The development, in the frame of the European ULTMAT project, of alloys offering at least 150°C surface temperature increase above the Ni-based superalloys' capability is presented. The expected achievement of the project is a thorough evaluation of the capability of refractory metal (Nb and Mo) silicide based multiphase materials to withstand enhanced temperature turbine service conditions (up to 1300°C). This is based on microstructural, mechanical, physical and environmental investigations in close connection with industrial scale material processing and component fabrication technologies. The paper presents an overview of the project results. Base materials are the metal/intermetallic ductile/brittle composites in the Nb-Nb5Si3 and Mo-Si-B systems. Improvements in high temperature creep resistance (up to 1300°C) as well as oxidation resistance (700°C to 1300°C) have been obtained. Processing routes have been developed (ingot and powder metallurgy) that allowed the manufacture of complex shaped parts.

The fracture and fatigue behavior of refractory metal silicide alloys/composites is significantly affected by the mechanical behavior of the refractory metal phase. This paper reviews some of the balance of properties obtained in the alloys/composites based on the Nb-Si system. Since some of the alloy/composite properties are dominated by the behavior of the refractory metal phase, the paper begins with a review of data on monolithic Nb and its alloys. This is followed by presentation of results obtained on Nb-Si alloys/composites and a comparison to behavior of some other high temperature systems.

Usually, single-phase intermetallics in bulk form can easily be crushed into powder by hammering. It was therefore quite a surprise when we found that a bulk sample of the monoclinic intermetallic compound Nb2Co7 could be extensively deformed at room temperature without shattering or fracturing. In a previous paper, results of microhardness, compression, tensile and bending tests were provided and discussed [1]. In order to understand the observed unusual deformation behavior of this intermetallic phase, its hitherto unknown crystal structure has been studied and the microstructure of undeformed and deformed samples has been analyzed in the present investigation by light-optical, scanning electron and transmission electron microscopy. Single-phase specimens deformed at very different strain rates (hammering and conventional compression testing) both show the occurrence of microcracks along grain boundaries which, in compression-deformed specimens, are strongly localized in extended shear bands oriented approximately 45° to the compression axis. The grains adjacent to the microcracks are heavily deformed whereas, away from the sheared regions, the samples remain free of any indication of plastic deformation.

The microstructural development of a forged Ti-43Al-4Nb-1Mo-0.1B (in at%) alloy during two-step heat-treatments was investigated and its impact on the tensile ductility at room temperature was analyzed. The investigated material, a so-called TNM™ gamma alloy, solidifies via the β-route, exhibits an adjustable β/B2-phase volume fraction and can be forged under near conventional conditions. Post-forging heat-treatments can be applied to achieve moderate to near zero volume fractions of β/B2-phase allowing for a controlled adjustment of the mechanical properties. The first step of the heat-treatment minimizes the β/B2-phase and adjusts the size of the α-grains, which are a precursor to the lamellar γ/α2-colonies. However, due to air cooling after the first annealing step, the resulting microstructure is far from thermodynamic equilibrium. Therefore, a second heat-treatment step is conducted below the eutectoid temperature which brings the microstructural constituents closer to thermodynamic equilibrium. It was found that temperature and duration of the second heat-treatment step critically affect the solid-state phase transformations and, thus, control the plastic fracture strain at room temperature. Scanning and transmission electron microscopy studies as well as hardness tests have been conducted to characterize the multi-phase microstructure and to study its correlation to the observed room temperature ductility.

A two-phase intermetallic alloy composed of Ni3Al (L12) and Ni3V (D022) was plasma-nitrided or -carburized in dependence of temperature and time. It was found that the hardness of the surface layer of the present intermetallic alloy was enhanced by both plasma-nitriding (PN) and -carburizing (PC), and primarily depended on treating temperature; the maximum surface hardness of the alloy was shown by nitriding at around 850 K and by carburizing at 1025 K. In addition, the hardened layers due to PN and PC effectively kept their hardness up to a high temperature. The XRD analysis revealed that vanadium nitride (VN) and vanadium carbide (VC) were formed in the surface of the nitrided and carburized alloy, respectively, suggesting that the enhanced surface hardness was attributed to the dispersion hardening due to the nitrides and carbides.

