稀有金属(英文版) 2017,36(08),659-665

Stability of NiCrAlY coating/titanium alloy system under pure thermal exposure

Xiao-Min Peng An-Ru Wu Li-Jun Dong You-Rui Tao Wei-Guo Gao Xiang-Lin Zhou

State Key Laboratory of Advanced Metals and Materials,University of Science and Technology Beijing

School of Mechanical Engineering,Hunan Institute of Engineering

收稿日期：4 May 2014

基金：financially supported by the National Natural Science Foundation of China (No.51101054);the Foundation of State Key Laboratory of Advanced Metals and Materials (No.2012-Z03);Hunan Provincial Natural Science Foundation of China (No.14JJ3132);

Stability of NiCrAlY coating/titanium alloy system under pure thermal exposure

Xiao-Min Peng An-Ru Wu Li-Jun Dong You-Rui Tao Wei-Guo Gao Xiang-Lin Zhou

State Key Laboratory of Advanced Metals and Materials,University of Science and Technology Beijing

School of Mechanical Engineering,Hunan Institute of Engineering

Abstract：

In this paper,NiCrAlY coating was deposited on TC4 titanium alloy using arc ion plating,and the stability of NiCrAlY coating/TC4 substrate system under pure thermal exposure was analyzed in a wide temperature range.During the thermal exposure,β→γ′→γphase transformation takes place in the NiCrAlY coating.The NiCrAlY coating phases totally decompose above the allotropic transformation temperature of TC4.The allotropic transformation ofα-Ti to β-Ti of the substrate significantly influences the stability of NiCrAlY coating/TC4 substrate system.NiCrAlY coating elements are mainly consumed by interfacial reactions below the allotropic transformation temperature of TC4.Above the allotropic transformation temperature of TC4,NiCrAlY coating elements are mainly consumed by element dissolution in β-Ti.The NiCrAlY coating totally degrades for the dissolution of coating elements in the substrate to form thickβ-Ti stabilized layer.

Keyword：

NiCrAlY coating; Titanium alloy; Stability; Pure thermal exposure; Degradation;

Author： Xiao-Min Peng,e-mail:tanfen1028@163.com;

Received： 4 May 2014

1 Introduction

Titanium and its alloys are extensively used in various industries with many specific properties.However,poor oxidation resistance and insufficient ablation resistance of titanium alloys at high temperature limit their wider application in high temperature and/or oxygen-rich atmosphere [ 1, 2] .Many studies were made to solve these problems by alloying [ 3, 4] or appropriate surface treatments [ 5, 6, 7] .

With good oxidation resistance,corrosion resistance and toughness,NiCrAlY coating is an ideal high-temperature coating for components of gas turbine engine system [ 8] .Recently,attempts have been made to protect titanium and its alloys using NiCrAlY coating.And studies show that NiCrAlY coating can obviously improve the oxidation resistance [ 9, 10] and hot corrosion resistance [ 11] of titanium alloy.Nevertheless,there is an incompatibility between NiCrAlY coating and titanium-based alloys under environmental attacks,which include oxygen-rich,corrosive and/or ablation atmospheres with high temperature.At high temperature,phase transformation of the coating and substrate,element interdiffusion and interfacial reaction take place in the NiCrAlY coating/titanium alloy system,which certainly deteriorate the mechanical properties of the system.Then,the stability of NiCrAlY coating is important for its application on titanium or its alloys at high temperature.At present,the relevant studies mainly focus on the oxidation resistance or interfacial reactions under oxygen-rich atmosphere of NiCrAlY coating/titanium alloy system [ 9, 10] .

