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MATERIALS SCIENCE
ArticleName Calculation and experimental investigations of structure and phase composition during solidification and thermal treatment of γ-alloy TNM, additionally alloyed with chromium
DOI 10.17580/tsm.2017.05.10
ArticleAuthor Akopyan T. K., Dashkevich N. I., Korotkova N. O., Dolbachev A. P.
ArticleAuthorData

1Institution of Russian Academy of Sciences A. A. Baikov Institute of Metallurgy and Materials Science, Moscow, Russia ; 2National University of Science and Technology “MISiS”, Moscow, Russia:

T. K. Akopyan, Researcher1,2, e-mail: nemiroffandtor@yandex.ru


National University of Science and Technology “MISiS”, Moscow, Russia:

N. O. Korotkova, Post-Graduate Student, Engineer of a Chair “Metal forming”
A. P. Dolbachev, Master's Degree Student, Engineer of a Chair “Casting Technologies and Art Treatment of Materials”

 

Interdepartmental Analytical Center, Moscow, Russia:

N. I. Dashkevich, Area Manager

Abstract

Using the calculation and experimental methods, we analyzed the phase composition and structure after solidification, cooling and subsequent thermal treatment, including hot isostatic pressing (HIP) of the γ-alloy TNM. Thermodynamic analysis, carried out in the Thermo-Calc program, revealed the β-solidification of the investigated TNM+C alloy (at. %: 43.00 Al, 4.46 Nb, 1.06 Mo, 0.83 Cr) and the basic TNM alloy (Ti43.5Al4Nb1Mo0.1B) (i. e. its solidification in equillibrium conditions is finished in the β-area). This analysis was followed by the solid-state transformations: polymorphic β→α transformation, segregation of secondary γ-crystals and eutectoid transformation α→α2+β+γ. Using the Sheil-Gulliver model, we analyzed the non-equilibrium solidification of the investigated alloy. Non-equilibrium solidification proceeded via two peritectic reactions: L+β and L+α→γ, which led to the appearance of the non-equilibrium α- and β-phases in addition to β-phase in the as-cast structure of the alloy. In contrast to the equilibrium conditions, the non-equilibrium crysiallization ended at a lower temperature, which corresponds to non-equilibrium solidus, reaching 1250 oС. Investigations in scanning and transmission electron microscope revealed that the microstructure of the alloy TNM+C in the as-cast state and after vacuum annealing at 1250 oC consists predominantly of lamellar γ/α2-colonies with the thickness of the separate plates of α2- and γ-phases less than 250 nm after annealing. The lamellar γ/α2-colonies are surrounded by areas of β and γ-phase. Qualitative and quantitative analysis of the alloy microstructures after HIP-treatment at 1250 oC and an excess pressure of 170 MPa showed several significant differences compared to the microstructures after vacuum annealing at the same temperature. The alloy structure got coarser, expressed by the eutectic colonies size increase with the thickness of the separate plates of α2- and γ-phases of 300–350 nm. In addition the phase ratio significantly changed. In particular, the amount of γ-phases increased by more than 5 times (from 6 to 34 vol.%) while the amount of α-phase reduced by almost half (82 to 40 vol.%). The higher content of the softest &-globular decreased the alloy hardness from 420 HV to 350 HV.

keywords Titanium aluminide, thermodynamic calculation, phase equilibria, microstructure, hot isostatic pressing, heat treatment
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