K. A. Kozlov

Deformation-induced dissolution of the intermetallics Ni3Ti and Ni3Al in austenitic steels at cryogenic temperatures

Abstract An anomalous deformation-induced dissolution of the intermetallics Ni3Al and Ni3Ti in the matrix of austenitic Fe–Ni–Al(Ti) alloys has been revealed in experiment at cryogenic temperatures (down to 77 K) under rolling and high pressure torsion. The observed phenomenon is explained as the result of migration of deformation-stipulated interstitial atoms from a particle into the matrix in the stress field of moving dislocations. With increasing the temperature of deformation, the dissolution is replaced by the deformation-induced precipitation of the intermetallics, which is accelerated due to a sufficient amount of point defects in the matrix, gained as well in the course of deformation at lower temperatures.

Comparative analysis of the kinetics of dissolution of oxides Y2O3 and Fe2O3 in the iron matrix upon mechanical alloying

The kinetics of low-temperature dissolution of oxides Y2O3 and Fe2O3 in an iron matrix during mechanical alloying has been studied using electron microscopy. It has been shown that the dissolution rate upon deformation of primary coarse oxides Fe2O3 in α iron (and, hence, saturation of the α matrix with oxygen) during treatment in a ball mill for up to 10 h is several times higher than the dissolution rate of Y2O3 oxides. The high-temperature (1100°C) annealing of a mechanoalloyed mixture of Fe + 1.5% Y + 1.35% Fe2O3 leads to the precipitation of 60% (of the total number of particles) secondary oxides 2–5 nm in size and only of 5–7% secondary nanooxides in a mechanoalloyed mixture of Fe + 2% Y2O3.

Solid-state mechanical synthesis of austenitic Fe-Ni-Cr-N alloys

The possibility of producing high-nitrogen nanostructured austenitic Fe-Ni-Cr-N alloys using solid-state mechanical synthesis in a ball mill has been studied. It has been shown that, in the matrices of iron and Fe-xNi (x = 6–20 at %) alloys with chromium nitrides, bcc and fcc Fe-Ni-Cr-N solid solutions are formed. With an increase in the concentration of nickel in the matrix, the amount of mechanically synthesized austenite grows. The subsequent heating of the mechanically synthesized specimens into the austenitic field of the phase diagram of Fe-Ni alloys leads to the α → γ transformation with the retention of the nanostructured Fe-Ni-Cr-N solid solution and precipitation of secondary nitrides CrN, which stabilize the structure.

Deformation-intensified atomic separation in bcc Fe–Mn alloys

The deformation-intensified atomic Mn-related separation of the bcc solid solution has been found in Fe100–xMnx alloys (x = 4.5–9.9) subjected to ball milling using Mössbauer spectroscopy. In the near surrounding of iron atoms, the atomic separation is similar to that observed upon the annealing of the alloys in a temperature range of 400–500°С. It has been found that the deformation-intensified atomic separation leads to the stabilization of the bcc phase with regard to the α → γ transformation, as well as to the expansion of the field of the existence of the bcc phase during heating.

Effect of aluminum on mechanical solid-state alloying of iron with nitrogen in ball mill

Mössbauer spectroscopy and X-ray diffraction analysis have been used to study mechanical solidstate alloying with nitrogen and chromium of iron and an Fe-3Al alloy in the process of the mechanical activation with chromium nitrides in a ball mill. It is shown that the deformation-induced dissolution of chromium nitrides in iron and Fe-3Al matrices results in the formation of substitutional chromium and interstitial nitrogen solid solutions. The alloying of iron with aluminum accelerates the process of the deformation-induced dissolution of chromium nitrides, but reduces the nitrogen content in the interstitial solid solution. Post-deformation annealing generally leads to the escape of aluminum from the matrix, the substitution of chromium for aluminum, and the formation of fine nitrides AlN.

Effect of the rate of cold plastic deformation on the kinetics of mechanical alloying of the Fe-35Ni-5Al alloy

The strain-induced dissolution of Ni3Al intermetallic particles in the matrix of the aging Fe-35Ni-5Al alloy has been found to become more intense with increasing rate of deformation in rotating Bridgman anvils. This regularity has been presumably explained by a disbalance in the kinetics of the dual process of strain-induced dissolution and formation of phases owing to an increase in the number of interstitial atoms and a retardation of the development of an alternative process of the precipitation of second phases with increasing rate of deformation.

Dynamic aging in an Fe–Ni–Al alloy upon megaplastic deformation. Effect of the temperature and deformation rate

The method of Mössbauer spectroscopy has been used to investigate the effect of the temperature and the rate of megaplastic deformation on the processes of dissolution–precipitation of intermetallic compounds in aging austenitic alloy with a composition of Fe–36Ni–9Al. It has been established that, upon deformation in revolving Bridgman anvils, in the temperature range of cryogenic temperatures (liquid nitrogen) up to 573 K, a change occurs in the character of phase transitions from atomic disordering and the dissolution of intermetallic compounds to their additional accelerated precipitation. The factor that affects the kinetics of the processes of dissolution–precipitation of intermetallic compounds in the metallic matrix is dynamic aging. Dynamic aging is activated with an increase in the temperature and a decrease in the deformation rate.

Solid-Phase Mechanical Alloying of BCC Iron Alloys by Nitrogen in Ball Mills

The methods of Mössbauer spectroscopy and X-ray diffraction analysis have been used to study the processes of a solid-phase alloying of the iron alloys with a bcc lattice by nitrogen that occur upon ball-mill mechanical activation in the presence of chromium nitrides. It is shown that a deformation-induced dissolution of chromium nitrides in the matrix of pure iron and in that of the alloys Fe–3Al and Fe–6V results in the formation of the substitutional chromium and interstitial nitrogen bcc solid solutions. An additional alloying of iron with aluminum or vanadium under the deformation dissolution of nitrides leads to the escape of aluminum and vanadium from the matrix and to a decrease in the nitrogen content characteristic of the interstitial solid solution proper due to the strong chemical bonding of alloying elements with nitrogen. The subsequent annealing leads to the decomposition of already formed solid solutions with the formation of aluminum, vanadium, and chromium nitrides of extreme dispersion.

Manufacturing of oxide-dispersion-strengthened steels with the use of preliminary surface oxidation

Regularities of deformation-induced dissolution of a surface layer of iron oxides in matrixes of iron-based alloys with bcc and fcc lattices have been studied by the methods of Mössbauer spectroscopy, transmission electron microscopy, and X-ray diffraction. A method of producing iron alloys strengthened by dispersed oxide nanoparticles and alloyed with elements possessing a high affinity to oxygen (titanium and yttrium) has been proposed, which implies a dynamic dissolution of a surface layer of iron oxides upon strong cold deformation and a precipitation of secondary yttrium and titanium nanooxides upon a subsequent high-temperature sintering of mechanically alloyed powders. There has been demonstrated a possibility of oxide strengthening of pure iron upon its interaction with air without introducing traditional alloying elements.

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.