Acta Metallurgica最新文献

The effects of various pre-annealing and surface polishing treatments on the crystallisation behaviour of amorphous Fe₄₀Ni₄₀B₂₀ have been investigated by a combination of differential scanning calorimetry, optical and transmission electron microscopy, and X-ray diffractometry. The results show that the crystallisation kinetics are not significantly affected by either pre-annealing or mechanical polishing alone. However, certain combinations of pre-annealing and surface polishing, typically pre-annealing at 350°C for 20 h followed by mechanical polishing with 1200 SiC, change the crystallisation process from bulk nucleation and near-spherical growth to surface nucleation and planar inward growth, with a corresponding sharp increase in the overall crystallisation rate, and a modification in the crystallisation rate law from Y = 1 − exp(− Kt³) to Y = K't. For both surface and bulk nucleation crystallisation mechanisms, transmission electron microscopy and X-ray diffractometry show a eutectic crystal structure, consisting of a fine scale lamellar mixture of orthorhombic (Fe, Ni)₃B and f.c.c. (Fe, Ni). The increase in the overall crystallisation rate after pre-annealing and surface polishing is caused by a much higher number of nuclei at the surface compared to the bulk, with little change in the eutectic crystal growth rate. At 390°C, the nucleation density is $\text{≈7 ×}\text{10}{}^{13}\text{m}{}^3$ for bulk nucleation and $\text{≈10}{}^{13}\text{m}{}^2$ for surface nucleation, while the eutectic crystal growth rate is ≈5 μm/s in both cases. The growth rate constants obey the Arrhenius law over the temperature range 381–396°C with an activation energy of ≈3.0 eV/atom. Surface nucleation of the crystallisation process in amorphous Fe₄₀Ni₄₀B₂₀ can be stimulated either by mechanical deformation of B-rich amorphous particles in the phase-separated, relaxed amorphous alloy structure, or by Ni enrichment at the amorphous alloy surface.

Copper single crystals oriented for single glide were fatigued at temperatures from 4.2 to 350 K in inert atmosphere. Fatigue behaviour was similar at all temperatures. Saturation stress decreased smoothly with increasing fatigue temperature from ≈85 MPa at 4.2 K to ≈27 MPa at 350 K. The effect of cycling rate on saturation stress was also measured. The observations suggest that the same fundamental mechanism, which must be mechanical in nature, operates throughout the temperature interval studied. The activation energy of the operating mechanism, calculated from the observed temperature and rate dependence of the saturation stress, increases linearly with temperature, from ≈0.175 eV at 77.4 K to ≈ 0.7 eV at room temperature. At 77.4 K (as at room temperature) fatigue life is shorter in reactive than in inert environment.

At temperatures above the flow stress peak in Ni₃Al the slip system is 〈110〉{001}. At higher temperatures still a second system, 〈010〉{001}, also operates. Both types of dislocation form glide loops which dissociate into $\text{1}\text{2}$ 〈110〉 APB-linked partials and both glide loops show ranges of elastic instability. Three types of dislocation may lower their energy further by a second dissociation on {111} planes: the screw 〈110〉 forms a Kear-Wilsdorf lock, the edge 〈110〉 forms a Lomer-Cottrell lock and the 45° 〈010〉 forms a B5 lock. Interactions between 〈110〉 and 〈010〉 dislocations give rise to two more dislocations with non-planar cores, the 〈110〉 lock and the 〈111〉 lock. All these locked dislocations are slow moving or immobile and all, except for the 〈111〉 lock, have been observed in the electron microscope. The formation of 〈110〉 and 〈111〉 locks and the mixing of the two slip systems through interactions between them give an explanation for the high-temperature work-hardening peak found in Ni₃Al.

Rapid surface resolidification using a high powered CO₂-laser has been performed on eutectic Al-32.7 wt% Cu at speeds between 0.2 and 8 m/s. By means of longitudinal cuts through the centre of the laser trace, the local growth rate has been measured by observation of the orientation of the microstructure using transmission electron microscopy. The various microstructures as a function of growth rate, are:

• 1.

  (a) regular lamellar eutectic α-Al/θ-Al₂Cu structure for growth rates below 20 cm/s with interlamellar spacing as fine as 17 nm;

• 2.

  (b) a new wavy eutectic α-Al/θ'-Al₂Cu morphology for growth rates of between 20 and 50 cm/s;

• 3.

  (c) a banded structure formed by alternating supersaturated α-Al solid solution and the wavy eutectic for growth rates greater than 50 cm/s.

