Abstract
Microstructure and mechanical properties of two TiAl-based alloys with nominal composition Ti-42.6Al-8.7Nb-0.3Ta-2.0C and Ti-41.0Al-8.7Nb-0.3Ta-3.6C (in at.%) were investigated and compared. The alloys were prepared by vacuum induction melting, followed by centrifugal casting. The as-cast samples were subjected to hot isostatic pressing and heat treatment consisting of solution annealing in β (Ti-based solid solution) phase field, cooling at a constant rate and stabilization annealing. The microstructure of the alloys consists of α2 (Ti3Al) + γ (TiAl) lamellar grains, single γ phase, coarse Ti2AlC particles, and irregular shaped α2 phase. The increase in the content of C at the expense of decreasing Al in the studied alloys affects solid-state phase transformation temperatures and leads to a decrease in size of grains and primary Ti2AlC particles, increase in the volume fraction of reinforcing carbide particles, decrease in the volume fraction of lamellar colonies, and widening of the grain boundaries. Long-term ageing at 800 °C has no effect on the grain size but leads to the formation of Ti4Al3Nb particles and increase in interlamellar spacing. The Vickers hardness, microhardness of lamellar grains, indentation nanohardness, and elastic modulus of the boundary γ phase decrease during ageing. The Ti-42.6Al-8.7Nb-0.3Ta-2.0C alloy shows improved creep resistance compared to that of Ti-41.0Al-8.7Nb-0.3Ta-3.6C and some reference TiAl-based alloys at a temperature of 800 °C and applied stress of 200 MPa.
Highlights
Lightweight intermetallic alloys based on ternary TiAl-Nb system are of great interest for applications in the aerospace, power engineering, and automotive industries due to their high specific strength, high melting temperature, good high temperature creep strength, and oxidation resistance [1,2]
This study aims to investigate and compare the microstructure and some mechanical properties of two TiAl-based alloys with nominal composition Ti-42.6Al-8.7Nb-0.3Ta-2.0C and its derivative
(in at.%) designated as C20 and C36, respectively, were prepared by vacuum induction melting in graphite crucibles followed by a centrifugal casting into a graphite mold
Summary
Lightweight intermetallic alloys based on ternary TiAl-Nb system are of great interest for applications in the aerospace, power engineering, and automotive industries due to their high specific strength, high melting temperature, good high temperature creep strength, and oxidation resistance [1,2]. Depending on chemical composition and applied processing techniques, TiAl-based alloys can be produced with different types of microstructure such as fully lamellar, nearly lamellar, duplex, and near gamma [3,4,5]. The duplex and near gamma alloys are characterized by higher room temperature ductility, tensile strength, and longer fatigue life than fully or nearly lamellar ones [6]. Better creep resistance of fully or nearly lamellar TiAl-based alloys has been related to highly anisotropic lath structure and reduction in interlamellar spacing [7,8]. The addition of carbon up to about 0.8 at.% contributes to precipitation strengthening through the formation of Molecules 2020, 25, 3423; doi:10.3390/molecules25153423 www.mdpi.com/journal/molecules
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