Effect of Hf additions on microstructure and mechanical properties of a Co-9Al-9W-2Ta alloy at room and high-temperatures

In The microstructural evolution, room/high-temperature mechanical properties of a Co-9Al-9W-2Ta alloy with 2, 5, 6, 9 at.% Hf additions (referred as to 2Hf, 5Hf, 6Hf and 9Hf alloy hereafter, and content of W+Hf = 9 at.% for all alloys) prepared by arc-melting were investigated. It was found that the as-cast 2Hf∼6Hf alloys showed a microstructure composed of Co-base solid solution γ phase (γ-CoSS) and eutectic of γ+ intermetallic compound Co₂₃Hf₆, and the 9Hf alloy was composed of primary Co₂₃Hf₆ and (γ+ Co₂₃Hf ₆) eutectic. While after 1170°C/8h solution and 800°C /100h aging, the cubic γ' phase with a size of 200nm∼700nm homogeneously and coherently precipitates on the γ matrix for the 2Hf ∼ 6Hf alloys, no γ' particles were found in the 9Hf alloy. The 2Hf alloy exhibits yield stress anomaly at temperatures above 600°C, and the temperature corresponding to the anomalies stress peak is at about 700°C However, the other three alloys show no yield stress anomalies.