Microstructure and properties of duplex delta-Al-3(Ti, V)/beta-(Ti, V) alloys

The microstructures of multiphase intermetallic alloys with compositions Al70Ti10V20 and Al62Ti10V28 based on the trialuminide Al3Ti have been characterized, following chill casting and postsolidification heat treatment, using a combination of scanning electron microscopy and transmission electron microscopy (TEM). Evidence of a eutectic reaction of the form L --> delta-Al-3(Ti, V) + zeta-Al8V5, not previously reported in the Al-Ti-V system, has been observed in both alloys solidified at sufficient levels of undercooling. The zeta phase is replaced by metallic beta-(Ti, V) phase during subsequent heat treatment in the range 1073 to 1273 K, and differential thermal analysis (DTA) of samples preannealed at 1173 K revealed an endothermic peak at similar to 1560 K, consistent with equilibrium eutectic melting of the form (delta + beta) --> L. Although the chill-cast alloys retained metastable intermediate high-temperature phases, duplex metallic-intermetallic microstructures, containing uniform fine-scale distributions of metallic beta-(Ti, V) solid solution in a delta-Al-3(Ti, V) intermetallic matrix, have been produced in both alloys during isothermal heat treatments at temperatures in the range 1073 to 1273 K. For both alloys, the bulk Vickers hardness of such microstructures remained in excess of that of binary Al3Ti, while in the Al62Ti10V28 alloy, where the increased volume fraction of beta phase took the form of a near-continuous network within delta matrix, there was evidence arising from indentation tests of a substantial improvement in the cracking resistance compared to both chill-cast ternary alloy and binary Al3Ti.