UBC Theses and Dissertations
The columnar to equiaxed transition in nickel based superalloys AM1 and MAR-M200+Hf Fernihough, John William
The columnar to equiaxed transition (CET) in Ni based superalloys AM1, electron beam remelted AM1 and MAR-M200+Hf was studied using bars instrumented with thermocouples to determine the critical solidification conditions leading to the CET. Two sets of experiments were conducted in the course of the work: the first set using investment cast moulds solidified in the experimental Bridgeman furnace at SNECMA, France. The second set of experiments consisted of directionally solidifying a bar using a copper chill for heat extraction, and were conducted at the University of British Columbia. Despite similar results of thermal gradient and solidification rate during each of the two sets of experiments, a CET was consistently obtained with the UBC experiments while not with the SNECMA experiments. The critical difference was found to be the shape of the liquid/solid interface, which was flat and horizontal in the UBC experiments but extremely curved concave upwards in the SNECMA experiments due to nature of the heat transfer. Hence the bars cast at SNECMA were dominated by secondary dendrite arm growth from the edge inwards. Comparing thermal results with the bar structure for the UBC experiments showed that the equiaxed zone always corresponded to a classical equiaxed thermal response marked by recalescence (a temporary rise in temperature) while the columnar zone corresponded to a monotonic descent in temperature. Application of a dendrite tip tracking program to the thermal data produced results in agreement with direct estimates of thermal gradient and solidification rate from the raw data. The CET was found to occur within a narrow range of solidification rates and thermal gradients, averaging 0.33 mm/sec and 5.45 °C/cm for EB remelted AM1, 0.322 mm/sec and 8.36 °C/cm for normal AM1, and 0.38 mm/sec and 5.56 °C/cm for MAR-M200+Hf No significant difference was found between normal and EB refined AM1. These results are in agreement with the Hunt CET model modified for superalloys by using a pseudo binary approximation for dendrite tip growth kinetics and an average nucleation undercooling of 3.7°C.
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