We have found that Ni3Al intermetallics shows catalytic activity for H2 production reaction, such as methane steam reforming. In addition to the single-phase Ni3Al, we recently studied the catalytic property of the Ni3Al/Ni two-phase alloy in foil form, and have found that the catalytic activity is not so high in the cold-rolled state. In case of atomized Ni3Al powder with a NiAl/Ni3Al two-phase structure in fact, it is possible to improve the catalytic activity for methane steam reforming by chemical pretreatment in acid and subsequent alkali solutions [1]. The reason was similarly due to the formation of fine Ni particles on the porous surface. These results show a possibility to improve the catalytic activity of Ni3Al/Ni two-phase alloy foils by the two-step chemical treatment similar as powder. In this study we carried out this chemical pretreatment on the surface of Ni3Al/Ni two-phase alloy foil and the catalytic properties for methane steam reforming were evaluated. Ni3Al/Ni two-phase alloy foil (Ni-18 at% Al) with a thickness of 30 μm was used. The chemical pretreatment consisted of two steps, the first step was acid leaching (HCl + HNO3, vol. ratio 3:1) and the second step was alkali leaching (NaOH solution, 20 wt%). Methane steam reforming was carried out in a conventional fixed-bed flow reactor as described in previous reports [1]. Prior to the reaction, the foil was reduced at 873 K for 1 hour by H2. Then isothermal test was carried out at 1123 K for 50 hours under steam-to-carbon ratio of 1. The surface morphology was analyzed using SEM and EDS. The chemically pretreated foil showed much higher catalytic activity for methane steam reforming than the non-treated foil. Especially, the catalytic activity rapidly increased during in the first several hours, and then slowly increased during the subsequent reaction. The maximum H2 production rate was about 30 L/min/m2. This value was 40 times higher than the non-treated foil. This result indicated that the catalytic activity can be improved by chemical pretreatment in acid and subsequent alkali solutions. The surface morphology observation revealed that acid leaching dissolved Ni3Al phase and alkali leaching dissolved Al mainly, resulting in the Ni-enriched surface structure. We consider that this Ni-enriched surface introduced by the two-step chemical treatment attributed to the initial catalytic activity. It also revealed that many fine Ni particles were formed on the surface during the reaction, and the amount of fine Ni particles increased with time. We consider that these Ni particles produced during reaction, contributed to the increase of catalytic activity during reaction. [1] Y. Ma, Y. Xu, M. Demura, D.H. Chun, G.Q. Xie, T. Hirano, Catal. Lett. 112 (2006) 31

The forging capabilities of two high-strength Fe3Al-based alloys have been evaluated. Based on these results one alloy has been used for forging steam turbine blades. Blades of about 600 mm length were successfully forged by a standard procedure otherwise employed for forging of 9-12 wt.% Cr steels. The forged steam turbine blades showed very good form filling, no pores and smooth surfaces. The blades were finished by cutting and grinding by standard procedures. The microstructure consists out of a Fe3Al matrix with additional Laves phase which predominantly precipitated on grain boundaries. The large Fe3Al grains in the cast precursors did only partially recrystallise during forging. This may be partially due to pinning of the grain boundaries by Laves phase precipitates. Cracks may form at those grain boundaries which are decorated with these brittle precipitates.

The microstructure in a Co-rich, Co-15 at.% Nb alloy was characterized in the as-cast condition. A predominantly lamellar eutectic morphology composed of a Co-Nb solid solution and the C15 Laves phase NbCo2 was confirmed by transmission electron microscopy. The C15 phase was heavily twinned, with only one variant of twins being present in the individual lamella, while the Co solid solution had the face centered cubic structure. In-situ heating to 600°C in the microscope confirmed the decomposition of the metastable Laves phase into a fine equiaxed, ˜10-20 nm grain size microstructure, and the product phase is the monoclinic Nb2Co7. The individual grains appear faulted. The matrix solid solution retained the fcc structure and no change in structure was observed on cooling to room temperature. Heating to temperatures as high as 1130°C leads to rapid grain growth in the Nb2Co7 phase, and the nucleation and growth of a few new grains within the original grains; however, the reverse peritectoid transformation previously reported, was not observed.

This paper presents an overview of the mechanical properties of the iron aluminide FeAl. Both the strength and ductility depend on a variety of parameters, including vacancy concentration, the environment (principally water vapor), alloy stoichiometry, temperature and grain size. The effects of alloying elements, particularly boron, are also briefly discussed. The review emphasizes how much of our current understanding of the strength and ductility of iron aluminides emanated from the discovery of the importance of both water vapor and thermal vacancies on these properties.

We have prepared Mg/Pd laminate composites with (Mg/Pd)=6, 3 and 2.5 atom ratios, by a super lamination technique. The homogeneous Mg-Pd intermetallic compounds, Mg6Pd, Mg3Pd and Mg5Pd2, are formed during the initial activation process. We investigated the hydrogen storage properties of these materials. The compounds can reversibly absorb and desorb a large amount of hydrogen, up to 1.46˜0.9 H/M, at 573 K. Except for the Mg5Pd2-hydrogen system, the pressure composition-isotherms show two plateaux. The mechanism of the phase transition during hydrogenation/dehydrogenation was analyzed by in-situ XRD measurements. These intermetallic compounds absorb and desorb hydrogen through reversible multistage disproportionation and recombination processes.

The ternary Nb-Cr-Al phase diagram exhibits extended phase fields of the cubic C15 and the hexagonal C14 Laves phases Nb(AlxCr1-x)2. A number of Nb-Cr-Al alloys were prepared by levitation melting and annealed at temperatures between 1150 and 1450 °C for up to 1500 h. Isothermal sections of the ternary Nb-Cr-Al phase diagram at 1150, 1300 and 1450 °C were obtained from electron probe microanalysis, X-ray powder diffraction and metallographic investigations in order to study the effect of Al on the stability and structure of the Laves phases. The C14 Laves phase in the Nb-Cr-Al system can dissolve up to 45 at.% Al by substituting Cr with Al on the two different crystallographic B-sites 2a and 6h of the C14 AB2 unit cell. The site occupations of the Al and Cr atoms on these two B-sites were determined by Rietveld analysis using the program FullProf. The experimental site occupation factors were compared to site occupation factors computed by a statistical mechanics approach based on first-principles electronic structure calculations. The experimental as well as the calculated site occupation factors indicate a preferred occupation of the 2a site by Al.

