The study of the Fe and Ni effect on the temperature of the VT14 alloy superplastic moulding

Metallurgy and Material Science


DOI: 10.34759/vst-2023-1-217-226

Аuthors

Postnikova M. N.*, Kotov A. D.**

National University of Science and Technology "MISIS", 4, Leninsky Prospect, Moscow, 119991, Russia

*e-mail: sitkina.m@misis.ru
**e-mail: kotov@misis.ru

Abstract

The complex goal of the research consisted in reducing the superplastic moulding temperature of the VT14 alloy by alloying with β-stabilizers with a high diffusion coefficient, which effectively lower the temperature of the β→α phase transformation to achieve optimal phase ratio under low temperature conditions. Fe and Ni wielding the high dif-fusion coefficient in the β-phase were selected as alloying elements. As the result, the effect of alloying by the β-stabilizers of various concentration on the microstructure evolu-tion, indicators of superplasticity, as well as on mechanical properties at the indoor temperature are being studied.

The alloys under study were additionally alloyed by the minor additive of boron (up to 0.1 wt.%) to enhance mechanical and technological characteristics by the fine crushing of the grain structure in the presented of the dispersed ToB particles while the molten crystallization, as well as while thermo-mechanical treatment. The results of the microstructure evolution analysis while annealing in the temperature range of 625–850 °C revealed that the Fe content growing from 0.5 to 2% and Ni from 0.5 to 1.8% led to the β-phase volume fraction growth, and, hence, shifting of the optimal temperature range to the lower temperatures.

Analysis of the uniaxial tension tests results with 1´10—3 s—1 velocity in the temperature range of 625–775 °C revealed that due to the β-transus temperature reduction and dif-fusion coefficient increase, the increase in the Fe and Ni content significantly improved the superplasticity indicators. Superplastic deformation of the modified alloys was characterized by the high values of the strain rate sensitivity coefficient m = 0.45–0.5 for the alloys with Fe and m = 0.5–0.6 for the alloys with Ni, as well as with high relative elongation of 500–1000% at the twice as low flow stress compared to the alloy without addi-tives. It was demonstrated as well that alloying by 0.9% Ni and 0.5% Fe was quite enough for ensuring high relative elongations and indicator m = 0.5 at the deformation temperatures of 700–775 °C, and temperature reduction to 625 °C, required concentration increasing of Ni up to 1.8% and Fe up to 2%.

Alloying allowed increasing the level of mechanical properties at the indoor temperature after the superplastic deformation at the temperature of 775 °C with the rate of 1´10—3 s—1. The increase of the β-stabilizers content contributed to the strength margin and yield stress margin by 100–250 MP, as well as minor plasticity reduction relative to the industrial VT14 alloy.

As the result, optimal alloys contents, wielding increased strength properties at minor plasticity reduction, and characterized by the high superplastic indicators under conditions of the lower temperatures (625—700°C) were proposed. These alloys are Ti-4Al-3Mo-1V-0,9Ni-0,1B and Ti-4Al-3Mo-1V-0,1B-0,5Fe-0,1B.

Keywords:

high-trength titanium alloys, VT14 alloy, superplasticity of titanium alloys, superplastic deformation temperature, β-stabilizers in titanium alloys

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