By hot extrusion below and above the α transus temperature equiaxed and predominantly lamellar microstructures were obtained in a Ti aluminide alloy. In order to examine a possible orientation dependence of mechanical properties the flow stress, the activation parameters of plastic deformation and the fracture toughness were determined parallel and perpendicular to the extrusion direction at room temperature and 700 °C. The observed anisotropy in these properties may be mainly attributed to anisotropies in the microstructures, in particular to a preferential alignment of lamellae in lamellar microstructures.

In ultrahigh vacuum (UHV), the ductility of FeAl decreases with increasing Al content and fracture becomes increasingly intergranular. Boron improves ductility by segregating to the grain boundaries and suppressing grain-boundary fracture. However, with increasing Al concentration, even the B-doped alloys become brittle and fracture intergranularly. Hydrogen gas at low pressures embritties FeAl, although not as severely as atmospheric moisture. Ductility is highest in UHV followed by that in O2, vacuum, and air.

High-purity stoichiometric NiAl single crystals have been prepared by crucible-free inductive zone melting and afterwards well annealed at temperatures below 1200 K. Internal friction measurements of torsionally deformed single crystals show two relaxation maxima at 500 and 800 K which anneal during the measurement. The first maximum can be assigned to the dislocation motion by kinkpair formation and the annealing to pinning of these dislocations by interstitial impurity atoms. The second maximum is attributed to the Snoek-Köster relaxation of dislocations in the presence of mobile interstitial impurity atoms and the annealing to the pinning of dislocations by vacancies. The kink-pair formation enthalpy in NiAl was estimated.

The tensile behaviour of Ni-32at.% Al-5at.% Fe and Ni-32at.% Al-12 at.% Cr two-phase intermetallic compounds has been studied in the 20–750°C temperature range. TEM studies of the defect structures have been performed. Both alloys exhibit a brittle behaviour at room temperature, while at high temperature the Ni-Al-Cr alloy appears to be much more ductile that the Ni-Al-Fe alloy. Various reasons for the limited ductility and the brittle-to-ductile transition in these alloys are discussed.

Some quaternary Ni3(Si, Ti) alloyed with transition elements V, Nb, Zr and Hf was prepared beyond their maximum solubility limits to investigate the effect of second-phase dispersion on moisture-induced embrittlement. V-added Ni3(Si, Ti) alloy contained ductile fcc-type Ni solid solution as the second-phase, while Nb-, Zr- and Hf-added Ni3(Si, Ti) alloys contained hard dispersion compounds as the second-phase. V- and Nb-added Ni3(Si, Ti) alloys did not display reduced tensile elongation in air, indicating that their second phases have the effect of suppressing the moisture-induced embrittlement. Possible mechanisms for the beneficial effect by the second phase on the moisture-induced embrittlement of V- and Nb-added Ni 3(Si, Ti) alloys are discussed in association with hydrogen behavior and deformation property in the constituent phases or at matrix/second-phase interface.

The microstructural stability of a directionally-solidified NiA1–9 at.% Mo quasi-binary alloy was investigated under conditions of thermal cycling between the temperatures 973 K and 1473 K utilizing time-temperature heating and cooling profiles which approximate potential engine applications. Two different microstructures were examined: a cellular microstructure in which the faceted secondphase Mo rods in the NiAl matrix formed misaligned cell boundaries which separated aligned cells approximately 0.4 mm in width and 5–25 mm in length, and a nearly fault-free fully columnar microstructure well aligned along the [001] direction. Both microstructures resisted coarsening under thermal cycling, but plastic deformation induced by thermal stresses introduced significant specimen shape changes. Surprisingly, the cellular microstructure, for which the cell boundary region apparently acts as a deformation buffer, exhibited better resistance to thermal fatigue than the more fault-free and better aligned columnar microstructure.

In the last few years, considerable progress been made in obtaining reproducible mechanical properties data for binary FeAl alloys. Two sets of observations are the foundation of this progress. The first is that the large equilibrium vacancy concentrations that exist in FeAl at high temperature are easily retained during cooling, and that these strongly affect the low-temperature mechanical properties. The second is that room-temperature ductility is adversely affected by water vapor. The purpose of this paper is to highlight our understanding of key phenomena and to show how an understanding of the factors which control the yield strength and fracture behavior has followed from the discovery of the above two effects.

Microstructure of TiAl produced by Laser Engineered Net Shaping (LENS) has been characterized using optical microscopy, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). It has been shown that the substrate has a significant effect on the microstructure deposited. Depending on operational parameters, either equiaxed γ-TiAl/α2 -Ti3AI or single phase α 2 microstructure can be obtained. In-situ heating experiments reveal that nucleation of lamellae often starts at grain boundaries in this retained α 2grains. By careful control of post heat treatment, an ultra fine lamellar microstructure may be obtained, which may significantly improve tensile property in these alloys [3].

