Fabrication of metal-matrix composites by application of the superplasticity effect

The production of continuous-fiber metal-matrix composites (MMCs) with improved mechanical properties by solid-state consolidation under superplastic conditions was investigated. The finite-element method was used to model MMC consolidation, which was experimentally studied on a sample of boronaluminum composite. It was shown that the deformation properties of a matrix exert considerable influence on metal flow geometry and its strain-stress state. The superplasticity effect provides the straightening of a matrix flow front and the localization of an intense deformation zone. It also facilitates the removal of pores in the final stage of hardening and considerably decreases local stresses on the fiber surface. The features of matrix material flow during consolidation of composites were considered. It was shown that the matrix deformation during composite consolidation takes place in the form of cooperative grain-boundary sliding and intragranular sliding. The mechanism of matrix deformation determines a type of fiber-matrix reaction at the interface, or, alternatively, the type of fiber-matrix interface interaction depends on the intensity of localized deformation in the given area. Identification of the interface structure was performed by acoustic emission.     

Use of the superplasticity phenomenon for developing a process for fabricating parts from magnesium alloys

Superplasticity of magnesium alloy ZK60A with an ultrafine-grain structure is investigated. It is shown that parts can be produced from this alloy by the method of superplastic gas forming.     

Molecular beam epitaxy of AlGaAs/Zn(Mn)Se hybrid nanostructures with InAs/AlGaAs quantum dots near the heterovalent interface

Features of the growth of InAs quantum dots in an Al_0.35Ga_0.65As matrix by molecular beam epitaxy at different substrate temperatures, deposition rates, and amounts of deposited InAs are studied. The optimum conditions for growing an array of low-density (≤2 × 10¹⁰ cm^?2) small (height of no more than 4 nm) self-organized quantum dots are determined. The possibility of the formation of optically active InAs quantum dots emitting in the energy range 1.3–1.4 eV at a distance of no more than 10 nm from the coherent heterovalent GaAs/ZnSe interface is demonstrated. It is established that inserting an optically inactive 5-nm GaAs quantum well resonantly coupled with InAs quantum dots into the upper AlGaAs barrier layer enhances the photoluminescence efficiency of the quantum-dot array in hybrid heterostructures.     

Short-range ordering and the abnormal mechanical properties of a Ni-20% Cr alloy

Effect of temperature on mechanical properties of a Ni-20% Cr alloy with an initial grain size of 100 μm in the temperature interval 150–1000°C (0.25–0.8 T _m) has been studied. It is shown that in the temperature range 400–600°C the alloy demonstrates a positive temperature dependence of both the flow stress and the coefficient of strain hardening and exhibits Portevin-le Chatelier effect (PLC). It has been established on the basis of a comparison of data of calorimetric studies and temperature dependence of mechanical properties that the unusual mechanical behavior of Ni-20% Cr alloy is a result of the occurrence of short-range order. The nature of the effect of the short-range order on the mechanical properties is discussed.     

The influence of texture on the mechanical properties of a superplastic magnesium alloy

Superplastic (SP) characteristics of a MA15 magnesium alloy subjected to tensile deformation in both textureless and textured states with identical fine grain structures have been studied. The existence of crystallographic texture is found to be the cause of the anisotropy of flow stress and plasticity. The shift of SP optimum strain rate towards higher values due to the strengthening of texture is observed. A special experiment has indicated that the difference of the alloy behaviour in textureless and textured states results from the operation of grain boundary sliding stimulated by the intragrain dislocation slip.     

Effect of γ′-phase content on preparation schedule for structure and superplasticity of high-temperature nickel alloys

Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 7, pp. 40–44, July, 1989.     

On the model of solid state joint formation under superplastic forming conditions

The peculiarities of solid state joint (SSJ) formation under conditions of superplastic forming (SPF) were investigated for the titanium alloy VT6S (Ti-6Al-4V). The influence of annealing and SPF on the change of state of the alloy surface was considered. A significant role of grain boundary sliding (GBS) in the formation of both surface microrelief and SSJ was established. It was shown that SSJ formation under SPF conditions is primarily a deformation process. Corresponding schemes of the appearance of surface microrelief and SSJ formation are proposed.     

Effect of second phase particles on grain refinement during equal-channel angular pressing of an Al-Mg-Mn alloy

The process of grain refinement under severe plastic deformation was examined in an Al-5.4% Mg-0.5% Mn-0.1% Zr alloy, which was subjected to equal-channel angular pressing (ECAP) in the strain interval from 1 to 12 at a temperature of ∼300 °C. It was shown that the size and distribution of the second phase particles precipitated under homogenization annealing strongly affect grain refinement. Extensive grain refinement under ECAP was provided by a dispersion of Al₆Mn particles with an average size of ∼25 nm that precipitated during the homogenization annealing at an intermediate temperature. The fully recrystallized structure with an average crystallite size of ∼0.55 μm evolves through continuous dynamic recrystallization. In contrast, homogenization annealing at a high temperature leads to the formation of coarse Al₆Mn particles with a plate-like shape. Under further ECAP, the formation of coarse recrystallized grains takes place in this material due to the discontinuous growth of recrystallized grains during the inter-pass annealing between the ECAP passes. The role of second phases in grain refinement is discussed in terms of pinning and driving forces for recrystallization.