The influence of microstructure on low temperature creep of ti-5 al-2.5 sn

Abstract

Creep tests were performed on normal grade Ti-5 Al-2.5 Sn at 194, 299, 339, and 422 K. Three processing histories were studied: bar stock and forgings made in theα andβ ranges. Creep stresses ranged from 40 to 90 pct of the 299 K tensile yield stress, σy. Not all combinations of stress and temperature gave reliable creep data. At 60 and 80 pct σy, the forged materials were more creep resistant than the bar stock, while at 90 pct σy all three materials were alike. The apparent activation energy for creep, about 37 kJ/mol, was about one-fourth the energy for selfdiffusion. Activation areas were about 10⇃²; thus the rate-controlling process at the stresses used was probably the overcoming of interstitial obstacles. Observations on thin foils showed that the bar stock and α-forged material had equiaxed grain structures, while theβ-forged material consisted of massive martensite(α’) plates. Films of retainedβ appeared to be present in manyα’ boundaries; this restricted slip to individualα’ plates. A microstructural rationale was constructed, suggesting that each material contained dislocation sources which differed in ease of operation. Theβ-forged material was fitted into this rationale by the observation that dislocations inα’ boundaries could apparently act as sources. It was concluded that the observed transient creep had arisen from exhaustion of the easiest sources at each stress level.