A study on the influence of Mo, Al and Si additions on the microstructure of annealed dual phase Cr-Ta alloys

The effects of additions of 5 at.% Mo, Al and Si on the long-term annealed microstructures of a two phase Cr-Cr2Ta alloy have been studied. Following 200 h at 1300 A degrees C, the lamellar eutectic constituent of all the alloys disintegrated into discrete particles of the Laves phase embedded within a Cr-rich solid solution phase, along with the formation of fine Laves phase precipitates. One of the predominant differences between the three alloying additions was the extent of the C14 to C15 polytypic transformation of the Cr2Ta-based Laves phase. With Mo and Al additions, the Cr2Ta Laves phase transformed from C14 to either C15 or intermediate hexagonal polytypes following 200 h annealing at 1300 A degrees C. In contrast, Si additions stabilised the C14 polytype, with no transformation to other polytypes observed after prolonged annealing at 1000, 1100 and 1300 A degrees C.