Defect Substructure Research Articles

EVOLUTION OF RAIL STRUCTURE-PHASE STATES AT CONTINUOUS SERVICE

The regularities of structure-phase states and defect substructure transformations of the rail surface layer with the 10 mm depth under continuous service (the passed tonnage of 1000 million ton of gross weight) have been revealed by the methods of modern physical material science. The microhardness profi les have been constructed and the reduction of the strength characteristics of the rolling surface after the rail operation on the railr ad has been revealed. Operation of the rails leads to the formation of the multilayer structure. It has been shown that the surface layer with the ~20 μm depth has a multiphase submicro- and nanocrystal structure and contains micropores and microcracks. The layer structure located at a distance of 2 mm from the rolling surface is morphologically similar to that of the steel before the operation and it is presented predominantly by perlite grains of lamellar morphology, grains of ferrite-carbide mixture and structurally free ferrite grains. The increased density of bend extinction contours at the depth of 2 mm from the rolling surface have been noted and it has been shown that maximum amplitude of stress fi elds is formed at the interphase globular particle-matrix density.

STRUCTURAL-SCALE LEVELS OF FATIGUE DURABILITY INCREASE OF STEELS AND ALLOYS BY ELECTRON-BEAM TREATMENT

The electron-beam treatment of different classes of steels (Fe– 0.1C–18Cr–10Ni–1Ti, Fe–0.2C–23Cr–18Ni, Fe–0.2C–13Cr, Fe– 0.76C–1V) and silumin (Al – 12 % Si) with the parameters: electron beam energy density 10 – 40 J/cm2, pulse duration 50 – 150 μs, pulse number 3 – 5, frequency 0.3 Hz) leads to the increase of cycle numbers up to the fracture in ~ 3.5 times. The studies of structure-phase states and defect substructure of these materials were carried out using the methods of scanning and transmission electron diffraction microscopy. It was shown that the increase in fatigue life of steel is due to the transformation of the structure of the surface layer of the material occurring during irradiation of samples high-intensity pulsed electron beam. It was suggested that the physical sense of the impact of multi-level structure-phase state on the mechanical properties of the surface layer of the material is in the redistribution of elastic energy as due to the interaction of the elastic fields of structural elements of different scale levels, and by reducing the scale level of plastic strain localization.

Features of Plastic Deformation and Fracture of a Dispersion-Strengthened V–4Ti–4Cr–(C, N, O) Alloy at Different Temperatures

4Using the methods of transmission and scanning electron microscopy, the defect substructure of a V–4Ti–4Cr– (C, N, O) alloy dispersion strengthened with nanosized oxicarbonitride particles and the features of its fracture after deformation via active tensile loading at the temperatures 20 and 800°C are investigated. It is shown that the most representative features of deformation at the elevated temperature is primarily activation of grainboundary mechanisms of deformation and fracture and secondly strain localization accompanied by crystallattice re-orientation. The reasons for these phenomena are discussed.

Increasing the fatigue life of steel and alloys by electron-beam treatment

The number of cycles to failure for 08X18H10T, 20X23H18, 2X13, and Э76Ф steel and Silumin (Al–12% Si alloy) may be increased by a factor of 3.5 by electron-beam treatment with the following parameters: energy density of the electron beam 10–40 J/cm2; pulse length 50–150 µs; 3–5 pulses; and pulse frequency 0.3 s–1. By scanning and transmission electron-diffraction microscopy, the structure and phase states and defect substructure of these materials may be investigated. The increase in the fatigue life of the steel is due to the transformation of the surface structure of the material under the action of the intense pulsed electron beam. In physical terms, the influence of the multilevel structure and phase state on the mechanical properties of the surface layer indicates redistribution of the elastic energy both on account of the interaction of the elastic fields of structural elements at different scale levels and on account of the reduction of the scale level corresponding to localization of the plastic deformation.

Steel fatigue life extension by pulsed electron beam irradiation

Extension (by a factor of 3.5) of fatigue life of steels of different structural classes has been established. The structural change, phase composition, defect substructure of the material surface layer at different scale levels (from micro to nano), and the suppression of processes contributing to the formation of zones of potential submicrocrack location have been analyzed by means of transmission electron microscopy.

