Binary Ti-Al samples containing from 46 to 54 at.% Al were solidified while undercooled by various amounts using electromagnetic levitation techniques. A detailed thermal history of these samples was obtained with sampling rates as high as 500 KHz during recalescence. This very high sampling rate was essential to resolve the thermal events. Primary α solidification was observed in samples containing from 51 to 54 at.% Al that were undercooled less than about 100 K at solidification. Primary β solidification was found for all undercoolings tested in samples containing less than 51 at.% Al and for undercoolings greater than about 100 K in samples containing 51 to 54 at.% Al. These first two observations are in agreement with results from Valencia et al. for Ti-45 at.% Al and Ti-50 at.% Al, respectively [4]. However, the third observation of β as the primary solidification structure at high undercooling and 54 at.% Al differs from Valencia's interpretation for Ti-55 at.% Al. The results in this study are consistent with the proposed McCullough Ti-Al phase diagram.

Ti-15Ta-48Al alloy samples were characterized after arc melting, controlled supercooling experiments and splat-quenching. It was found that primary b.c.c. (β) forms from the melt in all cases. The arc-button exhibits severe Ta segregation toward the dendrite cores which is substantially alleviated by rapid solidification. Increasing bulk supercooling of the melt reduces the size and scale of segregation and ultimately results in a single phase γ structure. Splat quenching also increases homogeneity and alters the post-solidification transformations in three ways: (i) it suppresses the typical precipitation of γt laths in α2, (ii) it inhibits the formation of σ, and (iii) the Ta-rich areas of the β phase transform to B2 instead of α + σ.

The intermetallic compound NiTi was cold rolled at room temperature. Amorphous bands were formed within the finely twined crystalline matrix after thickness reduction of 60%. Striking similarities were observed in microstructural morphology between amorphous bands and shear bands that are generally observed in heavily cold-rolled pure metals. We suggest from the present observations together with the reported results in other solid-state amorphization experiments that the amorphous bands are produced in the shear bands, and that amorphization is caused by mechanical instability against the shear stress.

Metastable phases, including nanocrystalline and amorphous structures, can be prepared by high energy cyclic deformation processes. In the present study, we compare the behavior of a stable congruent melting compound (Fe2Er Laves phase) with a mixture of pure elemental Fe and Er powders subjected to high energy ball milling. X-ray diffraction and transmission electron microscopy reveal similar results in both cases. In the early stages, a nanocrystalline fcc phase with lattice parameter a = 0.484 nm and a grain size of 6 nm is formed together with a bcc Fe-rich phase. Extended milling results in a nanoscale phase separation into Fe-rich and Er-rich crystallites with average grain sizes of 1.8-4 nm. Based on a lattice parameter analysis, the fcc phase was initially thought to be a metastable FeEr3 phase. Further studies revealed nitrogen gas in the milling vial had reacted with the powder during ball milling to produce the cubic ErN phase (“NaCl” structure with a lattice parameter of 0.4836 nm). Our experiments demonstrate that the steel vials for ball milling do not remain hermetically sealed during the milling process and a nitride phase can be formed easily if a catalyst for the dissociation of nitrogen molecules (such as Fe) exists in the system.

Alloys of (AlxM1−x)3Ti where M is V, Cr, Mn, Fe, Co, Nb, W, Fe + Ga or Fe + Ga + Mn were fabricated to form the Ll2 cubic phase, tested for ductility, and characterized by lattice parameter, microstructure, quantitative phase analysis, and phase composition. The elements V, Co, Nb, W, Fe + Ga and Fe + Ga + Mn are hereby newly found Ll2 formers in Al3Ti. Both X-ray diffraction and microprobe analysis were used to identify the phases, their volume fractions and compositions. The amount of ternary addition found in the Ll2 cubic phase varied between 4 and 12 atomic percent. This amount of ternary addition decreased as the atomic radius of the elemental addition increased. Pettifor's Mendeleev number was a useful guide in the selection of elements to form the Ll2 phase but did not correlate strongly with the amount of the ternary addition. For the DO23 phase of Al3Zr, the elements V, Mn, and Co were added to the list of Ni, Cu, Fe, and Cr as elements known to form the Ll2 phase. Little if any ductility was observed in these cubic Ll2 phases which readily cleaved with transgranular failure. To achieve ductility and toughness, more than transformation of these compositions from a lower to a higher symmetry crystal structure will be required.

