Papers by Author: B.K. Kardashev

Abstract: The Young’s modulus and internal friction of Be polycrystals (grain sizes 6-60 μm) prepared with a powder metallurgy technique were studied acoustically in both amplitude independent and-dependent damping ranges. The measurements were made by composite oscillator at resonant frequencies of longitudinal vibrations of about 100 kHz in the temperature range of 100-873 K. The data were used to get information about micro-flow properties at vibration stresses of 0,2-30 MPa. It was found that the micro-flow diagrams became non-linear at amplitudes higher than 5 MPa. Mechanical properties (acoustic micro-flow stress σ_y, yield point σ_0.2 and ultimate strength σ_В) as functions of grain size have shown clearly a correlation in the framework of Hall and Petch law at room temperature but this similarity is not complete: stresses σ_0.2 and σ_В are one order of magnitude higher and change more steeply with grain size as compared to σ_y. The similarity is not observed for the temperature dependences. The values of σ_0.2(Т) and σ_В(T) decrease smoothly at higher temperatures but σ_у(Т) demonstrates an unusual minimum at ~400 K. The different behavior of the acoustic and mechanical stresses proves, evidently, that the ultrasonic energy losses and the flow stress level are due to the different nature of obstacles for the vibration (near equilibrium positions) and translation movement of dislocations in beryllium.

Abstract: Acoustic measurements of Young’s modulus and internal friction at frequencies of longitudinal vibrations of about 100 kHz were made in the (5×10^-7 to 2×10^-4) vibration strain amplitude range for Al-Si alloys. Quasi-static bending stress-strain diagrams were studied as well. The samples of the alloy were prepared using a directional crystallization by Stepanov’s method. The compositions varied from 8 to 25 wt.%Si. The micro-plastic deformation of the materials (acoustic experiments) is explained by dislocation motion in grains of α solid solution. The micro-plastic yield stress has a minimum near the eutectic composition; the flow stress of plastic bending increases smoothly when the Si content increases.