Recrystallization and Grain Growth in NiAl
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I.
INTRODUCTION
Table I.
ALUMINIDE intermetallics, because of their strength, microstructural stability, and oxidation resistance at elevated temperatures, represent potential structural materials for use in advanced energy conversion systems. In polycrystalline form, however, these materials are generally brittle under ambient conditions and, therefore, are not utilized. Aluminides thus remain outside the realm of engineering materials. Brittleness is not an inherent characteristic, however. Single crystals are ductile and polycrystals may be, too, if their grains are fine enough. ~A key to utilization may then be techniques such as rapid solidification processing and thermal-mechanical processing for producing material having fine grains the boundaries of which are pinned by particles. This paper relates to thermal-mechanically induced grain refinement in NiAI, a B2 aluminide which melts at 1638 ~ and which retains complete order to its melting point. 2 Its purpose is to present the kinetics of recrystallization and grain growth of warm-worked, nickel-rich material.
II.
EXPERIMENTAL
A 6-ks ingot, 5 cm in diameter, was obtained from United Technologies, East Hartford, Connecticut. It was prepared from Ni and from AI shot by vacuum induction melting, followed by casting into a copper mold. Its chemical composition (in wt pct) was 30.7 AI, 0.02 Fe, 0.01 Si, 0.001 Mg, 0.0125 Zr, 0.002 C, 0.007 O, 0.0003 N, and ---0.003 each of Ti, Y, Mo, La, B, and Cr, balance Ni; i.e., 49 at. pct A1/51 at. pct Ni. The ingot was extruded to 6.4 mm diameter rod at the General Electric Co., Schenectady, New York. Table I lists the extrusion conditions and the hardness of the product. Two steps were used, one hot and the other warm. Following Step 1, the material was completely recrystallized (initial grain size =0.8 mm), whereas following Step 2 it was in a deformed state in which the grains were highly elongated along the extrusion direction and 'swirly' in appearance on transverse sections. Material in this state was used in the study of recrystallization. Slices =3 mm thick were taken from the 6.4 mm diameter rod and annealed in air. Subsequently, the specimens G. R. HAFF, formerly Masters Student, Thayer School of Engineering, is now Engineer, ODECO, New Orleans, LA, E.M. SCHULSON is Professor with Thayer School of Engineering, Dartmouth College, Hanover, NH 03755. Manuscript submitted December 8, 1981. METALLURGICALTRANSACTIONS A
Step 1 2
Extrusion Conditions and Hardness of the Extruded NiAI Rod
Extrusion Product Extrusion Area Temp. Reduction Diameter Grain Size* Hardness (~ Ratio (ram) (t~m) (Rc) ! 090 7:1 19 25 27 500 8:1 6.4 -39
'Determined by the method of linear intercepts and refers to the average spacing between grain boundaries on metallurgical sections
were mounted in bakelite, metaHographically examined, and measured for hardness. Upon sufficient annealing, recrystallization was observed in the form of lightly colored, equiaxed grains within a darkly etching, 'swirly' matrix of deformed grains. The volume fr
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