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 γ 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 γ grains resulting from casting defects.