V. A. Shabashov

New method of mechanical alloying of ODS steels using iron oxides

Processes of mechanical alloying of oxide-dispersion-strengthened reactor pressure-vessel steels by cold high-pressure torsion of a powder mixture of low-stable Fe2O3 (Fe3O4) iron oxides and the bcc matrix alloyed with Y and Ti have been investigated using Mössbauer spectroscopy, X-ray diffraction analysis, and electron microscopy. Some features of decomposition of iron oxides and phase transformations in the matrices synthesized by mechanical alloying with formation of solid solutions supersaturated with oxygen and various compounds of oxygen with iron and alloying elements, in particular, special nanooxides of yttrium and titanium have been established.

Nanostructure Formation and Phase Transformations in Nitrided Stainless Steel Kh18N8 during Severe Cold Deformation

The phase transformations and nanostructure formation in metastable stainless steel Kh18N8 are studied during nitriding and subsequent severe cold plastic deformation by high-pressure torsion in Bridgman anvils. Ion-plasma nitriding of the steel leads to the nitrogen saturation of austenite, CrN nitride formation, and the destabilization of the structure with respect to the γ-α transformation. Subsequent deformation is accompanied by the reverse α-γ phase transformation, the dissolution of nitrides, and the formation of nitrogen-supersaturated solid solutions and secondary nanonitrides. These fine particles restrict the grain growth in the matrix and stabilize the nanostructure formed.

Influence of the stressed state of the zone of friction contact on the formation of the structure of a surface layer and tribological properties of steels and alloys

An investigation has been performed of the influence of contact stresses, which appear in the zone of friction of iron-manganese alloys with 16.9–40.5 wt % Mn, wear-resistant high-manganese austenitic steels, and carbon steel U13, on the phase composition, structure, and strengthening of surface layers of these materials under conditions of dry friction at small (0.03 and 0.07 m/s) sliding velocities, when the frictional heating of the surface layer of the samples is virtually absent. A quantitative estimation of the arising compressive contact stresses (pressures) has been carried out. It is shown that the value of the compressive contact stresses approximately corresponds to the values of microhardness measured on the friction surface of the materials investigated. These compressive contact stresses initiate the occurrence a γ-ɛ martensitic transformation in the iron-manganese alloys with 16–40% Mn, which is characterized by a negative volume effect, but impedes the development of the ɛ-α and γ-α martensite transformations (in austenitic steels) which occur with an increase in the specific volume. The stresses in question favor the formation of nanocrystalline structures in a thin (≤10 μm thick) surface layer of friction steels and alloys. The contact tensile stresses following the contact compressive stresses initiate the formation and propagation of microscopic cracks in the surface layer of the friction materials, and activate the development of martensitic ɛ-α and γ-α transformations in this layer. It is shown that the structural and phase transformations initiated by contact stresses exert a substantial effect on the microhardness, friction coefficient, and resistance to adhesive wear of the steels and alloys under study.

Specific features of low-temperature phase transformations in nitrogen-Containing Cr-Mn-based steels

Physical and mechanical properties of nitrogen-containing austenitic steels of different alloying systems have been studied at temperatures from −196 to 700°C in the quenched state. It has been found that nitrogen-containing Cr-Mn-based steels undergo a paramagnetic to antiferromagnetic ordering (with a manifestation of invar properties), the ΔE effect, and a resistivity anomaly. It has been shown that the behavior of the temperature dependence of the yield stress in nitrogen austenite is determined by several factors. Along with specific features of the dislocation structure determined by a low energy of stacking faults, the strengthening of nitrogen austenite is influenced by its magnetic state.

Structural and phase transitions in nitrided layers of iron alloys during severe cold deformation

A nanostructuring procedure similar to that proposed previously for iron alloys with carbides, nitrides (γ′-Fe4N, TiN), and oxides, was implemented for X22 fcc alloy and X18H8 austenitic stainless steel. The procedure is based on the deformation-induced dissolution of disperse CrN nitride particles in the alloy matrices and the formation of supersaturated solid solutions of nitrogen, followed by the precipitation of secondary nanonitrides inhibiting the grain growth in the matrix during heating.

