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Rare Metals2018年第3期

Oxidation-induced damage of an uncoated and coated nickel-based superalloy under simulated gas environment

Shao-Lin Li Hong-Yu Qi Xiao-Guang Yang

School of Energy and Power Engineering,Beihang University

Beijing Key Laboratory of Aero-Engine Structure and Strength

收稿日期：12 July 2016

基金：financially supported by National Basic Research Program of China (No.2015CB057401);

Oxidation-induced damage of an uncoated and coated nickel-based superalloy under simulated gas environment

Shao-Lin Li Hong-Yu Qi Xiao-Guang Yang

School of Energy and Power Engineering,Beihang University

Beijing Key Laboratory of Aero-Engine Structure and Strength

Abstract：

Turbine blades and vans operated in an aggressive gas environment usually suffer from combined oxidation and cycle loading effects. The surface oxide layer will result in premature failure and lead to a significant reduction in the service lifetime. The effects of prior oxidation-induced damage under a simulated combustion-gas environment on the fatigue lifetime of the directionally solidified(DS) nickel-based superalloy DZ125 with and without an oxidation-resistant coating were presented. The fatigue lifetime of uncoated samples is adversely affected by prior oxidation exposure. The deterioration of fatigue lifetime in uncoated samples is associated with surface microstructural degradation, which occurs during prior exposure. However,the presence of MCrAlY coating is beneficial for the sample's lifetime under high stress. Further scanning electron microscopy(SEM) analysis demonstrates that the coating does not contribute to the initiation mode of fatigue cracks.

Keyword：

Gas environment; Fatigue; Lifetime; Coating; Superalloy;

Author： Hong-Yu Qi e-mail:qhy@buaa.edu.cn;

Received： 12 July 2016

1 Introduction

Directionally solidified (DS) nickel-based superalloys,such as DZ125,are used for the fabrication of turbine components(e.g.,blades and vans) in aircraft engines due to their excellentmechanical property and microstructure stability at practical application [ 1, 2, 3, 4, 5] .However,turbine blades and vans operated in an aggressive thermal environment are prone to reactions with atmospheric gas;thereby the surface layers,such as oxide layer andγ'-depleted layer,were formed on the component [ 6, 7] .In addition,cyclic mechanical loading is induced by repeated various phases,e.g.,takeoff,cruising and landing.Therefore,the presence of cyclic loading facilitates crack initiation on the surface layer due to oxide spike mechanism [ 8, 9, 10, 11, 12] .This phenomenon will result in premature failure and lead to a significant reduction in the service lifetime [ 13, 14] .

Previous studies focused on the mechanical behavior of DS nickel-based superalloy [ 15, 16, 17, 18, 19, 20] .However,few studies have been conducted on the influence of the microstructural deterioration induced by exposure to high-temperature oxidation on the mechanical behavior of DS super alloy.Although some works have been performed for fatigue [ 8] ,creep [ 21, 22] and tensile stress [ 23] ,the regular laboratory tests on air were not able to fully simulate the service environment of turbine parts.Moreover,oxidation-resistant coatings,such as MCrAlY,are extensively used for the surface protection of turbine components [ 24, 25, 26, 27, 28] .Nevertheles s,the exact mechanism of how such coatings exert their protective function remains unclear.

In the present paper,a newly developed burner rig was used to simulate turbine components service conditions and to evaluate the effect of oxidation with prior exposure on fatigue behaviors of the uncoated and MCrA]Y-coated nickel-based superalloy DZ125.

2 Experimental

The DS superalloy DZ125 was used in this study.The chemical composition (wt%) of this alloy is 0.1 C,8.9 Cr,10.0 Co,7.0 W,2.0 Mo,5.2 Al,3.8 Ta,1.5 Hf,0.015 B,and balanced Ni.All the substrate materials received standard heat treatment [ 29] .The fatigue samples with6 mm in diameter and 25 mm in gauge section were machined.The solidification of the [ 1] direction was along the length of the samples.Moreover,the coated samples were covered with MCrAlY coating by electron beam physical vapor deposition (EB-PVD).

To study the effect of the oxidation and MCrAlY coating on fatigue behavior,pre-exposure fatigue tests were carried out on uncoated and coated samples.Prior exposure at 980℃was performed for 25 h in a burner rig,as shown in Fig.1.After the set exposure time,the samples were removed from the burner rig,cooled down to room temperature,and then subjected to fatigue experiments.Loadcontrolled fatigue testing was performed in laboratory air using a triangular waveform with a stress rate of400 MPa·s^-1.The stress ratio was maintained at 0.These tests were performed at 980℃with maximum stresses(480,440,and 400 MPa).

Fractography and longitudinal section morphologies for uncoated and coated samples were determined and compared by scanning electron microscope (SEM,JSM 6010)and optical microscopy (OM,LEICA DM4000),respectively.Chemical analysis was performed using electron probe micro analyzer (EPMA,JXA-8100).

3 Results

3.1 Fatigue life

The influences of prior oxidation in a gas environment on the fatigue lifetime of uncoated samples (DZ125 superalloys) and unexposed samples are compared (Fig.2).For uncoated samples under applied maximum stress (σ_max)levels of 480,440 and 400 MPa,the lifetime (2N_f)decreases by approximately 31%,30%,and 23%,respectively,after prior exposure in a gas environment for25 h compared to those of the unexposed samples.This trend also indicates that changing the applied stress holds no determinate role in the effect of prior oxidation on the fatigue lifetime.

The fatigue lifetime of the uncoated and coated samples pre-exposed to a gas environment is also presented in Fig.2.Under the same conditions,the coated sample presents a longer fatigue lifetime than the uncoated samples under increased stress conditions because of the protective effects of the coating.However,the coating barely affects the lifetime under lower stress levels.

