Valentin G. Gavriljuk

Grain boundary strengthening in austenitic nitrogen steels

The effect of nitrogen and carbon on the strengthening of the austenitic steel Cr18Ni16Mn10 by grain boundaries is studied. It is established in accordance with previous results that, in contrast to carbon, nitrogen markedly increases the coefficient k in the Hall-Petch equation. Because of a pronounced planar slip induced by nitrogen and the absence of any noticeable segregation of nitrogen atoms at the grain boundaries, nitrogen austenite presents an excellent object for testing different existing models of grain boundary strengthening (pile-up of dislocations, grain boundary dislocation sources, work hardening). Based on the analysis of available data and measurements of interaction between nitrogen or carbon atoms and dislocations it is shown that the nitrogen effect can be attributed to a strong blocking of dislocation sources in grains adjacent to those where the slip started.

Hydrogen-induced equilibrium vacancies in FCC iron-base alloys

Abstract Dissolution of interstitials leads to an increase of equilibrium concentration of the site vacancies as a result of two main contributions: increase of entropy of solid solution and expenditure of energy for injection of the interstitial atoms. After hydrogen outgassing vacancies become thermodynairfically unstable and form dislocation loops which can be detected by means ofTEM. In our opinion, the concept of hydrogen-induced vacancies can be useful for interpretation of hydrogen-induced phase transformations and mechanism of plastic deformation ofhydrogenated materials.

Martensitic transformations and mobility of twin boundaries in Ni2MnGa alloys studied by using internal friction

Internal friction (IF) was measured in two non-stoichiometric Ni2MnGa alloys. Several IF peaks attributed to some restructuring of the martensitic lattice were observed below Ms. From the strain dependence of IF, the activation enthalpy for movement of twin boundaries between martensitic domains was estimated to be equal to 0.02–0.04 eV.

On a role of hydrogen-induced e-martensite in embrittlement of stable austenitic steel

Abstract Effect of the hydrogen-caused γ→e transformation on hydrogen embrittlement (HE) of AISI 310 type austenitic steel is studied. Using the alloying with silicon that increases the fraction and stability of the hydrogen-induced e-martensite, it is shown that the resistance to HE is improved. It is concluded that other factors, among them the activation enthalpy of hydrogen migration, are crucial for HE.

Nitrogen partitioning between matrix, grain boundaries and precipitates in high-alloyed austenitic steels

Nitrogen in austenitic steels is known to increase their resistance to sensitization treatments and to shift TTT diagrams to the right on the time scale, which suppresses the intercrystalline corrosion. It is also shown that nitrogen delays the precipitation of carbides and intermetallic phases, while, at the same time, chromium nitrides precipitate slower as compared to carbides. This paper aims at clarifying the question of possible nitrogen segregation at grain boundaries in austenitic steels. The nitrogen content in the chromium nitride Cr{sub 2}N and the intermetallic Laves phase was also measured as a test of the experimental technique used.

Influence of Nitrogen on Vibration Damping and Mechanical Properties of Fe-Mn Alloys

The addition of nitrogen to the iron-based FCC alloys significantly improves mechanical and corrosion properties. At the same time, nitrogen has been found to prevent clustering in these alloys and, as a result, to improve the chemical homogeneity of solid solutions. Based on these results, one can expect that the alloying by nitrogen will improve the mobility of interfaces providing a higher level of damping, along with its favorable influence on mechanical and corrosion properties. The present study aims at investigation of nitrogen effect on damping and mechanical properties of Fe-Mn based alloys.

Precipitates in Tempered Stainless Martensitic Steels Alloyed with Nitrogen, Carbon or Both

The precipitation in martensitic steels alloyed with (wt. %) ∼15Cr, ∼IMo, 0.62N or 0.6 %C or 0.29C+0.35N is studied after tempering in the range of 100-650°C by means of transmission electron microscopy (TEM), Mossbauer spectroscopy (MS), internal friction (IF) and dilatometry. It is shown that the substitution of carbon by nitrogen shifts the precipitation towards higher temperatures and expands the formation of stable interstitial phases over a broad temperature range. TEM studies reveal the precipitation of orthorhombic ξ-nitride and hexagonal e-nitride in the nitrogen martensite while cementite and e-nitride are formed in the nitrogen+carbon martensite. A higher mobility of dislocations in the nitrogen martensite was observed by means of IF. The fraction of retained austenite (RA) in as-quenched martensite is increased in the order: C→N→C+N, and the stability of RA against decomposition during tempering is changed in the same order. According to MS data, the distribution of chromium atoms is characterised by a tendency to clustering in the carbon martensite and to short range ordering in the nitrogen and nitrogen+carbon martensites. The different precipitation behaviour of martensites during tempering is explained by the nitrogen effect on iateratomic bonding and short range atomic order in solid solutions.

Steel of Highest Fracture Energy

The strength of common steels is considerably raised by thermomechanical treatment, heat treatment or cold working, however at the expense of toughness, which is usually defined as the work to fracture, i.e. the fracture energy. In part it depends on the loading conditions, e. g. the ratio of the hydrostatic stress and the equivalent shear stress as well as on the thermal activation contained in the temperature and velocity of loading. A drastic change of loading conditions (e.g. a sharp notch or a subzero temperature) may displace a ductile steel from the upper shelf of toughness to the brittle lower shelf. The present study is therefore based on uniaxial tensile tests to reveal the toughening of new steels at the upper shelf. In addition ISO-V notch impact tests are performed to indicate an embrittling transition range. It is the aim of this investigation to overcome the brittleness of some high-strength steels and develop a steel of high strength and high toughness.

Nitrogen and carbon in austenitic and martensitic steels: atomic interactions and structural stability

The effect of nitrogen and carbon on the structure of austenitic and martensitic steels is discussed. A correlation is shown between the electronic structure, atomic distribution and thermodynamic stability of Fe-based alloys. An increase in stability of solid solutions and a delay of precipitation is observed in the order of alloying with carbonRTnitrogenRTcarbon+nitrogen. Experimental data are presented for C, N and C+N steels.

Mechanism of hydrogen-induced phase transformations in metals and alloys

Abstract Theory and experiment are presented to prove that hydrogenation of austenitic steels leads to a significant increase of concentration of the thermodynamic equilibrium host-lattice vacancies. The theory predicts the vacancy-induced loss of the phase stability, which creates a basis for a new mechanism of hydrogen-induced phase transformations. It is shown that an external pressure can compensate for the effect of interstitials on vacancy formation and vacancy-induced loss of the phase stability. Along with formation of the eH martensite in hydrogen-charged stable austenitic Cr18Ni16Mn10 steel, a high density of dislocation loops has been observed by means of TEM, which is evidence of the high concentration of vacancies which lose equilibrium and form plane vacancy discs when hydrogen leaves the sample. In accordance with the theory, it is shown that gaseous hydrogenation under high external pressure does not induce γ → eH transformation, although the same hydrogen concentration has been obtained as in the case of cathodic charging of this steel, which had led to formation of eH martensite.