Austenite grain refinement during load-biased thermal cycling of a Ni49.9Ti50.1 shape memory alloy

A near-equiatomic NiTi shape memory alloy was subjected to a variety of thermomechanical treatments including pure thermal cycling and load-biased thermal cycling to investigate microstructural evolution of the material under actuating conditions. In situ and post mortem scanning transmission electron microscopy (STEM) was used to study the effects of stress on the development of defect substructures during cycling through the martensitic transformation. High temperature observations of the austenite phase show rapid accumulation of dislocations and moderate deformation twinning upon thermomechanical cycling. Additionally, TEM-based orientation mapping suggests the emergence of fine crystallites from the original coarse austenite grain structure. A possible mechanism is proposed for the observed grain refinement based on the crystallographic theory of martensite transformation.

Formation of Gradient Multiphase Nanostructured Surface Layers Using the Electron-Ion-Plasma Combined Method

The data of the study of the elemental and phase composition, the condition of the defect substructure, mechanical and tribological characteristics of titanium alloy VT6 subjected to electroexplosive carburization with a sample of titanium diboride powder and the subsequent treatment by high-intensity pulsed electron beam is presented. It has been established that the main mechanisms responsible for an increase in the mechanical and tribological properties of the material are solid, dispersion, and grain boundary.

Evolution of the structure and phase states of rails in prolonged operation

The transformation of the structural and phase states and defect substructure of the surface layer (depth up to 10 mm) in rails during prolonged operation (with a total load amounting to 1000 million t) is analyzed on the basis of metal physics. The microhardness is plotted, and decrease in strength of the rail’s contact surface after prolonged operation is noted. In rail operation, a multilayer structure is formed. The surface layer (about 20 μm) has a multiphase submicrocrystalline and nanocrystalline structure; it contains micropores and microcracks. The structure at a distance of 2 mm from the contact surface is morphologically similar to the steel structure before operation: it consists primarily of pearlite grains (mainly plates), mixed ferrite-carbide grains, and structure-free ferrite grains. The density of the flexural extinction contours increases at a distance of 2 mm from the contact surface. The amplitude of the stress field is greatest at the phase boundary between a globular particle and the matrix.

THE EVOLUTION OF THE PHASE CONSTITUTION AND DEFECT SUBSTRUCTURE OF RAIL STEEL SURFACE LAYERS UNDER FATIGUE

Quantitative analysis of the evolution of phase composition and defect substructure of rail steel surface layers with the structure of plate perlite and nanosize martensite under fatigue of loading is carried out by the methods of transmission electron microscopy. Possible reasons of fatigue fracture of rail steel with different structure are discussed.

FORMATION OF STRUCTURE, PHASE COMPOSITION AND DEFECT SUBSTRUCTURE IN BULK QUENCHED RAILS OF SPECIAL CATEGORIES

Using transmission electron microscopy methods the layer by layer analysis of low temperature reliability and increased wear resistance and contact durability rails head is carried out and quantitative creation regularities of structure phase states and defect substructure formed according to diffusion and shear of γ → α-transformation on rail cross section are established.

INCREASE OF THE FATIGUE LIFE OF THE RAIL STEEL BY ELECTRON BEAM TREATMENT

Studies of structure, phase composition, defect substructure, irradiation morphology and fatigue fracture surfaces of rail steel treated by high intensive electron beam with energy density 10 and 20 J/sm2 are carried out by methods of scanning and transmission electron diffraction micros- copy. It is shown that the reason of fatigue life increase (in 2.5 times) after irradiation is the formation of toothed separation boundary “strengthened layer – matrix”.

The Structure of the Surface Alloy Formed as a Result of High-speed melting of the System Film (TiCu)/Substrate (Al)

Alloying of the surface layer of aluminum with titanium (Ti + Cu) has been carried out. The studies of the elemental and phase composition, the state of the defect substructure of the surface layer modified with an electron beam have been realized. It is shown that the liquid phase alloying of aluminum allows one to form a multiphase submicrocrystalline structure with high strength and tribological properties in the surface layer.

