Kinetics of grain boundary migration in dilute CuSi bicrystals

Kinetics of grain boundary migration in dilute copper-silicon bicrystals have been investigated by the reversed-capillary technique at temperatures and silicon solute additions ranging from 275–400°C and 50–680 ppm, respectively. Grain boundary displacements increase with increasing time and temperature and are characterized by an apparent activation energy, which increases from 27 to 35 kCal/mole with increasing silicon content. Results are in excellent quantitative agreement with theories of grain boundary migration and indicate that the controlling mechanism is thermally activated transport of silicon across the moving boundary.     

Segregation to and structure of [001] twist grain boundaries in CuNi alloys

The segregation, thermodynamic, and structural properties of [001] twist boundaries in CuNi alloys have been examined within a wide range of misorientations and temperatures. Cu always segregates to the boundary. The concentration of the first layer adjacent to the boundary increases monotonically with misorientation and no obvious cusps are observed. All other thermodynamic properties vary smoothly with the misorientation, with the exception of the vibrational entropy of the boundaries without segregation. The unsegregated vibrational entropy shows a large peak at the misorientation corresponding to the Σ17 boundary and two minima around the Σ13 and Σ5 boundary orientations. The concentration distribution within the plane of the grain boundaries can be described by the same structural unit model established for [001] twist boundaries in pure materials. Regions of large tensile stress show greater segregation than do regions of compressive stress. Regions of large shear stress tend to show reduced segragation compared with regions of small shear stress.     

Precipitate stability in AlCuMgAg alloys aged at high temperatures

A study has been made of the thermal stability of the Ω phase in AlCuMgAg alloys aged at high temperatures (200 to 350°C). This phase, which precipitates as thin plates on the {111}_α planes, has been shown to be replaced by the equilibrium precipitate θ (Al₂Cu) after prolonged ageing (e.g. 2400 h at 250°C). Measurements have been made of the thickening behaviour of the Ω plates and the various orientations and morphologies of the θ phase have been characterised. Whilst there is some evidence for the direct allotropic transformation of Ω to θ, it is concluded that a gradual dissolution/re-precipitation mechanism dominates the changes to microstructure at these high temperatures. Although magnesium and silver are known to segregate to the Ω phase, they were not detected in association with θ. Rather they were found to partition to sites of the S phase (Al₂CuMg) which forms as a minor precipitate under these ageing conditions.     

Kinetics of the amorphous to icosahedral phase transformation in AlCuV alloys

Single phase icosahedral samples are obtained by annealing melt-spun Al₇₅Cu₁₅V₁₀ amorphous alloys. The kinetics of this amorphous to icosahedral phase transformation were measured isothermally and nonisotheramally by differential scanning calorimetry and from changes in the electrical resistivity. Transmission electron microscopy and energy dispersive X-ray analysis indicate that the transformation proceeds polymorphically by nucleation and growth, ruling out a “micro-quasicrystal” model of the glass in this system. A standard Johnson-Mehl-Avrami analysis of the isothermal, transformation data yields Avrami exponents in the range 2–2.5, which are inconsistent with a polymorphic transformation. These anomalous Avrami exponent arise from an inhomogeneous distribution of quenched-in nuclei. Fits are made to a kinetic model assuming a constant nucleation rate and growth on these quenched-in nuclei. An analysis of the nucleation rates obtained from these fits gives an estimate for the interfacial energy between the icosahedral phase and the glass of 0.002 J/m² ⩽ α ⩽ 0.015 J/m², demonstrating that the short range order must be similar on both sides of the interface.     

Microstructural characteristics of quasicrystalline phases in alloys of AlCuFeCo

An investigation of the structural characteristics of quasicrystalline phases obtained in quaternary alloys is carried out. The coexistence between the quasicrystalline phases phases is analyzed. HREM images and diffraction patterns which correspond to the decagonal and icosahedral phases show pronouned deviations from the perfect decagonal and icosahedral symmetries. These effects are more pronounced when the specimen is annealed. Both kind of quasicrystalline phases show planar faults and dislocation-type of defects. Planar faults show the same image contrast features as in the crystalline case. Evidence is presented based on image contrast characteristics of two different types of planar faults. Dislocations in these phases show no evidence of spliting into partials.     

