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 ...

a b s t r a c tA b-solidifying TiAl alloy with a nominal composition of Tie43.5Ale4Nbe1Moe0.1B (in at.%), termed TNMÔ alloy, was produced by a powder metallurgical approach. After hot-isostatic pressing the microstructure is comprised of fine equiaxed g-TiAl, a 2 -Ti 3 Al and b o -TiAl grains. By means of two-step heat-treatments different fine-grained nearly lamellar microstructures were adjusted. The evolution of the microstructure after each individual heat-treatment step was examined by light-optical, scanning and transmission electron microscopy as well as by conventional X-ray and in-situ high-energy X-ray diffraction. The experimentally evaluated phase fractions as a function of temperature were compared with the results of a thermodynamical calculation using a commercial TiAl database. Nano-hardness measurements have been conducted on the three constituting phases a 2 , g and b o after hot-isostatic pressing, whereas the hardness modification during heat-treatment was studied by macro-hardness measurements. A nano-hardness for the b o -phase is reported for the first time.

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.

Intermetallic g-TiAl based alloys are a class of novel, light-weight structural materials with attractive mechanical properties for advanced high-temperature applications. Due to their low density (4 g cm À3 ), their high yield and creep strength up to 800 8C and their good oxidation resistance they have the potential to replace the heavier Ni-based superalloys (8 g cm À3 ) in industrial and in aviation gas turbines as well as in automobile engines. [1] Conventional titanium aluminide alloys consist of tetragonal g-TiAl (L1 0 structure; P 4/m m m) and small small amounts of hexagonal a 2 -Ti 3 Al (D0 19 structure; P 6 3 /m m c). Through special heat treatments various microstructures can be established in these two phase alloys to optimize their mechanical properties. [2] The most restricting factor for a broad industrial implementation of titanium aluminides is their low ductility that also limits their workability. A promising design strategy to overcome the brittleness and to improve the hot workability is to induce the formation of more ductile phases by adding ternary alloying elements. The body-centered cubic (bcc) high-temperature b-Ti(Al) phase (A2 structure; I m 3 m) can act as a ductilizing phase in TiAl alloys because it provides a high number of independent slip systems. In recent years several authors have reported that stabilizing the b phase by alloying elements such as Nb, Mo, Ta, or V, significantly improves the hot workability. [3][4][5] Additionally, novel types of microstructures can be achieved exploiting the ternary solid state transformations. [3,4] In spite of this progress, the exact pathway of phase transformations and thus the evolution of microstructures in b phase containing TiAl alloys are not fully understood up to now. At lower temperatures, the disordered bcc b phase can transform to ordered cubic b o -TiAl phase (B2 structure; P m 3 m). However, calculated and experimental transition temperatures show large discrepancies. [6,7] In high-Nb containing TiAl alloys b and/or b o can decompose to ordered hexagonal v o -Ti 4 Al 3 Nb phase (B8 2 structure; P 6 3 /m m c). [8,9] The formation of an orthorhombic phase (B19 structure; P m m a) is reported in Al-lean and Nb-rich TiAl alloys and is interpreted as a transition structure between the cubic b and/ or b o and the orthorhombic O-Ti 2 AlNb phase (C m c m). [4] The crystallographic data of all phases mentioned above and relevant for this work are listed in Table 1.Ordered phases, such as b o and v o , are often assumed to be detrimental to ductility due to their low crystal symmetry. Otherwise the orthorhombic O phase is known to be relatively ductile and even v o containing TiAl alloys show good plastic formability at 800 8C. [8] Thus, with respect to alloy design and processing, it is of high importance to know which kind of additional phase will be formed and which further phase transformations occur during processing and service.In recent years intermetallic g-TiAl based alloys with additional amounts of the ternary b phase have a...

The application of titanium (Ti) based biomedical materials which are widely used at present, such as commercially pure titanium (CP-Ti) and Ti-6Al-4V, are limited by the mismatch of Young's modulus between the implant and the bones, the high costs of products, and the difficulty of producing complex shapes of materials by conventional methods. Niobium (Nb) is a non-toxic element with strong β stabilizing effect in Ti alloys, which makes Ti-Nb based alloys attractive for implant application. Metal injection molding (MIM) is a cost-efficient near-net shape process. Thus, it attracts growing interest for the processing of Ti and Ti alloys as biomaterial. In this investigation, metal injection molding was applied to the fabrication of a series of Ti-Nb binary alloys with niobium content ranging from 10wt% to 22wt%, and CP-Ti for comparison. Specimens were characterized by melt extraction, optical microscopy, X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), and transmission electron microscopy (TEM). Titanium carbide formation was observed in all the as-sintered Ti-Nb binary alloys but not in the as-sintered CP-Ti. Selected area electron diffraction (SAED) patterns revealed that the carbides are Ti2C. It was found that with increasing niobium content from 0% to 22%, the porosity increased from about 1.6% to 5.8%, and the carbide area fraction increased from 0% to about 1.8% in the as-sintered samples. The effects of niobium content, porosity and titanium carbides on mechanical properties have been discussed. The as-sintered Ti-Nb specimens exhibited an excellent combination of high tensile strength and low Young's modulus, but relatively low ductility.

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