Mechanical Loss Associated with Stress Anomaly in Ni 3 Al and Ni 3 (Al, Ta) Single Crystals

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Mat. Res. Soc. Symp. Proc. Vol. 578 © 2000 Materials Research Society

of the complex stacking fault (CSF) energy with composition, which has a direct influence on the cross-slip ability, and a fair correlation has been recently established between the flow stress and the CSF energy value of various Ni 3A1 alloys [9]. In a first attempt of comparing mechanical loss spectra obtained with different Ni 3A1 alloys, the present paper reports mechanical spectroscopy studies performed on binary Ni75A125 and ternary Ni 75A124Ta, single crystals. The mechanical loss and the dynamic shear modulus behaviors have been investigated over a large temperature range that extends from 100 K up to 1300 K. For both alloys, particular attention has been focused on the temperature range of the flow stress anomaly (i.e. below 700 K). As in the previous studies [4, 5], virgin and pre-deformed single crystalline specimens have been used. The results obtained for the two compositions are compared and discussed in the framework of the superkink models, which have been proposed for explaining the anomalous behavior of the Ni 3Al phase. EXPERIMENTAL A single crystalline rod of binary Ni 3A1 having the stoichiometric composition was produced at the National Research Institute for Metals, Tsukuba, Japan. It was uni-directionally grown at a rate of 25 mm/h by using a floating-zone method. The preparation and growth procedures are reported elsewhere [10, 11]. The Ni 3(A1,Ta) single crystals with a nominal composition of Ni 75A124Tal were provided by Prof. D. P. Pope at the University of Pennsylvania, USA. Flat rectangular specimens of sizes of 25 mm x 2 mm x 0.45 mm oriented along the crystallographic direction, have been spark eroded from the rods and further carefully polished with successively finer grades of abrasive papers for removing the surface damage layers. The mechanical loss (damping) and the shear modulus evolutions were investigated as a function of the temperature in the range 100 K - 1300 K by using an inverted torsion pendulum. This pendulum has been specially designed for allowing large oscillation amplitudes and it can be configured into two working modes: free-decay and forced vibrations. In the free-decay mode, a torsional oscillating strain is initially imposed to the specimen up to a given strain amplitude at which the applied excitation is stopped. The mechanical loss is then obtained from the waveform analysis of the free decay of the residual vibrations by the Fourier transformation method [ 12, 13]. The shear modulus (G) can be derived from the pendulum frequency (f) by using the relation G=

8;r 21 2



where 1, w and t are the length, width and thickness of the specimen, respectively, I is the pendulum momentum of inertia and 03is a geometrical factor [14]. In the forced-vibrations mode, the inertia masses are removed and the specimen is subjected to an alternating stress at imposed frequency. The mechanical loss corresponds to the phase lag tan 0 (0 being the mechanical loss angle) between the stress excitati