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Influence of various external factors on microhardness and plastic deformation peculiarities of metals and ionic – covalent crystals


Author: Diana Leu
Degree:doctor of physics and mathematics
Speciality: 01.04.07 - Condensed matter physics
Year:2005
Scientific adviser: Daria Grabco
doctor habilitat, professor, Institute of Applied Physics of the ASM
Institution:

Status

The thesis was presented on the 15 April, 2005
Approved by NCAA on the 23 June, 2005

Abstract

Adobe PDF document1.33 Mb / in romanian

Summary

The investigation results of hardness modification of the wide series of materials under concentrated load action and their plastic deformation peculiarities studied in the same conditions are presented in the thesis. The purpose of the thesis was to appreciate the influence of a lot of factors (external and internal) on modification of the mechanical properties of investigated materials under concentrated load action: nano - , micro- and macroindentation.

The detailed study of this influence on the following materials: Fe alloys, steel, cast-iron, α-brass, ancient Fe, and FS-GaN, revealed that the existing in scientific literature data concerning the deformation regularities of ionic and ionic-covalent single crystals are valid for the polycrystals with the metallic and ionic-covalent chemical bond, as well. For the first time, the presence of the rotational plastic deformation in the region around the indentations on the metals was revealed in this work.

The analysis of the Hv=f(P) curves pointed out an important modification of hardness with the load increase (P). For the first time, it was established that the curves Hv=f(P) for the metallic samples can be divided into three specific intervals:

By the study of microstructure of the investigated metals and the deformed region around the indentations it was demonstrated that the existence of the mentioned three intervals indicates that the indentation process in the metals occurs due to different mechanisms of the plastic deformation. They are: for the Ist interval – the intragranular translation sliding: microlevel, for the IInd interval – the translation/rotation sliding mechanism: mesolevel; for the IIIrd interval – the crystal behavior is like to a homogeneous solid: macrolevel. It is really important that the simultaneous presence of the translation and rotation deformation modes leads to their interaction that has an important contribution into the plasticity and brittleness of crystals.

For the first time, the mechanical properties and deformation peculiarities under nano- and microindentation of the FS – GaN (single crystals and polycrystals) have been in detail studied. It was shown, that because of a strong covalent bond together with low dislocation mobility the FS –GaN crystals possesses a high hardness and an enough high brittleness. It was demonstrated that at small loads (~P=10 mN) the indentation formation on the FS – GaN can be ensured due to only the translation/rotation mechanism without the frail destruction. Both the single crystals and policrystals of FS-GaN relax by fracture formation under increase of the applied loads.

The presence of two specific regions of the plastic deformation: (i)- the zone with a low hardening (D>2,5÷3,0 d), peripheral zone, and (ii) - the zone with a strong hardening (D&lp;2,5÷3,0 d), quasidestruction zone, where the main plastic movement and rotation processes take place, was detected in the neighbourhood of the indentations.

The mentioned specific structural regions of the deformed zone were revealed by the morphological study. The regular evolution of these substructures leads to the appearance, creation and modification of the various plastic structural levels: micro-, meso- and macrolevels.