel element; Co-Al-W base superalloy; γ′ phase; microhardness