Microstructure/Processing Relationships in High-Energy High-Rate Consolidated Powder Composites of Nb-Stabilized Ti 3 Al
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MICROSTRUCTURE/PROCESSING RELATIONSHIPS IN HIGH-ENERGY HIGH-RATE CONSOLIDATED POWDER COMPOSITES OF Nb-STABILIZED Ti3 A1 + TiAl C. Persad, B.-H. Lee, C.-J. Hou, Z. Eliezer and H.L. Marcus, Center for Materials Science and Engineering, The University of Texas at Austin, Austin, Texas 78712. 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 Is" 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 Ti 3A1 is transformed to alpha two with an intermediate beta two phase. In composite blends of TiAl powders mixed with Nb-stabilized Ti 3Al powders a 10 gtm 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. INTRODUCTION The Ti-Al system offers a rich variety of phases and microstructures. Many of its
equilibrium phases are disordered solutions with wide composition ranges, or simple ordered structures based on the disordered solutions [1]. Non-equilibrium processing such as the production of powders and ribbons by rapid solidification alters the phase boundaries [2,3]. Alloying additions further complicate the precise prediction of phase fields and it becomes practical to adopt the use of a qualitative Ti-Al pseudo-binary [4]. At the focus of alloy design and development effort remain the binary intermetallics: TiAl (gamma) and the Ti3 A1 (alpha two). Lipsitt has reviewed the history, progress, and potential uses of these materials in aircraft turbine engines [5,61. The TiA1 intermetallic is of lower density (3.76 vs 4.15- 4.70 gm/cm 3) but is also less ductile at room temperature (1-2 vs 2-5 percent elongation) than the Ti3A1 [5]. The density advantage of TiAl is enlarged when the influence of common ternary additions such as Nb, Mo, and Ta to Ti3 Al is taken into account. Powder-based composites of Ti3Al + TiAl have therefore been designed to take advantage of the ductility of the Ti 3A1 as a matrix and the low density of the TiAl as a dispersed constituent. In addition to the increased specific strength and potential thermal stability, these composites attempt to exploit the lattic
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