Accommodation stresses and structural-phase transitions in Fe-Ni alloys upon compression in Bridgman anvils

Mössbauer spectroscopy was used to study polymorphic phase transitions in iron and Fe-Ni (15, 25, 32 at % Ni) alloys in situ in Bridgman anvils under a quasi-hydrostatic compression to 25 GPa. A connection has been established between the type of polymorphism and degree of transformations on the one hand and the hyperfine parameters, magnetic texture, and stresses in the structure of the initial and resultant phases on the other hand. In iron and in the Fe-15 % Ni alloy, a baroelastic change in the direction of the magnetic texture of the phase has been revealed, which is caused by the appearance of oriented accommodation stresses between the and phases. In the range of compositions of 15–25 at % Ni, the appearance of a hyperfine magnetic structure of spectra of the high-pressure phase has been found.

Phase transformations in the hematite-metal system during mechanical alloying

Mössbauer spectroscopy and X-ray diffraction are used to show that the phase transformations in hematite α-Fe2O3-metal (M = Fe, Ni, Ti, Zr) powder mixtures induced by severe cold plastic deformation in ball mills occur via the formation of M-Fe-O solid solutions, redox reactions with the reduction of metallic iron, and the formation of secondary MxOy oxides and MxFey intermetallics. Mechanical activation in a ball mill is compared to that under high-pressure shear in Bridgman anvils. The transformations that take place in a ball mill are found to have several stages and to be accelerated.

Dissolution of the Fe4N nitride in the nitrided layer of iron upon cold deformation by shear under pressure

Mössbauer spectroscopy, X-ray diffraction, and transmission electron microscopy were used to study structural and phase transformations upon cold (300 K) deformation by shear under pressure in thin layers of nitrides Fe4N formed on the surface of bcc iron. Strain-induced dissolution of nitrides in bcc iron with the formation of bcc and fcc solid solutions supersaturated with nitrogen and secondary Fe16N2 and Fe4N nitrides was found. The dispersiveness of nitride phases in the layers deposited on bcc iron determines the accelerated kinetics of cold mechanosynthesis and the possibility of the formation of nanocrystalline iron-based solid solutions supersaturated with nitrogen and containing secondary nitrides.

Structure and phase composition of oxide dispersion strengthened fcc alloys prepared via strong deformation in the Fe2O3-Fe-Ni-M (M = Ti, Zr) systems

Deformation-induced Fe 2 dissolution in fcc Fe-Ni-M (M = Ti, Zr) alloys has been studied by Mössbauer spectroscopy, X-ray diffraction, and transmission electron microscopy. The results indicate that high-pressure shear deformation in Bridgman anvil cells and ball milling lead to dissolution of the low-stability oxide Fe2O3 in the fcc matrix and the formation of metallic solid solutions and secondary oxides of the alloying elements. This enables preparation of oxide dispersion strengthened Fe-Ni alloys and grain size reduction of the fcc matrix. The formation of secondary oxides occurs more actively during ball milling than during high-pressure shear deformation because of the more significant local heating of the mixture and the larger specific surface area and higher reactivity of the powder.

Structural transformations in wear resistance of iron- and cobalt-based amorphous alloys during abrasive wear

The wear resistance and structural changes in a number of amorphous alloys based on iron and cobalt and in high-carbon tool steels are studied during wear by a fixed abrasive (crondum, Carborundum) at room temperature and −196°C. The abrasive wear resistance of the amorphous alloys is shown to be 1.6–3.1 lower than that of the high-carbon tool steels having a similar hardness. The relatively low level of the abrasive wear resistance of the amorphous alloys is assumed to be caused by strain softening of their surface during wear. A nanocrystalline structure is found to form in local microvolumes in a thin deformed surface layer of the alloys.