The atomistic processes during fracture of NiAl are studied using a new embedded atom (EAM) potential to describe the region near the crack tip. To provide the atomistically modeled crack tip region with realistic boundary conditions, a coupled finite element - atomistic (FEAt) technique [1] is employed. In agreement with experimental observations, perfectly brittle cleavage is observed for the (110) crack plane. In contrast, cracks on the (100) plane either follow a zig-zag path on (110) planes, or emit dislocations. Dislocation generation is studied in more detail under mixed mode I/II loading conditions.

In an effort to understand the deformation mechanism in high temperature B2 intermetallics, atomistic simulations were carried out for dislocation cores in a series of compounds exhibiting the B2 structure (FeAl, NiAl, CoAl). A comparison was made on the basis of core structures, dislocation splittings and Peierls stress values. The (110) and (112) γ surfaces were computed for these three compounds. The importance of the APB values and the maximum shear faults for explaining the dislocation behavior is discussed.

Recent studies of single crystals and bicrystals indicate that dislocations of the type a<011> are important, and may actually control, deformation at intermediate temperatures (above the brittle-to-ductile transition temperature) in hard-oriented NiAl. In the present work, the fine structure of a<011> dislocations has been examined using both high resolution and diffraction-contrast transmission electron microscopy. Evidence has been found for the decomposition of a<011> dislocations into two a<001> dislocations. The initial driving force for the decomposition is due to core effects, as revealed by molecular statics and dynamics Embedded Atom Method calculations. Additional decomposition occurs by a combination of climb and glide. A continuum-based dislocation model is introduced which incorporates these relevant microstructural features.

The microstructures of NiAl-Fe (Ni, 50.3at % Al, 0.28 at% Fe) polycrystals tested in HCF (high cycle fatigue) below and above the DBTT (ductile to brittle transition temperature) viz., 673 K, 823 K, 873 K and 928 K at different stress amplitudes were observed by transmission electron microscopy. The microstructure consisted of low energy dislocation networks and dislocation cells. The fundamental features of the dislocation structures are described. Misorientation angles between cells were measured from Kikuchi patterns obtained from cells through microdiffraction. The misorientations across the cell walls were found to increase with increase in the stress amplitude to which the material was subjected during cycling. The mechanisms for the formation and evolution of these structures are discussed on the basis of existing theoretical models. The implications of these substructures on the mechanical properties are also discussed.

The dislocation configurations produced by room and high temperature compression of <100> oriented single crystals of binary NiAl and in those containing iron and hafnium additions have been analysed and compared to those obtained by hardness indentation and TEM insitu tensile tests. Kinking occurs during room temperature compression such that <100> dislocations are activated in all cases but the iron-containing alloy also exhibited a large density of <111> screw dislocations. The latter however, appear immobile when they are created by hardness indentations of thin foils, while only pile-ups of <100> segments can propagate. Similarly, although different slip systems are present after high temperature compression, only <100> dislocation segments have been confirmed to be mobile after room temperature hardness indentation of these predeformed thin foils. The improvement in ductility observed at room temperature in the predeformed specimens of the binary and the iron containing alloys has been attributed to the increased production of these mobile <100> dislocations.

Dynamic strain aging has been characterized in standard purity NiAl single crystals during compression testing. The activation energy for the process suggests carbon and oxygen as the most likely agents. Experimental data is compared with a recent model and implications for the fracture behavior of NiAl are considered.

The strain aging behavior of three polycrystalline NiAl alloys has been investigated at temperatures between 300 and 1200 K. Yield stress plateaus, yield stress transients upon a tenfold increase in strain rate, work hardening peaks, and dips in the strain rate sensitivity (SRS) have been observed between 700 and 800 K. These observations are indicative of dynamic strain aging (DSA) and are discussed in terms of conventional strain aging theories.

In the present study, single-edge notched bend specimens of NiAl single crystals were tested in {100}<010>, {101}<101=, and {101}<010} orientations before and after heat-treating the notched specimens at 1000°C for one hour. The fracture toughness data for the non heat-treated specimens were found to be consistent with the previous results. An increase in the fracture toughness of NiAl was observed in all orientations studied upon heat-treating the notched specimens. The toughness ratio for the two orientations obtained from the heat-treated notched specimens was found to be 1.2. This ratio is in agreement with the reported stress analysis considering the crack kinking in {100} oriented specimens. A detailed SEM analysis revealed that the electric discharge machine (EDM) cutting of the notch caused the formation of sharp microcracks at the notch front and also created internal stresses in the vicinity of the notch. The increase in toughness upon heat treatment is attributed to the modification of EDM damage.

