In-Situ Synthesis of Particle Dispersed Nanocrystalline NiAl By Cryomilling
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IN-SITU SYNTHESIS OF PARTICLE DISPERSED NANOCRYSTALLINE NiAl BY CRYOMILLING
Benlih Huangt, J. Vallonelt, C. F. Kleintt, and M. J. Luton"t I Department of Materials Science and Eng., Rutgers University, Piscataway, NJ 08854. It Corporate Research, Exxon Research and Engineering Company, Annandale, NJ 08801.
ABSTRACT The present research was carried out to study the cryogenic synthesis NiAI from elemental powders with the view of forming particle dispersed NiAI. It was found that nanocrystalline NiAl with a crystallite size in the range 5nm to 10nm is produced during cryomilling. Additionally, the nanocrystalline NiAI maintains the fine crystallite size when annealed at high temperatures. It is thought that the resistance of the microstructure to coarsening is due to the presence of nano-scale particles of AIN formed by reaction between milling medium, liquid nitrogen, and aluminum. The transformation of Ni and Al to ordered NiAI was studied using x-ray diffraction. This showed that cryomilling not only produces a solid solution but also can induce in-situ ordering of stoichiometric NiAI. The B2 NiAI structure forms during milling after the elemental powder blend is cryomilled for about 40 hours. It also forms when the "mechanical" solid solution is annealed at room temperature for about twelve hours.
INTRODUCTION Mechanical alloying is a ball milling process by which elemental or pre-alloyed powders are rolled, flattened, fractured and welded to synthesize novel materials. The process is carried out either in an attritor, a shaker or a conventional ball mill. Various intermetallics and amorphous materials have been synthesized. Recently, cryomilling, or cryogenic slurry mechanical alloying process was introduced to decrease the grain size of oxide dispersion strengthened alloys [1]. The rationale for cryogenic processing is that decreasing the milling temperature leads to an increase in the strain energy stored in the material, which, in turn, increases the density of recrystallization nuclei during subsequent annealing. In this way significant grain refinement is achieved. Cryomilling in liquid nitrogen has been shown to cause the in-situ formation of aluminum nitride or oxy-nitride particles in aluminum and dilute aluminum alloys[l]. Here the dispersoids are found to be typically 2 to 10 nm in diameter and with a mean spacing 50 to 100 rn. The consolidated alloys have an extremely fine grain size, typically of the order of 50 nm. Due to its high melting temperature, 1638°C [2], relatively low density, 6 g/cm 3 [3] and resistance to high temperature oxidation, the intermetallic compound NiAl is of interest for aerospace applications. Considerable effort has been made to improve the mechanical properties. Recently, Whittenberger et al [4] employed cryomilling to disperse yttria a prealloyed NiAl matrix. Although 0.2 % of yttria was added, the predominant second phase present in the consolidated alloy was AlN. In these alloys about 10 vol.% of AIN is present. Compression test between 927'C and 1127 0 C of extruded
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