Peculiarities of internal friction in boron carbide

Abstract

Internal friction (IF) study of the arc‐melted and hot‐pressed boron carbide is offered. At freqeuncies of torsional oscillations of 1–10 Hz the relaxation maxima at 470 and 780–800 K and the hysteretic maximum at 630 K have been revealed in arc‐melted specimens. Hydrogen annealing suppresses the hysteretic maximum and decreases the value of activation energy of the intense maximum at 470 K from 1.15 to 0.90 eV. Amplitude dependence of internal friction and dynamic shear modulus was used to evaluate the energy of detachment of point defects from linear structural imperfections. The existance of multiple twins on the (001) planes, significant amplitude dependence of the internal friction and shear modulus, relatively low values of activation energy of (∼1.0 eV) and the relaxation frequency factor of (∼1012 s−1) in boron carbide certificate of the relaxation origin of IF maximum at 470 K due to twin boundary movement. Accounting on the analysis of computer simulated twins, the twinning direction and specific crystallographic shear upon twinning on the (001) planes, necessary for the theoretical evaluation of the maximum characteristics were determined. Within the limits of the model of surface twinning dislocation, the internal friction mechanism is offered, according to which the relaxation on processes at about 470 K are due to formation and expansion of ring flat kinks under the effect of the, temperature and external stresses. Using the values of crystallographic shear and shear modulus, the critical radius of twinning dislocation rings, the shear modulus along twin boundaries and halfwidth of a single kink are determined.