G. V. Talanova

Phase transformations at high pressures and temperatures and the structure of a hypoeutectic 1Ni-99Al alloy

The phase transformations in a hypoeutectic 1Ni-99Al alloy are studied by differential barothermal analysis in the temperature range up to 750°C at a compressed argon pressure up to ∼100 MPa. The Al matrix of the initial alloy is found to be saturated by micropores at a concentration of 3.7 × 1010 cm−3. After melting and solidification in a compressed argon atmosphere, the micropore concentration increases to 3.2 × 1011 cm−3. As a result of melting and solidification at a high pressure, the initial fine-grained structure of the alloy with an average grain size of 16 μm transforms into a coarse-grained structure during dendritic solidification. The processing of electron-microscopic images is used to determine the volume content of intermetallic compound Al3Ni in the Al matrix. The liquidus temperature of the alloy at 100 MPa increases by 10°C, and the solidus temperature is 5°C higher than the eutectic transformation temperature in aluminum-rich Al-Ni alloys. The solid-phase decomposition of the supersaturated solid solution of nickel in aluminum occurs at 630°C. At 100 MPa, the field of solid solutions of nickel in aluminum extends to 1.2 at % Ni as compared to the Al-Ni system at atmospheric pressure. The lattice parameters of Al and Al3Ni are found to increase in the alloy solidified at 100 MPa. The microhardness of the Al matrix in the alloy is measured after a barothermography cycle. A portion of the Al-Ni phase diagram is proposed for a pressure of 100MPa in the nickel content range 0–4.3 at %.

Barothermal analysis and structure of the eutectic Al-Ni (2.7 at % Ni) alloy

Phase transformations of the 2.7Ni + 97.3Al eutectic alloy have been characterized by differential barothermal analysis (DBA) at temperatures of up to 750°C and pressures of up to ∼100 MPa. The results demonstrate that the Al matrix of the as-prepared alloy was saturated with micropores. After melting and crystallization in compressed argon, the micropore density increased. As a result of melting and solidification, the fine-grained structure of the as-prepared alloy transformed to a macrocrystalline, dendritic structure. Electron microscopy was used to determine the volume fraction of the intermetallic phase NiAl3 in the Al matrix. Supersaturated solid solutions of nickel in aluminum decompose at 626°C to give a mixture of Al〈Ni〉 and NiAl3. At 100 MPa, the nickel-aluminum solid-solution range extends to 2.7–2.8 at % Ni, and the nickel content at the eutectic point may reach 3.1–3.3 at %. The Al and NiAl3 in the alloy solidified at 100 MPa had reduced lattice parameters. We determined the microhardness of the Al matrix after DBA.

Thermography of the phase transformations in nickel-based eutectic alloys at high pressures and temperatures

The phase transformations in STEMET 1301A and STEMET 1311 nickel solder alloys are studied by differential barothermal analysis and metallography at temperatures up to 1100°C and pressures up to 120MPa. When the pressure increases, the temperature of the onset of crystallization of the initial amorphous STEMET 1301A alloy decreases and that of the STEMET 1311 alloy increases. At high pressures, the solidus temperatures of the alloys changes insignificantly and the liquidus temperature of the STEMET 1301A alloy increases by 30°C. The solidification ranges of both alloys at 120 MPa are found to broaden substantially.

Thermography of the phase transformations in a hypoeutectic Al-7% Si-0.5% Mg silumin at high pressures and temperatures

The phase equilibria in a hypoeutectic AK7 aluminum alloy (Al-7% Si-0.5% Mg silumin) are studied by differential barothermal analysis in the pressure range 5–131 MPa at temperatures up to 960 K. The baric dependences of the liquidus temperature during heating and the solidus and liquidus temperatures during cooling are found to have a minimum at 5 MPa. The solidus temperature during heating increases sharply as the pressure increases from 5 to 53 MPa and remains almost unchanged at a pressure p ≥ 53 MPa. The baric coefficients of the solidus and liquidus temperatures in the pressure range 5 ≤ p ≤ 53 MPa achieve ~0.6 K/MPa, which is much higher than the baric coefficient of the melting temperature of pure aluminum. A pT phase diagram is proposed for the AK7 alloy.

Thermographic investigation of the phase equilibria in an aluminum-based alloy at high pressures and temperatures

The phase equilibria in a commercial aluminum alloy are studied by thermography, particularly, by differential barothermal analysis, in the pressure range from 10 to 150 MPa at temperatures below 960 K. The liquidus temperature is found to increase insignificantly with the pressure, and the solidus temperature increases jumpwise at a pressure of 60 MPa. The phase pT diagram of the aluminum alloy and a eutectic model of the baric dependences of the solidus and liquidus temperatures are proposed.

