On Thermal Fatigue of Nickel-Based Superalloys

Abstract

The thermal fatigue (TF) behaviour of two single crystal nickel-based super-alloys, SRR99 and CMSX-4, is reported. Single edge wedge specimens are rapidly heated by induction heating of the wedge tip to a maximum temperature between 1000‡C and 1175‡C and cooled to 200‡C by forced air. A constant cycle period is employed for all experiments. The strain distribution along the edge of the TF specimens is measured. Changing the induction frequency leads to different strain ranges. CMSX-4 shows crack initiation always on cast microporosities. SRR99 presents an additional oxidation/spalling/re-oxidation mechanism under low strain loading. An “integrated approach” combining TF and TMF (thermo-mechanical fatigue) is proposed. The applied TMF temperature-strain cycle is deduced from the measured TF-cycle. Under this new temperature-strain cycle the crack initiation life and the total life in TMF for SRR99 are compared with the TF results.

Specific Aspects of Isothermal and Anisothermal Fatigue of the Monocrystalline Nickel-Base Superalloy CMSX-6

“…Thus, a comparison of the cyclic deformation of TMF tests with data obtained in typical thermal fatigue tests on wedge-shaped specimens with much shorter cycle times is problematic. Further complications arise from the fact that the damage mechanisms and the process of crack propagation are quite different in the two cases, compare [24]. One step to improve the comparability would be to perform the TMF tests on hollow specimens in the hope to increase the strain rate by about one order of magnitude, compare [25].…”

Thermal fatigue of single-crystalline superalloy CMSX-4®: a comparison of epitaxial laser-deposited material with the base single crystal

“…1. This geometry, used in earlier work in our laboratory [3][4][5][6] can be regarded as a simplified version of the shape of the leading edge of a turbine blade, one TF-cycle simulating the start up and shutdown of the engine.…”

“…The cycling is interrupted periodically for crack length measurement and sample inspection by scanning electron and optical microscopy. Data are plotted in the form of the length of the longest crack detected, a, measured perpendicularly to the edge on the side where the crack appears longest, as a function of the number of cycles, N. The thermal fatigue crack initiation and failure lives are, as in earlier work [5], defined as the number of cycles N i and N f corresponding to crack lengths of 0.1 and 1 mm, respectively. A detailed description of the testing procedure can be found in [3].…”

Role of heat-flux density and mechanical loading on the microscopic heat-checking of high temperature tool steels under thermal fatigue experiments

“…Testing of real components under service conditions is expensive. Laboratory in-house TF experiments are generally developed to assess the behaviour of materials [4,7,15]. Different unconstrained specimens (with various sizes and geometries) and heating and cooling systems are used.…”

“…where a is the mean thermal expansion, DT is the temperature amplitude (DT = T max À T min ), and E is the mean modulus of elasticity. Therefore, these are the ''mechanical'' strains and stresses that must be considered as criteria for crack initiation and propagation [15]. This paper deals with the influence of the TF test conditions on the microscopic heat-checking pattern formed on the surface of a X38CrMoV5 steel specimen.…”