EXPERIMENTAL AND THERMODYNAMIC ANALYSIS OF DIFFERENCES IN PHASE TRANSFORMATION OF p-(Ni,Pt)Al COATING DURING ISOTHERMAL AND CYCLIC OXIDATION

摘要

Ni-base superalloys used for aero-engine blades operate during thousands of hours at elevated temperatures up to 1,100 °C and even higher. In order to resist to such extreme conditions, Pt-modified nickel aluminide, P-(Ni,Pt)Al, coatings are widely used for the protection of Ni-base superalloys against corrosion and oxidation [1]. However, the protective properties of the coating get degraded by its transformation into y'-Ni3Al phase. Such transformation is caused by the loss of Al due to both Al consumption for the formation of (X-Al_2O_3 thermal grown oxide (TGO) and Al diffusion from the Al-rich coating to the superalloy substrate. It was showed that β-(Ni,Pt)Al to γ'-Ni_3Al phase transformation occurred differently whether it happens during isothermal or cyclic oxidation. In isothermal conditions, the transformation was characterised by a continuous β/γ' transformation front moving from the superalloy substrate towards the surface, whereas thermal cycling favoured γ'-Ni_3Al precipitation at β-(Ni,Pt)Al grain boundaries (GBs) [2-4]. Such a difference in phase transformation leads to a different coating behaviour during further mechanical tests [5, 6]. Nevertheless, the reasons for such a difference in β-(Ni,Pt)Al to γ'-Ni_3Al phase transformation in isothermal and cyclic oxidation conditions are not yet understood. In this study, a special attention was paid to the measurements of the phase compositions constituting the coating deposited on Ni-base single-crystal AMI alloy. Oxidation was carried out for different durations in isothermal and cyclic conditions at 1,100 °C. Afterwards, the differences in β-(Ni,Pt)Al to γ'-Ni_3Al phase transformation and phase compositions were analysed. Systematic concentration measurements using a Noran Si(Li) EDS (energy-dispersive X-ray spectroscopy) system installed on a LEO 1450VP SEM and a CAMECA SX100 EPMA (electron probe microanalysis) showed that both P-(Ni,Pt)Al and γ'-Ni_3Al phases became rapidly homogeneous and that the β-(Ni,Pt)Al to γ'-Ni_3Al phase transformation should be thus interface-controlled. While a homogenisation of the β-phase was widely reported, this is yet undocumented for γ'-phase. Although the homogenisation was found for both phases, the compositions of β-(Ni,Pt)Al and γ'-Ni_3Al phases were found to be dependent on oxidation conditions. For example, for the shortest oxidation time (Nmin hours and cycles; duration of each cycle was 1 hour with 45 min dwell at 1,100 °C) Pt content in P-phase was measured to be 1.3 at% higher than that measured after the cyclic oxidation (Fig. 1). Further oxidation led to a decrease in this difference (difference in Pt content in P-phase reduced to 0.5 at%). The results of equilibrium calculations at 1,100 °C showed that measured β-(Ni,Pt)Al and γ'-Ni_3Al compositions differed from those obtained by Thermo-Calc software using TCNI7 database. Namely, higher Cr, W and Mo contents were measured in the β phase in comparison with calculated ones. On the other hand, thermodynamic calculations suggested the γ'-stabilisation with temperature decrease from 1100 °C. This result should be one of the reasons for γ'-precipitation at β GBs during thermal cycling. Moreover, the influence of temperature decrease on equilibrium Pt content in β-phase could explain as well lower Pt concentration measured in β-phase during cyclic oxidation than that obtained for isothermal oxidation.