Structure formation and mechanical properties of heat-resistant alloy based on titanium aluminide under heat treatment

Аuthors

Umarova O. Z.^*, Pozhoga V. A.^**, Buranshina R. R.^***

Moscow Aviation Institute (National Research University), 4, Volokolamskoe shosse, Moscow, А-80, GSP-3, 125993, Russia

*e-mail: umarovaoz2014@gmail.com
**e-mail: vasi89@mail.ru
***e-mail: buranshina2012@yandex.ru

Abstract

Titanium intermetallic Ti₂AlNb (orthorhombic phase) based allows are promising materials for gas-turbine engine elements manufacturing operating at the temperatures of 650 — 700°С instead of heat-resistant steel due to their high specific properties, and also intermetallic super- - and  -alloys possessing low technological plasticity.

Orthorhombic alloys phase composition and structure strongly affect the final mechanical and technological properties of semi-finished products, which can be controlled by certain of thermo-mechanical and thermal treatment modes. Thus, the purpose of this study consisted in studying the effect of heat treatment on the structure and properties of heat-resistant alloy based on Ti₂AlNb titanium intermetallic.

In this work, the effect of various heat-treatment modes on the structure, hardness and mechanical properties of the VTI-4 alloy based on Ti₂AlNb titanium aluminide was studied. The samples were subjected to heat treatment, X-ray diffraction and metallographic analyses. Besides, the hardness of samples was measured by Rockwell method, and mechanical tensile tests were carried out at room temperature.

Based on conducted studies, data on the temperature ranges of phase regions in the alloy was obtained, and a scheme for the two-stage heat treatment was designed. It was stated, that the structure and hardness of the alloy are greatly affected by the cooling rate between the first (high-temperature) and the second (low-temperature) treatment stages. Increasing of the cooling rate from 0.01 K/sec to 10 K/sec resulted in fine-dispersed orthorhombic phase formation; the alloy hardness increased by 5 HRC units, and the strength grew by 100 MPa while maintaining a satisfactory level of 4 — 6% for the plastic properties. The cooling rate after the low-temperature stage had no effect on the alloy structure and hardness.

It was shown also, that temperature reduction of isothermal holding in the low-temperature stage by 50°C resulted in the tensile strength increase by 80 MPa, and plasticity decrease by 3%.

Designed VTI-4 alloy heat treatment modes on the example of rod semi-finished product allowed form in the alloy structure with different size of structural components. The obtained results allow also predict changes in the strength and plastic properties of other types of VTI-4 alloy semi-finished products according to the need for further forming operations.

Keywords:

titanium aluminide, Ti2AlNb, orthorhombic alloy heat treatment, titanium aluminide structure and phase composition

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