The misfit between 120° rotational variants of the lamellar structure in γ-phase TiAl with the ordered L10 lattice is analysed. It is shown that such a misfit on the (111) interface plane, which has a shear character, can be accommodated by one, two or three arrays of interface intrinsic dislocations. One example of interface dislocation structure observed by transmission electron microscopy is presented.

The hot corrosion resistance of a single crystal Ni-48Al-1Ti-0.5Hf-0.2Ga‡ alloy was examined in a Mach 0.3 burner rig at 900C for 300 hours. The combustion chamber was doped with 2 ppmw synthetic sea salt. The hot corrosion attack produced a random mound morphology on the surface. Microstructurally, the hot corrosion attack appeared to initiate with oxide-filled pits which were often broad and shallow. At an intermediate stage, the pits increased in size to incorporate unoxidized Ni islands in the corrosion product. The rampant attack stage, which was observed only at sharp sample corners, was characterized by rapid inward growth of alumina in finger-like protrusions incorporating significant amounts of Al-depleted Ni islands. Aluminum consumption in the oxide fingers resulted in the growth of a γ' layer ahead of the advancing oxide fingers.

Vacancy-like defects in CoAl in the composition range 48.5 at.-% <Cco, < 53 at.-% are investigated by means of positron lifetime spectroscopy and Doppler-broadening measurements. The observed lifetimes in the annealed samples confirm that defects are quenched-in during the production of the samples. The values of the positron lifetime and the S-parameter decrease with increasing Co concentration. After quenching from 1400°C or 1600°C an increase in the positron parameters isobserved. Long-time annealing of the Co-rich sample shows a dramatic decrease of the positron lifetime to the expected bulk lifetime.

Two unconventional methods of cold-work: (a) controlled mechanical (ball) milling (under shear conditions), and (b) shock-wave (explosive) loading at ˜5 GPa shock pressure were applied to two cubic intermetallics: (a) Mn-stabilized cubic (L12) titanium trialuminide (AI3Ti(Mn)) and (b) B2 iron aluminide (FeAl). The long-range order (LRO) parameter S of both intermetallics decreases and their lattice constants increase with increasing milling time providing strong evidence that the mechanical milling disorder is due to antisite-atom pairs. A nanocrystalline structure is being formed in both intermetallics upon milling. A shock-wave loading gives rise to only a partial disorder of both A13Ti(Mn) and FeAl but their lattice constants are not affected (no point defects formation). Annealing at 600°C results in the restoration of LRO and lattice constants as well as microstructural evolution in Fe-45A1.

Point defect behavior in B2-type FeAI alloys is investigated from a thermodynamic point of view, based on the Bragg-Williams method. The model is developed by taking new defect formation mechanisms, random vacancy distribution (RVD), and antisite atom recovering (ASAR), into consideration, which were proposed based on the current findings in in situ neutron and X-ray diffraction studies for the B2 FeAl. The condition for appearance of the RVD and ASAR states is given. Application of this model to B2 FeAl alloys shows that the RVD-like behavior is reproduced in the Ferich composition region and also a rapid increase in vacancy concentration observed in the Al-rich region can be interpreted by the ASAR process by antisite Al atoms.

A simple statistical analysis of the distribution of superkinks along a single superdislocation is presented. This analysis is then used to estimate the rate of self-immobilization of superdislocations. The self-immobilization rate decreases exponentially with increasing superdislocation length. The implications for experiment are discussed.

An industrial Ti-46.5A1–4(CrNb,Ta) alloy, with and without boron additions, has been successfully rolled to sheet 1.5mm thick. The microstructure of the sheet material has been characterised as a function of boron content and subsequent heat treatments. It is observed that rod-like (Ti-Ta)B precipitates are effective in refining the grain structure in both as-rolled and subsequently heat treated material. As-rolled microstructures rapidly cooled from the α + γ phase field contain unstable α2, which transforms to B2 phase on annealing below the α + γ tranus. Annealing in the α-phase field can result in a designed fully lamellar microstructure.

The formation energies of intrinsic point defects and the interaction energies of possible defect pairs in NiA1 are calculated from first principles within an order-N, locally self-consistent Green's function method in conjunction with the multipole electrostatic corrections to the atomic sphere approximation. The theory correctly reproduces the ground-state properties of the off-stoichiometric NiAl alloys. The constitutional defects (antisite Ni atoms in Ni-rich and Ni vacancies in Al-rich NiAl) are shown to form ordered structures in the ground state, in which the defects of the same kind tend to avoid each other at the shortest separation distance on their sublattice. A mean-field theory is applied to calculate the equilibrium concentrations of thermal defects. The statistics of thermal defects is interpreted in terms of dominant composition-conserving complex defects which are shown to be triple defects in Ni-rich and nearly stoichiometric NiAl. In the Al-rich region a novel thermal excitation dominates where two constitutional Ni vacancies are replaced by one antisite Al atom. The number of vacancies, as well as the total number of point defects decrease with temperature in Al-rich NiAl. The boundary between the two regions is treated analytically. The vacancy concentration exhibits a minimum in its temperature dependence at the boundary. Similar analysis is applied to study constitutional and thermal defects in Ni3Al. The calculated effective vacancy formation energy in Ni3Al as a function of concentration is in excellent agreement with recent experimental data.

