Thermo Mechanical Treatment of Nickel and Titanium Aluminides

Abstract

Polycrystalline Ni3Al and TiAl are attractive materials for high temperature structural applications due to their stability in oxidizing and sulphidizing environment upto700 0 C. They possess significantly higher specific stiffness and similar specific strength as that of super alloys. Hence, these materials can replace super alloys for high temperature applications (~900°C). TiAl has lesser density and can be used for reducing component weight up to 50% and suitable for aerospace and automobile (high performance vehicles) sectors. The major difficulty for putting Ni3Al for engineering applications is its extremely low ductility and inter-granular fracture at ambient temperatures. TiAl, apart from the said brittleness it also suffers from high temperature corrosion. However the brittleness of these aluminides can be reduced by micro-alloying and by subjecting them to Thermo Mechanical Treatments, TMT. This paper deals with the recrystallization studies on nickel aluminides, deformed to different extents by rolling. The average grain size dependence with the % elongation is evaluated in the grain size range of 10-35micron. For the nickel aluminide deformed for 50% by rolling, the variation of resistivity and hardness with annealing time is determined. The homogenized TiAl samples were cold worked and annealed at 1000 0 C. Since the aluminide suffers from low ductility at room temperature, an arbitrary parameter, electrical resistivity, was chosen. Corresponding hardness values were also obtained. Finally a qualitative determination of ductility was made by studying the flow behavior of alloy around the hardness indentation. Thus a correlation was developed between resistivity, hardness and ductility values. It was then to some extent possible to investigate the TMT cycles on the microstructure and hence on the ductility of the TiAl without going for the actual tensile tests.