I. G. Kabanova

Formation of an L10 superstructure in austenite upon the α → γ transformation in the invar alloy Fe-32% Ni

Structure of a metastable austenitic invar alloy Fe-32% Ni preliminarily quenched for martensite and subjected to α → γ transformation using slow heating to various temperatures (430–500°C) with the formation of variously oriented nanocrystalline lamellar austenite, which was subjected to an additional annealing at 280°C (below the calculated temperature of ordering of the γ phase), has been studied electron-microscopically. An electron diffraction analysis revealed the presence of an L10 superstructure in the disperse nickel-enriched nanocrystalline γ phase both after annealing at 280°C and in the unannealed alloy immediately after α → γ transformation upon slow heating to 430°C.

Detection of the ɛ phase and the Headley-Brooks orientation relationships upon α → γ transformation in the Fe-32% Ni alloy

The structure of an Fe-32% Ni alloy preliminarily quenched for martensite and subjected to α→ γ transformation upon a slow heating to different temperatures (430–500°C) has been studied by the electron-microscopic method. There has been observed an intermediate ɛ phase with an hcp lattice and rarely encountered Headley-Brooks bcc/fcc orientation relationships, which differ from the Kurdjumov-Sachs relationships. The networks of reflections of the ɛ phase have been observed in electron-diffraction patterns of the Fe-32% Ni alloy after both a slow heating to 430°C without annealing and a slow heating to 500°C with a subsequent annealing at 280°C; the Headley-Brooks relationships between the α matrix and the γ phase, which are typical for increased temperatures of phase transformations, have been observed in the samples after a slow heating to 500°C with annealing.

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.

Martensitic transformations and magnetic-field-induced strains in Ni50Mn50−x Gax alloys

Concentration dependences of the temperatures of forward and reverse martensitic transformations in Ni50Mn50−xGax alloys (x = 19–25) and features of the jumpwise elongation ε induced by magnetic field H in a single crystal of the Ni50Mn28.5Ga21.5 alloy have been studied. A single-variant state of martensite in the single crystal was formed by compression under the action of both a reference magnetic field and mechanical loading. It has been shown that when employing uniaxial mechanical compression, several large jumps (whose nature is associated with the appearance of structural defects hindering the displacement of boundaries of martensite twins) arise in curves of the single-crystal elongation induced by an applied perpendicular magnetic field H⊥.

Structural Mechanism of Reverse α → γ Transformation and New Functional Properties of Fe-Ni Austenitic Alloys

The structure of the metastable austenite Fe–32%Ni alloy quenched for martensite and subjected to the α→γ transformation at slow heating (to 593–773 K) with the formation of a nanocrystalline austenite of different orientation was investigated. Electron diffraction analysis has revealed that nickel-enriched nanocrystalline γ-phase obtain ordered L10 superstructure. Together with the γ-phase, there was detected the disperse ε-martensite with a hexagonal close-packed (hcp) lattice. Along with the realization of ordinary orientation relationships (ORs) of Kurdjumov-Sachs, between a matrix α-phase and an ordered γ-phase in the sample of Fe-32%Ni alloy after slow heating to 773 K there have been established Headley-Brooks Ors. Cyclic transformation promotes the improvement of many functional properties of steels (strength, coercive force, controlling of the thermal expansion coefficient and others).

Kinetics and the mechanism of the realization of the reverse α-γ transformation in the metastable Fe-Ni-Ti alloys: II. Electron-microscopic examination of the alloy structure

The mechanism of the formation of reverted austenite has been investigated by studying the structure of samples annealed in the temperature interval of the reverse α-γ transformation at consecutively rising temperatures. A hypothesis on the cause for the transition from the isothermal to athermal kinetics in the middle of the interval of the reverse α-γ transformation is suggested. The heat liberation in the middle part of the transformation range and an essential influence of this effect on the structure of reverted austenite in the N25KhT2 alloy has been confirmed experimentally.

Structural heredity in the U-6Nb alloy and conditions for its elimination

The conditions have been determined for the manifestation of structural heredity in the U-6 wt % Nb alloy with the restoration of the size and shape of grains of the initial high-temperature γ phase in the course of the forward γ → α″ martensitic transformation upon cooling and the reverse α″ → γ transformation upon heating. Contrary to iron-based alloys, the restored γ phase does not undergo recrystallization due to phase-transformation-induced deformation (phase naklep) upon subsequent high-temperature heating. The elimination of structural heredity with a noticeable grain refinement (by almost an order of magnitude, down to 10–20 μm) in the high-temperature γ phase occurs in the process of the repeated quenching from 700°C after the use of one of the preliminary heat treatments (cold deformation of the α″ martensite, the recrystallization of the deformed α″ phase, the high-temperature aging of the initial α″ martensite, and eutectoid decomposition).

Types of twins and the dislocation structure of α uranium in the initial and explosion-deformed states

Transmission electron microscopy is used to study and analyze the twin and dislocation structures of commercial-purity uranium samples in the initial (undeformed) state and after severe deformation induced by shock loading by plane waves with various intensities. As the shock loading intensity increases, the density of chaotically distributed dislocations and twins first increases, and, then, polygonization processes develop and result in a subgrain structure. Crystallographic analysis of the initial and deformation twins in uranium reveals predominant twins of the compound type {130} and rare {172} and {176} second-type twins.

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.

Electron microscopic study of austenite microtwins and their influence on the crystallographic features of the pearlitic transformation

The crystallographic relationship between the structural components in high-carbon 120G4 steel after a partial isothermal pearlitic transformation has been studied by TEM. It has been found that a high density of microtwins and stacking faults was observed in the retained austenite. A new type of crystallographic relationship has been detected—parallelism of close-packed directions and planes of the structural components of fine-plate pearlite to one of the variants of twinned austenite.