Acta Metallurgica et Materialia最新文献

An aluminum single crystal with the axial direction of [211̄0 was fatigued in push-pull at the constant resolved shear stress amplitude 4 MPa, frequency 20 Hz room temperature. Microcracks, microvoids, macrobands, extrusions and intrusions were observed on the side-surface containing the Burgers vector b. Most microcracks were opened, and were within, but approximately perpendicular to, PSBs. Slip steps were found in the extrusions and intrusions. There was net irreversible slip in one direction in most PSBs. Some short cracks along the PSBs on the side-surface were also observed at 5 × 10⁶ cycles. These observations indicate that, without the aid of the surface roughness of PSBs, cracks can still be nucleated, and that, apart from the notch effect of a PSB, there are other factors controlling crack initiation in single crystal aluminium. There may be an internal tensile stress existing in a PSB in the direction of b, and a shear stress applied by the specimen grips in the specimen due to the irreversible slip in one direction in PSBs. These stresses and the applied stress are responsible for the formation of microcracks, microvoids, extrusions, intrusions and macrobands on the side-surface.

Microstructural and textural evolution during rolling were investigated in (112)[11$\text{1}$] single crystals of Al, Cu and homogenous supersaturated All.9wt%Cu. After a rolling degre of 30% the initial C-orientation (112)[11$\text{1}$] of all three materials has rotated towards the so called D-orientation (4411)[1111$\text{8}$]. While in the non-shear banding Al the D-orientation remains stable up to high rolling degrees, in the shear banding materials Cu and AlCu it rotates back to the initial C-orientation simultaneously with the formation of shear bands. This orientation change is explained by a rigid body rotation due to the special geometry of a deformation with unidirectional shear bands. With the onset of shear band formation also strong orientation scatterings about tthe transverse direction appear in the pole figures. These scatterings are located inside the shear bands as well as in their vicinity. They are due to the strong shear deformation and the resulting reaction stresses occurring in the shear bands and in their vicinity, respectively.

Hydrogen induced void nucleation of 310 stainless steel was investigated. Experimental results indicated that hydrogen promoted void nucleation. A new model of hydrogen induced void nucleation was proposed. The basic idea of this model is that hydrogen induces void nucleation not only by promoting microcrack nucleation but also by promoting the transition of microcrack to microvoid; hydrogen also increases the stability of a microvoid by forming hydrogen pressure in the microvoid and by decreasing the void surface energy.

A directional dependence of the corrosion fatigue behavior of Fe-2.7wt%Si alloy Σ5[100]/(021) bicrystals was observed in a film forming solution. To understand this directional dependence, dislocation nucleation analyses at the opposite crack tips based on the Rice-Thomson concept and continuum elastic-plastic analyses, within single crystal theory, of the near tip fields by finite element method were performed. Explanations of brittle/ductile fatigue crack growth under corrosive conditions and the local crack tip rotation within the interface are offered based on these analyses.

A fracture model for spheroidized hypereutectoid steels is developed based on the concept that the stress in the ferrite matrix is the driving force for crack initiation at grain boundaries within the coarse carbides. The ferrite matrix fracture stress, δ_f,ferr is calculated by averaging the ferrite stress using upper and lower bound concepts, and by utilizing the fracture strength of the carbide. The analyses and results indicate that the fracture behavior follows a classical fracture mechanics relation in that the fracture strength is a unique function of the reciprocal of the square root of the carbide particle size with δ_f,ferr equal to zero at infinite carbide (crack) size. It is concluded that the fracture strength of the iron-iron carbide composite is enhanced by: (i) increasing the strength of grain boundaries within carbides; (ii) decreasing the average carbide size, and (iii) increasing the carbide volume fraction.

The stability of an infinitely long screw 〈111〉 superdislocation dissociated into APB-coupled superpartials on {110}, {112}, or a combination of both, has been analysed under the approximation of linear elasticity theory of dislocations. In the absence of an applied load, the variation of the configurational energy of the dislocation pair in equilibrium has been examined analytically as a function of two basic material parameters, i.e. the elastic anisotropic parameter along 〈111〉M and the ratio of {112} vs {110} APB energies λ = γ₁₁₂/γ₁₁₀. It is found that although there is no tor1ue force on the screw partials on both {110} and {112}, the torque force is present at intermediate angular positions and acts in the direction of stabilizing {112} dissociation and destabilizing {110} dissociation. In general dissociation on {112} is favoured energetically if λ > M^1/3. Since M > 1 in anisotropic elastic media, ranging from M = 1.09 for FeAl to M = 1.64 for AuCd, favourable {112} dissociation does not require γ₁₁₂ < γ₁₁₀. For compounds with $\text{M > 1.22(√}\text{3}\text{2}\text{)}$, the screw superdislocation may dissociate into stable non-planar configurations with the APB lying partly on {110} and partly on {112}, forming a locked structure.