Abstract
The details of creep and substructure in powder metallurgy Ta were investigated in the temperature range 1523–1623 K and at stresses extending from 35 to 120 MPa. The experimental data show that the apparent creep behaviour of Ta is different from that of pure metals with regard to two characteristics: the stress exponent is about 7 and the activation energy for creep (502kJ mol−1) higher than that for self-diffusion (416.5kJ mol−1). An analysis of the data reveals a threshold stress for creep, which strongly depends on temperature. An examination of creep substructure by means of transmission electron microscopy shows the presence of frequent interactions between dislocations and nanometre particles in the interiors of equiaxed subgrains whose average size decreases with increasing stress. The dislocation-dispersion particle interaction configurations resemble those reported for dispersion-strengthened alloys. It is suggested that these particles were introduced in Ta as a result of its processing by powder metallurgy and that their interaction with moving dislocations gives rise to the threshold stress creep behaviour observed in the metal. When the threshold stress is incorporated into creep analysis and plots, the true creep behaviour becomes consistent with that ascribable to high-temperature climb. The results demonstrate that creep in Ta is sensitive to processing conditions, since earlier data on Ta which were produced by the electron-beam process do not exhibit threshold creep behaviour.