Behavior of nickel-base superalloy single crystals under thermal-mechanical fatigue

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I.

INTRODUCTION

C O O L E D turbine blades in jet engines must have good resistance to thermal-mechanical cyclic loadings in order to withstand the superposition of centrifugal loads and thermal stresses during takeoff and landing operations. These components have long been designed simply using isothermal low-cycle fatigue (LCF) and creep tests. However, the synergy between fatigue damage and time-dependent phenomena, such as creep or oxidation, can be much stronger under thermal transient conditions than under isothermal creep-fatigue loading. Thermalmechanical fatigue (TMF) tests are thus especially suited to simulate the behavior of critical areas of components and are far superior to creep-fatigue tests. The advantages and limitations of the TMF test have been discussed elsewhere. [1'2'3~ There is no temperature gradient across the specimen section, and the stress is induced by a mechanical strain applied to the specimen to simulate the constrained thermal expansion of a component part. The major limitation is that, in order to avoid a temperature gradient in the section, cycle periods can be rather longer than thermal transients in actual components. In TMF testing, the phasing of strain and temperature can be arbitrarily varied. Most authors have been using two basic mechanical strain v s . temperature cycles: the "in-phase" cycle, where the mechanical strain is maximum at maximum temperature; and the "out-of-phase" cycle, where the mechanical strain is maximum at the minimum temperature, t4,5,6~ The phasing of strain and temperature can vary to a great extent according to component geometry, engine type, e t c . In our group, "faithful" cycles are preferred; such cycles often have a large hysteresis and simulate more closely conditions experienced in service, tl'7j Turbine blades are generally made from cast superalloys. Although both creep and oxidation can interact with fatigue in so-called creep-fatigue tests or TMF tests, superalloys are very susceptible to oxidation effects. This has been emphasized by Coffin tS~and by later E. F L E U R Y and L. RI~MY are with the Centre des Mat6riaux, I~cole des Mines de Paris, U R A 8 6 6 - - B . P . 87, Paris, France. Manuscript submitted October 2, 1992. METALLURGICAL AND MATERIALS TRANSACTIONS A

a u t h o r s . 19,1~ Various damage models have been proposed to account for the interaction between oxidation and fatigue in these a l l o y s . [9'12"13] Directionally solidified single crystals have been introduced in advanced engines to increase performance. Nickel-base superalloy single crystals have better creep resistance, much higher thermal fatigue resistance, and higher incipient melting temperatures than conventionally cast alloys. [14-171They are cast with the [001] direction of the face-centered cubic (fcc) lattice of the matrix along the main directions of the blade. However, due to geometry and temperature gradient, components can actually experience fairly high stresses along crystallographic directions different from the [001] direction. Only a few st

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