In preview work [ 7] ,the interfacial reactions of NiCrAlY coating/titanium alloy were studied in detail after the vacuum heat treatment at 750-900℃for a long period.But deeper work should be done in a wider temperature range to further understand the stability and degradation of the NiCrAlY coating.It was reported thatγ'-Ni₃Al phase that increased the microhardness of the coating precipitated in NiCrAlY coating after heat treatment [ 12] .But the detailed phase transformation behavior and strengthening mechanism were unclear.Recently,Liang et al. [ 13] studied the phase stabilities of the cast NiCrAlYRe coating alloy in the temperature range of 800-1200℃by removing the factor arising from coating-substrate interdiffusion.But,the systematic reports about the stability of the NiCrAlY coating/titanium alloy,which should include phase transformation,interfacial behavior and element interdiffusion behavior,are limited.What is more,it is meaningful to further study the degradation behavior and the relationships between the degradation and the thermal exposure temperatures of NiCrAlY coating/titanium alloy system.In this paper,the stability of NiCrAlY/TC4 system under pure thermal exposure,which includes phase transformation,interfacial behavior and element interdiffusion behavior,was studied systematically in a wider temperature range of 650-1050℃.The phase transformation of NiCrAlY coating was studied in great detail using transmission electron microscopy (TEM).And the study focused on the degradation mechanism of the NiCrAlY/TC4 system at different temperatures.This work is helpful to understand the degradation of the coating/substrate system when NiCrAlY coating is used to protect titanium alloy in high-temperature atmospheres.

2 Experimental

TC4 (Ti—6Al-4V,wt%) alloy plates were cut into coupons with dimension of 15.0 mm×10.0 mm×2.5 mm.After polishing up to 500 grit abrasive paper,followed by cleaning in lye and distilled water,ultrasonic cleaning in acetone and drying in cold air,the coupons were loaded into an arc ion plating facility to deposit the Ni-30Cr-12A1-1Y (wt%) coating.Prior to deposition,the arc ion plating chamber was flooded with high-purity argon and evacuated to a background vacuum of 5×10^-3 Pa.Then,the surface of coupons was further cleaned by ion bombardment for 5-10 min with negative bias up to 600-800 V and arc current of 80-120 A.The deposition was carried out at negative bias voltage of 160-180 V and arc current of 80-120 A.

The coated coupons were heat-treated in vacuum furnace with a vacuum of 1.33×10^-3 Pa (heating rate less than8℃.min^-1) and then cooled down to room temperature.The coated coupons were pided into groups and held for 3 h in the vacuum furnace at 650,750,870,950 and 1050℃.

The microstructure was observed with scanning electron microscopy (SEM,Sirion200) and transmission electron microscopy (TEM,TecnaiG²20).The cross sections of NiCrAlY/TC4 coupons before and after thermal exposure were etched for 20 s using a mixture of HN0₃ and HF.The phase composition was characterized by X-ray diffraction(XRD,Rigaku D/Max 2500).Electron probe microanalysis(EPMA,EPMA-1610) and SEM equipped with energydispersive spectrometer (EDS,GENES IS 60E) were used to investigate the element distribution.Electron-transparent foils were ground parallel to the coating with a thickness of～30μm and then punched into disks with 3 mm in diameter for plasma thinning.The thickness of the coatings was determined by ball cratering (Calotest) with 10-mmdiameter steel ball.

3 Results and discussion

3.1 Phase stability of NiCrAlY coating

Figure 1 shows XRD patterns of NiCrAlY coating surface with and without vacuum heat treatment at different temperatures for 3 h.As shown in Fig.la,as-deposited coating is composed ofγ-Ni (fcc),α-Cr (fee) andβ-NiAl (B2).After vacuum heat treatment at 650℃,γ'-Ni₃Al phase forms in NiCrAlY coating (Fig.lb).At 750℃,NiCrAlY coating is also composed ofγ-Ni,α-Cr,β-NiAl andγ'-Ni₃Al (Fig.lb).But the intensity ofγ'-Ni₃Al diffraction peak becomes stronger than that of 650℃,suggesting thatγ'-Ni₃Al content increases significantly.However,after vacuum heat treatment at 870 and 950℃,the intensity ofγ'-Ni₃Al diffraction peak becomes weaker than that of 750℃,which indicates thatγ'-Ni₃A1 content decreases.According to the change of the diffraction peak intensity,γcontent decreases.Butβ/αcontent increases with the appearance ofγ'After vacuum heat treatment at 1050℃,only CrTi₄ (JCPDS PDF65-6818,a=b=c=0.3210 nm) is observed on the surface of NiCrAlY coating with the disappearance of all the initial coating phases.CrTi₄ is considered as a relatively Crrichβ-Ti solid solution because its crystal structure and parameters are similar to those ofβ-Ti.

