Elastic, micro- and macroplastic properties of polycrystalline beryllium

The Young’s modulus and the internal friction of beryllium polycrystals (size grain from 6 to 60 μm) prepared by the powder metallurgy method have been studied as functions of the amplitude and temperature in the range from 100 to 873 K. The measurements have been performed using the composite piezoelectric vibrator method for longitudinal vibrations at frequencies about 100 kHz. Based on the acoustic measurements, the data have been obtained on the elastic and inelastic (microplastic) properties as functions of vibration stress amplitudes within the limits from 0.2 to 30–60 MPa. The microplastic deformation diagram is shown to become nonlinear at the amplitudes higher than 5 MPa. The beryllium mechanical characteristics (the yield strength σ0.2, the ultimate strength σu, and the conventional microscopic yield strength σy) obtained with various grain sizes are compared. At room temperature, all the parameters satisfactorily obey the Hall-Petch relationship, although there is no complete similarity. The temperature dependences are quite different, namely: σ0.2(T) and σu(T) decrease monotonically during heating from room temperature to higher temperatures; however, σy(T) behaves unusually, and it has a minimum near 400 K. The different levels of stresses and the absence of similarity indicate that the scattering of the ultrasound energy and the formation of a level of the macroscopic flow stresses in beryllium occur on dislocation motion obstacles of different origins.