Microstructure and mechanical properties of VT25U/TiB composite prepared in situ by casting and subjected to hot forging

R. Gaisin1, V. Imayev1, R. Imayev1
1Institute for metals superplasticity problems of Russian academy of sciences
The work was devoted to study of microstructure and mechanical properties of discontinuously reinforced composite materials based on Ti/TiB fabricated in situ using common casting. A two-phase titanium alloy VT25U was taken as a matrix material. The boron addition in an amount of 1.5 wt.% corresponding to 8 vol.% TiB was found optimal. To obtain axially aligned TiB-whiskers and the most creep resistant matrix condition, isothermal two-directional hot forging leading to drawing of the workpiece along third direction followed by annealing in the β and α+β temperature range was performed. The mechanical properties of the novel VT25U/TiB composite were studied in tension parallel to the predominant orientation of the TiB-whiskers. Along with tensile tests, creep tests were carried out and the obtained properties were compared with those of the matrix alloy subjected to near the same forging and heat treatment. The composite material demonstrated appreciably higher (by 22-50%) strength and creep resistance at T=550-600°C in comparison with those of the matrix alloy while retaining acceptable ductility (δ=3% at room temperature). The density normalized strength of VT25U/TiB at T=500-700°C was higher than that of both the matrix VT25U alloy and all conventional titanium alloys with the operating temperature of T=500-600°C. Microstructural examination showed high adhesion strength of interfacial boundaries between the matrix and the TiB-whiskers, which is retained up to T=700°C that correlates with the mechanical properties. Fracture surface observations suggest that the main failure mechanism of the VT25U/TiB composite is fracture of TiB-whiskers followed by ductile failure of the matrix.
Received: 15 May 2017   Revised: 17 May 2017   Accepted: 17 May 2017
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