Defect structures induced by deviation of composition from stoichiometry and site preference of additional elements are predicted for B2-type aluminides (NiAl, CoAl and FeAI) consisting of 8A-group elements by comparing formation energies of particular atomic configurations under the pseudo-ground state. Interaction energies between atoms and vacancies are estimated using a modification of Doyama’s relationship. The results show that vacancy-type defects are formed in NiAl and CoAl only when the total composition of Al and additional elements occupying the Al-site is more than the stoichiometry, whilst with less than the stoichiometry, substitutional defects are formed. In FeAI, only substitutional defects are formed in either case of deviation. Similar to Al, Si occupies only the Al-site, although elements in 1A- and 8A-groups of the periodic table mostly occupy only the Ni-, Co- and Fe-site. The other elements occupy either Al-site or the site of 8A-group elements, or both, depending on the composition.

The site-distributions of Fe in four B2-ordered NiAl-based alloys with Fe concentrations of 10%, 2%, and 0.5% have been determined by ALCHEMI (atom-location by channeling-enhanced microanalysis). Site-distributions have been extracted with standard errors between ∼1.5% (10% Fe concentration) and ∼6% (0.5% Fe concentration). The results show that Fe has no strong site-preference in NiAl and tends to reside on the site of the stoichiometrically deficient host element.

An improved ALCHEMI analysis procedure is outlined. The analysis explicidy addresses the phenomenon of ionization delocalization, which previously complicated the determination of site-distributions in aluminide intermetallics, leading to inaccurate and oftentimes nonphysical results. The improved ALCHEMI analysis also addresses the presence of anti-site defects. The data acquisition conditions have been optimized to minimize the sources of statistical and systematic error. This optimized procedure should be suitable for all analyses of B2-ordered alloys. Several analyses at different channeling orientations show that the extracted site-occupancies are robust as long as the data are acquired at orientations that are remote from any major pole of the crystal.

Atom probe field ion microscopy (APFIM) has been used to characterize NiAl microalloyed with molybdenum and zirconium. Field ion images and atom probe analyses revealed segregation of zirconium to dislocation strain fields and ribbon-like morphological features that are probably related to dislocations. These results provide direct experimental evidence in support of the suggestion that the tremendous increase in the ductile-to-brittle transition temperature (DBTT) in zirconium-doped NiAl is due to pinning of dislocations by zirconium atoms. Atom probe analyses also revealed segregation of zirconium to grain boundaries. This result is consistent with the change from an intergranular fracture mode in undoped NiAl to a mixture of intergranular and transgranular fracture mode in zirconium-doped NiAl. The NiAl matrix was severely depleted of the solutes molybdenum and zirconium. Small Mo-rich precipitates, detected in the matrix and grain boundaries, are likely to contribute to the significant increase in the room-temperature yield stress of microalloyed NiAl through a precipitation hardening mechanism.

This paper presents the results of atomistic studies of grain boundaries in NiAl B2 alloy. The interatomic forces are described by Finnis-Sinclair type N-body potentials, and are fitted to properties of NiAl. The results show that the structure, energy and cohesive strength of a grain boundary depend strongly on its chemistry, and a grain boundary possessing more Al is the weakest. Energies of antisite defects at the grain boundary ∑5 {210} are also calculated, and the results suggest that Al has much larger tendency to segregate at a grain boundary than Ni does.

The atomic structure of the Σ = 5 (310) [001] grain boundary in NiAl has been determined by a synergistic approach combining high resolution electron microscopy (HREM) and atomistic structure calculations. A bicrystal of controlled orientation was produced by diffusion bonding and imaged with the electron beam parallel to the [001] tilt axis. The results showed that the material remains chemically ordered up to the boundary plane. Atomistic structure calculations employed N-body empirical potentials developed for the NiAl phase to examine the changes in interfacial energy due to the incorporation of various point defects at the grain boundary. A self-consistent model structure was determined which was of lowest energy and produced calculated images which matched experimental images of the boundary. Monte Carlo simulations confirm the stability of this structure at finite temperatures.

An overview of recent advances in ductile-phase toughening of multiphase intermetallics is presented. The fracture characteristics of aluminides, silicides, chromides, and their composites are reviewed to elucidate relationships between microstructure, toughening mechanism, and fracture resistance. The roles of crack-tip blunting, trapping, bridging, and interface delamination in instigating fracture resistance are considered. Prospects for improving fracture toughness are discussed in conjunction with relevant microstructural variables such as the volume fraction, size, morphology, constraint, and work-of-fracture of the ductile phase.

