Influence of High Pressures on the Solidification of the Al86Ni6Co4Gd2Tb2 Liquid Alloy

Abstract

The purpose of this work is to study the possibility of the formation of new phases in the Al₈₆-Ni₆Co₄Gd₂Tb₂ alloy upon rapid solidification of its high-temperature melt under high pressure. Samples for research were obtained under high pressures of 3, 5, and 7 GPa in a high-pressure chamber of the “toroid” type. The chamber consists of flat hard alloy anvils pressed into steel rings. Alget stone was used as a pressure-transmitting medium. Heating and melting of the sample was carried out by passing an alternating current through the sample placed in a hexagonal boron nitride crucible. High pressure punches served as current leads. The temperature value was calculated on the basis of the thyristor readings, according to the current power. Cooling of the melts was carried out at a rate of 1000 deg/s, the temperature of the melt before quenching was 1500°C. Experimental scheme: pressure setting → pulse heating → holding at a set pressure and temperature → cooling without depressurization to room temperature → high pressure reduction to atmospheric. The microstructure of the samples of the alloy of the eutectic composition Al₈₆Ni₆Co₄Gd₂Tb₂, obtained depending on the quenching temperature (1500°C) and high pressure (3, 5, and 7 GPa), has been investigated by the methods of X-ray diffraction analysis, optical and electron microscopy. Cooling rate 1000 deg/s. Due to the combination of a high solidification rate and mechanical compaction under high pressure, samples of an alloy of the composition Al₈₆Ni₆Co₄Gd₂Tb₂ with a fine structure and high density were obtained. At pressures of 5–7 GPa, the formation of new phases was noted in the alloy: Al₃(Gd/Tb)* (of the Al₃U type), with a primitive cube structure (cP4/2) with a lattice parameter a = 4.285 ± 0.002 Å and Al₈(Ni/Co)₄Gd* (of the Al₈Cr₄Gd type) with a tetragonal structure (tI26/1) with parameters a = 8.906 ± 0.003 Å and c = 5.150 ± 0.003 Å. Studies have shown that the average microhardness of a sample obtained, in particular, under a pressure of 7 GPa, is high (~1700 MPa) due to solid solution and precipitation hardening. This is almost 2 times higher than in the original sample. The results obtained show the fundamental possibility of using the method of solidification of the melt under high pressure to change the level of properties of aluminum alloys used in industry without changing their chemical composition by modifying the structure and changing the composition of the structural components of the sample.