EFFECT OF INTERSTITIAL CONTENT ON HIGH-TEMPERATURE FATIGUE-CRACK PROPAGATION AND LOW-CYCLE FATIGUE OF ALLOY-720

Alloy 720 is a high-strength cast and wrought turbine disc alloy curre ntly in use for temperatures up to about 650-degrees-C in Allison's T8 00, 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. Stringering was attri buted to relatively higher boron and carbon levels. These interstitial s are known to affect creep and ductility of superalloys, but the effe cts on low-cycle fatigue and fatigue crack propagation have not been s tudied. Recent emphasis on the total life approach in the design of tu rbine discs necessitates better understanding of the interactive fatig ue crack propagation and low-cycle fatigue behavior at high temperatur es. 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 chara cteristics of the original composition. Improvement in strain-controll ed low-cycle fatigue life was achieved by fragmenting the continuous s tringers via composition modification. The fatigue crack propagation r ate was reduced by a concurrent reduction of both carbon and boron lev els to optimally low levels at which the frequency of brittle second p hases was minimal. The changes in composition have been incorporated f or production disc forgings.