Investigation of the Young's modulus of TiB needles in situ produced in titanium matrix composite
Introduction
Titanium matrix composites have been investigated over a long time for their aerospace and aeronautics applications. Most of the studies concerned titanium reinforced SiC monofilaments. These monofilaments exhibit a high specific strength and Young's modulus. The main drawbacks are their relatively high cost as well as the mechanical and chemical instabilities with the titanium matrix during the fabrication of the composite or in service. The chemical instability promotes a reaction zone at the fiber/matrix interface and the coefficient of thermal expansion mismatch promotes residual stresses in the brittle reaction zone. These deficiencies have limited the field of applications for these composites. In this study, another type of reinforcement for titanium-based matrices is evaluated. TiB has been identified as possessing the most appropriate balance of thermochemical stability, good mechanical properties, and thermal expansion [1], [2], [3]. However, the mechanical properties of TiB are not well known and available only by analysis of two-phase materials. One study assumed that TiB Young's modulus is the same as ZrB2 (550 GPa) [4], [5]. Another adopted 371 GPa, which is a value measured by vibration impulse excitation on Ti/TiB composite [6].
In this paper we propose one Ab initio approach, based on the calculation of the electronic structure of TiB, to predict the Young's modulus of the TiB whiskers. The ASW method is used. Ti/TiB composites are produced by a powder metallurgy route. The Young's modulus of TiB is experimentally evaluated via micro-indentation tests performed directly on the TiB whiskers, and from bend tests on the Ti/TiB composites. Computational and experimental values of the Young's modulus of TiB will be compared. The chemical and the mechanical stability of the Ti/TiBW composites is verified after some long duration heat treatments at high temperature (50 h, 1100 °C).
Section snippets
Calculation of the electronic structure of TiB
The calculation of the electronic structure of TiB was undertaken using the ab initio self-consistent Augmented Spherical Wave (ASW) method [7] which gives rise to describe the electronic properties of a material starting from those of its atomic constituents. The calculations are based on the well established density functional theory in which the effects of exchange and correlation are treated in the local density approximation within the scheme of Von Barth and Hedin and Janak [8], [9].
Experimental
Ti/TiB composites were produced by a powder metallurgy route involving (1) blending, (2) consolidation by hot unidirectional pressure (900 °C, 80 MPa, 120 min), and (3) annealing at 1100 °C with different durations to form TiB needles by reaction between one boron precursor and the Ti matrix. Source materials consisted of fine titanium powder (<44 μm) and very fine TiB2 powder (<10 μm) for the boron source.
The microstructure evolution of the Ti/TiB composites was characterized by measuring the
Conclusion
It was demonstrated from this study that the TiB compound could be a very good candidate for the reinforcement of the titanium matrices (Ti or Ti-64). This compound has a 20% higher stiffness than the SiC and exhibits a very good chemical stability at very high temperature with titanium without any reaction zone. The composite can be prepared by a powder metallurgy route. TiB2 powders are cold blended with the titanium or titanium alloy powders. A first step of hot compaction is necessary to
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