Part I – January 1969 - Communications - Elastic Constants of Ni3AI Between 80° and 600°K

AN intermetallic compound, Ni3A1, has an ordered fcc structure of type Ll2, and shows peculiar dependence of the yield stress upon temperature; i.e., the yield stress increases by a factor of six upon raising the temperature from 77" to 900K1, 2 This compound is technologically important, since many nickel-base superalloys owe their high-temperature strength to the peculiar yield stress behavior of Ni3Al. The only study of the elastic behavior of this compound was made by Davies and Stoloff,1 who determined Young's modulus of poly crystalline Ni3A1 between 300" and 730°K. The determination of single-crystal elastic constants of Ni3A1 is therefore of interest in connection with dislocation dynamics and is reported in this paper. A single-crystal specimen, 1 by 1 by 3 cm, was cut from an ingot generously supplied by Dr. B. H. Kear of Pratt & Whitney Aircraft. Following initial machining with a Servomet and a surface grinder, the specimen was etched and annealed at 1100°C for 5 hr in vacuum followed by furnace cooling. Two parallel faces were oriented normal to within ±l deg of a [I101 crystallographic axis by the back-reflection Laue technique. The parallel faces were hand-lapped, etched, and polished by the usual metallographic procedures. Final thickness along the [I101 axis was (0.9325 * 0.0005) cm. Chemical analysis gave an average composition of 87.35 pct Ni. Total impurity content is esimated to be no more than 0.1 pct. The elastic constants were calculated from measurements of ultrasonic waves which were propagated along the [110] axis. A longitudinal wave and two shear waves whose displacements are aligned along the [ 110], [li~], and [001] directions, respectively, are necessary in order to completely specify the three elastic constants. Two 3/6-in. quartz transducers were bonded to the parallel faces of the sample with ~ac-seal.3 Although this bond was sensitive to thermal shock, it performed satisfactorily over the entire temperature range for both longitudinal and shear wave propagation. A frequency of 5 mHz was employed in the method of Williams and Lamb4 in order to measure the absolute value of the ultrasonic velocity. The pulse-overlap technique of Mcskimin5 was also used at this frequency in order to determine the shift in the ultrasonic velocity