HIGH TEMPERATURE MECHANICAL SPECTROSCOPY IN ORDERED NI-3(AL,TA)

Mechanical loss measurements were performed on polycrystalline and sin gle crystalline samples (of three different orientations) of a Ni-3(Al ,1%Ta) alloy with the L1(2) ordered structure. Measurements were condu cted in torsion pendula, on as-cast samples and samples predeformed ei ther at 300 K or at 1300 K, and revealed the occurrence of two main ph enomena: (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 stron gly 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 p oint defects, to the complex dissociation mode of superdislocations an d to the change in dislocation mechanisms occurring at the peak temper ature in flow stress (about 800 K) in relationship with a change in ac tive slip systems.