N. V. Kazantseva

Microstructure and plastic deformation of orthorhombic titanium aluminides Ti2AlNb. IV. Formation of the transformation twins upon the α2 → O phase transformation

X-ray diffraction and transmission electron microscopy were used to study the O-phase formation in the Ti-25.6% Al-13.9% Nb-0.3% Mo-0.3% Zr alloy upon the B2 → O phase transformation. The formation of pseudotwins is shown to be possible in the B2 phase in the Ti-Al-Nb alloys. It is found that the O phase upon the B2 → O phase transformation does not undergo twinning, and all twins observed in the alloy belong to the intermediate metastable B19 phase. The orthorhombic O phase is formed upon ordering inside the B19-phase twins and save their boundaries.

Study of the Ti-Al-Nb Phase Diagram

Differential thermal and X-ray diffraction analyses as well as optical and transmission electron microscopy were used to study orthorhombic Ti2AlNb-based alloys, such as Ti-22 at. % Al-26.6 at. % Nb, Ti-23.5 at. % Al-21 at. % Nb, Ti-24.6 at. % Al-22 at. % Nb. These alloys in the initial state contain an orthorhombic O phase (Ti2AlNb) ordered in terms of all three elements. The differential thermal analysis was performed at different heating and cooling rates, namely, 40 and 80 K/min. The solidus and liquidus temperatures of the Ti-22 at. % Al-26.6 at. % Nb, Ti-23.5 at. % Al-21 at. % Nb, Ti-24.6 at. % Al-22 at. % Nb alloys have been measured; these are TS = 1707 and TL = 1737°C, TS = 1703 and TL = 1730°C, and TS = 1694 and TL = 1728°C, respectively. The temperature boundaries of phase transformations occurring in these alloys have been determined. The activation energy of phase transformations has been calculated using the Johnson-Mehl-Avrami equation. The results obtained are compared with the available literature data. Causes of the discrepancy between equilibrium phase diagrams of the Ti-Al-Nb system reported by different authors are analyzed.

The structure and magnetic properties of a heat-resistant nickel-base alloy after a high-temperature deformation

The structure of a turbine blade made of the ChS-70V alloy has been studied after operation in an experimental regime at 880°C. A considerable change in the structural state of the alloy indicates the presence of an extremely high level of stresses in the material. During the operation, the magnetic susceptibility of the alloy increases by two orders of magnitude. The possible structure objects responsible for a change in the magnetic susceptibility are the complexes of superstructure intrinsic stacking faults inside the intermetallic phase.

Phase diagram of the Co–Al–W system. structure and phase transformations near the Co3(Al, W) intermetallic composition range

Low-temperature portion of the polythermal section for the Co–Al–W system in the vicinity of the Co3(Al, W) intermetallic composition has been studied experimentally using electron microscopy and hightemperature X-ray diffraction analysis. Low-temperature structural phase transformations and temperature ranges of the existence of phases have been determined. The morphology of Co3(Al, W) intermetallic particles was studied as a function of the tungsten content in alloys.

Study of magnetic properties and structural and phase transformations in the Co-19 at % Al-6 at % W alloy

The Co-19 at % Al-6 at % W alloy prepared by two methods of melting in an inert atmosphere, namely, by arc melting followed by casting into a water cold copper mold and induction melting followed by casting in a ceramic (Al2O3) mold, has been studied. It was found that the phase composition of the alloy and its magnetic properties depend on the cooling rate of ingot after melting. Samples are ferromagnetic up to 800°C; the specific magnetization equal to σ= 10 emu/g is retained up to ∼700°C. The Curie temperatures of all phases found in the alloy have been determined. It was found that the formation of the Co7W6 phase in the alloy increases the coercive force of the alloy, whereas the saturation magnetization demonstrates a 1.5-fold decrease.

Use of mechanoactivation for obtaining hydrides of titanium aluminides

The hydrogen storage capacity of titanium aluminides Ti(Al,Nb) (β0) and Ti3 (Al,Nb) (α2) subjected to mechanoactivation has been investigated. It has been found that the mechanoactivation in a hydrogen atmosphere makes it possible to obtain hydrides of titanium aluminides with a hydrogen concentration of up to 1.8 wt % at room temperature without enhanced requirements for purity and pressure of the supplied hydrogen. The release of hydrogen from such samples in a vacuum begins at a temperature of about 175°C.

Structure and mechanical properties of an Ni3Al single crystal upon high-temperature deformation

Structure and strength properties of single-crystal 〈001〉 samples of Ni3Al have been studied in the as-grown and homogenized state during tensile tests in the temperature range of 1150–1250°C. At the strain rate of 1.32 mm/min (2 × 10−5 m/s), the samples are in the state of superplasticity. The basic mechanism of relaxation is dynamic recovery; in some regions of the sample, recrystallized grains are formed. At 1250°C, coarse twins are observed in the zone of fracture, which indicates the “switching on” of additional slip systems necessary to guarantee the relaxation process.

Structure and magnetic properties of a Ni3(Al, Fe, Cr) single crystal subjected to high-temperature deformation

The structure and magnetic properties of the Ni3(Al, Fe, Cr) single crystal subjected to high-temperature tensile deformation to failure at 850–900°C have been studied. No recrystallized grains and metastable phases were found. The rupture zone of the alloy subjected to deformation (at 900°C) to the highest degree demonstrates the fragmentation accompanied by rotation of atomic layers and changes of the chemical composition in the nickel and aluminum sublattices. Magnetic studies of the alloy have shown the existence of two Curie temperatures for samples cut from the rupture zone. Samples cut away from the rupture zone exhibit no additional magnetic transitions; twines and planar stacking faults in the alloy structure. The alloy deformed to the lower degree of deformation (at 850°C) also demonstrates twins; no ferromagnetic state was found to form.

Study of the martensitic transformation in the Co–9 at % Al alloy

Phase transformations in the Co–9 at % Al have been investigated after slow furnace cooling. It has been shown that the structure and phase composition of the alloy after slow cooling do not correspond to the equilibrium phase diagram of the alloy of this chemical composition. It has been established that the α → ε martensitic transformation does not require overcooling and occurs even during a slow cooling of the alloy. It has been found that the formation of 4H modulated martensite is a specific feature of the binary alloys of cobalt and is not connected with the rate of their cooling. The Curie temperatures for the B2, α, and ε phases have been determined.

Investigation of the structure of two heat-temperature nickel-based alloys after high-temperature deformation

The structural tests of turbine blades made from two commercial high-temperature nickel-based alloys were carried out after their test operation at elevated temperature and stresses. Both alloys contain 40% of a hardening intermetallic phase and the upper operating temperature of both alloys is limited to 900°C, but they have a different resistance to high-temperature deformation.