Microstructure and Mechanical Properties of RF Plasma Sprayed Titanium Aluminide Sheet
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MICROSTRUCTURE AND MECHANICAL PROPERTIES OF RF PLASMA SPRAYED TITANIUM ALUMINIDE SHEET J.A. GRAVES, S.J. VEECK, G.W. WOLTER AND B.K. ZUIDEMA Howmet Corporation, Applied Research Division, 1500 S. Warner Street, Whitehall, MI 49461. ABSTRACT Alloys based on intermetallic compounds such as Ti 3Al (alpha-two), offer excellent elevated temperature strength and creep resistance. These same properties, however, combined with the susceptibility of the materials to interstitial element contamination, limit the use of "conventional" thermomechanical processing (TMP) in the production of sheet product forms. Plasma spraying of intermetallic alloys offers an alternative, near net shape processing path to produce homogeneous sheet products, having fine grained, equiaxed microstructures. This paper details the microstructural evolution and associted mechanical properties of Ti-24Al-llNb (at.%) processed to sheet form using RF plasma spraying. The response of the microstructure/tensile properties of the plasma sprayed alpha-two to various controlled heat treatments is discussed. INTRODUCTION Alloys based on the intermetallic compound Ti3Al (alpha-two) are well suited to aerospace applications due to their relatively low density and retention of strength/creep resistance to temperatures approaching 650°C [1]. The processing of titanium aluminides into useful forms such as sheet product, remains a significant technical challenge however, due to the high temperatures required and the sensitivity of the material to contamination. Induction plasma spraying offers an approach for net shape processing which is suitable for the fabrication of monolithic and continuous fiber reinforced composites in sheet form. While the interstitial element (e.g. oxygen) content of the plasma sprayed sheet is higher than typical ingot metallurgy products, the resulting fine grained microstructure is uniform in chemistry with isotropic mechanical properties. The use of an induction plasma reactor allows the spraying of relatively coarse powder (+200 micrometer), thereby limiting the increase in interstitial contamination during processing to 100-300 ppm. This paper describes the microstructure and mechanical properties of a representative alphatwo alloy processed into sheet form using induction plasma spray techniques. EXPERIMENTAL APPROACH A representative alpha-two alloy of nominal composition Ti-24Al-l1Nb (at.%) was selected for evaluation. While this composition can yield a variety of metastable microstructures, recent results indicate that under equilibrium conditions this alloy comprises an ordered hexagonal (D01i structure) alpha-two phase in combination with a Nb rich body centered cub c (B2 structure) beta phase [2,3]. Powder produced by the plasma rotating electrode process (PREP) was obtained from Nuclear Metals, Inc., having a measured composition of Ti24.1Al-10.8Nb (at.%). The interstitial content of the starting powder was measured as 975 0, 79 N, 18 H and 13 C (ppm by weight). The powder was sprayed using a radio frequency induction plasma
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