Formation mechanisms and mechanical properties of interfacial intermetallic compounds

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In the Ti-Al binary system, three different intermetallic compounds, namely Ti_3Al, TiAl and Al3Ti are formed. These three intermetallic compounds have potential as new light-weight high-temperature materials and thus their mechanical properties have been intensively investigated. In particular two-phase materials composed of TiAl and a small volume fraction of Ti_3Al have been attracting considerable attention. When Ti-Al alloys with nearly equiatomic compositions are cast, the alloys exhibit the lamellar structure composed of TiAl and Ti_3Al lamellae, The lamellae of Ti_3Al is often thin and sandwiched between the two neighboring TiAl lamellae. The TiAl/Ti_3Al lamellar structure can thus be used as a useful vehicle for studying effects of interfacial compound on the mechanical properties of interface. The Ti_3Al phase is much stronger than the TiAl phase and the large difference in strength between TiAl and Ti_3Al results in the large strain accumulations and incompatibilities at the TiAl/Ti_3Al interface. Thus, the TiAl/Ti_3Al/TiAl sandwich structure is brittle when it is stressed along the direction perpendicular to the interface. Various elements were added to TiAl/Ti_3Al two-phase alloys and their partition behavior between the two constituent phases was investigated to find elements which reduce the difference in strength between TiAl and Ti_3Al. Tin is found to exhibit such an beneficial effect on the interface. However, the results of the mechanical testing of Sn-bearing TiAl/Ti_3Al alloys were not as good as expected. This is because the Sn-bearing Ti_3Al phase exhibits a lower stringth than the binary Ti_3Al phase but it is still brittle.Interface between MoSi_2 and SiC was also investigated and the two phases were found to be matched in an orientation relationship which was different from that reported previously.