Monte-Carlo simulations (MCS) and the path probability method (PPM) were used to study disorder→order transformations in bcc alloys having the AB3 stoichiometry. Both methods used an explicit vacancy mechanism of ordering and an activated-state rate theory for the vacancy jumps. We studied the evolution of short-range order (SRO) as well as B2 and D03 long-range order (LRO) in alloys that began as random solid solutions. The growth rates of SRO and LRO were significantly higher for the PPM than for the MCS. We attribute this difference to improper handling of correlated vacancy motions in the PPM. The PPM also suffered from an artificial incubation time for the initiation of LRO. Both the MCS and the PPM showed that SRO has a tendency to develop in two stages. In the first stage there is a quick relaxation of the SRO by itself. In the second stage, which occurs with a longer time constant, the SRO and LRO grow simultaneously. Parametric plots of one order parameter against another, here termed “kinetic paths”, are discussed. A variety of different kinetic paths through the B2 and D03 order parameters can be predicted theoretically, depending on the choice of interatomic potentials. This range of calculated kinetic paths is broad enough to encompass our experimental results of SRO and LRO evolution in Fe3Al.

Alloys near the Fe3Al stoichiometry were prepared far from thermodynamic equilibrium by rapid solidification methods. These highly disordered materials were then annealed at low temperatures to develop B2 and D03 order. Both shortrange and long-range order (SRO and LRO) were measured by Mössbauer spectrometry and by x-ray diffractometry, respectively. The SRO was found to evolve in two stages: an initial quick growth of the SRO alone, followed by a slower growth of SRO along with LRO. As expected, at lower temperatures the absolute rates of growth of order were suppressed, but more interestingly, the relative rates of growth of B2 and D03 long-range order parameters were different at different temperatures of annealing. When the B2 order parameter is graphed against the D03 order parameter, at lower temperatures the “kinetic path” through this graph shows that B2 order evolves relatively more rapidly than D03 order.

We report angle-resolved uv photoemission measurements from ordered and disordered Cu3Au. We observe considerable changes in the photoemission spectra across the order-disorder transition which we attempt to correlate with recent electronic structure calculations. Overall, we find reasonable agreement between theory and experiment although we note some important discrepancies.

We have studied the photoemission from the (110) faces of two disordered Ni-Fe alloys containing 25% and 50% Fe over the photon energy range 20 – 45 eV with the aim of investigating their electronic structure. We observe a number of features that disperse with photon energy and we indicate their possible origin with reference to the one-electron Bloch spectral densities calculated using the SCF-KKRCPA.

The position sensitive atom-probe (POSAP) is capable of extremely high resolution (subnanometer) microanalysis. Its 3-dimensional capabilities are unique and offer a new opportunity to study the topology and structure of materials. This paper is an initial overview of the new types of parameters and analysis that this technique makes possible.

The properties of many advanced alloys are derived from extremely fine-scale microstructures. This poses interesting questions about the measurement of composition on this scale. The phase separation of model Fe-Cr alloys has been been studied with the atom-probe. Statistical techniques have been used to estimate the composition and compare the results with the predictions of linear and non-linear theories of spinodal decomposition and the distributions obtained from Monte-Carlo calculations.

A nickel base superalloy, Alloy 718, has been studied by analytical electron microscopy in order to trace the development of the complex microstructure which is produced during a typical multistage thermal treatment. The distribution of δ γ″, γ′ and Laves phases was found to be strongly dependent on aging treatment.

The aging of β′ NiAl precipitates in ferritic Fe-Ni-Al alloys has been studied by transmission electron microscopy (TEM) and atom-probe field-ion microscopy (APFIM). The addition of Mo alters the lattice parameter of the phases and segregation of Mo to the interface between the matrix and the particles may alter the interfacial energy. The compositions of the matrix, precipitates and interfaces have been measured by TEM and APFIM. The results are compared.

