Mechanical Behavior of a Directionally Solidified Nickel-based Superalloy Su 247LC DS at Elevated Temperature

Abstract

Directionally solidified Nickel-based superalloys are the workhorse for the Jet Engine turbine rotor blades and nozzle guide vanes (NGV) applications. Su 247LC DS is the material used for turbine rotor blades and NGVs of one of the gas turbine engines. This thesis aims to study elevated temperature deformation of this material as experienced by the alloy during its service exposure with particular attention to the effect of /’ eutectic content. Directionally solidified cast rods were subjected to varying extents of homogenization and solution treatment and two-step aging cycles (1080C and 870C) to arrive at different microstructures. A higher temperature final aging temperature (1000C instead of 870C) was selected to introduce a version of the microstructure (1250ST1000A (4% eutectic-48%’)) that would represent an overshoot of temperature during the service life of the jet engine components. Four different combinations of /’ eutectic and ’ precipitate microstructures (1260STA (2% eutectic-55%’), 1250STA (4% eutectic-55%’), 1254FSTA (6% eutectic-48%’) and 1250ST1000A (4% eutectic-48%’)) were selected in the present study. Mechanical testing, namely, monotonic tensile and tensile creep tests were conducted for all the four microstructures in a temperature range of 700-980C which is the typical operating range for the turbine blade and vane material. Fully reversed strain controlled low cycle fatigue (LCF) and stress controlled high cycle fatigue (HCF) tests were performed for three microstructures, namely, 1260STA (2% eutectic-55%’), 1250STA (4% eutectic-55%’) and 1254FSTA (6% eutectic-48%’) at a temperature of 700C over a range of strain amplitudes (0.7-1%) and stress amplitudes (400-600MPa) respectively. Creep tests were conducted over a temperature range of 700-980C while the stress level was varied between 250-900MPa. Monotonic tensile strengths (yield and ultimate tensile) were found to enhance with decreasing eutectic content and increasing ’ precipitate volume fraction with an exception to the 1250ST1000A (4% eutectic-48%’) microstructure. This microstructure achieved by following higher final aging temperature exhibited the lowest tensile strength over the entire test temperature range. Anomalous yield behavior was exhibited by all the microstructures in the temperature range of 700-850C. This was followed by a sharp reduction in strength level with a further increase in test temperature. Fracture of the samples was characterized by carbide particle cracking and slip initiation on octahedral slip planes. The dislocations were mostly restricted within  channels at 700C. Left over dislocation loops were occasionally observed within ’ precipitates. At, 850C, misfit dislocations were found to develop. The isothermal LCF life was found to reduce monotonically with increasing imposed strain amplitudes for all the three microstructures. Lower strain amplitude tests (strain amplitude level 0.8%) exhibited a marginal strengthening during the initial few cycles followed by a stabilized stress response. A gradually increasing stress response was observed for higher strain amplitude (0.8%) tests. The fatigue crack origin was associated with surface and near-surface casting porosity along with cracked carbide particles. Large carbide particle cracks were found to propagate into the matrix and activate slip on octahedral slip planes. The overload failure area decreased gradually as the imposed strain amplitude was reduced. The crack propagation region was characterized by faceted feature revealing fatigue crack propagation along well-defined slip planes. Samples tested with higher strain amplitudes exhibited wider hysteresis loop and hence higher plastic strains. Even though no significant difference was evidenced with respect to fatigue life and deformation characteristics as a function of eutectic content, the microstructure with highest eutectic content and lowest ’ precipitate volume fraction (1254FSTA (6% eutectic-48%’)) revealed highest plastic strain amplitudes over the entire strain amplitude range. The trend in fatigue life and fracture characteristics in high cycle fatigue was found to be similar to that observed in case of LCF behavior of the alloy. The tensile creep behavior of the alloy was found to be influenced by stress level and test temperature. The lower temperature and higher stress level tests (700C/900MPa and 760C/800MPa) exhibited dominance of primary creep, whereas the higher temperature lower stress level tests (980C/250MPa) revealed deformation by rafting. Initiation of rafting was observed at 850C (stress level 550MPa). Shearing of ’ precipitates were frequently observed at lower test temperatures (700C and 760C). The LMP plot showed a linear dependence on stress for all the four microstructures. Microstructures with similar ’ volume fraction but different eutectic content exhibited similar deformation behavior over the entire test range. Experimental evidence suggested a better homogenization and solution heat treatment process was beneficial in reducing the /’ eutectic content that resulted in a higher volume fraction of ’ precipitates in microstructures. The microstructures with highest precipitate content exhibited the highest tensile strength and creep resistance. The deformation microstructure did not reveal any direct role of /’ eutectic phase.