National University of Science and Technology MISiS, Moscow, Russia:
N. O. Korotkova, Post-Graduate Student of the Department of Metal Forming
A. P. Dolbachev, Engineer of the Department of Metal Forming

¹National University of Science and Technology MISiS, Moscow, Russia ; ²Baikov Institute of Metallurgy and Materials Science, Moscow, Russia.

T. K. Akopyan, Research Fellow of the Department of Metal Forming^1,2, e-mail: nemiroffandtor@yandex.ru

Using both thermodynamic calculations and experimental analyses, the phase composition and microstructure of the new TNM-type alloy with the following actual compositions, % (at.): TNM – 40.05 Al, 4.51 Nb and 1.05 Mo have been analysed. According to thermodynamic calculation, equilibrium solidification of the new alloy proceeded via the β-phase, i. e. L → L + β → β, followed by the solid-state transformations, the main of which is the eutectoid reaction α → α₂ + β + γ. The microstructure of the alloy in the as-cast state and after high-temperature vacuum annealing was examined by means of scanning electron microscopy and electron microprobe analysis. Experimental studies have shown that the main structural component is the lamellar γ/α₂-colonies (with a thickness of individual plates of about 300 nm) surrounded by the β- and γ-phase crystals, the total fraction of which does not exceed 30% (vol.). According to the electron microprobe analysis, β-phase contains the largest amount of molybdenum, while the concentration of aluminium is the smallest. In contrast, the γ/α₂-colonies (initial α-phase) and γ-phase have higher aluminium and minimum molybdenum content. The observed results are in a good agreement with the results of the thermodynamic calculation. Experimental quantitative analysis of the phase fraction after vacuum annealing at 1250 ^oC of the new alloy has revealed that the total amount of -phase does not exceed 8% (vol.), what practically corresponds to the base TNM (Ti43.5Al4Nb1Mo) alloy after similar heat treatment. However, the base alloy also contains up to 14% of the γ-phase, which is not observed in the new alloy. After annealing at 1100 ^oC, new alloy contains up to 24% (vol.) β-phase and 4% (vol.) γ-phase, while the base TNM alloy contains up to 14% (vol.) β-phase and 21% (vol.) γ-phase. In general, we can conclude that a reduction of aluminum content from 43.5 to 40.0% (at.) leads to a considerable increase in the amount of β-phase and decrease the γ-phase content. It is necessary to note, the calculated quantitative data of the phase fraction depending on the alloy annealing temperature are far from experimentally obtained data. However, a qualitative analysis of the change the phase relationship depending on the annealing temperature or chemical composition of the alloy, as well as the chemical composition of the phases is available.

This research project has been sup ported by the Russian Foundation for Basic Research (RFBR) grant for the implementation of the research project № 16-33-01108 (мол_а).

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