Crystal growth and high-temperature properties of intermetallic compound nickel aluminide

Investigations have been performed to the studying single crystal growth mechanism of intermetallic compound Ni3Al and to understand its mechanical behavior in single crystal and polycrystalline form at elevated temperatures under an oxidizing environment. Two crystal growth techniques, electron beam zone melting and Bridgman method, are combined together to grow single crystals in this work. The effects of the concentration of aluminum (23.5 at.%Al, 24 at.%Al, and 25 at.%Al, superheat of melt, and growth rate on the crystal growth process have been studied. Different compositions of Ni3Al and crystal growth conditions produce different microstructures. Formation of single crystals of Ni3Al with 23.5 at.%Al, is easier than for the other two alloys. Single crystal formation always accompanies with dendrite growth for the alloys with 23.5 at.%Al, and 24 at.%Al. Crystal growth of stoichiometric Ni3Al (25 at.%Al, inclines towards facet growth. Low index faces are preferred growth orientations for these three alloys. Optimum superheat and growth rate for growing crystals have been obtained. Ni3Al with 23.5 at.%Al, has the highest optimum superheat range among the three. The crystal growth mechanisms for these different Ni 3Al alloys have been discussed comprehensively.;Single crystal samples were homogenized before mechanical tensile tests. Polycrystalline materials were made from deformed and recrystallized single crystal materials. This has excluded the effect of impurities on grain boundaries. Mechanical tensile tests were accomplished under dry argon environment. The results of mechanical property tests on single crystal and polycrystal Ni 3Al indicate that preoxidization does not alter the anomalous yield behavior of Ni3Al. This evidence implies that dislocation cross slip mechanisms still apply for oxidized materials. There is no obvious discrepancy in strength and ductility between preoxidized and non-oxidized single crystal of Ni3Al. However, the strength of non-oxidized samples is slightly better than that of preoxized ones. On the contrary, ductility of non-oxidized samples is slightly lower than that of preoxidized ones. This indicates that Ni3Al single crystal will keep its performance under an oxidizing environment as in a non-oxidizing environment at high temperature. The mechanical tensile test results for polycrystalline materials show that oxide scales have little effect on the strength and ductility of Ni3 Al in the test temperature range. Neither NiO nor Al2O 3 or NiAl2O4 at grain boundaries is considered to have contribution to the properties.