Papers by Author: K. Sapozhnikov

Abstract: The internal friction of the hyperstabilized martensite demonstrates very low values, both above and below the nominal martensitic transformation temperature, due to a pronounced pinning effect. Over a wide temperature range it is comparable with the level of damping in the parent phase. A study of the temperature dependence of the non-linear ultrasonic internal friction and its strain amplitude hysteresis indicates that the diffusion, assisted by dislocations/interfaces, is quite pronounced and in Ni-Fe-Ga and Cu-Al-Be alloys it operates at temperatures around 20 K. The renucleation of the lamellar parent phase during the reverse martensitic transformation close to 600 K is accompanied by an internal friction peak which demonstrates a substantial transitory contribution. After renucleation of the parent phase the samples recover a conventional martensitic transformation with the internal friction level in the martensite comparable to the one in non-stabilized samples. Observations of a relaxation peak in the parent phase of different alloys for temperatures just below the renucleation stage of the reverse transformation point to the essential role of diffusion in the nucleation of the parent phase in hyperstabilized martensites.

Abstract: Microstructural changes induced in a carbon fiber – magnesium matrix composite during thermal cycling in the range of 100 - 360 K are detected by an ultrasonic technique. The composite was comprised of Mg-2wt.%Si alloy matrix reinforced with long unidirectional carbon fibers (volume fraction of about 30%). Temperature variations of the elastic modulus of the composite are largely determined by elasticity of the carbon fibers stressed by the thermally expanded/contracted matrix. Anelastic properties of the composite (internal friction and modulus defect) are caused by dislocation motion in the matrix. Temperature spectra of anelasticity of the composite are controlled by a competition between creation of fresh mobile dislocations under the action of thermal stresses and immobilization of the fresh dislocations by atmospheres of mobile point defects.

Abstract: Dynamics of structural defects was investigated in situ during quasistatic deformation of polycrystalline Al-1wt.%Si-0.3wt.%Mg and Al-12wt.%Si-0.3wt.%Mg alloys by means of simultaneous measurements of internal friction and acoustoplastic effect. The alloys were subjected to different heat treatments after quenching: natural ageing at room temperature (T4 treatment) and peak ageing at 433 K for 8 hrs (T6 treatment). This enabled us to study the effect of different microstructure components – solute clusters (for the T4 treatment), GP zones and β (( precipitates (for the T6 treatment), coarse Si particles (for Al-12wt.%Si-0.3wt.%Mg) – on the irreversible and reversible components of dislocation motion under the combined action of oscillatory and quasistatic stress. Based on the data obtained, conclusions have been drawn about microstructural mechanisms of the acoustoplastic effect.