Changes in the degree of long-range order (LRO) have been investigated in B2-ordered intermetallicFe-44.8at%Al by electrical resistivity measurement (resistometry). Order variations were observed during isochronal annealing for different states of initial thermal treatment. A complicated behaviour of resistivity changes with temperature is observed when starting with a small degree of disorder. Very slight re-ordering is found already at about 470K and is surprisingly followed by a slight disordering above 530K. Only for very long annealing times the expected ordering tendency is observed in this temperature range. Above 710K re-ordering is continued and reversible equilibrium changes of order with temperature are observed above 830K.

Phase stability of κ-phases (perovskite-type phases) was studied in the third-period transitionmetals (M: Mn to Cu) - aluminum (Al) - carbon (C) systems. The concentrations of transition metals (CM) were systematically changed from 60mol%Mn to 20mol%Ni+40mol%Cu along the periodic table under the constant nominal alloy compositions of CMl+CM2=60mol%, CA1=60mol% and Cc=20mol%. Alloys were prepared using mechanical alloying and hot pressing. Constituent phases were investigated by X-ray diffraction analysis, scanning electron microscopy and electron probe microanalysis. It is confirmed that Mn3Al(C,O)0,95, Fe3Al(C,O)0.87, Co3AI(C,O)0. 6 and Ni3Al(CO)0.26 exist in their ternary systems where oxygen is introduced during the process. Also, only one κ-phase is recognized in an alloy having an intermediate composition in every system. This suggests that continuous solid solutions between these κ-phases are formed in the pseudo-ternary systems. Composition dependence of lattice parameter is discussed.

In situ straining experiments on NiAl, NiAl-0.2at% Ta, γ-TiAl, and MoSi2 in a high-voltage electron microscope showed a transition from the obstacle controlled dislocation motion or the Peierls mechanism at low temperatures to either an unstable or viscous motion at high temperatures. It is suggested that the viscous motion is due to the formation of point defect atmospheres around the dislocations, which cause additional drag and may be responsible for the flow stress anomaly in some of these materials. The atmospheres may be of an extrinsic or an intrinsic nature. A new model is proposed for the origin of intrinsic atmospheres assuming that the energy of a dislocation in an intermetallic alloy may be lowest if the dislocations contain a number of point defects in their core. The dragging of atmospheres may lead to an “inverse” dependence of the strain rate sensitivity on the strain rate, as observed experimentally. The macroscopic deformation data of the studied materials are discussed in terms of the model.

We have investigated a microstructure evolution of a Ti-48Al-3.5Cr (in at.%) alloy at high-temperatures (>1473K). In the alloy annealed at 1673K for 1.8ks, followed by air-cooling, a characteristic microstructure with a feathery fashion was uniformly formed. From a cooling-rate-controlling study, it was found that formation of the feathery structure is accomplished during continuous cooling from 1673K to 1573K, within the α+γ two-phase region. Transmission electron microscopy revealed that the feathery structure is composed of lamellar colonies (5–10µm) which are crystallographicaly tilted slightly (a few degree) with their neighbors. A surprising fact is that lamellae in each colony are mostly the γphase with few α2 phase less than 5% in volume. This suggests that the feathery structure is a metastable product and has not resulted from the α → α+γ transformation above 1573K. Instead, the feathery structure formation should be attributed to the non-equilibrium α → γtransformation which occurs at high-temperatures with a small degree of supercooling. We discuss this interesting phase transformation in terms of the α→γ massive transformation, based on the continuous-coolingtransformation (CCT) diagram constructed for the present alloy.

NiAl-based hybrid composites containing 5, 15 and 30 volume percent of A12O3 fibers in addition to AIN dispersion particles were fabricated via a powder metallurgy route. The strengthening mechanisms associated with the short A12O3 fiber in NiAl-(A1N)dispersion,-(A12O3)fiber, composite were studied with an emphasis on the effect of thermal residual stress on the compressive strength at room temperature and 1300K. At 300K, the yield strength of the composites was predicted using a linear superposition of the monolithic yield strength, direct fiber strengthening and tensile matrix residual stresses. The prediction shows good agreement with the measured data. The majority of the room temperature strengthening is attributable to the residual stress. At 1300K, the strengthening was achieved only by the load sharing of the fibers and there is no direct influence from the process-induced thermal residual stresses since they were completely relaxed at this temperature.

Intermetallic compound Co3A1Cx, or called ic-phase, has been reported to assume the E21, or Perovskite structure. In the present work its crystal structure is critically reinvestigated in the two-phase alloys containing κ-phase in the Co primary solid solution matrix, α(Co), using conventional electron diffraction. It is shown that the crystal structure of κ-phase is a derivative of E21 being a cubic structure composed of eight E21 sub-unit cells in a half of which the body center sites are not occupied by carbon atoms. As a result, its space group is Fm3m and the chemical formula should be Co3AIC0.5. It is also found that the lattice parameter of the phase is about twice as large as that of α(Co). Orientation relationship of the κ-phase with the matrix α(Co) is found to be similar to the case for the γ' phase with the fcc γmatrix in a Ni-base superalloy since three orthogonal axes of κ-phase are parallel to those of α(Co). Lattice misfit under this orientation relationship between the two phases is found to be about 2.5%.

