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In this research, the microstructural and chemical composition changes of thermally grown oxide (TGO) and bond coat (BC) layer were investigated after cyclic oxidation of Pt–Al–YSZ coating at 950°C. First, the Pt–Al layer was coated by the high-activity-low-temperature (HALT) method on the Rene-80 substrate, and then the YSZ top coat (TC) layer was coated by the atmospheric plasma spraying (APS) method on the HALT Pt–Al layer. Cyclic oxidation was performed in ambient air for 50 cycles. After cyclic oxidation, the Pt–Al layer was divided into upper and lower regions. Upward diffusion of Al from the Pt-Al layer to TGO led to the transformation of β to γ′ and γ in the lower region, rumpling in the upper region, and internal oxidation at the interface of the upper and lower region of the Pt–Al layer. The TGO layer included mixed oxides of NiO, Cr2O3, and NiCr2O4, along with discontinuous Al2O3 veins. Depletion of Al from Pt–Al led to the ratcheting of the TGO, the formation of NiAl2O4 at the Pt–Al/TGO interface, and, subsequently, the nucleation of lateral microcracks in the upper region of the Pt–Al. A mechanism for changing the chemical composition of TGO after cyclic oxidation was proposed. [ABSTRACT FROM AUTHOR]

The impact of a Pt-Al bond coat on the high-cycle fatigue (HCF) behaviour of fourth-generation single crystal superalloys at 760℃ and 900℃ was investigated. The Pt-Al bond coat was found to be almost negligible under low stresses at 760℃; however, the bond coat effect was detrimental at 900℃ or under high stresses at 760℃. At 760 °C with increasing high-amplitude stress, the tip cracks preferentially nucleated from defects within coat and propagated inwards, accelerating the fracture. The fatigue life of the coated alloys was considerably decreased at 900 °C, which was attributed to the damage accumulated in the bond coating via oxidation, crack-induced oxide cracking and interfacial microstructure degradation. An Elevated temperature led to the crack-induced oxides shifting from a small fan-shape to a large umbrella-shape, increasing the density of slip bands and quantity of secondary cracks in the substrate. Furthermore, more Ru diffused outwards at 900℃ than that at 760℃, which caused the disappearance of L-C dislocation locks and tertiary γ′ phases and aggravated TCP phases precipitation in the substrate at 900℃. Ultimately, to assess the degradation of the HCF life, an empirical life prediction method was developed, and the calculations results well matched the test results.

High-cycle fatigue tests of Pt–Al bond coated and bare single crystal superalloys were conducted at 700 °C and 800 °C, aiming at investigating the effect of bond coat on the high-cycle fatigue properties of a single crystal superalloy. The impact of bond coat was studied merely on the tensile and creep properties of this superalloy, but was not found on the high-cycle fatigue behavior of this alloy. The experimental results showed that the bond coat was found to be beneficial at 800 °C or under low stresses at 700 °C owing to the fact that cracks were retarded by the fine grains in the IDZ, the P-type rafted γ′ phases in the substrate or the coat/substrate interface, respectively. However, the bond coat was detrimental under high stresses at 700 °C owing to the rapid propagation of the early cracking and the brittle nature of bond coat. Moreover, EBSD results showed the all bond coat cracks mainly initiated at the surface of the coated specimens and started from grains with {123} slip system within bond coat. Fatigue tested samples at 800 °C showed a comparatively higher Schmid factor value than that at 700 °C. Following the fatigue test, TEM tests indicated that in the superalloy close to the bond coat, the dislocation networks formed due to the activity of various slip systems at 700 °C; under 450 MPa at 800 °C in this region, a special a0 superdislocation with two segments of compact core was observed in the rafted γ′ phases.

[Display omitted] • The Pt-Al bond coat was detrimental at 900 ℃ or under high stresses at 760 ℃, while was negligible under low stresses at 760 °C. • At 760 °C, the protection of crack-induced oxide disappeared and the tip cracks formed with amplitude stress increasing. • At 900 °C, the fatigue property degradation was related to the coat oxidation, oxide cracking and interfacial microstructure degradation. • An empirical life prediction method on coated superalloy was developed, matching well with the test results. The impact of a Pt-Al bond coat on the high-cycle fatigue (HCF) behaviour of fourth-generation single crystal superalloys at 760℃ and 900℃ was investigated. The Pt-Al bond coat was found to be almost negligible under low stresses at 760℃; however, the bond coat effect was detrimental at 900℃ or under high stresses at 760℃. At 760 °C with increasing high-amplitude stress, the tip cracks preferentially nucleated from defects within coat and propagated inwards, accelerating the fracture. The fatigue life of the coated alloys was considerably decreased at 900 °C, which was attributed to the damage accumulated in the bond coating via oxidation, crack-induced oxide cracking and interfacial microstructure degradation. An Elevated temperature led to the crack-induced oxides shifting from a small fan-shape to a large umbrella-shape, increasing the density of slip bands and quantity of secondary cracks in the substrate. Furthermore, more Ru diffused outwards at 900℃ than that at 760℃, which caused the disappearance of L-C dislocation locks and tertiary γ′ phases and aggravated TCP phases precipitation in the substrate at 900℃. Ultimately, to assess the degradation of the HCF life, an empirical life prediction method was developed, and the calculations results well matched the test results. [ABSTRACT FROM AUTHOR]

An evaluation was conducted on the impact of a Pt–Al bond coat on the creep behaviours of a second-generation Ni-based single crystal superalloy at 750 °C/820 MPa, 850 °C/630 MPa, 1038 °C/137 MPa and 1100 °C/112 MPa, respectively. As revealed by the creep results, the creep behaviours of the coated superalloy were inferior to those of the bare superalloy to some extent under all testing conditions. The maximum deterioration in the strain-to-fracture and time-to-rupture was observed at 1100 °C/112 MPa (7.3%) and 750 °C/820 MPa (74.7 h), respectively. Following the creep test, deconvolution calculations were performed, which indicated that the creep stress for all core superalloys was as low as approximately 1.7–1.9% of the applied stress while the bond coat stress was 4.0–10.4% that at the minimum creep rate. Moreover, TEM analyses revealed that within the superalloy substrate adjacent to the bond coat, there were fewer stacking faults and an abundance of dislocation debris observed within γ′-Ni 3 Al at 750 °C/820 MPa. The misfit of the γ/γ′ phase was insignificant and the dislocation networks became irregular at 1100 °C/112 MPa. With an increase in temperature from 750 °C to 1100 °C, the fracture mechanism of both the substrate and bond-coat was found to shift from quasi-cleavage to micro-void coalescence. The quasi-cleavage cracks occurring within the bond coat increased the number of connections with creep cracks to cause spreading from the interior of the samples, thus resulting in premature failure at 750 °C/820 MPa and 850 °C/630 MPa. However, the degradation at 1038 °C/137 MPa and 1100 °C/112 MPa was ascribed to the increasing thickness of the inter-diffusion zone, as well as the volume of the blocky γ′-Ni 3 Al phase inside the bond coat and topologically close-packed phases in the superalloy near the bond coat. [ABSTRACT FROM AUTHOR]