Dispersed short range order in gold-silver alloys

The diffuse intensity of X-rays was measured on two gold-silver alloys containing 50 and 75 at.% gold, resp., after annealing at 300, 250 and 200°C. From these data the short range order (SRO) parameters a, were calculated. It was found that these parameters increase only very slightly with decreasing temperature and even after irradiation with 2 MeV electrons at 55°C, almost no further increase was observed. In contrast to these findings are the results of measurements of the electrical resistivity which indicate that the atomic mobility in these alloys is sufficiently large to allow the observation of structural changes at temperatures as low as 100°C. However, the thermodynamic equilibrium state of SRO is obtained only for temperatures as low as about 200°C without irradiation. The equilibrium state at 55°C is achieved after a 2 MeV electron irradiation of about 1000 min at this temperature. It is further found that an increase of the degree of order in the gold-silver alloys results from two processes having a rate ratio of about 10⁵; one causes a decrease, the other an increase of the electrical resistivity. The evaluation of the experimental SRO parameters and the electrical resistivity leads to an atomic arrangement which is best decribed by the dispersed particle model in the gold-silver alloys with particle sizes of about two lattice constants.     

The classification of short range order by electron microscopy

After discussion of the various modelling concepts, a broad classification of short range order (sro) is suggested, differentiating between uniform and bounded ‘micro-domains’ and unbounded ‘static concentration wave packets’. It is explained why this classification is physically useful, and how transmission electron microscopy (TEM) imaging provides a direct means of differentiating between these two models. The analogy between this model classification and the microcrystallite-amorphous problem in the classification of ‘glassy’ structures is discussed.Criticisms [1] of our earlier work [2] on electron diffraction information obtained for Ni₄Mo have been carefully analysed. We could find no reason to change our view of the nature of s.r.o. in this particular material. Our essential conclusion is that it is unrealistic to describe sro in quenched Ni₄Mo as of microdomain type.     

Monte Carlo simulation of short range order relevant to CuAu

The Monte Carlo simulation method is employed to determine the Cowley-Warren short range order parameters for the first twelve nearest-neighbor shells on the Ising model relevant to CuAu. The Monte Carlo results are used to determine the free parameter in a mean-field-like class of theories described by Clapp and Moss. The ability of these theories to predict ratios between pair potentials is tested with our results. In addition, evidence of a region of heterophase fluctuations is presented in agreement with X-ray diffuse scattering measurements on Cu₃Au.     

Determination of long range order in Ni-Base ternary alloys by X-ray anomalous diffraction using synchrotron radiation

Long range order (LRO) parameters in three ternary Ll₂ alloys [Ni₇₅Al₁₅Ti₁₀, Ni₇₀Al₂₀Cr₁₀ and (Ni₃Fe)_96.4Cr_3.6] have been measured using a new method. This method takes advantage of the variation in atomic scattering factors for X-rays near absorption threshold and opens a new area of research with the development of synchrotron facilities. The results show that Ti substitutes mostly, but not exclusively, for Al in NiAlTi. In NiAlCr, Cr occupies both Ni and Al sites. In NiFeCr, the order is not perfect; the Cr atoms are distributed on both types of sites, with a slight preference for Fe sites.     

Kinetics of short range order formation in Au-15 at.% Ag under electron irradiation

The short range order (SRO) induced resistivity change was measured during isothermal irradiation with 3 MeV electrons after quenching from 523 K. In the case of low temperature and high beam current density the resistivity follows a first order kinetics due to a recombination induced constant concentration of the ordering defect. The effective activation energy of the relaxation time of the SRO formation could be evaluated as 0.38 eV which is exactly half of the migration energy of the vacancy as measured in quenching experiments. This shows that the vacancy is the faster defect and therefore causes most of the ordering. Deviations from the first order kinetics of the SRO induced resistivity change could be quantitatively analyzed as change in the vacancy concentration. Its dependence on time, beam current density and irradiation temperature could be successfully described by numerical parameters. A reasonable magnitude of the parameters and complete quantitative agreement between down-quenching, upquenching and irradiation experiments was achieved.     

