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Alloy 720 is a high strength cast and wrought turbine disc alloy now in use for up to about 1200°F in Allison’s T800, T406, GMA 2100 and GMA 3007 engines. In the original composition intended for use as turbine blades, large carbide and boride stringers formed and acted as preferred crack initiators. The stringering was attributed to relatively higher boron and carbon levels. These interstitials are known to affect creep and ductility of superalloys, but the effects on low cycle fatigue and fatigue crack propagation have not been studied. Recent emphasis on the total-life approach in the design of turbine discs necessitates better understanding of the interactive fatigue crack propagation and low cycle fatigue behavior at high temperatures. The objective of this study was to improve the damage tolerance of Alloy 720 by systematically modifying boron and carbon levels in the master melt, without altering the low cycle fatigue and strength characteristics of the original composition. Improvement in strain-controlled low cycle fatigue life was achieved by fragmenting the continuous stringers via composition modification. The fatigue crack propagation rate was reduced by a concurrent reduction of both carbon and boron levels to optimally low levels at which the frequency of brittle second phases was minimal. The changes in composition have been incorporated for production disc forgings. Superalloys 1992 Edited by S.D. Antolovich, R.W. Stusrud, R.A. MacKay, D.L. Anton, T. Khan, R.D. Kissinger, D.L. Klarstrom The Minerals, Metals & Materials Society, 1992

Effect of Interstitial Content on High Temperature Fatigue Crack Propagation and Low Cycle Fatigue of Alloy 720