High Temperature Mechanical Spectroscopy in Ordered Ni₃(Al, Ta)

Abstract
Mechanical loss measurements were performed on polycrystalline and single crystalline samples (of three different orientations) of a Ni₃(Al, 1%Ta) alloy with the L1₂ ordered structure. Measurements were conducted in torsion pendula, on as-cast samples and samples predeformed either at 300 K or at 1300 K, and revealed the occurrence of two main phenomena : (a) The presence of a well-defined internal friction peak at a temperature of about 950 K for a frequency of 1 Hz. This relaxation peak has an activation energy of about 3 eV and its amplitude is strongly dependent on sample orientation and predeformation conditions. (b) An internal friction background, which amplitude at high temperatures (above 1100 K) mainly depends on strain amplitude and predeformation conditions. Both phenomena appear closely related to the presence of point defects, to the complex dissociation mode of superdislocations and to the change in dislocation mechanisms occurring at the peak temperature in flow stress (about 800 K) in relationship with a change in active slip systems.