The glide properties of <001> dislocations have been studied by in situ straining experiments at and below room temperature, with the aim of studying slip, cross-slip, Peierls friction forces, and pinning at small obstacles. Most results are in a good agreement with atomistic calculations. It is concluded that unpinning from small extrinsic obstacles is probably the rate controlling mechanism in this temperature range and in the soft orientation.

A new generation of wrought γ-TiAl alloys having a good balance of properties at both low and high temperatures has recently been developed. Addition of light elements (C, Si) to the base alloy of this system (K5 material) has been shown to increase the creep strength of the material significantly. The microstructure of the materials with superior creep performance was found to consist of wellaligned rows of fine precipitates, delineating partially dissolved α2 laths. These precipitates were found to interact strongly with dislocation structures. This paper studies the mechanisms for α2 dissolution and fine particle precipitation in annealed samples of the K5 series alloys. Microstructural design issues that would allow for optimization of the particular strengthening mechanism will also be discussed.

Among the high-temperature intermetallic systems, transition-metal silicides are attractive because of their high melting temperatures (many greater than 2273 K) and potential oxidation resistance. In particular, Mo5Si3 exhibits a very high melting point (2453 K) and also has a solubility range of 2 – 3 atomic percent, which can aid in processing and alloy design strategies. The focus of this study is to evaluate the processing and properties of Mo5Si3 and Mo5Si3-base ntermetallics. For the optimal baseline comparison, high-purity single crystals have been fabricated, and thermal and elastic properties have been experimentally measured. Although Mo5Si3 has a strong thermal expansion anisotropy, ts elastic anisotropy factors and the Poisson's ratios indicate that Mo5Si3 is less anisotropic in elasticity. The combination of the thermal and elastic properties has been employed to calculate the thermal residual stress and to explain the potential for grain boundary cracking during processing. Room temperature Vickers indentation tests of Mo5Si3 have been performed. The orientation dependence of hardness and fracture toughness of Mo5Si3 single crystals have been obtained. The corresponding deformation and fracture modes have been revealed by microscopy studies. Finally, micro- and macroalloyed Mo5Si3 with aluminum and boron will be briefly explored with property assessments.

Secondary Ion Mass Spectrometry has been used to study the distribution of elements in and near grain boundaries in boron-free and boron-doped Ni76Al24 alloys with and without ∼220 wt. ppm of deuterium. In boron-free alloys, sulfur was distributed about the grain boundaries in both deuterium- free and deuterium-charged samples. The distribution of deuterium followed that of sulfur and was segregated to grain boundaries. In the boron-doped material, sulfur was not found at most grain boundaries in the uncharged material, but was in the charged material. No deuterium was found at the grain boundaries in the boron-doped material. It is proposed that in the boron-free material it is the synergistic effect of sulfur and hydrogen that is responsible for the environmental sensitivity of this alloy. In boron-doped material, boron segregation to the grain boundary prevents sulfur, and to some extent hydrogen, segregating to the grain boundary.

The effect of different oxide particles on the microstructure and the high temperature creep behavior of ODS ternary L12-(Al,Cr)3Ti intermetallic compounds, which were prepared by high energy ball milling, was analyzed. Particular attention was paid to the development of the dispersoid size and distribution after the milling process, the consolidation and the heat treatment.

X-ray diffraction (XRD) and transmission electron microscopy (TEM) of the as-milled powders indicate that the ordered matrix can nearly be amorphizied during milling. Moreover, independent of the type of oxide the dispersoids with a mean diameter ≤ 20 nm are homogeneously distributed in the metallic matrix. Using hot pressing under protective atmosphere the powders were compacted to nearly full density, followed by heat treatment. This results in grain sizes varying between 200 nanometers and several micrometers. However, due to the nature of the dispersoids used different particle diameters were found after consolidation and annealing. Although known to be the thermodynamically most stable oxide the initial Y2O3 powder has formed a mixed yttrium-aluminum oxide during manufacturing and, hence, significant particle growth has been observed. While CeO2 shows a similar behavior, the line compound A12O3 remains stable, but the average particle size of this oxide also increases during the heat treatment. First preliminary mechanical tests supported by TEM observations show the significant influence of the grain size on the creep resistance at high temperatures.

Modifications of alloy chemistry are often used to tailor the intrinsic flow behavior of structural materials. Models of solution strengthening, high temperature yield stress and creep must relate the effects of chemistry to the mechanisms that influence these material properties. In ordered alloys, additional information regarding the crystallographic site occupancy of ternary elements is required. In this study relaxed structures and energies for intrinsic and substitutional point defects are calculated using a first principles plane-wave-pseudopotential method. Calculated defect energies are used to predict the density and site preferences of solid-solutions (Si, Cr, Nb, Mo, Ta and W) in ³TiAl. Size and modulus misfit parameters are calculated and the interaction of these defects with a dissociated ordinary screw dislocation evaluated within anisotropic elasticity theory. The derived interaction strength is then related to solid-solution strengthening for these defect centers. Predicted solid-solution effects are in good agreement with experimental observations for the binary alloy.

