A model for the FCC→HCP transformation,its applications,and experimental evidence

A model for the FCC→HCP transformation is proposed. It is envisaged that the dislocation reaction may govern the nucleation of six-layer HCP crystal. A macroscopic HCP region forms when these nuclei, located at different levels within a localized slipped region, growth into each other. The observed orientation dependence of the strain-induced martensite and the coexistence of HCP and twinned regions have been rationalized in terms of the proposed model. In addition, the supporting evidence has been developed by examining the crystallographic features of faulted regions in partially transformed Co-6.25 wt pct Fe alloy by transmission electron microscopy. M. L. GREEN, formerly with Bell Laboratories, Murray Hill, NJ     

Out-diffusion of the noble component during the initial stage of the oxidation of γ′-Ni3Al

Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) studies of the oxidation behaviour of single crystalline γ′-Ni₃Al revealed that Ni diffuses through the oxide layer to the surface of the material during oxidation at low oxygen partial pressure. Small pure nickel particles (diameter ∼ 50 nm) form on top of the oxide scale. In this paper, this observation is explained on the basis of a micromechanical model. The model assumes that at intermediate temperatures the prevailing transport mechanism is the diffusion along the oxide/metal interface caused by compressive stresses generated during oxide growth. Relaxation of internal stresses as well as the formation of the small Ni-particles on top of the oxide surface are controlled by interface diffusivity. Other possible stress relief mechanisms can be excluded by theoretical estimates.     

A new selection criterion for APA papers?

Comments on an incident which appears to involve the application of criteria other than scientific merit to a research contribution. The author includes a letter explaining why his paper with Porter, Conrad, and Mason, "Chronic avoidance behavior, gastrointestinal pathology, and death in the Rhesus monkey," was rejected from inclusion at the opening meeting of an APA convention. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

A process model for age hardening of aluminium alloys—II. Applications of the model

The process model for ageing of aluminium alloys, developed in Part I [H. R. Shercliff and M. F. Ashby, Acta metall. mater.38, 1789 (1990)], is applied to a number of heat treatments, establishing a basis for such problems as the prediction of the strength loss in the heat-affected zone of welds. First, the reheating of previously aged material is considered. Heat treatment using a parabolic thermal cycle is then modelled in terms of an equivalent isothermal treatment, and extension to weld thermal cycles is considered. Finally the isothermal models are presented as novel “iso-yield diagrams”, which are useful for evaluating the data from thermal cycles and have potential as process diagrams.     

Microstructural investigation on the precipitation of α″ nitrides and α″ → γ′ nitride transformation in ion-nitrided pure iron

The precipitated characteristics of α″-Fe₁₆N₂ nitrides in the diffusion layer of ion-nitrided pure iron were investigated with transmission electron microscopy (TEM) and high-resolution electron microscopy (HREM). Three sets of α″ nitrides, whose habit planes are (100)_α,(010)_α,(001)_α, respectively, do not precipitate simultaneously from the diffusion layer, which is different from the normal homogeneous precipitation in Fe-N alloys. Unlike the typical disc-shaped morphology reported widely, the α″ nitrides in the diffusion layer appear as ribbonlike slices. They grow on {001}_α matrix planes with a parallel orientation relationship, and the direction of their length is parallel to the <110>_α direction. The interface between the α″ nitride and α matrix and a 7 deg [111]/(112) low-angle-tilt grain boundary in the α″ nitride were examined with HREM. The distributions of dislocations at the interface and the grain boundary were investigated. During microstructural examination, it was observed that a γ′-Fe₄N nitride could grow on an α″ nitride directly. The orientation relationship during the α″ → γ′ nitride transformation was determined as to be (001)_γ//(110)_α,[110]_γ//[111]_α.     

Modeling of γ/γ′ phase equilibrium in the nickel-aluminum system

A theoretical model is proposed for the determination of phase equilibrium in alloys taking into consideration dissimilar lattice parameters. Volume dependent pair interactions are introduced by means of phenomenological Lennard-Jones potentials and the configurational entropy of the system is treated in the tetrahedron approximation of the cluster variation method. The model is applied to the superalloy relevant nickel-rich, γ/γ′ phase region of the Ni-Al phase diagram. The model predicts reasonable values for the lattice parameters and the enthalpy of formation as a function of composition, and the calculated phase diagram closely approximates the experimental diagram.     

Heterogeneous nucleation during the titanium αaiβ transformation studied by Synchrotron X-ray topography

This study is an in situ and “real time” approach of the nucleation problem during the αaiβ titanium phase transition by Synchrotron X-Ray Topography. The preponderant part played by the strains in the nucleation initiation and the behaviour of isolated matrix dislocations, at the transformation temperature, in the α phase as well as in the β phase, has been shown.     

