V. A. Zavalishin

Effect of composition and temperature on the redistribution of alloying elements in Fe-Cr-Ni alloys during cold deformation

Deformation-induced redistribution of components in a steel Kh11N30 was shown to decrease up to zero as the deformation temperature increases from 0 to 300°C. The maximum Curie temperature of deformation-induced ferromagnetic clusters formed in steels Kh11N30, Kh12N30, and Kh15N38 is the same and is equal to ∼160°C. The formation of 3–5-nm particles of an ordered L10 or L12-type phase whose amount is 5 to 10 vol % was found by field-ion microscopy.

Comparative analysis of corrosion cracking of austenitic steels with different contents of nitrogen in chloride- and hydrogen-containing media

The structural state and the resistance to stress-corrosion cracking (SCC) at constant loads have been studied using samples with a grown crack by the method of the cantilever bending on quenched austenitic stainless steels of the 20Cr-6Ni-11Mn-2Mo-N-V-Nb (Kh20N6G11M2AFB) type, with different contents of nitrogen (0.17, 0.34, 0.43, and 0.50 wt % N). The tests were conducted in a 3.5% aqueous solution of NaCl (without providing polarization) and in a similar solution under cathodic polarization, which causes the formation of hydrogen. It has been shown that, in a chloride solution without polarization, the steels do not undergo SCC for 2000 h. In the case of significant cathodic polarization via employment of a magnesium protector, there was revealed a brittle character of fracture upon SCC in all steels. It has been shown that steel with a nitrogen content of 0.43 wt % possesses the maximum absolute values of rupture stresses under the conditions of cathodic polarization.

Structural mechanism of reverse α → γ transformation and strengthening of Fe-Ni alloys

Fe-32% Ni alloy subjected to slow heating to a temperature below As at a rate of 0.01 K/min demonstrates the untwinning and appearance of an intermediate ɛ phase with an hcp lattice and lattice parameters a = 2.535, c = 4.132 Å, and c/a = 1.63. Slow heating to 430–490°C leads to the formation of nanocrystalline austenite enriched in nickel, which substantially increases the hardness of martensite. The formation of austenite in the Fe-32% Ni alloy, which is a mixture of martensite with 20–30% nanocrystalline austenite, during its rapid heating to 600°C occurs via the bulk mechanism with short-range atomic diffusion. In this case, the diffusion does not eliminate the concentration micro-inhomogeneity of the alloy in nickel but leads to the reorientation of γ-phase nanocrystals, almost eliminates the dislocation structure, and removes the strengthening by phase hardening.

A new approach to creation of high-strength austenitic steels with a controlled shape-memory effect

A new approach to creation of high-strength manganese austenitic steels with a controlled shape-memory effect was realized as a result of carbide dispersion hardening. The formation of VC nanocarbides differing in dispersity stabilizes or destabilizes austenite with respect to the formation of deformation-induced ɛ martensite, which makes it possible to control the magnitude of shape-memory effect and the degree of strengthening.

Acoustic detection of stress-corrosion cracking of nitrogen austenitic steels

Structural changes and resistance to the stress-corrosion cracking of the nitrogen-bearing austenitic steels 04Kh20N6G11M2AFB and 09Kh20N6G11M2AFB (with 0.04 and 0.09 wt % C, respectively) after different treatments, including thermomechanical action, quenching from 1200°C, and aging at 700°C for 2 and 10 h, have been studied. It has been shown that aging at 700°C of the air-melted austenitic steel 09Kh20N6G11M2AFB leads to a decrease in the strength of samples with an induced crack upon the cantilever bending in air and in a 3.5% aqueous solution of NaCl as compared to the strength of the steel 04Kh20N6G11M2AFB-EShP with a smaller carbon content after high-temperature mechanical treatment or quenching from 1200°C. The smallest resistance to stress-corrosion cracking is observed in the samples of 09Kh20N6G11M2AFB steel after 10 h of aging, which is accompanied by the most intense acoustic emission and by brittle intergranular fracture. This is explained by the high rate of the anodic dissolution of the metal near chromium-depleted grain boundaries due to the formation of continuous chains of grain-boundary chromium-containing precipitates of carbides and nitrides.

Catastrophic corrosion cracking of Cr-Mn-V aging austenitic steels

Investigations were carried out into the processes of corrosion cracking in a 3.5% aqueous solution of NaCl of 40Kh4G18F and 35Kh12G24F2M austenitic steels hardened by carbide aging, in comparison with Kh12N18T3 and Kh18G20A0.5 austenitic steels subjected to intermetallic and solid solution hardening, respectively. The Cr-Mn-V steels were characterized by a catastrophic reduction of corrosion cracking resistance at loads ofσ=(0.1-0.9)σair associated with weakening of grain boundaries during precipitation of large carbides VC and Cr23C6. An increase of the grain size is accompanied by a further reduction of corrosion resistance caused evidently by enrichment of the grain boundaries with impurities. The acoustic emission method was used to examine the kinetics of defect formation in examined steels in corrosive medium. The results showed advancing cumulation of microcracks under loading in the aging Cr-Mn-V steels under the effect of salty and distilled water which leads to catastrophic failure in holding for 1–4 h. Microcrack cumulation under loading at σ = 0.9σair in a 3.5% aqueous solution of NaCl was insignificant in steels with intermetallic (Kh12N18T3) and solid solution (Kh18G20A0.5) hardening because no second phase precipitated at the grain boundaries.

Precipitation-hardening stainless steels with a shape-memory effect

The possibility of obtaining the shape-memory effect as a result of the γ → ε → γ transformations in aging stainless steels strengthened by VC carbides has been investigated. Regimes are given for strengthening aging (at 650 and 720°C) for stainless steels that predominantly contain (in wt %) 0.06–0.45C, 1–2V, 2–5Si, 9 and 13–14Cr. The values of reversible deformation e (amount of shape-memory effect) determined after heating to 400°C in samples preliminarily deformed to 3.5–4% vary from 0.15 to 2.7%, depending on the composition of the steels and regimes of stabilizing and destabilizing aging.

Using an intermediate nanocrystalline γ phase for producing austenitic steels with a controllable thermal expansion coefficient

The metastable austenitic iron alloy with 31.3 wt % Ni (N31) has been used to show the possibility of the formation of a nickel-concentration inhomogeneity in a fine-grained austenite due to an α → γ trans-formation under the condition of a preliminary formation of a nickel-enriched intermediate nanocrystalline γ phase. The thermal expansion coefficients (TECs) in the range of −100 to +300°C have been estimated in concentrationally inhomogeneous steel N31 after various heat treatments. The conditions necessary to ensure the possibility of controlling the TEC in wide limits have been found.

Effect of a nanosized state on the magnetic properties of Ni3(Al,Fe) and Ni3(Al,Co)

An analysis of the magnetic properties of the binary Ni3Al intermetallic compound (unalloyed and alloyed with iron and cobalt) after high-pressure torsion has been carried out. For all of the alloys studied, the deformation using ten revolutions of the anvils at 10 GPa results in the formation of a nanocrystalline condition. After the deformation, a significant decrease takes place in both the magnetic susceptibility of the Ni3Al alloy and magnetization of the Ni3Al-Co intermetallic compound. The initially ferromagnetic Ni3Al-Fe alloy becomes paramagnetic at room temperature.

Calorimetric effects of structural changes occurring during ordering of the equiatomic alloy CuAu

The calorimetric method was used to confirm the results of an electron-microscopic investigation indicating the complex multistage process of ordering of the equiatomic alloy CuAu, which includes relaxation phenomena as the inherent part of the very ordering process.