Observations of directional gamma prime coarsening during engine operation
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I. I N T R O D U C T I O N S T R E S S induced directional coarsening of 3,' in single crystal nickel-base superalloys has been studied extensively in numerous laboratories during the last decade. t1'2'3] In alloys having y' with a smaller lattice parameter than the 3, (a negative misfit parameter), it has been shown that the stress induced coarsening will cause a microstructure consisting of alternating lamellae of 3' and 3''. These lamellae form perpendicular to the axis of the applied stress. This behavior has been called rafting. The studies have generally been concerned with either the beneficial effects of directional coarsening on creeprupture behavior or the kinetics of directional coarsening in isothermal laboratory conditions. Two alloys (NASAIR 100 and Alloy 3) having a negative misfit parameter, developed by the Garrett Turbine Engine Company under NASA Contract NAS3-20073 (MATE), have been observed to form 3'' lamellae in laboratory creep tests, t4'5] Turbine blades made of these alloys were tested in a ground-based engine as part of the Garrett program to evaluate the potential benefits of the single crystal alloys to small turbine engines. Examination of the blades after the engine test was completed showed that the alloys had formed a lamellar structure of 3' and 3,' during the engine operation. [6] The work reported here was initiated to examine in more detail the directional coarsening response of these alloys in particular, to determine the range of stress and temperature over which directional coarsening might occur in actual turbine engine operations, and to make observations relating to directional coarsening during the conditions of turbine engine operation. II. B A C K G R O U N D
Solid turbine blades made from the two alloys were run in a series of ground-based engine tests performed by the Garrett Turbine Engine Company in a specially modified TFE731 turbofan engine, t6"71These tests were
S. DRAPER and D. HULL, Materials Engineers. and R. DRESHFIELD, Senior Materials Engineer. are with the NASA Lewis Research Center, Cleveland. OH 44135. Manuscript submitted March 30. 1988. METALLURGICAL TRANSACTIONS A
performed to evaluate the suitability of the single crystal alloys for use in this class of small turbine engines. Initially, a performance test was conducted to evaluate the effect of the modified TFE731 turbofan engine on engine performance. The performance test started at the maximum power setting. The power then was stepped down in increments to 50 pct of the maximum power, with holds at each power setting of approximately 10 minutes. The total time of the test was approximately 1.5 hours. The balance of the engine test consisted o f the four 50 hour segments shown in Figure 1. The first segment was to evaluate potential high cycle fatigue (Figure l(a)) problems by operating the engine at many speeds to determine if harmonic conditions might exist which could cause fatigue damage to the blades. The second segment, called a stress-rupture test, evaluated maximum continuous temperature
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