Acta Metallurgica最新文献

A theory is developed to predict the long range order parameter, composition and temperature at the interface of a chemically ordered phase as a function of interface velocity and liquid composition during rapid crystal growth. It extends the solute trapping theory of Aziz to a solid phase consisting of two sublattices. The engulfment of atoms randomly on the two sublattices by the rapidly moving liquid-solid interface is balanced against the interdiffusion across the interface that attempts to restore local equilibrium. With increasing interface velocity the theory predicts a progression from the solidification of a phase with equilibrium long range order parameter and with equilibrium solute partitioning to the solidification of a disordered crystalline phase with the same composition as the liqiud. Predictions for solids with free energy functions in which the order disorder transition is first or second order show that the decrease of order parameter to zero with increasing interface velocity will be discontinuous or continuous respectively. Also solute trapping can occur at either a higher or a lower growth rate than disorder trapping depending on the free energy function.

Intergranular cavitation during creep in low carbon Astroloy, a typical nickel-base superalloy with a γ-γ' microstructure, was studied in a temperature range of 0.60–0.65 T_m. Because the grain boundary carbides could be distributed quite uniformly on all grain boundaries regardless of their structure, cavitation which results from them was also very uniform. The cavity nucleation rate was found to be directly proportional to the average creep rate. Throughout their growth the cavities retained a spherical-caps shape and their overall growth rate correlated best with a model of unconstrained growth.

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.

Measurements made during fatigue at very low temperatures show that electrical resitivity saturates with the flow stress. The net crystal defect content therefore reaches an equilibrium value. Isochronal anneal of the fatigued crystals showed no reduction of resistivity below 80 K, and <4% reduction at 100 K. The fundamental fatigue mechanism operating at low temperature is thus unlikely to involve point defect recovery processes. The resistivity then decreases with increasing anneal temperature; at 300 K ≈40% remains. There is no correlation between resistivity anneal data and PSB profile shapes. Dislocation density in PSB walls, calculated from resistivity, exceeds that for which description in dislocation terms is valid; the material is dislocation-saturated. Such PSB wall material should behave as a perfect sink for dislocations, providing a natural explanation for fatigue saturation.

Plastic deformation of ferritic stainless steel alloy AL 29-4-2 has been found to severely decrease the diffusivity and permeation of hydrogen and to increase the solubility somewhat. These changes are compared to the effects of deformation on these quantities in austenitic alloys. The differences in behavior of these classes of stainless steels are interpreted in terms of trapping and phase transformation behavior. The uniformly deformed material is studied in an effort to simulate the state of material at the tip of a hydrogen embrittlement crack. The observed differences in crack propagation behavior in the alloys can be rationalized in terms of hydrogen transport and solubility in the deformed state.

Small precipitate f.c.c. Fe-Co particles in a Cu matrix transform martensitically into b.c.c. particles by simple cooling a Cu-1.4 mass% Fe-0.6 mass% Co alloy to the liquid nitrogen temperature. Transmission electron microscopy has revealed that the small transformed particles are free from dislocations around them. However, the transformation strains create elastic fields in and around the particles. Successive annealing results in the occurrence of the diffusional relaxation of the elastic fields as envisaged by the changes in the Moiré fringes in the Fe-Co particles. The mechanism and kinetics of the diffusional relaxation are analyzed both experimentally and theoretically. It is found that diffusion at the interfaces between the Cu matrix and Fe-Co particles causes the relaxation and that the activation energy of the interfacial diffusion is 1.6eV.

Melt-spun glassy Fe-22.5Al-10Zr metal (concentration in atomic percent) is an interesting material for high temperature oxidation study due to its high crystallization temperature, 650°C, and its capacity to form a protective oxide layer. We have applied analytical electron microscopy on cross-section samples to study the composition variation near the two surfaces of the melt-spun ribbon. An aluminum depleted layer was detected using X-ray energy dispersive spectroscopy ≈200 nm beneath the He-cooled surface. No composition variation was observed as a function of depth below the wheel-cooled surface. The crystallization behavior of this material both below and above the crystallization temperature was also studied. Crystallization begins with the nucleation of α-Fe particles via a primary crystallization mechanism followed by the formation of a metastable phase, Fe₈Al₃Zr, via polymorphic crystallization. At 700°C, above the crystallization temperature, the Fe₈Al₃Zr transformed to the equilibrium phase, Fe₁₀Al₄Zr₃, by solid state reaction.

A systematic examination of microbands developed in various materials, including pure Al, Cu, Ag, Nb metals and Al-Mg, 6061 Al and Al-Li-Cu alloys, deformed dynamically or quasi-statically to intermediate strains has been conducted. Based on extensive characterization of microbands using transmission electron microscopy, the characteristics of microbands do not appear to be strongly dependent on crystal structure, material properties, strain level or deformation path. This finding suggests that the formation mechanism of microbands may be similar in a variety of f.c.c. and b.c.c. metals and alloys. A possible microband formation mechanism is proposed, based on a concept of the further development of coarse slip bands (or dislocation tangles on glide planes), which is consistent with experimental observations. The model involves first the generation of polarized dislocations on primary slip systems, followed by an annihilation process for the primary dislocations in the central portion of a band structure, forming double dislocation walls parallel to the primary slip planes. Misorientation inside the double walls is believed to be caused by the directional shear of primary dislocations. Secondary slip is then induced by the internal stresses in the region between the double walls. Finally a stable dislocation configuration is created as a result of the interaction between the primary and secondary dislocations. The proposed model is in agreement with several observed phenomena, including the constancy of microband thickness, tilt axis, shear sense, and uniform shear strain over the cross-section of a microband. The roles of stacking fault energy, solute atoms and precipitates on microband formation are also discussed.