Diffusion along migrating and stationary grain boundaries in the CuAg system

The diffusion along a migrating random grain boundary of a Cu-3.8 at.% Ag bicrystal has been investigated by discontinuous precipitation and dissolution. The measurements have been carried out over an extensive temperature range. The reaction front velocities varied between 1.8 × 10⁻¹⁰ and 2.7 × 10⁻⁶ m/s. The values of sδD_b (s = segregation factor, δ = grain boundary width and D_b = grain boundary diffusion coefficient) have been determined from the precipitation data according to different models. All of the four models applied yield sδD_b values which are of the same order of magnitude as the sδD_b values of stationary grain boundaries. The best correspondence is given by the model of Turnbull resulting in the following Arrhenius parameters: pre-exponential factor (sδD_b)₀ = 5.80 × 10⁻¹¹ m³/s and effective activation energy Q^ef_b = 144.1 kJ/mol. Based on the numerous available data which permit a relevant comparison, we conclude that the diffusivities of migrating, sliding and stationary grain boundaries in the CuAg system are of the same order of magnitude. This conclusion is in accord with the general behavior of other binary systems.     

A study of phase decomposition in CuNiFe alloys

A study of phase decomposition in two CuNiFe alloys was realized by AP-FIM. It was possible to confirm that phase decomposition takes place via spinodal decomposition in this alloy system, as the amplitude of the composition modulation increased with ageing time without practically any change in the wavelength of the modulation. It was also observed that the morphology of the decomposed phases is related to the coherency-strain energy as predicted by Cahn's theory of spinodal decomposition. By analysis of the data of the modulation wavelength, it was possible to estimate the coherent spinodal temperature to be 800 ± 25 and 900 ± 25 K in Cu46 at.% Ni4 at.% Fe and Cu48 at.% Ni8 at.% Fe alloys, respectively. In the early stage of decomposition, the change in modulation wavelength showed a time exponent as small as about 0.07. On the other hand, in the coarsening stage the change in the modulation wavelength agreed well with the LSW theory of thermally activated growth. The activation energy for this coarsening process was determined to be 216 ± 10 and 232 ± 10 kJ/mol in the Cu46 at.% Ni4 at.% Fe and Cu8 at.% Fe alloys, respectively. The compositions of the decomposed phases are consistent with the miscibility gap in the calculatedequilibrium CuNiFe phase diagram.     

Direct observation of coarsening in AlCu alloys

Observations have been made on coarsening of θ-CuAl₂ precipitates at the surface of AlCu alloys. One specific area was examined over extended periods so that dissolution and growth of individual precipitates could be followed. It was found that nearly all precipitates shrank on ageing, even those larger than the average particle size. Only the largest 8% increased in dimension. A simple theory was used which gave good agreement with the observed dissolution kinetics of all precipitates except the very largest. This theory assumes a solute flux from the precipitate to a larger precipitate located a significant distance away. The theory gave a mean value for interfacial energy of 0.81 J/m². Particles were observed to spheroidise during ageing, and by the time their dimension had decreased 50%, they were completely spherical. Particle coalescence took place in a few cases but was generally of minor importance in the overall coarsening process.     

Addition rules and the contribution of δ' precipitates to strengthening of aged AlLiCu alloys

The room temperature yield strength of two alloys, Al-2.30 wt% Li-2.85 Cu-0.12 Zr (the 2–3 alloy) and Al-2.90 Li-0.99 Cu-0.12Zr (the 3-1 alloy), was investigated as a function of aging time at 160 and 190°C. Reversion experiments were used to separate the contributions of the δ' and T₁, precipitates to the overall strength of the aged samples. The reversion heat treatments nearly completely dissolved the δ' precipitates, leaving the dispersion of T₁ precipitates essentially undisturbed and enabling its contribution, Δτ_T1, to be measured experimentally. Assuming linear additivity of the precipitation hardening contribution, Δτ_p, and all the others (due to grain size strengthening, solid solution strengthening, etc.), the δ' contribution, Δτ_δ', was estimated using a generalized addition rule of the type Δτ_p^q = Δτ_δ'^q + Δτ_T1^q, where q is a constant (the shear stresses, Δτ, were obtained from the measured yield stresses using data on the textures of the alloys). These values of Δτ_δ', were then analysed according to a recent version of the theory of order strengthening which requires that the trailing dislocation be pulled through the sheared ordered precipitates immediately on encountering them. It was found that for the underaged alloys only q = 1.4 yielded values of the APB energy on {111}, γ_apb, that were identical for both the 3-1 and 2–3 alloys (0.150 J/m²). Using this value of γ_abp, the theoretically predicted values of Δτ_δ' at peak strength and the δ' particle size at peak strength were in very good agreement with the experimentally measured values, and the predicted spacings of the dislocation pairs were in excellent agreement with those measured. It is demonstrated that the value of q is not merely an empirical constant required to bring theory and experiment in agreement, but is justified by the results of computer simulation experiments on strengthening by point obstacles of two distinct strengths. The possible role of other strengthening mechanisms is discussed.