Microstructure and creep strength of different γ/γ′-strengthened Co-base superalloy variants

Abstract

The influence of W, Ta, Ti, Nb, V, Si, Mo, Ir and Cr on the high temperature properties of γ/γ′-strengthened Co-Al-W superalloys was investigated. All alloys exhibit a γ/γ′-microstructure with remarkably differing γ′-volume fractions. W, Ta, Ti, Nb, V increase the γ′-volume fraction and γ′-solvus temperature. An increased W content and alloying of additional elements except of Ir decreased the liquidus temperature. First creep experiments revealed creep strength comparable to polycrystalline Ni-base superalloys and importance of the grain boundary strengthening. Online submission version:The influence of W, Ta, Ti, Nb, V, Si, Mo, Ir and Cr on the high temperature properties of gamma/gammaprime-strengthened Co-Al-W superalloys was investigated. All alloys exhibit a gamma/gammaprime-microstructure with remarkably differing gammaprime-volume fractions. W, Ta, Ti, Nb, V increase the gammaprime-volume fraction and gammaprimesolvus temperature. An increased W content and alloying of additional elements except of Ir decreased the liquidus temperature. First creep experiments revealed creep strength comparable to polycrystalline Ni-base superalloys and importance of the grain boundary strengthening.Conventional Co-base superalloys are suitable materials for use in a corrosive environment at high temperatures like in a gas turbine. However, the high temperature strength of these classic Co-superalloys alloys can not compete with the excellent high temperature properties of γ/γ′-strengthened Ni-base superalloys [1]. Recently a ternary compound Co 3 (Al,W) with the L1 2 structure was discovered by Sato et al [2]. This led to the development of a new class of high temperature Co-base superalloys with a γ/γ′-microstructure similar to Ni-base superalloys [3,4]. Further investigations by Suzuki et al. [5,6] revealed the occurrence of a flow stress anomaly similar to Ni-base superalloys. The flow stress at the peak temperature increases by addition of Ta. Moreover, it was found that Ta stabilizes the γ′-phase and increases the γ′-solvus temperature. Shinagawa et al. [7] showed that boron enhances the ductility of the Co-9Al-9W (at.%) system by strengthening the grain boundaries. Ab-initio calculations [8] confirmed experimental results [9] that the L1 2 Co 3 (Al,W) compound is ductile in nature and can be used as a hardening phase.In this study several polycrystalline γ′-hardened Co-base superalloys containing additional elements were investigated by means of differential scanning calorimetry and scanning electron microscopy. Mechanical properties were examined by the first creep experiments on this new alloy class in compression. In the present paper the influence of alloying elements on the evaluated properties is discussed.The composition of the alloys under investigation and the abbreviations used to name the alloys subsequently are given in Table 1. The alloy selection was made with the aim to investigate the influence of various alloying elements on the thermophysical properties of the alloy system like γ′-solvus ...

Coarsening behaviour and interfacial structure of γ′ precipitates in Co-Al-W based superalloys

“…The aged specimens were mechanically ground, polished and etched for 3 s using a Spar etchant solution [25] (100 ml of distilled water, 100 ml of 32% HCl, 10 ml of 65% HNO 3 and 0.3 ml of Spar etchant, with 1-methoxy-2-propanol as the main constituent).…”

Creep Strength and Microstructure of Polycrystalline γ' - Strengthened Cobalt-base Superalloys

“…Even though the γ/γ' two-phase field in the Co-Al-W phase diagram [2] is relatively small, minor additions of other alloying elements to the ternary composition are feasible and can improve the high temperature properties like yield and creep strength as well as the oxidation resistance [7,9,10]. In general, the partitioning behavior of the alloying elements is similar to Ni-base superalloys [22].…”

“…Additional investigations of the long term stability were conducted on solution heat treated samples encapsulated in argon atmosphere and aged at 750 °C and 900 °C for 1000 hrs, respectively. [9,14]. Compositions marked with "*" could not be determined using EDS.…”

Vacuum induction melting and vacuum arc remelting of Co-Al-W-X gamma-prime superalloys

“…This alloy system has the potential for use in high temperature gas turbine applications because such alloys can have high gammaprime contents (up to 90 vol.%) with a high gammaprime solvus temperature [2] and correspondingly higher temperature capability than many Ni-base superalloys. Co-Al-W-X alloys have been produced that have high temperature strength comparable to Waspaloy [1], better creep resistance than many Ni-base disk or blade alloys [3], and a high gamma prime solvus [2,4] temperature. Such performance is suitable for turbine disk or casing applications.…”