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Peculiarities of Formation of Micro- and Mesoscopical Levels of Plastic Deformation of Crystals under Concentrated Load Action


Author: Olga Şikimaka
Degree:doctor of physics and mathematics
Speciality: 01.04.07 - Condensed matter physics
Year:2005
Scientific adviser: Dumitru (decedat) Ghiţu
doctor habilitat, professor, Institute of the Electronic Engineering and Nanotechnologies
Scientific consultant: Daria Grabco
doctor habilitat, professor, Institute of Applied Physics of the ASM
Institution:

Status

The thesis was presented on the 22 April, 2005
Approved by NCAA on the 23 June, 2005

Abstract

Adobe PDF document0.62 Mb / in romanian

Summary

The results of the investigation of a wide range of crystals KCl, LiF, CaF2, MgO, GaP, Si, Bi-Sb, ZnIn2S4, SiO2 at micro- and nanoindentation are presented in this work. This fact made it possible to trace the development of the deformation process in dependence on the type of crystal structure of material and, especially, on the different dislocation mobility. Complex investigations of the deformation zones around the indentations, including the study of dislocation structure, fine surface relief by using AFM, microstructure and roentghen analyses, behavior of the nanoindentation curves h(P) show, that the decrease of dislocation mobility leads to the intensification of the rotation plasticity and the implication of other mechanisms of plastic deformation, such as, twinning, phase transition, compression of material. The conversion from translation plasticity to the rotation-translation plasticity means the conversion to a higher level of plastic deformation, the mesolevel, when the possibilities of the previous microscopical level are exhausted.

The comparison of deformed zones around indentations made by micro- and nanoindentation methods was performed in the thesis for the purpose to reveal common and specific peculiarities. The calculations showed that the strain extent made by Berkovich indenter (nanoindentation method) is bigger than that made by Vickers indenter (microindentation method), especially at the initial stage of indenter penetration. This fact causes more pronounced contribution of rotation plasticity and crack formation during the nanoindentation process as compared with microindentation one.

Besides these two factors – (i) dislocation mobility decrease, determined by the type of crystal structure and (ii) increase of strain extent, determined by the type of indenter, it was established two more factors, furthering the intensification of rotation processes – (iii) decrease of deformation temperature and (iv) increase of loads applied to the indenter.

The quantitative and qualitative analysis of indentation recovery was carried out in this work, as well. It made possible to reveal two characteristic stages of recovery process: elastic, during unloading and plastic, after complete unloading. It was defined and argued the contribution of each of these stages to the entire recovery process according to the structural type of material, especially, with the type of atomic bond in crystals.

The microhardness–load and nanohardness–load dependences studied in this work, showed the general tendency to hardness increase with load decrease. According to the obtained results it was proposed an explanation of this effect on the basis of existence of two mechanisms of dislocation formation during plastic deformation at micro- and nanoindentation: by multiplication of existent dislocations and by new dislocation generation. The change of their relative quantity in the deformed zone with the diminution of this zone leads to the considerable changes of stress values and, consequently, hardness values.