Effect of Pt-Al bond-coat on the tensile deformation and fracture behaviors of a second-generation SX Ni-based superalloy at elevated temperatures

Abstract

An investigation was conducted into the influence of applying PtAl bond-coat on the tensile deformation and fracture behavior of second-generation single crystal superalloy substrate with the temperature ranging between room temperature RT and 1100 °C. In the meantime, the development of a two-layer PtAl bond-coat, consisting of an outer layer of single-phase β-(Ni,Pt)Al and an inter-diffusion zone of refractory metal precipitates, has also been discussed. As revealed by the results, the application of PtAl bond-coat contributed to a noticeable deterioration in both yield strength and ultimate tensile strength of the superalloy substrate at elevated temperatures. Nevertheless, the elongation of coated superalloy was shown to be 17.2% (at RT) and 10.4% (at 750 °C) lower and 9.6% (at 1100 °C) higher compared to uncoated samples. Following tensile test, the TEM analyses indicated that there were onsets of various slip systems, the traditional superlattice stacking faults were retarded at intermediate temperature and the dislocation debris could be increased to develop dislocation networks at higher temperature in the region of superalloy near the bond-coat. As the temperature was on the rise, the fracture mechanism of coated superalloy shifted from tearing ridges and dimples to quasi-cleavage and then to micro-void coalescence. While that of PtAl bond-coat was transformed from brittle to ductile rupture. Within the temperature range from RT up to 750 °C, a combination of long crack lengths and greater crack penetration depths into bond-coat caused more deterioration in the tensile properties of superalloy. When the temperature rose above 750 °C, the application of PtAl bond-coat improved the ductility of the substrate, as a result of the ductile failure in the bond-coat which hindered the development of through-thickness cracks.