A microstructural study of creep in short fibre reinforced aluminium alloys

The present study investigates creep in two aluminium-based short fibre reinforced metal matrix composites which differ in magnesium content. Creep in the two MMCs is the result of two interrelated processes. During primary creep load is transferred to the fibres. This eventually causes rupture of the fibres and controls tertiary creep. During primary creep a work hardened zone forms around the fibres from where stresses are induced into the fibres. The work hardened zone can be represented by a system of dislocation loops. A recovery mechanism is proposed which involves the movement of dislocation loops to the fibre ends where they shrink. This process contributes to the total creep strain and is dependent on the build up of the interface. Adding magnesium to the aluminium alloy increases the number and size of particles in the fibre/matrix interface. This results in a longer effective recovery path and increases creep strength.     

First-principles investigation of perfect and diffuse antiphase boundaries in HCP-based Ti-Al alloys

First-principles thermodynamic models based on the cluster expansion formalism, Monte Carlo simulations, and quantum-mechanical total energy calculations are employed to compute short-range-order (SRO) parameters and diffuse-antiphase-boundary energies in hcp-based α-Ti-Al alloys. Our calculations unambiguously reveal a substantial amount of SRO is present in α-Ti-6 Al and that, at typical processing temperatures and concentrations, the diffuse antiphase boundaries (DAPB) energies associated with a single dislocation slip can reach 25 mJ/m². We find very little anisotropy between the energies of DAPBs lying in the basal and prism planes. Perfect antiphase boundaries in DO₁₉-ordered Ti₃Al are also investigated and their interfacial energies, interfacial stresses, and local displacements are calculated from first principles through direct supercell calculations. Our results are discussed in light of mechanical property measurements and deformation microstructure studies in α-Ti-Al alloys. This article is based on a presentation made in the symposium entitled “Defect Properties and Mechanical Behavior of HCP Metals and Alloys” at the TMS Annual Meeting, February 11–15, 2001, in New Orleans, Louisiana, under the auspices of the following ASM committees: Materials Science Critical Technology Sector, Structural Materials Division, Electronic, Magnetic & Photonic Materials Division, Chemistry & Physics of Materials Committee, Joint Nuclear Materials Committee, and Titanium Committee.     

First-principles investigation of perfect and diffuse antiphase boundaries in HCP-based Ti-Al alloys

First-principles thermodynamic models based on the cluster expansion formalism, Monte Carlo simulations, and quantum-mechanical total energy calculations are employed to compute short-range-order (SRO) parameters and diffuse-antiphase-boundary energies in hcp-based α-Ti-Al alloys. Our calculations unambiguously reveal a substantial amount of SRO is present in α-Ti-6 Al and that, at typical processing temperatures and concentrations, the diffuse antiphase boundaries (DAPB) energies associated with a single dislocation slip can reach 25 mJ/m². We find very little anisotropy between the energies of DAPBs lying in the basal and prism planes. Perfect antiphase boundaries in DO₁₉-ordered Ti₃Al are also investigated and their interfacial energies, interfacial stresses, and local displacements are calculated from first principles through direct supercell calculations. Our results are discussed in light of mechanical property measurements and deformation microstructure studies in α-Ti-Al alloys. This article is based on a presentation made in the symposium entitled “Defect Properties and Mechanical Behavior of HCP Metals and Alloys” at the TMS Annual Meeting, February 11–15, 2001, in New Orleans, Louisiana, under the auspices of the following ASM committees: Materials Science Critical Technology Sector, Structural Materials Division, Electronic, Magnetic & Photonic Materials Division, Chemistry & Physics of Materials Committee, Joint Nuclear Materials Committee, and Titanium Committee.