The toughening effect of ligaments in a fully lamellar Ti-48Al-2Cr-2Nb alloy has been determined by evaluation of J integrals of the crack tip and crack wake regions from surface strain maps. The crack tip fracture toughness KtiP was found to vary with lamellae orientation and ranged from Ktip = 13 MPa√m for inter-lamellar fracture to Ktip = 19.5 MPa√m for translamellar fracture The bridging contribution to fracture toughness was found to arise from a bimodal distribution of ligament sizes. Fracture surface reconstructions showed a distribution of large hinges that initiated at grain or subcolony boundaries and smaller ligaments associated with delaminations within a grain. The stress-displacement functions for these two ligament populations have been determined from a large scale bridging analysis of the resistance curves along with fractographic information. The small ligamerts appear to account for the initial rapid rise in crack growth resistance; and the large hinges, for the slow continued rise in toughness at large crack extensions.

A preliminary investigation was conducted for developing a thermomechanical procedure to refine the lamellar colony size of a fully lamellar structure and improve the balance of the mechanical properties of TiAl based alloys. The main experimental results are as follows: 1) The lamellar colony size in a fully lamellar structure can be reduced to 50 μm using a proper thermomechanical procedure. 2) The influence of colony size on the compressive yield stress of the fully lamellar structure is complex. In the large colony size region, the yield stress can be described by the Hall-Petch relation. In the small colony size region, the yield stress decreases as the colony size decreases. 3) The TiAl alloy, with a fine fully lamellar structure, has the best balance of tensile properties and fracture toughness.

Titanium aluminides with a lamellar microstructure consisting of the intermetallic phases ֱ2 (Ti3Al) and γ(TiAl) suffer from brittleness at ambient temperatures but exhibit at the same time a relatively high fracture toughness. This discrepancy indicates particular processes stabilizing crack propagation in the lamellar microstructure. In this context, the toughening mechanisms were investigated in (α2 + γ) TiAl alloys which contained different volume fractions of lamellar colonies. The fracture toughness for crack propagation parallel or across the lamellar interfaces was estimated by using chevron-notched bending bars. Electron microscope studies were performed to characterize the related processes of crack tip plasticity. Special emphasis was paid to the crystallography of crack propagation and to the interaction of crack tips with lamellar interfaces. Accordingly, the lamellar morphology derives some of its toughness from interface-related processes which stabilize crack propagation by deflecting the crack tip and providing the necessary dislocation sources for crack tip shielding in the process zone ahead of the crack tip.

A sufficient Si addition to TiAl matrix has led to TiAl + Ti5Si3 dual phase alloys, showing coupled-growth microstructure. Compression tests at R.T. as well as high temperatures indicated that the yield stress increased with increasing Ti5Si3 volume fraction, and decreased at higher temperature. The reinforcement from Ti5Si3 phase was obvious, while high Si and Al contents resulted in low ductility. The fracture surfaces were quasi-cleavage. Further research should concern with the adjustment of the shape and amount of the second phase.

Subgrain boundaries (subboundaries) in a creep deformed investment cast Ti-48Al-2Nb-2Cr specimen were characterized. A multi-stress drop test was performed at temperature 765 °C and stresses from 276 MPa to 103 MPa. The stress exponent n = 7. Subboundaries were observed both in equiaxed γ grains and within γ laths in lamellar grains. Dislocations within the subboundaries are characterized to be ordinary 1/2<110] dislocations. Subboundaries are found to occur on many crystallographic planes and no preferred crystallographic planes are found for subboundary formation in the crept TiAl specimen. Misorientations across the subboundaries are less than 1°. Analysis of subboundary formation mode indicates that the creep deformation in TiAl is pure metal type.

Enhanced work hardening of the phases in the lamellar microstructure has been cited as an explanation for the lower minimum creep rates of a two-phase TiAl/Ti3Al lamellar alloy compared with the minimum creep rates of the individual TiAl and Ti3Al single-phase alloys tested between 980 K and 1130 K. This proposition is confirmed by TEM observations. Thermal and thermomechanical exposure result in the microstructural evolution, which increases the minimum creep rate (εmin) of the lamellar alloy. The effect of microstructural evolution on εmin will be discussed in the present paper.