Impede Dislocation Motion Research Articles

Theory of dislocation-solute atom interactions in solid solutions and related nonlinear anelasticity

A theory for dislocation-solute atom interactions in solid solutions has been developed which allows one to calculate the nonlinear dislocation strain-amplitude-dependent internal friction. The suggested model accounts for different modes of dislocation-solute atom interactions: (i) Solute atoms distributed in the dislocation glide plane interact with the dislocation core and represent short-range obstacles for the dislocation motion; (ii) Solute atoms situated away from the dislocation glide plane create relatively weak long-range elastic stress fields, also impeding dislocation motion. We assume that dislocations move in a two-component system of obstacles which differ with respect to the thermodynamics of dislocation--point-defect interactions. Namely, dislocations overcome short-range obstacles under the combined action of applied stress and thermal energy, whereas relatively weak long-range obstacles are surmounted athermally. The model predicts a complicated multistage behavior of the nonlinear internal friction in the strain amplitude--temperature--solute concentration domain, which is in excellent agreement with recent experimental data.

High-temperature oxidation of a particulate SiC-aluminium metal-matrix composite

Metal-matrix composites provide a relatively new and often a cheap way of strengthening metals and particularly of increasing the elastic moduli. The advantages of metals as matrices are their high tensile and shear moduli (both much closer to the fibres contained), high melting point, resistance to moisture, dimensional stability, joinability and high ductibility and toughness. In dispersion-strengthened and also in particlereinforced composites (particles of 1 to 10#m) the matrix is the major load-bearing constituent and in the fine dispersion (particles of < 1 #m) the second phase is present primarily to impede dislocation motion with little load bearing by the particles. In particlereinforced composites the metallic matrix and the particles share the applied load with a volume fraction of particles usually in the region of 20 to 30%. The sharing of the applied load involves some load transfer between the particles and the matrix and this involves primarily the interface between the particle and the metallic matrix. Thus the precise chemistry and crystallography of this region are crucial parameters in the subsequent mechanical and chemical performance of the composite. Possible applications for such materials include the automobile industry, where both connecting rods and pistons might be fabricated from metal-matrix composites, and the aerospace industry for load-bearing compression struts between the inner cabin shell and the outer aircraft skin. The improved properties of composites for these applications such as increase in modulus, improved wear and fatigue resistance are currently being exhaustively examined and aluminium alloys containing approximately 10#m silicon carbide grit are good candidate materials. Although these applications involve operation in

Microhardness response of nitrogen-implanted and X-irradiated lithium fluoride

The microhardness behaviour of {001} faces of LiF has been investigated before and after both nitrogen-ion implantation and implantation followed by X-irradiation. Implantation alone produced no changes in either hardness or dislocation etch-pit rosette sizes on the surface, though radial indentation fracture was promoted. Implantation followed by X-irradiation produced high densities of colour centres deep into the crystal, much deeper than the implantation damage depth (∼0.2μm). Hardness and rosette sizes were affected to depths of ∼150μm, which was less than the colouration depth. Thus the “colour centres” producing the colour are probably different from those impeding dislocation motion.

The dependence of creep rate on microstructure in a γ′ strengthened superalloy

The principal creep strengthening mechanism in superalloys is the ability of γ′ precipitates to impede dislocation motion. In this report various theories of dislocation creep are reviewed, with particular emphasis on the contribution of precipitates to the ‘friction stress’ resisting dislocation motion. Creep data is presented for the nickel-base superalloy IN-738, which is used as a blading material in the first row high pressure stage of gas turbines. At high stress (315 MPa at 1123 K) it is shown that dislocations may overcome γ′ precipitates by cutting whereas at low stress by-pass of precipitates involves diffusion-controlled climb. A model is developed which considers the contribution to the friction stress of the back stress resulting from the climb process, which is used to explain the decrease in creep resistance of this alloy as a consequence of precipitate coarsening at the operating temperature.

Tensile Properties of Thin, Evaporated Gold Films

Evaporated gold films in the 500–15 000 A thickness range were prepared on rocksalt substrates. Depending on the temperature of the substrate during deposition, the films were completely, partially, or randomly oriented with respect to the (100) plane of the rocksalt, as determined by x-ray diffraction. The stress-strain curve of the free films was investigated and was found to be quite steep up to very high stresses, but plastic deformation and creep were observed even at relatively low stresses. The ultimate tensile strength of the films is from 2 to 4 times that of the annealed bulk material and is not a function of film thickness. The Young's moduli are found to be normal. Comparing the stress-strain behavior of evaporated films with that of whiskers suggests that the former owe their high strength to a high concentration of defects which were uniformly quenched in during the evaporation process, and which greatly impede dislocation motion and multiplication, and not to an abnormally high stress required to nucleate dislocations which is commonly observed in whiskers.