Toughening of TiAl composites by NiAl additions

Abstract

Intermetallic alloys based on the TiAl or NiAl phases are promising as potential materials for high temperature structural applications. Limited ductility and toughness of these materials at ambient temperatures, however, has hampered their acceptance. Only modest improvements have been achieved by alloy and microstructural control, even though extensive investigations have been carried out in these areas. Evidence of room temperature ductility at unusually fine grain sizes has been observed in intermetallics and this could lead to further improvements. Mechanical alloying is a method known to produce ultrafine-grained materials. Although it is common to mechanically alloy elemental powders, there are few studies where prealloyed powders have been mechanically alloyed. In this study ultrafine-grained composite intermetallics are produced by milling mixtures of prealloyed titanium aluminide and nickel aluminide powders. Milling at cryogenic temperatures promotes fracture and suppresses inter-particle solid state welding. The low temperatures also suppress the formation of new phases and minimize the contamination that inevitably occurs during room temperature milling. Retaining the ultrafine grain size is difficult during consolidation. The conventional method for limiting grain growth is to incorporate inert dispersoids. Although this has been found to increase strength, a severe reduction in ductility generally results. An alternative is tomore » produce a multiphase material to limit grain growth. Ductility should not be affected to the same degree as for materials with fine brittle dispersoids. This is the rationale for milling and subsequently consolidating a mixture of two different prealloyed intermetallic phases.« less