Decomposition of the γ solid solution in a nickel-based cast alloy at a high uniform pressure

The formation of γ′ particles in an as-cast nickel alloy at barothermal treatment temperatures of 1235, 1260, and 1320°C and a pressure of 175 MPa for an action time of 2 h (isobar-isothermal holding (IIH)) is studied by quantitative metallography using optical and electron microscopy and image processing computer programs. An analysis of images of the dendritic structure of the alloy, which is formed by the morphology and sizes of γ′ particles, demonstrates an increase in the degree of homogeneity of the alloy with the IIH temperature. The concentration of γ′ particles is determined in the initial alloy and after barothermal treatment (BTT) at temperatures of 1235, 1260, and 1320°C, and the volume fraction of the γ′ phase is found to significantly decrease as the IIH temperature increases. An activation mechanism is suggested for the formation of the nucleation centers of γ′ particles in the γ solid solution, and the activation energy of the dissolution/coalescence of γ′ precipitates is determined. The fraction of nonequilibrium γ′ particles is determined in the material in the initial state and after BTT at three IIH temperatures. The precipitation of γ′ particles is characterized by a bimodal character with the formation of nanoparticles in the initial material and upon BTT at temperatures of 1235 and 1260°C.

Effect of the Thermal Component of Barothermal Treatment on the Carbide Phase Component of a Cast Nickel Alloy

Electron-probe microanalysis, scanning electron microscopy, and optical quantitative metallography are used to study the behavior of the carbides in a corrosion-resistant nickel superalloy during a barothermal action at a fixed pressure and exposure time (190 MPa and 210 min, respectively) and a sequential increase in the barothermal treatment (BTT) temperature from 1235 to 1320°C. The BTT temperature is found to significantly affect the chemical composition of the carbides, and this effect manifests itself in an increase in the titanium, molybdenum, or tungsten content and a decreases in the nickel or chromium content. The granulomteric composition of the carbide skeleton in the alloy is studied: it is found to depend substantially on the temperature of barothermal action. A model is proposed to describe the change in the chemical composition of the carbide structural constituent of a cast nickel alloy at a high pressure and temperature.

Effect of barothermal processing on the structure and properties of a WC-Ni3Al alloy

We have studied the effect of barothermal processing (BTP) on the properties of samples prepared by sintering mixtures of tungsten carbide and aluminum nickelide powders (92 vol % WC + 8 vol % Ni3Al) in vacuum and hydrogen. X-ray diffraction data indicate that BTP influences the unit-cell parameters of the WC and improves the mechanical properties of the alloy. In particular, it increases its fracture toughness, bending strength, compressive strength, and fracture work. BTP at 1450°C and 150 MPa produces the most significant changes in the properties of the alloy.

Microstructure and properties of the eutectic 12Si–Al alloy subjected to barothermal treatment

A binary 12Si–Al alloy is subjected to barothermal treatment (hot isostatic pressing) at a temperature of 560°C and a pressure of 100 MPa for 3 h. This treatment is shown to result in a high degree of homogenization in the chemically and structurally heterogeneous initial alloy. As follows from the morphology of silicon microparticles, barothermal treatment of the 12Si–Al alloy leads to thermodynamically promoted silicon dissolution in the aluminum matrix up to ~10 at % with the formation of a metastable supersaturated solid solution, which decomposes upon cooling. The process of removal of porosity, which results in the formation of a high-density homogeneous material, is analyzed. After a cycle of barothermal treatment, a bimodal size distribution of the silicon phase constituent forms in the 12Si–Al alloy at an average microparticle size of 2.7 μm and an average nanoparticle size of 36 nm. The linear thermal expansion coefficient of the alloy decreases after barothermal treatment, and the microhardness of the eutectic alloy is determined after this treatment. Barothermal treatment of the 12Si–Al alloy is shown to be an effective tool for the removal of microporosity, achieving a high degree of homogenization, and forming a near-optimum bimodal size distribution of the silicon structural constituent, which is comparable with or even exceeds the results of conventional heat treatment of the material at atmospheric or lower pressure.

Effect of barothermal processing on the microstructure and properties of Al–10 at % Si hypoeutectic binary alloy

We describe barothermal processing (hot isostatic pressing) of an Al–10 at % Si binary alloy for 3 h at a temperature of 560°C and pressure of 100 MPa. The results demonstrate that this processing ensures a high degree of homogenization of the as-prepared alloy, which is chemically and structurally inhomogeneous. The morphology of the silicon microparticles in the material suggests that heat treatment of the Al–10 at % Si alloy at 560°C and a pressure of 100 MPa leads to a thermodynamically driven, essentially complete silicon dissolution in the aluminum matrix and the formation of a metastable, supersaturated solid solution, which subsequently decomposes during cooling. We analyze the associated porosity elimination process, which makes it possible to obtain a material with 100% relative density. Barothermal processing of the Al–10 at % Si alloy is shown to produce a bimodal size distribution of the silicon phase constituent: microparticles 1.6 µm in average size and nanoparticles 43 nm in average size. Barothermal processing is shown to reduce the thermal expansion coefficient of the alloy, and the microhardness of the two-phase alloy is determined. Based on the present results, we conclude that barothermal processing is an effective tool for eliminating microporosity from the Al–10 at % Si alloy, reaching a high degree of homogenization, and producing a near-optimal microstructure, which surpasses results of conventional heat treatment of the material at atmospheric and reduced pressures.