Powder processing of ductile materials for high pressure studies: application to intermetallic alloys
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ABSTRACT Ductile materials are difficult to powderize for use in high pressure work. The potential of different techniques (gas-atomization, mechanical alloying, ball milling and subsequent annealing) for preparing suitable powders of some aluminides is investigated. Compression of Ti46 AI54 and NiAl prepared by these methods yields equation of state parameters in good agreement with determinations by other methods.
INTRODUCTION Compression of polycrystalline materials even in hydrostatic or quasi-hydrostatic pressure media creates high non-isotropic stresses at grain contacts independent of the pressure device [1]. The stresses can cause large deviations from equations of state (representing hydrostatic volume compression) and extensive plastic deformation above the yield point of ductile samples (work hardening). Hydrostatic compression studies of powders thus require them to consist essentially of single crystals (and to be embedded in a hydrostatic pressure medium). For ductile materials, such powders may be difficult to produce. Recent advances in powder processing make it possible to prepare micro- or nanocrystalline powders of ductile metals and alloys by mechanical milling [2,3] and microcrystalline particles by gas-atomization [4]. Annealing or recrystallization of milled samples may produce powders of micrometer-sized particles consisting of just a few or only a single crystal. This study investigates the potential of these processing techniques for the production of samples of some intermetallic alloys (TiA1, NiAI, Ni 3AI) suitable for high pressure studies.
BACKGROUND METAL ALUMINIDES The nickel and titanium aluminides are characterized by low weight to strength ratios, high melting points and good oxidation resistance [5]. They are of thus of interest as structural materials for high temperature applications in the aerospace industry and for stationary gas turbines of electric power plants. Their usefulness is limited by low tensile ductility and poor fracture toughness at room temperature and inadequate creep resistance at high temperatures. Experimental and theoretical studies of microalloying have attempted to find ways to improve these properties. Hydrostatic compression studies are of interest since they simulate the size effect of alloying elements and allow for a determination of the bulk modulus for comparison with theoretical calculations. The very properties that make the aluminides of interest for industrial applications create problems in powder preparation and in high pressure studies. These properties are their 161
Mat. Res. Soc. Symp. Proc. Vol. 499 01998 Materials Research Society
high elastic stiffness, strong elastic anisotropy and their susceptibility to shear deformation by introduction of defects (stacking or twin faults, anti-phase boundaries) because of their dense packing. It is for these reasons that high pressure studies of these materials are lacking. POWDER PROCESSING FOR PREPARING MICRO- AND NANOCRYSTALLINE SAMPLES The gas-atomization process belongs to the category of
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