The Mo-Si phase diagram exhibits a Mo5Si3-MoSi2 eutectic at the 54% Si composition. Since the terminal phases have comparable melting points and are equidistant from the eutectic composition, there is the possibility of obtaining lamellar microstructures in this system. In addition, if the alloys are directionally solidified, there is the further possibility of obtaining aligned lamellae. In this study, a high temperature (xenon-arc-lamp) optical floating zone furnace is utilized to directionally solidify Mo-Si alloys of the eutectic composition. Growth conditions are systematically varied to investigate their effects on the solidification microstructure. Growth rates and rotation speeds are identified that result in lamellar microstructures.

Iron aluminides with compositions from Fe3A1 to FeAl show an anomalous stress rise at temperatures about 600–800K. Many models have been suggested to explain this behaviour, with the suggestion of hardening by vacancies presently receiving much attention. An alternative model is suggested here, valid for those alloys where the simultaneous reduction of DO3order does not interfere, based on the density of constrictions produced on mobile <111> superdislocations and their subsequent pinning by decomposition or by their cross slip to produce jogs. The exponential increase in stress with temperature can be related to the pinning point density and the absolute values of strengthening correspond well with those predicted by this model. The orientation dependence of the stress anomaly and the tension-compression asymmetry can be related to the role of shear stresses in assisting constriction of the dislocation partials by modifying the core configuration and by assisting the expansion of the new, decomposed dislocations.

Nb-15A1-20V alloys with about 45% by volume of an A15 phase in a B2 matrix have been subjected to creep deformation at temperatures between 1000°C and 1200°C and stresses from 80 MPa to 240 MPa in vacuum. The creep curves show a rapid transition from primary to steady-state creep with a well-defined activation energy. Microstructural observations show that the creep deformation occurs mainly in the B2 matrix and is controlled by dislocation mechanisms. Samples which had been coldrolled and annealed to reduce the size of A 15 precipitates and increase the yield strength were found to have lower creep resistance than those with unrefined microstructures. The significance of these observations for the creep mechanisms is discussed.

In this work, superplastic behaviours in Ti-33A1–3Cr-0.5Mo (wt%) γ-TiAl alloys with two different initial microstructures of near gamma (NG) and duplex (DM) structure were investigated with respect to the effect of testing temperatures and strain rates. At 1050°C and a strain rate of 8×10–5 S–1, a maximum elongation of 570% was observed for NG-TiAl and a maximum elongation of 467% for DM-TiAl. The relations of flow stress and strain rate sensitivity vs. strain rates at different temperatures were also determined by incremental strain rate tests. The results showed that the value of strain rate sensitivity is higher and the flow stress is lower for NG than those for DM at the same condition. The microstructural evolution during superplastic deformation was examined and correlated to the mechanical properties for these two alloys. The influence of microstructure on the superplastic behaviours of γ-TiAl alloys, and possible superplastic deformation mechanisms were finally discussed.

This paper will discuss the relationship between microstructure and creep behavior in hightemperature niobium-silicide based in-situ composites. The creep behavior of composites generated from binary Nb-Si alloys, and higher order alloys containing Mo, Hf and Ti additions, will be described. In-situ composites were tested in compression at temperatures up to 1200°C and stress levels in the range 70 to 280MPa. It was found that the Hf concentration can be increased to 7.5 with little increase in creep rate, over that for the binary Nb3Si-Nb composite, but at higher concentrations the creep rate is increased at stress levels higher than 21OMPa. At stresses less than 21OMPa the Ti concentration can be increased to 21 without a detrimental effect on creep performance, but at higher concentrations there is a substantial increase in the creep rate.

Development of α 2-Ti3A1 based dual phase alloys have shown some promising potentials in property improvement by introducing Ti5Si3 sulicide phase into the matrix via Si alloying. However, the presence of coarse network of Ti5Si3phase formed by eutectic reaction in the as-cast state also embrittles the alloy. Both hot rolling and powder metallurgy are considered to be the possible ways to refine the Ti5Si3 phase in the matrix. Two Ti-Al-Si-Nb alloys whose Si contents are 2 and 5 at. % respectively were arc melted into ingots and then hot rolled to sheets in this investigation. Optical metallographic examination correlates the microstructures of the as-cast and as-rolled alloys with the different rolling amounts, showing that the coarse silicide network is broken into small particles after hot rolling. Mechanical property testing from room temperature to 8000°C indicates that the strength and plastic elongation of the hot-rolled alloys are much higher than those of the as-cast ones. The data obtained in this investigation are comparable with those obtained in the P/M processed specimens. Fracture surfaces of the alloys are also examined.