A statistical design of experiments (DOE) strategy was implemented to optimize alloys based on the Ni-33AI-31Cr-3Mo eutectic system using small amounts of potential strengthening elements (Hf, Si, Ta, Ti). Following the analysis of the DOE results, several alloys were selected for directionally solidification (DS) utilizing a modified Bridgeman technique. The as-grown alloys were microstructurally examined by optical and scanning electron microscopy. They were also evaluated for fracture toughness at room temperature and compressive properties at 1300K. The microstructures and mechanical properties of these DS DOE alloys are discussed and compared to the directionally solidified Ni–33Al–3lCr–3Mo base composition.

The behavior of jogged screw dislocations in γ-TiAl alloys have been investigated with largescale molecular dynamics (MD) simulations. We find a new mechanism for formation of pinning points in jogged screw dislocations. We also find that the critical height for the jogs in ±[110] directions on the (001) plane to move nonconservatively is between 3r0 and 4r0, where r0 is the nearest neighbor distance of aluminum atoms. Interstitials and vacancies are created during the nonconservative motions of the jogs. In addition, the formations of dislocation dipole and loops around the jogs are also observed.

Ir should be used as an effective oxygen diffusion barrier (ODB) for ultrahigh temperature structural materials since If exhibits extremely low oxygen diffusivity. Oxidation resistance of Ir is, however, not good due to formation of gaseous oxide, IrO3, over 1390K. In this study, the improvement of oxidation resistance was aimed through alloy design of alloying with Al. IrAl is expected to form a self-healing multifunctional layered structure composed of Ir layer as ODB and A12O3 layer as a protective oxide (PO) on the Ir layer. Ar arc-melted IrAl alloy was crushed into powder, followed by hot pressing and heat treatment to remove Ir formed by eutectic reaction. Oxidation behavior was measured using thermogravimetry (TG) and differential thermal analysis (DTA) under the conditions of (1) dynamic heating of 0.167K/s and (2) isotherms at 1273K to 1673K in O2 environment. It was found that oxidation resistance is much improved by alloying with At and that the designed structure (PO/ODB) is formed on the IrAl substrate. Compressive mechanical properties were investigated from RT to 1873K: both the strength as a function of normalized temperature and specific strength are higher than those of pure Ir, NiAl, TiAl and Ni3A1. IrAl is promising as a advanced smart coating material equipping good oxidation resistance as well as high temperature strength.

Cyclic strain-rate change tests were performed by using binary, stoichiometric Ni3Al single crystalline specimens with different tensile orientations at 400 K. In all the specimens, the flow stress was independent of strain rate in steady state and exhibited a temporary change by the strain-rate change. The characteristics of the temporary stress change, the initial stress change and transient time, were independent of orientation. Based on the multiplication-immobilization model, we concluded that the immobilization mechanism, the Kear-Wilsdorf locking mechanism, is independent of orientation in binary, stoichiometric Ni3Al.

The deformation mechanisms active in hard orientation compressed pst-TiAl have been studied by transmission electron microscopy (TEM). The observed deformation modes involved soft superdislocation and hard ordinary dislocation slip, and hard twin systems. The transfer of these twin shears occurred across all types of (γ/γ)-interfaces. Transfer mechanisms have been determined on the basis of crystallographic analyses of the TEM data. The active modes of deformation were consistent with macroscopic shape changes observed for the pst-TiAl when the twin shear transfer mechanisms were considered. Based on these observations it has been proposed that the strength and ductility of polycrystalline fully-lamellar TiAl may be improved by reducing the width of both γ- and α2-lamellae.

Nb-Al-N and Nb-Si-B powder compacts were prepared by spark plasma sintering, and their microstructure, mechanical properties and oxidation behavior were investigated. Adding nitrogen was easily done by blending or mechanically alloying Nb and AIN powders and then sintering them. The addition of nitrogen caused the formation of Nb2N in all the Nb-Al-N compacts and Nb2N and Nb3Al2N in compacts with high aluminum and nitrogen contents. The highest room-temperature hardness and the highest yield stress at 1473K were observed for compacts consisting of Nb2N and Nb3Al2N and those consisting of Nb2N and Nb2Al, respectively. Nb-Si-B compacts were prepared from elemental powders. Two or three of NbB2, Nb5Si3, Nb5Si3B2 and NbSi2 phases were identified as constituent phases of Nb-Si-B compacts depending on composition unless a large amount of silicon is consumed by forming SiO2. Contributions of NbB2, NbSi3 and Nb5Si3B2 phases to room-temperaturehardness and yield stress at 1973K were much larger than those of NbSi2. However, the oxidation resistance of Nb-Si-B compacts increased with increasing the volume fraction of NbSi2.The oxidation resistance of NbSi3B2 was better than that of NbSi3, but was not as good as that of NbSi2.