Primary creep at 750 °C in two cast and PM Ti48Al48Cr2Nb2 alloys

The primary creep of two Ti48Al48Cr2Nb2 alloys produced by cast (C) and powder metallurgy (PM) routes is investigated at 750 °C, under 150 and 80 MPa, respectively. A higher primary strain and a longer duration are measured for the former alloy, which also exhibits a high scatter in the primary stage from one specimen to another. From strain change dip tests, it is evident that internal stress is higher in the C than in the PM alloy. Transmission electron microscopy carried out on crept specimens shows that the deformation is due to ordinary dislocations moving by mixed climb and that large (gamma) grains contain a high density of these dislocations. According to these experimental results, it is proposed that primary creep involves a homogenisation stage in terms of the alloy resistance. The scatter measured in the C alloy is related to the number of (gamma) grains resulting from casting defects.