Microstructure and Mechanical Properties of In Situ TiB/TiC Particle-Reinforced Ti-5Al-5Mo-5V-3Cr Composites Synthesized
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TION
INTEREST in the fabrication of discontinuously reinforced titanium matrix composites (TMCs) has grown steadily in recent years due to their outstanding properties, such as high specific strength, hardness, and stiffness combined with heat and/or corrosion resistance.[1–4] Previous investigations have shown that titanium and its alloys can be successfully reinforced by ceramic particles.[5] In particular, the literature reports that the most effective strengthening can be achieved through particle reinforcement with titanium carbide particles TiCp and whisker-shaped titanium boride TiBw.[5–13] TMCs with particle reinforcement STEFFEN GRU¨TZNER, LUTZ KRU¨GER, and MARKUS RADAJEWSKI are with the Institute of Materials Engineering, TU Bergakademie Freiberg, Gustav-Zeuner-Str. 5, 09599 Freiberg, Germany. Contact e-mail: Steff[email protected] CHRISTIAN SCHIMPF is with the Institute of Materials Science, TU Bergakademie Freiberg, Gustav-Zeuner-Str. 5, 09599 Freiberg, Germany. INES SCHNEIDER is with the Bundeswehr Research Institute for Materials, Fuels and Lubricants (WIWeB), Institutsweg 1, 85435 Erding, Germany. Manuscript submitted September 14, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
based on TiCp and TiBw exhibit excellent mechanical properties such as high stiffness,[6,14,15] hardness [6,15,16] and/or wear resistance.[17,18] Such ceramic reinforcements can be created in situ within the matrix by means of powder metallurgy processes. One important advantage of the in situ method is that the reinforcement surfaces do not contain contaminants such as oxygen or nitrogen, thus facilitating stronger bonding at the matrix/reinforcement interface.[5] Due to the high reactivity of titanium with boron, carbon or nitrogen, reactive powders such as B4C, TiB2, CrB, MoB, BN, or even elemental boron and carbon can be added for in situ synthesis of the reinforcing particles.[11] Common fabrication techniques include reactive hot pressing (RHP), hot isostatic pressing (HIP), spark plasma sintering (SPS), and hot extrusion. In the case of B4C, for instance, and due to the high temperatures required during processing, the following exothermic reactions are evident[5]: 5Ti þ B4 C ! 4TiB þ TiC
½1
3Ti þ B4 C ! 2TiB2 þ TiC
½2
Thermodynamically, the Gibbs free energy DG of reaction (1) is more negative than that of reaction (2) in the temperature range between 1200 K and 2200 K. Hence, TiB rather than TiB2 is formed preferentially according to reaction (1).[5] The detailed chemical reactivity and microstructural evolution between pure titanium and B4C in the temperature range between 1000 C and 1600 C are investigated by Mogilevsky et al.,[19] Brodkin et al.,[20] Zhao et al.,[21] and Vallauri et al.[22] Their studies concur in showing that, depending on the temperature, the formation of distinct reaction layers with defined sequences is evident. In the temperature range between 1000 C and 1300 C, in particular, the resulting layer sequence within the Ti/B4C reaction layer is described as Ti-TiC0.5-TiB-TiB2
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