Single-crystal elastic constants of Co3(Al,W) with the cubic L12 structure have been experimentally measured by resonance ultrasound spectroscopy at liquid helium temperature. The values of all the three independent single-crystal elastic constants and polycrystalline elastic constants of Co3(Al,W) experimentally determined are 15~25% larger than those of Ni3(Al,Ta) but are considerably smaller than those previously reported. Two-phase microstructures with cuboidal L12 precipitates being well aligned parallel to <100> and well faceted parallel to {100} are expected to form very easily in Co-base superalloys because of the large value of E111/E100 and cij of Co3(Al,W). This is indeed confirmed by experiment. Values of yield stress obtained for both [001] and [¯123] orientations of L12/fcc two-phase single crystals moderately decrease with the increase in temperature up to 800°C and then decrease rapidly with temperature above 800°C without any anomaly in yield stress. Slip on {111} is observed to occur for both orientations in the whole temperature range investigated.

The formation of precipitate depleted zones (PDZ) near grain boundaries in Mg-based alloys strengthened by precipitation hardening is deemed as detrimental to the material since wide depleted zones may affect the mechanical and corrosion properties of the alloy. Experimental investigation of PDZ evolution in Mg-Zn-Sn-alloys aged at different temperatures for different times was conducted by SEM and TEM and by measuring the microhardness of near grain boundary zones at low loads. It was found that at early stages of aging (175°C, ≤1 day) the hardening is caused by formation of MgZn2 needle- and T-like MgZn2/Mg2Sn particles. The neargrain boundary zone is harder than the grain matrix, due to large round MgZn2 and Mg2Sn particles formed at grain boundaries. Increasing the aging was found to decrease the hardening in the matrix as well as in the near-grain boundary zones due to the dissolution of MgZn2 needles and the coarsening of T-like particles. Substantial microhardness decrease in the near-grain boundary zone (from 70 to ~30 HV) found at low loads (10 gr) was connected with the formation of PDZ. This was confirmed by TEM and SEM studies. Further aging, at 225°C for 1-8 days, leads to the formation of "crusts" of enlarged T-like particles around depletion zones. As a result, the microhardness of PDZ's measured at higher loads (25 and 50 gr) increases up to ~ 60÷70 HV close to the ones measured in the grain matrix.

While only a minor phase constituent, the deformation behavior of the hexagonal α2-Ti3Al phase, significantly affects the mechanical properties of two-phase TiAl based alloys. We have used conventional and high-resolution transmission electron microscopy to investigate the fine structure of pyramidal plane glide dislocations, with Burgers vectors of the type b=<2c+a>, in α2-Ti3Al after room temperature compression of binary polysynthetically twinned TiAl normal to the lamellar interfaces to nominal plastic strains of 1%-7%. We report atomic resolution observations of non-co-planar dislocation core configurations for <2c+a>-dislocations and show that translamellar deformation twins active in the majority γ-TiAl phase play an important role in facilitating pyramidal plane slip in α2-Ti3Al in the lamellar two-phase alloys.

Three kinds of Mo-Si-B ternary alloys and a 1 at.% Al added Mo-Si-B alloy with the compositions near Mo-8.7 at.% Si-17.4 at.% B that is in the Mo5SiB2 and Mo two-phase compositional region were produced by Ar arc-melting followed by the heat treatment at 1800 °C for 24 h. These alloys have the characteristic fine microstructure composed of small Mo solid solution (Moss) particles in the Mo5SiB2 (T2) matrix with the primary phase (Moss or T2 depending on composition). The volume fraction of the Moss particles ranges from 25.5 to 30.5 % and its average size from 3.0 to 6.4 μm in the fine microstructure of the alloys. Micro cracks were introduced by Vickers hardness tests into the microstructures, and their propagation is disturbed by the small Moss particles. Thus, each hardness value seems to relate to the cracking behavior around each indent. On the other hand, Vickers hardness values do not show correlation with the volume fraction of the Moss particles, but clearly decrease with increasing the average particle size of Moss. Therefore, it should be concluded that the increase in the particle size of Moss could enhance the toughness of the Mo5SiB2/Mo-based alloys effectively by ductile phase toughening.

A significant grain refinement is achieved for a Ti-45Al (at.%) alloy via the addition of a novel Ti-Al-TiB2 master alloy. This refinement is attributed to a low lattice mismatch of TiB2 with the primary solidifying phase and its thermodynamic stability. Model calculations allow for the prediction of the grain size as a function of cooling rate, alloy constitution and particle size distribution and content. However, the model predictions are smaller than the experimentally observed grain size since the fraction of effective particles is overestimated.