Microstructure/Processing Relationships in High-Energy High-Rate Consolidated Powder Composites of Nb-Stabilized Ti₃Al + TiAl

Abstract

A new approach to powder processing is employed in forming titanium aluminide composites. The processing consists of internal heating of a customized powder blend by a fast electrical discharge of a homopolar generator. The high-energy high-rate “1MJ in 1s” pulse permits rapid heating of an electrically conducting powder mixture in a cold wall die. This short time at temperature approach offers the opportunity to control phase transformations and the degree of microstructural coarsening not readily possible with standard powder processing approaches. This paper describes the consolidation results of titanium aluminide-based powder composite materials. The focus of this study was the definition of microstructure/processing relationships for each of the composite constituents, first as monoliths and then in composite forms. Non-equilibrium phases present in rapidly solidified TiAl powders are transformed to metastable intermediates en route to the equilibrium gamma phase. The initial single phase beta in Nb-stabilized Ti₃Al is transformed to alpha two with an intermediate beta two phase. In composite blends of TiAl powders mixed with Nb-stabilized Ti₃Al powders a 10 μm thick interfacial layer is formed on the dispersed TiAl. Limited control of post-pulse heat extraction prevents full retention of the rapidly solidified powder microstructures.