Fig.1 XRD patterns of NiCrAlY coating surface with and without vacuum heat treatment at different temperatures for 3 h:a 2θrange of 30°-120°and b 20 range of 36°-45°

Figure 2 shows bright-field TEM images and selected area electron diffraction (SAED) patterns (insets) of the NiCrAlY coating before and after vacuum heat treatment at different temperatures for 3 h.As shown in Fig.2a,asdeposited poly crystalline NiCrAlY coating is composed ofγ-Ni,β-NiAl andα-Cr.The grain boundaries of as-deposited coating are indistinct,which may be associated with the formation of residual stress during the coating deposition and/or sample preparation.After vacuum heat treatment at 750℃,γ'-N1₃Al (fine dark particles in Fig.2b) precipitates inγ-Ni grain (Ni 93.2 at%,Al 4.5 at%and Cr 2.3 at%by EDS) according to Fig.2b and its inset.At 950℃,fine granularγ'-Ni₃Al phase also can be observed inγ-Ni grains as shown in Fig.2c.SAED pattern of the inset in Fig.2c shows the typical twin reflection on the{111}plane along<112>direction ofγ/γ'.Certainly,γ'-Ni₃A1 phase may precipitate alongβ-NiAl grain boundary [ 14] .From the comparison of Fig.2b and c,the distribution density ofγ'-Ni₃Al (fine dark particles in Fig.2b,c) in NiCrAlY coating after vacuum heat treatment at 750℃is higher than that of 950℃,which indicates that the content ofγ'-Ni₃Al decreases after vacuum heat treatment at 950℃.The precipitatedγ'-Ni₃Al phase of NiCrAlY coating after vacuum heat treatment is coherent with that ofγ-Ni substrate according to the insets in Fig.2b,c,which is beneficial to strengthening NiCrAlY coating.Therefore,reference shows that the best precipitation hardening effect can be obtained after vacuum heat treatment at 700-750℃for NiCrAlY coating [ 12] .

According to the discussion above,it is reasonable to consider that in isothermal atmosphere,βγ'phase transformation happens in NiCrAlY coating according to the reactionγ+β→γ'+α[^l5],which is usually associated with the Al diffusion from the NiCrAlY coating to the substrate [ 16] .Theγ'-Ni₃Al phase forms inγ-Ni substrate or alongβ-NiAl grain boundary [ 14] .The fine and dispersingγ'-Ni₃Al particles formed inγ-Ni are coherent withγ-Ni substrate,which is beneficial to strengthening NiCrAlY coating.On the other hand,the decrease inβphase content may impair the high-temperature oxidation resistance of NiCrAlY coating to a certain degree due to the higher A1 content ofβphase.With the increase in vacuum heat treatment temperature,γ→γphase transformation takes place to formγphase with lower A1 content with the diffusion of A1 from the coating to the substrate.Element Al is consumed by interfacial reactions to form Al-containing products (such as Ti₃Al [ 17] ) or dissolution inαandβ-Ti substrate for its high solid solubility inαandβ-Ti.At higher temperature (1050℃),with the diffusion of coating elements to the substrate,NiCrAlY coating phases totally decompose for the depletion of coating elements,which will be discussed in detail in the following sections.Because NiCrAlY coating phases possess different properties,the phase transformations during the vacuum heat treatment influence the coating properties,such as mechanical properties,oxidation resistance and hot corrosion resistance.For example,β→γ'→γphase transformation will result in a reduced Young,s modulus of the coating due to the different Young’s modulus values ofβ,γ'andγphases [ 14, 18] .

