Acta Metallurgica最新文献

The effect of neutron irradiation (9.4 × 10²² n/m² at 395°C) on solute segregation to grain or interfacial boundaries in several iron base alloys doped with P, Cu and/or C has been investigated using scanning Auger microscopy. It was found by fracture surface Auger analyses that while neutron irradiation enhanced intergranular segregation of S in a Cu-doped alloy with the absence of P segregation, it mitigated S segregation and promoted P segregation in P containing alloys. The quantity of segregated P was much greater for the irradiated alloys than for the thermally 1000 h aged alloys. The P-doped alloy showed a stronger effect of neutron irradiation on the enrichment of P segregation than the P-Cu-doped and P-C-doped alloys. Argon sputtering experiments indicated that the segregation profiles for S and P were more broadly and narrowly distributed during irradiation, respectively. A remarkable transition of the P segregation profile was observed near the interface of particles lying along grain boundaries where S was preferentially segregated because of the competitive segregation between S and P. The mechanism for neutron irradiation-induced solute segregation is discussed in light of inverse Kirkendall effects and the formation of defect-solute complexes arising from the dynamic interaction between solute and defect fluxes. The relationship of intergranular solute segregation to embrittlement is presented.

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.

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.

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.

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.

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 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.

For fatigue at temperatures from 4.2 to 350 K, crystals in saturation comprise PSB and matrix regions, however, details of PSB morphology are temperature-dependent. There are three main types of profile. Type I: at very low temperatrures, mildly bulged PSBs comprising smooth extrusions. Type II: from ≈15 to ≈250 K, triangular bulges reaching > 10 μm in height. Type III: above 300 K, non-bulged PSBs comprising both intrusions and extrusions. Transition regions with PSBs of mixed character are found between the distinct types. Room temperature is near the end of a transition region, PSB morphology therefore depends on fatigue conditions. PSB profile development is independent of fatigue environment; crack propagation is environment dependent. There is no correlation betwen PSB morphology and point defect mobility.

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.