Stress relaxation and constant displacement rate tensile tests were performed on poly-crystalline GTD111 alloy material removed from General Electric MS6001B first stage combustion turbine buckets. Samples were examined in the standard heat treated condition, thermally exposed at 900°C for 5000 hours and from service run buckets. Creep rates of the material were measured and evaluated directly in terms of temperature capability at 850°C and 900°C. Stress relaxation tests done at 0.8% total strain indicated that the creep rate properties in the service exposed airfoil were an order of magnitude higher than the material properties in the standard heat treated condition measured in the root form. In terms of temperature capability, the creep rate properties of the service run airfoil material had decreased by the equivalent of almost 40°C. The stress relaxation test method was demonstrated to be a very useful tool in quantifying the degradation of creep properties in service run components. Creep data that would require years to gather using conventional creep tests was generated in a few days. This now makes realistic life assessment and repair / replace decisions possible during turbine overhauls. The test method’s unique ability to measure changes in creep rate over a large stress range, enabled the technique to distinguish between changes in creep strength due to (normal) microstructural evolution from the combined effects of microstructural evolution and strain related creep damage. A method for estimating standard constant load creep rupture life from the stress relaxation creep rate data is also presented along with time-temperature parameter correlations. The data sets examined in this study indicate that creep rupture lives can be estimated within a factor of three from the stress relaxation data. The information and analysis techniques described in this paper are directly applicable to metallurgical life assessment evaluations and the re-qualification of repaired General Electric buckets in Frame 3, 5, 6, 7 and 9 engine models.

Creep strength and fracture resistance are two properties that are critical in the selection and optimization of high temperature materials. These same properties may be progressively impaired by service exposure and are, therefore, key to the assessment of remaining life of high temperature components. In recent years, a methodology based on accelerated measurements of these properties has been developed. The two critical properties are decoupled in this approach which allows a clearer separation of the effects of microstructural evolution, damage development and environmental attack. The evaluation of creep strength is illustrated using extensive data on conventionally cast (IN738), directionally solidified (GTD111) and monocrystalline (CMSX-4) superalloys. It is shown that the properly defined creep strength is not sensitive to section size for any of the alloy classes. The well known effect of section size on rupture life of conventionally cast alloys is thus an effect on fracture resistance rather than creep strength. The comparisons with long term creep rupture test data show good agreement in all cases indicating that the accelerated testing is quite capable of yielding both comparative and design data. With the added advantage of a separate fracture resistance criterion the new methodology should be broadly useful.

Standard size and miniature specimens of IN738 were taken from a service exposed turbine blade and vane for comparative stress relaxation testing at 800C, 850C and 900C. Base data taken from root section material were used to construct stress vs. creep rate parametric curves which could be used directly in design. Up to five decades in creep rates were obtained at each temperature from tests lasting less than one day. The data were also presented in the form of stress vs. predicted times to 0.5% creep which compared well with available long time creep data. Differences were noted in specimens taken from different locations in the airfoil regions which probably resulted from differences in grain size or orientation. Based on these measurements it was concluded that there was no significant effect of section size on creep strength as defined by this test, and that the alloy was quite insensitive to prior deformation and thermal exposures. A life management procedure, using a combination or creep strength evaluation based on the stress relaxation test and a separate fracture evaluation measurement, is outlined in which end of useful life is defined in terms of minimum acceptable performance levels.

A new approach to tensile creep testing and analysis based on stress relaxation is described for sintered silicon nitride. Creep rate data covering up to five orders of magnitude were generated in tests lasting less than one day. Tests from various initial stresses at temperatures from 1250C to 1350C were analyzed and compared with creep rates measured during conventional constant load testing. It was shown that at least 40% of the creep strain accumulated under all test conditions was recoverable, and that the deformation could properly be described as viscoelastic/plastic. Tests were conducted to establish the level of repeatability and the effects of various thermomechanical histories. It was shown that none of the prior exposures led to significant impairment in creep strength. The results were used for three different grades to establish the value of the accelerated test to compare creep strengths for acceptance and for optimization. Several useful correlations were obtained between stress and creep rate. The systematic creep rate dependence as a function of loading strain prior to relaxation provided a possible basis for design in terms of a secant modulus analysis.