Effect Of Alloying Elements On Strengthening Phase Of Co-8.8Al-9.8W Superalloy

Since Ni-based superalloys exhibit excellent high-temperature strength and creep resistance, which attributed to the γ/γ’two-phase strengthening mechanism. Therefore, high temperature structural materials for aerospace and energy applications have long been dominated by Ni-based superalloys. But after decades of incremental improvement for high temperature performance, the operating temperatures are nearing their theoretical limit value. In 2006, an analogous γ/γ’ two-phase micro structure was observed in the Co-A1-W ternary system with liquidus and solidus temperatures 50～150℃ higher than Ni-based superalloys.Thus, it can obtain higher operating temperature than the Ni-base alloy by introducing the γ/γ’strengthening mechanism in the conventional cobalt-based alloy. The strengthening mechanism is one of the developing trend of the cobalt base superalloy. However, in the process of developing Co-A1-W alloy still faces many challenges, where the urgent problem is to improve the γ’phase dissolution temperature (ideal temperature is 1300℃) and keep the stability of γ/γ’two-phase micro structure in high operating temperature after long aging time. The volume fraction of γ’ strengthening phase maintains between 50% and 70%. Based on the foregoing, in this paper, on the basis of previous research of Co-A1-W ternary alloy the main research work is as follows:(1) Effects of five alloying elements on micro structure and phase transition temperature have been investigated in this study by adding 2at.% content of Ni、 Nb、 Ta、 Ti and Mo in Co-8.8A1-9.8W ternary system. The results show that in addition to the 2Ti alloy, alloys as-cast showed inter granular phase and the morphology and composition of intergranular phase were different. After the heat treatment of solid solution and aging for five alloys, the Differential Thermal Analysis(DTA) test results show that the stability of y’phase improved by adding Nb, Ta and Ti elements in Co-8.8A1-9.8W alloy and the effect of Ta is most significant. while Ni and Mo elements reduced the dissolution temperature and stability of γ’phase. The typical of γ/γ’two-phase micro structure is generated in all alloys. Except for Ni and Mo elements, the other three elements no obvious change the cubic morphology of y’strengthening phase in Co-8.8A1-9.8W alloys. Furthermore, the volume fraction of y’phase decreased by addition of Ni and Mo elements and the Ni is more significant, while the volume fraction of y’phase increased by addition of Nb, Ta and Ti elements. When the aging treatment time increased from 50h to 100h at 900 ℃ different forms of secondary precipitates appears in five alloys,which reduced the stability of alloys.(2) The effects of nickel contents(5%、15%、25% and 35%, atomic fraction) on microstructural evolution and γ’strengthening dissolution behavior of Co-8.8Al-9.8W alloys have been investigated. The results show that with the increasing of Ni content, γ’solvus temperature increase at different degree and the volume fraction of γ’phase increase gradually. When the nickel content in Co-8.8A1-9.8W alloy reached 25%, γ’phase transformation temperatures increase to 1100℃. However, the solidus temperatures and y’phase morphology of four Co-Al-W base alloys are nearly unchanged. The typical of γ/γ’two-phase micro structure is generated in four alloys after the heat treatment at 900℃ for 50h. When prolonged aging at 900℃ for 100h, the y’phase volume fraction decreases and there is an obvious phenomenon of the y’ phase become coarsen. The microhardness of the four alloys after heat treatment at 900℃ for 50h and 100h 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 volume fraction of γ’phase decreased and coarsening of γ’phase, which led to the hardness of Co-8.8A1-9.8W alloys reduced.