Microstructure and Interfaces in TiB 2 /Ti-46Al-3Cr Alloy Composites
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ABSTRACT A ternary titanium aluminide alloy, Ti-46AI-3Cr (at%), was discontinuously reinforced with 5 vol% titanium diboride (TiB 2), by an in-situ synthesis technique, resulting in a two phase y(TiAl) (mainly) and Ct2 (Ti3A1) matrix with randomly dispersed TiB 2 particle. Interfaces of TiB 2 -TiAl were investigated by Analytical Electron Microscopy (AEM) and High Resolution Electron Microscopy (HREM). No consistent crystallographic orientation relationship was observed between TiB 2 particle and TiA1 matrix, and there was no evidence of alloying elements (such as Cr) segregation or interphase formation at the TiB 2 -TiAI interface. HREM results indicated that no semi-coherent interface between TiB 2 and TiAI has been observed. There existed a thin amorphous layer (0.5 to 1.3 nm) at the TiB 2 -TiAI interface, which may accommodate the large lattice misfit across the interface and enhance the interfacial bonding. INTRODUCTION TiB 2 particle reinforced y(TiAl) alloy composites are a new category of intermetallic composites with improved toughness, creep resistance, and oxidation resistance at high temperatures [1]. They usually have a two-phase ot2 + y matrix (Ti 3AI (DO 19 ) and TiAI (LI 0 ) structures, respectively), which contains TiB 2 particles [2]. These composites can be fabricated by the XD (exothermic dispersion) process, during which TiB 2 particles are formed in-situ in the molten metal matrix [3]. Usually in the composites, such as TiB 2 /TiAJ composites expected to be used at high temperatures, the nature of the bonding and the extent of any reaction zone between the particle and the matrix are the crucial aspects to determine the mechanical properties of the composite materials. Therefore, it is important to investigate the atomic structure and chemical compositions across the interface, and the relationships between the microstructure and mechanical properties of the composite materials. Recently, the interfaces of TiB 2 /NiAI [4, 5] and ct-Al 20 3/NiAl [4] have been investigated. A semicoherent interface between a-Al2 0 3 and NiAI was observed, but no consistent semicoherent interface between TiB 2 and NiAl has been observed. Si segregation was detected at the TiB 2/NiAl interface, but no segregation was detected in the a-AI2 0 3/NiA] interface. A thin interface amorphous layer existed between TiB 2 and NiAl. All of the above mentioned results were based on the AEM and HREM observations. The purposes of present study were to characterize the microstructure and interfaces in the in-situ synthesized TiB2/Ti-46AI-3Cr alloy composites. EXPERIMENTAL PROCEDURES In-situ composites of TiAI - 5 vol% TiB 2 were fabricated by using the following procedures: In the molten Ti-B master alloy, ingot was cast by adding Al and alloying element Cr to get the required composition: Ti-46A]-3Cr (at%), then isothermally hot forged at the (a 2 + Y) two405 Mat. Res. Soc. Symp. Proc. Vol. 410 01996 Materials Research Society
phase temperature range. The metallographical samples were prepared by etching in solution: 50 ml H
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