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 ...

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The influence of various alloying elements on the creep properties of polycrystalline Co-base superalloys hardened by a ternary L1 2 compound, Co 3 (Al,W) (γ'-phase), was investigated. A Ti containing quaternary alloy shows creep strength similar to Ni-base superalloys IN100 and IN713C at 850 °C and strongly superior to conventional Co-base superalloys as Haynes 188.The activation energy for creep between 850 and 950 °C is similar to the polycrystalline Nibase superalloy IN 100 in the same temperature range. Strengthening of the grain boundaries by third phase precipitates was found to be crucial for the mechanical properties. This can be achieved either by precipitation of borides or by additional intermetallic phases which precipitate due to oversaturation. During compressive creep at 850°C only a slight tendency for directional coarsening occurs, while at 950°C distinct γ/γ′-rafts perpendicular to the external compressive stress axis are formed which indicate a positive lattice misfit even at 950°C.

A method for finding the optimum alloy compositions considering a large number of property requirements and constraints by systematic exploration of large composition spaces is proposed. It is based on a numerical multi-criteria global optimization algorithm (multistart solver using Sequential Quadratic Programming), which delivers the exact optimum considering all constraints. The CALPHAD method is used to provide the thermodynamic equilibrium properties, and the creep strength of the alloys is predicted based on a qualitative numerical model considering the solid solution strengthening of the matrix by the elements Re, Mo and W and the optimum morphology and fraction of the γ′-phase. The calculated alloy properties which are required as an input for the optimization algorithm are provided via very fast Kriging surrogate models. This greatly reduces the total calculation time of the optimization to the order of minutes on a personal computer. The capability of the multi-criteria optimization method developed was experimentally verified with two new single crystal superalloys. Their compositions were designed such that the content of expensive elements was reduced. One of the newly designed alloys, termed ERBO/13, is found to possess creep strength of only 14 K below CMSX-4 in the high-temperature/low-stress regime although it is a Re-free alloy.

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