Acta Metallurgica最新文献

The effect of surface stress on the equilibrium conditions at crystal-melt, coherent crystal-crystal and greased crystal-crystal interfaces is investigated by using a variational method to test for equilibrium. In all three cases, the interface between the phases is modelled as a Gibbsian dividing surface, and the excess internal energy associated with the interface is allowed to depend on both the deformation of the interface and the crystallographic normal to the interface. The position of an interface can vary due to both deformation at the interface and transformation between the two phases at the interface (accretion), and so we define a special variation that accounts for both. Thus, surface stress appears explicitly in both the force and energy balances at crystal-melt and coherent crystal-crystal interfaces. In particular, an interfacial strain energy term appears in the energy balance at these interfaces; this term gives the energy of deforming the interface against the force associated with the surface stress, and is a new result from this analysis. Anisotropy also appears in this energy balance through a term that can be expressed by using Cahn and Hoffman's ξ-vector. Finally, it is shown that a greased crystal-crystal system differs from crystal-melt and coherent crystal-crystal systems in that two independent deformations and crystallographic normals can be defined at a greased interface. However, by partitioning the excess energy associated with a greased interface between these deformations and normals, one can reduce the equilibrium conditions at a greased interface to those that obtain if the two crystals would interact only through a thin fluid layer at the interface.

The addition of a dispersed ductile phase to a brittle material can lead to significant increases in fracture resistance compared to the untoughened matrix material. Often the important mechanism appears to be bridging by intact ductile ligaments behind the advancing crack tip. Although a framework for predicting toughness enhancements from bridging mechanisms exists, the required detailed model of ligament deformation which would provide the load-extension relation for a typical ligament has not been available. In this paper, numerical modeling of a plastically deforming ligament constrained by surrounding elastic matrix material is performed and the relevant toughness enhancement information extracted. Comparison is made to model experiments as needed to investigate such deformation processes as well as to toughnesses measured for technologically important composites. The results suggest that debonding along the interface between the ligament and the matrix may enhance the toughening effect of a ductile phase.

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 dependence of the tensile fracture stress on crack length in dilatationally phase-transforming ceramics is studied by modeling the evolution of the transformed regions around the tips of finite cracks during crack growth. The presence of the transformation is found to reduce the stress required for crack-growth initiation. However, the peak, or “ultimate”, tensile stress is found to occur during subsequent crack growth, and the transformation-strengthening, that is, the increase of tensile strength due to phase transformation, is found as a function of initial crack size.

According to the mesh length theory of workhardening, low-temperature strain rate effects in materials exhibiting a dislocation cell structure are due to the simultaneous operation of a number X of super-critically bowing links in the average, typically roughly equiaxed, cell of diameter L. The corresponding length of mobile dislocations per cell is then LX. Defining $\text{g =}\text{L}\text{l}\text{̄}$, with $\text{l}\text{̄}$ the average link length in the cell walls, dislocation cell breakdown is expected for X_max ≈ 0.3 g², on the criterion that at the most 50% of all cell wall dislocations may be simultaneously destabilized and therefore at most 10% of all possible dislocation sources may be activated simultaneously since each destabilizes four adjoining links besides itself. When X = X_max, the mobile dislocation density within the cells is about 30% of that in the walls, but significant interactions and thus extra hardening is expected only if the dislocation density in the cell interiors is about sixteen times the cell wall dislocation density. Therefore the mobile dislocations can add little, if anything, to the permanent strain hardening. However, on account of the decrease of average source length with increasing X a transient increase of flow stress and workhardening coefficient arises which typically amounts to a few percent per ten-fold increase of strain rate. Mobile dislocations remaining in the cell interiors decrease the elastic modulus and can give rise to anelastic creep as well as to recovery effects. Numerical estimates are in good agreement with observations.

We present a novel heuristic “universal” formula for the scaled structure function following a quench into the miscibility gap which gives very good fits to a variety of experimental observations. The single adjustable parameter γ needed to fit data for alloys, binary fluids, polymer mixtures and computer simulation curves depends essentially only on the fraction of the volume of the minority phase. Minimizing the ratio of “surface area to volume” of the minority phase predicts a rough morphology of the system—its local character changes from spherical isolated droplets to interconnected plate-like objects as the minority fraction increases. By relating γ to this microstructure we obtain the value of γ correctly to within 10%.

The flow stress of L1₂-type Ni₃(Si, Ti) by the compression test was measured as a function of temperature, strain rate, orientation and alloy composition. The critical resolved shear stress (CRSS) increased from 77 K, reached a peak, and then decreased rapidly with increasing temperature. The single crystals with orientations close to [011] and [1̄11] showed the operations of octahedral slips (111) [1̄01] below the peak temperature and of cube slips (001) [1̄10] above the peak temperature, whereas those with orientation close to [001] showed the operations of octahedral slips {111} over the entire temperatures. The CRSS depended on the orientation and alloy composition below the peak temperature but on the orientation and the strain rate above the peak temperature. The peak temperature was dependent on the orientation but almost independent of the strain rate and alloy composition. It is suggested that the deformation mechanisms for Ni₃(Si, Ti) single crystals are quite similar to those for other Ni-based L1₂ single crystals such as Ni₃Al, Ni₃Ga and Ni₃Ge; at low temperatures the dislocation movement of the superpartials dissociated on (111) plane with APB became sessile by micro cross slip to (001) plane. At high temperatures, two deformation modes were operative, depending on the orientation. The deformation of the single crystals with orientations far from [001] is due to the Peierls-Nabarro mechanism of (001) [1̄10] slip, whereas the deformation of the single crystals with orientations close to [001] is due to the intrusion of the “diffusive” process of (111) [1̄01] slip.

The diffusion in $\text{Fe}{}_{\mathrm{1-x}}\text{B}{}_{\mathrm{x}}\text{Fe}{}_{\mathrm{1-y}}\text{B}{}_{\mathrm{y}}$ and $\text{Co}{}_{\mathrm{1-x}}\text{Zr}{}_{\mathrm{x}}\text{Co}{}_{\mathrm{1-y}}\text{Zr}{}_{\mathrm{y}}$ amorphous bi- and multilayers was investigated. The dependence of the diffusion coefficient on concentration was observed. This effect was explained taking into account the concentration dependence of mixing enthalpy correlated with chemical short range order in amorphous state.

The high creep strength of tempered martensite ferritic steels results from the presence of a dislocation network of subgrain boundaries (SGBs) which is stabilized by carbides. In these microstructures creep accelerated particle coarsening is observed at creep rates of the order of 10⁻¹⁰ s⁻¹ (long-term creep). The microstructural explanations presented in this study, are based on the close contact between particles and SGBs:

• 1.

  (i) the difference in the thermodynamic potential on SGBs perpendicular and parallel to the stress axis results in short range pipe diffusion fluxes over distances of the order of the mean subgrain size (0.5 μm) which are not enforced during stress free ageing;

• 2.

  (ii) at high temperatures, recovery processes can result in a decrease of the dislocation density within SGBs, which results in a decrease of pipe diffusion along SGBs. In the presence of a stress higher dislocation densities in SGBs (higher pipe diffusion fluxes) can be maintained than without stress (ageing);

• 3.

  (iii) during primary creep carbides interrupt “knitting” reactions between “free” dislocations and SGBs. As a result carbides “come in contact” with additional pipe diffusion paths (pinned dislocations). At low stresses the “pinned” dislocations stay in contact with the “pinning” carbides throughout the creep life. These “additional” matrix/carbide pipe diffusion paths do not form during ageing (absence of primary creep). All three effects can contribute to “creep accelerated particle coarsening” in tempered martensite ferritic steels.