Olga Semina

Evolution of Al–19·4Si alloy surface structure after electron beam treatment and high cycle fatigue

Abstract Processing of Al–19·4Si alloy by high intensive electron beam has been carried out, and multiple increase in fatigue life of the material has been revealed. Investigations of structure and surface modified layer destruction of Al–19·4Si alloy subjected to high cycle fatigue tests to fracture have been carried out by methods of scanning electron microscopy. The factors responsible for the increase in fatigue life of Al–19·4Si alloy have been revealed and analysed.

Surface layer structure degradation of rails in prolonged operation

By methods of optical, scanning and transmission electron microscopy and microhardness measurement the transformation regularities of structure-phase states, defect substructure, fracture surface and mechanical properties of rail surface layer up to 10 mm deep in process of long-term operation (passed tonnage of gross weight 1000 mln. tons) were revealed. According to the character of fracture and level of structure imperfection the three layers were detected: surface, transition and boundary ones. It has been shown that the surface layer ~20 μm in thickness has a multiphase, submicro- and nanocrystalline structure and it contains micropores and microcracks. The increased density of bend extinction contours at 2 mm depth from the tread contact surface was noted, and it was shown that the maximum amplitude of stress fields was formed on the interphase boundary the globular cementite particle–matrix. The evaluation of stress fields was done.

Structure-phase states evolution in rails during a long operation

By methods of scanning and transmission electron microscopy the transformation regularities of structure-phase states, defect substructure, fracture surface of rail surface layer up to 10 mm deep in process of long-term operation (passed tonnage of gross weight 1000 mln tons) were revealed. It has been shown that the surface layer ∼20 μm in thickness has a multiphase, submicro- and nanocrystalline structure and it contains micropores and microcracks. The increased density of bend extinction contours at 2 mm depth from the tread contact surface was noted. The analysis of structure morphological constituents and internal stress fields, created by intra- and interphase boundaries after long operation was carried out. It was shown that the maximum amplitude of stress fields was formed on the interphase boundary the globular cementite particle–matrix. The evaluation of stress fields was done.

Long-term operation of rail steel: Degradation of structure and properties of surface layer

By methods of optical, scanning and transmission electron diffraction microscopy and microhardness and tribology parameters measurement the changes regularities of structure-phase states, defect substructure of rails surface after the long term operation (passed tonnage of gross weight 500 and 1000 mln. tons) were established. It is shown that the wear rate increases in 3 and 3.4 times after passed tonnage of gross weight 500 and 1000 mln. tons, accordingly, and the friction coefficient decreases in 1.4 and 1.1 times. The cementite plates are destroying absolutely and cementite particles of around form with the sizes 10–50 nm are forming after passed tonnage 500 mln tons. The appearance of dynamical recrystallization initial stages is marked after the passed tonnage 1000 mln tons. The possible mechanisms of established regularities are discussed. It is noted that two competitive processes can take place during rails long term operation: 1. Process of cutting of cementite particles followed by their carrying out into the volume of ferrite grains or plates (in the structure of pearlite). 2. Process of cutting, the subsequent dissolution of cementite particles, transition of carbon atoms to dislocations (into Cottrell atmospheres), transition of carbon atoms by dislocations into volume of ferrite grains or plates followed by repeat formation of nanosize cementite particles.

Physical nature of rail strengthening in long term operation

Regularities of changes in structure-phase states and the defect substructure of rail surface layers up to 10 mm along the fillet in long-term operation (the gross tonnage 1000 mln tons) were determined by methods of transmission electron diffraction microscopy and by measuring microhardness. The possible reasons of the observed regularities were discussed. It was noticed that two competitive processes may proceed in rail operation: (1) cementite segregation followed by their carrying to the volume of ferrite grains or plates (in the pearlite structure) and (2) cutting, subsequent dissolution of cementite particles, transition of carbon atoms at dislocations (Cottrell atmospheres), and carbon atom transfer by dislocations into the volume of ferrite grains (or plates) followed by the formation of cementite nanoparticles. A qualitative analysis of rail hardening mechanisms at different distance from the tread surface along the fillet after long-term operation was done. It was shown that hardening had a mult...

The role of electro-explosion alloying with titanium diboride and treatment with pulsed electron beam in the surface modification of VT6 alloy

The paper presents the results of the investigation of VT6 titanium alloy subjected to electro-explosion alloying with TiB2 and irradiation with pulsed electron beam. It was established that electro-explosion alloying resulted in a high level of roughness of the surface layer with high adhesion of the modified layer and matrix. Further irradiation of the material with electron beam resulted in the smoothing of the surface of alloying and formation of a porous structure with various scale levels in the surface layer. It was also established that the energetic exposure causes the formation of a gradient structure with a changing elemental composition along the direction from the surface of alloying.

Degradation of structure and properties of rail surface layer at long-term operation

ABSTRACT The microstructure evolution and properties variation of the surface layer of rail steel after passed 500 and 1000 million tons of gross weight (MTGW) have been investigated. The wear rate increases to 3 and 3.4 times after passed 500 and 1000 MTGW, respectively. The corresponding friction coefficient decreases by 1.4 and 1.1 times. The cementite plates were destroyed and formed the cementite particles of around 10–50nm in size after passed 500 MTGW. The early stage dynamical recrystallisation was observed after passed 1000 MTGW. The mechanisms for these have been suggested. The large number of bend extinction contours is revealed in the surface layer. The internal stress field is evaluated. This paper is part of a themed issue on Materials in External Fields.

The Influence of Electron Beam Treatment on Al-Si Alloy Structure Destroyed at High-Cycle Fatigue

By scanning and transmission electron diffraction microscopy method the analysis of structure-phase states and defect substructure of silumin subjected to high-intensity electron beam irradiation in various regimes and subsequent fatigue loading up to failure was carried out. It is revealed that the sources of fatigue microcracks are silicon plates of micron and submicron sizes that are not soluble in electron beam processing. The possible reasons of the silumin fatigue life increase under electron-beam treatment are discussed.

Increase in fatigue life of steels by electron-beam processing

Increase in fatigue life (by 3.5 times) of steels of different structural classes has been determined. By methods of transmission diffraction electron microscopy the variation of structure, phase composition and defective substructures parameters of material surface layer at different scale levels (from micro to nano) and the suppression of processes contributing to the formation of regions being the potential site of submicrocracks’ formation has been analyzed.

Phase Composition and Defect Substructure of Strengthening Layer Surfaced on Low Alloyed Steel

The microstructure and microhardness distribution in surface of low carbon Hardox 450 steel coated with alloyed powder wires of different chemical compositions are studied. It is shown that the microhardness of 6-8 mm thickness surfaced layer exceeds that of base metal by more than 2 times. The increased mechanical properties of surfaced layer are caused by the submicro and nanoscale dispersed martensite, containing the niobium carbides Nb2C, NbC and iron borides Fe2B. In the bulk plates a dislocation substructure of the net-like type with scalar dislocation density of 1011 cm-2 is observed. The layer surfaced with the wire containing B possesses the highest hardness. The possible mechanisms of niobium and boron carbides formation in surfacing are discussed.