Effect of Al and W Contents on the Solidification and Solution Microstructure of Novel γ/γ′ Cobalt‐Base Superalloys

Abstract

The solidification and heat treatment characteristics of superalloys are key to engineering and practical applications. The solidification and solid solution behavior of γ/γ′ cobalt‐base superalloys Co–30Ni–xAl–(15 − x)W–5Cr–1Ta–4Ti (x = 7, 9, 11 abbreviated as 7Al8W, 9Al6W, and 11Al4W, respectively) are studied using directional solidification experiments. The results indicate that there are Laves phase, (γ + γ′)e eutectics, and (β + γ′)e eutectics in the interdendritic regions for the 7Al8W alloy. The solidification sequence of the 7Al8W alloy is L → L1 + γ → L2 + γ + Laves → L3 + γ + Laves + (β + γ′)e →L4 + γ + Laves + (β + γ′)e + (γ + γ′)e → γ + Laves + (β + γ′)e + (γ + γ′)e. As the Al content increases and the W content decreases, the Laves phase fraction in the alloy decreases and the (β + γ′)e eutectics fraction increases. The Laves phase completely disappears in the 11Al4W alloy. An unremovable μ phase forms in the 7Al8W alloy during solution treatment. The 9Al6W alloy and 11Al4W alloy can exhibit a γ single‐phase microstructure after single‐stage solution treatment (1250 °C for 1 h) and two‐stage solution treatment (1210 °C for 1 h + 1275 °C for 1 h), respectively. These experimental results are helpful for composition optimization, solidification, and solution treatment parameter determination of novel γ/γ′ cobalt‐base superalloys.

Crystallization behavior of ternary γ–γ′ Co–Al–W alloy

“…Zhou et al [15] investigated the effect of Ta and Ti on the directional solidification characteristics of novel γ-γ′ Co-based superalloys. The same authors published the work concerning effect of Al and W contents on the solidification and solution microstructure of new cobalt superalloys [16]. Sani et al [17] performed the research on effect of homogenization heat treatment on segregation and microstructure of γ-γ′ Co-based superalloys.…”

“…The heat treatment aspects of ternary Co-Al-W alloys were investigation by Tomaszewska et al [18]. Except arc melting and VIM/VAR methods, works concerning directional solidification [15,16,19] and a single crystal solidification [20] of γ′-strengthened Co-based superalloys are available.…”

Quantitative and Qualitative Assessment of As-Cast Microstructure and Microporosity of γ-γ′ Co-based Superalloys

“…[14][15][16] Considering fabrication of c-c 0 Cobased superalloys, the investigators are often focused on microsegregation, partition coefficient, solidification path and other. [17][18][19] In most of cases, the microstructural analysis is qualitative. However, the information about new class of superalloys could be supplemented with data concerning quantitative description of microstructure.…”