A process model for age hardening of aluminium alloys—I. The model

Process modelling techniques are used to describe the changes in yield strength due to age hardening of heat-treatable aluminium alloys. A model for the isothermal ageing curve is developed. This is demonstrated for a number of alloys and the success of the approach is assessed. Applications and a new diagram, showing the variation of strength with temperature and time, are described in an accompanying paper.     

On the deformation of the 〈100〉 orientated γ′ strengthening phase of the CMSX2 superalloy

Both conventional macroscopic deformation tests and in situ straining experiments were devoted to the plastic behaviour study of the 〈100〉 orientated γ′ strengthening phase of the CMSX2 superalloy. Tensile test experiments, performed at various temperatures, reproduce the anomalous variation of the flow stress. Specimen unloading/reloading experiments show that the flow stress anomaly does not ensue from the exhaustion of the mobile superdislocation population. Stress relaxation experiments suggest that at both low and high temperatures, the superdislocation motion is controlled by a thermally activated mechanism, while in the temperature range around 300°C, the laws of thermal activation fail and instabilities occur on both the stress/strain and stress relaxation curves. In situ straining experiments, performed at 25°C and 320°C, lead to a microscopic scale approach of the deformation. At room temperature, a locking/unlocking mechanism controls the superdislocation motion. At 320°C, in addition to the locking/unlocking mechanism, local pinning points (either of extrinsic or intrinsic nature) affect the superdislocation motion. The in situ experiments permit us to propose various superdislocation core configurations so as to account for the experimental facts described above.     

Stacking fault formation in γ′ phase during monotonic deformation of IN738LC at elevated temperatures

Deformation behaviour of IN738LC in two γ′ morphologies under monotonic loading at elevated temperatures has been studied. Mechanisms of dislocation-γ′ precipitate interaction have been identified. Dislocation reactions leading to stacking fault formation within γ′ have been characterized.     

On the elastic mismatch in the order-disorder transformation and solid state amorphization of intermetallic compounds—II. Criteria for the solid-amorphous transformation in intermetallic compounds

Using simple thermodynamic considerations for the Gibbs-free energies of the crystalline and amorphous states a general condition for the solid-state amorphization of intermetallic compounds by irradiation or mechanical impact was derived. It was obtained that the amorphization is possible if the maximum elastic energy, ΔH_el^m, stored during the (chemical) order-disorder transition is larger than ΔH_top^a, the enthalpy of the topological change of amorphization. This condition can be rewritten into the form: the amorphization is not possible if the order-disorder temperature is less than the melting temperature. T_c < T_m, and if T_c >T_m—in contrast to Johnson's condition—the possibility of the amorphization depends on the relative magnitude of the elastic mismatch energy and the chemical energy of ordering. If their ratio is large enough the amorphization is possible. Detailed calculations, carried out for a large number of compounds, led to a satisfactory agreement with the experimental observations.     

Application of Computational Thermodynamics to the Design of a Co-Ni-Based γ′-Strengthened Superalloy

A currently available commercial Calphad thermodynamic database was utilized to investigate its applicability to alloy design in the new class of Co-Ni-based γ′-strengthened high-temperature alloys. A simple primary design criterion was chosen: maximize the γ′ solvus temperature in the six-component Co-Ni-Al-Ti-W-Ta system while ensuring no formation of secondary, potentially deleterious phases. Secondary design considerations included the effects of alloying elements on equilibrium γ′ volume fraction and on solidus and liquidus temperatures. The identified composition, Co-30Ni-9Al-3Ti-7W-2Ta-0.1B (expressed in mole percent), representing a conservative estimate of the maximum allowable concentrations of alloying additions Al, Ti, W, and Ta, was subsequently produced and characterized. The experimentally measured γ′ solvus temperature of the new alloy was 1491 ± 3 K (1218 ± 3 °C), about 35 K (35 °C) above any previously reported two-phase γ−γ′ Co-(Ni)-based alloy. No secondary phases were observed in the alloy after annealing at temperatures between 1173 K and 1473 K (900 °C and 1200 °C). Additional alloy compositions with experimentally measured γ′ solvus temperatures in excess of 1533 K (1260 °C) were also identified employing the same basic approach. The efficacy of currently available thermodynamic databases in their application to Co-based γ′-strengthened superalloy development is discussed, including expanding design efforts to include additional alloying elements, as well as specific areas for improvement of future databases.