An atom probe field ion microscopy and transmission electron microscopy characterization has been performed on the intragranular precipitates that are formed in a niobium-modified nickel-base superalloy 718 after the various stages of a multistage heat treatment. Atom probe results indicate that there is a wide variation in the compositions of the DO22-ordered γ″, the Ll2-ordered γ′ precipitates and the γ matrix with heat treatment and with position in the microstructure.

A series of Al-25 (Nb,Sc), at.%, and Al-25 (Zr,Sc), at.%, alloy buttons were arccast in argon and homogenized for 2 h in vacuum at temperatures ranging from 1473 to 1573K. X-ray powder-diffraction indicated that almost all the Nb in Al-25 Nb (DO22) had to be replaced by Sc in order to obtain the Ll2 structure. In the case of Al3Zr much less Sc was required - the single phase Ll2 composition is approximately Al16Zr8Sc.

Recent calculations [1,2] show that once the tetragonality is properly included the phase stability of the trialuminide transition-metal binary alloys can be understood in terms of their electronic density of states. The dominating feature is a deep minimum in the density of states just below the major transition metal d-band peak. The exact position of the minimum changes with structure type (i.e., Ll2, DO22, or DO23, and with c/a). The alignment of the Fermi energy with the minimum appears to determine the equilibrium structure. The results of linearized-muffin-tin-orbital [3] (LMTO) electronic structure calculations are compared to the rigid band model and checked against the experimentally determined phase boundaries.

Extended defects, such as dislocations and grain boundaries, control a wide variety of material properties and their atomic structure is often a governing factor. A necessary precursor for modeling of these structures is a suitable description of atomic interactions. We present here empirical many-body potentials for alloys which represent a very simple scheme for the evaluation of total energies and yet reflect correctly the basic physical features of the alloy systems modeled. As examples of atomistic studies we show results of calculations of the core structures of screw dislocations in Ll2 compounds. The same potentials have also been used to calculate structures of grain boundaries in these compounds. The deformation and fracture behavior of Ll2 alloys is then discussed in the light of grain boundary and dislocation core studies.

The interaction between a screw dislocation and symmetric [1 1 0] tilt boundaries under the influence of an applied shear stress was investigated by atomistic simulation. Many-body potentials representing Cu and Ni3Al were used for the description of the interatomic forces. A comparison will be made with in-situ observations in a transmission electron microscope of the interaction of screw dislocations with a coherent twin boundary.

Since the early studies on bcc materials, computer simulation of dislocation core structures has been recognized as a valid means of understanding yield behavior with temperature. Many of today's high temperature ordered alloys do have yield anomalies and computer simulation of core structures is a basic ingredient in the corrent theories for the anomalous yield behavior in Ll2 alloys. In the case of B2 type alloys there are very few of these studies. The present work deals with computer simulation of core structures in B2 NiAl. An embedded atom type potential is used to simulate interactions and several slip systems are considered, particularly those having [100] and [111] slip vcctors that are the experimentally observed.

First-principles total-energy calculations of the elastic constants, shear fault energies, and cleavage energies of TiAl are presented. We find a large elastic shear anisotropy along the [011] direction, and high APB energies due to the strong cohesion between Ti and Al layers. Shear faults of SISF, SESF, and twin boundary are predicted to be prevalent due to their low energies. The anomalous temperature dependence of flow stress is explained by the cross-slip pinning and fault dragging mechanisms. The intrinsic brittleness of TiAl is discussed in terms of the low mobility of 1/2[110] dislocations.

The first principles ASW (augmented spherical wave) method is used to predict the elastic properties of Al-base and Ni-base fcc alloys. It is shown that the elastic modulus increase or decrease of the fcc alloys is closely correlated with the change in the lattice constants and that the results for various solute atoms can be summarized in the simple curves, depending on the species of the solute atoms. We demonstrate that the theoretical calculations for the binary alloys can be used for the design of the (multi-component) fcc alloys with desired elastic properties and lattice constants.