3.2 Surface layer characterization

Prior exposure in a gas environment causes the formation of an oxide layer on the surfaces of both the uncoated and coated samples (Fig.3;Table 1).Moreover,the number of macroscopic cracks in the coated samples is larger than that in the uncoated samples.These cracks can be attributed to the extremely easy cracking under cyclic loading caused by the mismatch in material properties (e.g.,Young modulus,yield strength) in the coating and matrix material [ 29] .

Fig.2 Fatigue lifetime of uncoated and coated samples pre-exposed to a gas environment

Fig.1 Diagrammatic sketch a and experimental apparatus b of a jet fuel burner rig facility for prior exposure experiment

Fig.3 SEM images of surface morphologies of a uncoated and b coated samples pre-exposed for 25 h

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Table 1 Major chemical compositions of surface layers in uncoated and coated samples pre-exposed for 25 h

3.3 Longitudinal sections characterization

The micrographs in Fig.4 show longitudinal sections of the uncoated and coated samples pre-exposed for 25 h at980℃.An oxidation scale of～10μm thickness is found on the surface of uncoated samples (Fig.4a),thereby indicating that severe damage occurs on the sample surface.Theγ'-depleted layer is also observed underneath the surface oxidation layer.The EPMA result shows that Al content in theγ'-depleted layer is minimal (Fig.5a).The presence of the surface damage in these samples could be attributed to the fatigue cracks that originate from the surface.

Under coated conditions,no such severe surface damage is observed because of the oxidation-resistant coating applied on these samples (Fig.4b).However,cracks are also observed at the surface of the coated samples because of the cyclic loading.The oxide layer is identified as NiO and Al₂O₃ oxides,which are formed on the sample surface(Fig.5b).

3.4 Fractography

Figures 6 and 7 show the fracture surface morphologies of the uncoated and coated samples,respectively.This result suggests that at high stress amplitudes,the fatigue crack initiates at defects (such as voids and loose)(Figs.6b,7b).By contrast,surface crack nucleation occurs at low stress amplitudes both for uncoated and coated samples (Figs.6d,7d),indicating that the coating holds no effect on the fatigue initiation behavior.

4 Discussion

4.1 Effect of simulated gas prior exposure on fatigue lifetime

The influence of prior exposure on fatigue behavior is the most evident effect on the surface and subsurface micro structures.When uncoated samples were exposed to980℃for 25 h in the simulated combustion-gas environment,an oxide layer with～10μm in thickness is observed on the surface (Fig.4a).These brittle oxidation layers are easily cracked in the presence of cyclic loading(Fig.3a).Gordon et al. [ 8, 9] demonstrated that the crack nucleation was caused by the coalescence of micro-cracks that emerged from the protective oxidation layer.

Moreover,the presence of a 16-μm-thickγ'-depleted layer beneath the surface oxidation layer is another factor that affects both nucleation and propagation processes.This layer consists of equiaxed grains with the grain boundary perpendicular to the loading direction [ 23, 30] .Therefore,the cracks are rapidly propagated throughγ'-depleted layer and finally into the substrate.Research on Rene 80 also reported that theγ'-depleted zone formed during prior oxidation significantly influenced the fatigue behavior [ 31] .

Fig.4 OM images of longitudinal section for a uncoated and b coated samples pre-exposed for 25 h

Fig.5 EPMA analyses of major elements through surface layer thickness of pre-exposed a uncoated and b coated samples

Fig.6 SEM images of fracture surface of uncoated samples:a whole fracture surface and b crack initiation region under high stress(σ_max=480 MPa);c whole fracture surface and d crack initiation region under low stress (σ_max=440 MPa)

Fig.7 SEM images of fracture surface of coated samples:a whole fracture surface and b crack initiation region under high stress(σ_max=480 MPa);c whole fracture surface and d crack initiation region under low stress (σ_max=440 MPa)

Therefore,the shortened fatigue lifetime of the exposed samples relative to that of the unexposed samples can be ascribed to the surface deterioration,as well as the reduction of the sample bearing area because of the oxidationinduced damage.

4.2 Effect of coating on fatigue lifetime

Under the coated condition,the effect of coating on fatigue crack initiation is only few microns in thickness (Figs.6,7).Nevertheless,the coating can affect the fatigue lifetime,and such effect depends on the applied fatigue loading.Under the high fatigue loading,the fatigue lifetime of the coated samples lengthens compared with that of uncoated samples (Fig.3).As previously mentioned,the oxidation andγ'-depleted layers are formed at the surface of uncoated samples during prior exposure.Thus,the lengthening could be ascribed to the coating that maintains the load-bearing cross section by preventing surface damage during prior exposure,which should protect the material from oxidative damage.However,both uncoated and coated samples acquire a relatively lengthened lifetime under low stress conditions (Fig.3).The samples require longer exposure duration under fatigue loading and hightemperature environment.Therefore,the protective effect of the coating is not highlighted,and the coating appears to only minimally affect the fatigue lifetime.

5 Conclusion

The surface layer of the pre-exposing uncoated sample,which includes the oxidation and y'-depleted layers,facilitates the highly rapid initiation of surface cracks during subsequent fatigue cycling and reduces the effective net area of salt-coated samples.The fatigue lifetime is significantly reduced for the uncoated samples with pre-exposure for 25 h relative to that of the uncoated samples without prior oxidation exposure.The coating does not contribute to the initiation mode of the fatigue crack.The coating changes the fatigue lifetime of the coated samples with respect to that of the uncoated samples.Under high stress,the coating effectively enhances the surface damage resistance of the coated samples during prior exposure,thereby resulting in higher fatigue lifetimes than those of the uncoated samples.Under low stress,the effects of the coating are not enhanced and the same fatigue lifetime is retained.