QUANTITATIVE-ANALYSIS OF MICROSTRUCTURES PRODUCED BY CREEP OF TI-48AL-2CR-2NB-1B - THERMAL AND ATHERMAL MECHANISMS

A gamma-based TiAl alloy with equiaxed microstructure and fine grain s ize has been studied to analyze the deformation mechanisms responsible for the creep behavior. The microstructures produced by creep and hig h temperature deformation have been examined by TEM to obtain informat ion about the different aspects characterizing the primary and seconda ry stages of creep. Mechanical twinning has been confirmed to occur in a fraction of the grains that never exceeds 50% while 1/2[110] disloc ations are active within all the gamma grains. The twins are only resp onsible for a small amount of strain, but they lead to a subdivision o f the microstructure and determine (directly or indirectly) the harden ing process observed during the primary stage of creep. We have propos ed that during the secondary stage the creep rate is determined by the unblocking of pinned dislocations by processes such as a pipe diffusi on or cross slip that allow thermally activated glide of 1/2[110] disl ocations on (001) planes.