Modification of the structure and phase state of a ferrite-cementite composition by an electron beam

The phase composition and defect substructure formed in carbon steel subjected to a microsecond high-current e-beam have been studied by the thin-foil electron diffraction microscopy. It has been shown that the mechanism and the degree of globular cementite dissolution, phase composition and the morphology of the appearing structure depend on the distance from a melt spot and are determined by the temperature field gradient. As an electron beam approaches the "spot" (the increase in steel temperature), the evolution of the material volume containing cementite particles is accompanied by the formation of structures that alternate in a regular manner.

Mechanisms of dynamic rearrangement of the defect substructure of industrial steels

The initial stage of dynamic recrystallization initiated in steels by a high-current electron beam has been studied using the TEM method. It has been shown that the degree of dynamic recrystallization is dictated by the value of the stacking fault energy γSF. In steels with relatively low values of γSF (15−20 mJ/m2), centers of dynamic recrystallization develop by the mechanism of pair coalescence of subgrains, no matter what the crystalline lattice type is. The increase in γSF up to 35−40 mJ/m2 causes a changeover in the mechanism of multiple subgrain coalescence. At much higher values of γSF, dynamic recrystallization evolves by migration of local regions of large-angle grain boundaries.

Structure, phase composition, and defect substructure of differentially quenched rail

Differential quenching of rail by compressed air is a promising hardening method. Transmission electron microscopy is used for layer-by-layer analysis of differentially quenched rail. Quantitative parameters of the structure, phase composition, and dislocational substructure are determined and compared for different quenching conditions. Differential quenching of rail by compressed air in different conditions is accompanied by diffusional γ → α transformation. Three morphologically distinct components are formed: grains of plate pearlite, structure-free ferrite, and ferrite-carbide mixture. Gradient behavior is noted in the resulting structure: the state of the surface layer in the rail steel depends not only on the quenching conditions but also on the direction of observation and the depth of the layer being analyzed. Dislocational substructure is obtained; not only dislocational chaos, but also reticular, cellular, and fragmented dislocational substructure.

Combined electron-ion-plasma doping of a titanium surface with yttrium: Analyzing structure and properties

A titanium surface layer is modified with yttrium by combining electroexplosive doping and high-intensity electron-beam irradiation. The element and phase content, defect substructure, and mechanical and tribological characteristics are investigated for the doped layer. Saturating the surface titanium layer with yttrium, oxygen, and carbon atoms is shown to produce a metalloceramic layer reinforced with oxides and titanium and yttrium carbides that results in a multifold increase in microhardness and reduces the friction coefficient and wear rate in the modified layer.

Structure and properties of surface layers obtained due to titanium-surface alloying by yttrium via combined electron-ion-plasma treatment

The surface layer of titanium is modified via a combined method involving electroexplosive alloying by yttrium and subsequent high-intensity electron-beam irradiation. The elemental and phase composition, defect substructure, and mechanical and tribological characteristics of the alloyed layers are investigated. The titanium-surface layer is revealed to be saturated with yttrium, oxygen, and hydrogen atoms, which give rise to the formation of a metal-ceramic layer hardened with titanium and yttrium oxides and carbides and stimulate a manifold increase in the microhardness and a decrease in the friction coefficient and wear rate of the modified layer.

Superior Quality Rails: Structure-Phase States and Defect Substructure

Using transmission electron microscopy methods the layer by layer analysis of the bulk hardened superior quality rails is carried out and the quantitative parameters of structure, phase state and defect substructure gradients are established. It is shown that the interface boundaries globular cementite particles-matrix are the possible places of microcracks initiation.

Plastic Deformation Localization of Low Carbon Steel: Hydrogen Effect

In the present study, the features of plastic strain localization and structure of polycrystalline low carbon steel Fe-0.07%C were investigated after hot rolling and electrolytic hydrogen saturation. The method of double-exposure speckle photographyhave been used for identification of main types and parameters of plastic flow localization at different stages of strain hardening.Shown the effect of hydrogen on changing of defect substructure and cementite morphology by using optical and electron microscopy.

Structural and phase states of bulk-quenched rail and differentially quenched rail

For high-quality rail bulk-quenched in oil and DT350 rail differentially quenched in different conditions, the low-temperature reliability, wear resistance, and contact-fatigue strength are subjected to layer-by-layer analysis along the central axis and at the shoulder, by up-to-date physical methods. On the basis of the results the structure, phase state, defect substructure, and internal stress fields are compared.