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

Abstract

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 %.