Diffusion induced grain boundary migration (DIGM) was investigated in the Ni substrate of Ni-Cu diffusion couples. After annealing at 615°C for 26 h, TEM/STEM revealed:

• 1.

  (1) dislocation walls at the initial grain boundary positions,

• 2.

  (2) a small misorientation (0.1–0.5°) between the DIGM zone and the Ni matrix,

• 3.

  (3) highest Cu concentrations in the DIGM zones adjacent to the original grain boundary positions,

• 4.

  (4) steps and line defects at the DIGM boundaries and

• 5.

  (5) grain boundary faceting caused by DIGM.

• 1.

  (1) a decrease in the dislocation densities in the DIGM zones, in the matrix and at the original grain boundary positions

• 2.

  (2) a change in Cu concentration profiles across the DIGM zones. The experimental results are interpreted in terms of current DIGM theories.

Based on SIMS analyses, X-ray diffraction patterns, scanning and transmission electron microscopy observations and chemical analyses at various scales, the mechanisms of LASER surface alloying of Ni coating of Al-based alloy have been investigated. The formation of Al₃Ni primary dendrites in the case of the low thickness of the initial Ni coating is reported and a double dendritic mode is observed in the case of the thicker coating, Al₃Ni₂ followed by Al₃Ni dendrites. A segregation in Si has been observed just below the exposed surface and related to the dendritic solidification mode. The optimized surface treatment parameters have been experimentally determined as a function of the initial thickness of the Ni coating.

The intent of this article is to apply recent solutions for the mechanics of cracks at and near bimaterial interfaces to rationalize crack trajectories observed by experiment and to provide a basis for interpreting measurements of the interface fracture energy, Γ_i. It is demonstrated that the choice of test specimen governs the tendency of cracks to either remain at interfaces or deviate away, based on considerations of the phase angle of loading, ψ. It is further revealed that the measured interface fracture energy may be strongly influenced by the crack trajectory, as governed by ψ, through crack shielding and plasticity effects. Consequently, interfaces do not typically have unique fracture energies, but instead Γ_i depends on ψ which, in turn, is influenced by the test method.

The importance of elastic anisotropy in the substitutional bainite (α₁ plate) formation at defects in copper base alloys has been emphasized in this work. It has been argued that a defect such as a dislocation in an elastically anisotropic medium could lower its energy by reducing the energy factor K (elastic anisotropy and thus composition dependent) which in turn would lead to higher elastic anisotropy around the defect through solute reduction. Proof for this is provided through TEM evidence, monitoring the elastic anisotropy dependent bend angles of directionally unstable dislocations in β Cu-Zn-Al as a function of isothermal ageing which eventually leads to bainite formation. The role of elastic anisotropy in promoting both solute diffusion and shear and its implications on the initial stages of bainite formation are discussed.

The energetics of two different models leading to the formation of a row of prismatic loops from the fluctuation and subsequent break-up of dislocation dipoles or elongated dislocation loops have been evaluated as a function of fluctuation amplitude and wavelength. Break-up most likely occurs by spontaneous pinching off of small circular loops at the ends of closed-ended dipoles or elongated loops. The theoretical results agree well with experimental observations of the dislocation substructure in Ti⁴⁺-doped sapphire (α-Al₂O₃) deformed by basal slip.

A system that consists of a misfit precipitate transforming from an infinite matrix is analyzed to determine the effect of elastic fields on a morphological stability analysis. Elastic fields enter the analysis through the boundary condition that sets the solute concentration at the interphase interface. Elastic fields influence the stability results when the shear modulus of the precipitate is different from that of the matrix; they act to stabilize the interface and favor spherical growth shapes if the shear modulus of the precipitate is greater than that of the matrix, and to destabilize the interface and favor non-spherical growth shapes in the alternative case. These elastic effects are especially pronounced at small supersaturations. For the case of a hard precipitate growing in a soft matrix, elastic fields can, at small supersaturations, absolutely stabilize the interface against any given perturbation wavelength. In the absence of capillarity, elastic fields spoil the shape preserving growth of an ellipsoidal precipitate, a solution found by Ham when both capillarity and elastic fields vanish. When a precipitate is undergoing unstable growth, elastic fields shift the value of the fastest growing harmonic such that stabilizing elastic fields favor long wavelength perturbations and relatively smooth growth shapes, while destabilizing elastic fields favor short wavelength perturbations and relatively rough growth shapes.