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.

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The influence of several alloying elements on the microstructure and creep properties of polycrystalline Co-base superalloys hardened with the ternary L1 2 compound, γ'-Co 3 (Al,W), are presented in this work. The stability of the γ/γ'-microstructure was investigated by long term aging at temperatures of 750 °C and 900 °C. High contents of chromium and iron destabilize the γ/γ'microstructure. Additions of nickel widen the γ/γ'-two phase field. Discontinuous precipitation is observed only in the iridiumcontaining alloy during aging. In all other alloys no formation of further intermetallic phases are found even after 1000 hrs aging at 750 °C and 900 °C. Boron has been added for the formation of grain boundary strengthening intermetallic phases which leads to an adjacent γ'-depleted zone. Nanoindentation shows that titanium improves its hardness. The creep strength of the studied alloys is sufficiently better than that of commercial carbide hardened polycrystalline Co-base superalloys and already close to γ'hardened Ni-base superalloys. An alloy containing 2 at.% titanium exhibited a creep strength comparable with the Ni-base alloy IN100. The microstructure after creep tests at 950 °C showed rafting perpendicular to the compressive load axis, confirming the positive lattice misfit of the alloys at high temperatures which was determined with high energy X-ray diffraction.

a b s t r a c tTernary Co-Al-W alloys with W contents between 8 and 11 at.% were investigated with respect to element partitioning, c 0 solvus, c 0 volume fraction, lattice mismatch and creep properties. W is enriched in the c 0 precipitates and an increasing W content stabilizes this phase. The lattice mismatch is positive up to 900°C and rafting occurs during creep with a preferred direction in accordance with this positive lattice mismatch.

Plastic deformation mechanisms in a crept L1 2 hardened Co-base superalloy are investigated by transmission electron microscopy. The deformation happens mainly in the cobalt solid solution matrix by slip of ordinary a/2<101>{111} dislocations and rafting of the  precipitates perpendicular to the external compressive stress axis is observed. Dislocation structures are early stages of the development of / interfacial dislocation networks. Furthermore,  precipitates are sheared by partial dislocations generating stacking faults which sometimes extend over matrix channels and neighboring  precipitates. Due to the positive lattice mismatch at creep temperature the dislocation activity in matrix channels perpendicular to the compressive external stress axis is less pronounced compared to parallel channels.

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