Fig.2 Bright-field TEM images and SAED patterns (insets) of NiCrAlY coating with and without vacuum heat treatment at different temperatures for 3 h:a as-deposited,b 750℃and c 950℃

3.2 Interface stability of NiCrAlY coating/titanium alloy

Figure 3 shows the cross-sectional backscattered electron(BSE) images of NiCrAlY coating/titanium alloy interface with and without vacuum heat treatment.As shown in Fig.3a,as-deposited NiCrAlY coating is continuous and tightly bound to the substrate without cracks and pores.And the coating is about 20μm in thickness.Just as shown in Fig.3b,c,dentate-like interfacial reaction product forms near the NiCrAlY coating/titanium alloy interface after vacuum heat treatment at 650 and 750℃.After vacuum heat treatment below the allotropic transformation temperature of titanium,Ni₃(Al,Ti)/TiNi/Ti₂Ni/Ti₃Al interface products form in turn from the NiCrAlY coating to titanium alloy substrate at the interface.Near or above the allotropic transformation temperature of titanium,Cr begins to take part in the interface reaction andα-TiCr₂phase is detected [ 7] .

According to the EPMA results (Position 1 in Table 1),the dentate-like phase is Ti₂Ni.As shown in Fig.3a-c,there are no obvious changes of the TC4 substrate near the coating/substrate interface after vacuum heat treatment at650 and 750℃.At 870℃(Fig.3d),the thickness of interface reaction layer increases,but the coating thickness decreases obviously with more violent interface reactions and element diffusion.After vacuum heat treatment at950℃,the coating thickness decreases to lower than10μm,as shown in Fig.3e.It can be observed from Fig.3d and e that the thickness of interface-reactioneffected layer between the coating and substrate increases rapidly from 40μm at 870℃to 200μm at 950℃.The Ni-and Cr-rich interface-reaction-effected layer is considered asβ-Ti stabilized layer according to EPMA results(Positions 2-7 in Table 1).As shown in Fig.3f and EPMA results (Positions 8-10 in Table 1),NiCrAlY coating disappears and only a thick interface-reaction-effected layer can be observed after vacuum heat treatment at 1050℃.It is worth noting that Kirkendall voids belt forms between the coating and substrate after vacuum heat treatment at/above 870℃for the violent element interdiffusion of the coating and substrate.At 1050℃,the Kirkendall voids aggregate to form pores as shown in Fig.3f.The appearance of Kirkendall voids and pores in the coating/substrate interface or in the substrate is certainly harmful to the properties of coating/substrate system,e.g.,adhesion of the coating and substrate and mechanical properties of the substrate.What is more,cracks,which may form during the vacuum heat treatment or the sample preparation,appear in the interface-reaction-effected layer after vacuum heat treatment at 950℃(Fig.3e).So it is enough to prove that titanium alloy substrate turns more brittle after the dissolution of coating elements (Ni,Cr and Al) in substrate.

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Table 1 EPMA results of NiCrAlY/TC4 interface after vacuum heat treatment as shown in Fig.3 (at%)

Fig.3 Cross-sectional BSE images of NiCrAlY/TC4 interface with and without vacuum heat at different temperatures for 3 h:a as-deposited,b 650℃,c 750℃,d 870℃,e 950℃and f 1050℃

According to the discussion above,it is reasonable to consider that NiCrAlY coating elements (Ni,Cr and Al) are mainly consumed by interfacial reactions between the coating and the substrate below the allotropic transformation temperature of TC4 alloy (650 and 750℃).When the heat treatment temperature is above the allotropic transformation temperature of the alloy,the transformation ofα-Ti toβ-Ti takes place in TC4 substrate.Moreover,Ni and Cr areβ-Ti stabilizing elements for titanium.Then,owing to the much higher solid solubility of Ni,Cr and Al inβ-Ti than that inα-Ti [ 17, 19] ,NiCrAlY coating elements are mainly consumed by element dissolution inβ-Ti to formβ-Ti stabilized layer above the allotropic transformation temperature of TC4 alloy.With the depletion of coating elements,NiCrAlY coating totally degrades to form a thick coating-element-richβ-Ti stabilized layer after vacuum heat treatment at 1050℃.

3.3 Elements stability of NiCrAlY coating/titanium alloy

Figure 4 shows cross-sectional BSE image and element line distribution of as-deposited NiCrAlY/T℃4 interface.As shown in Fig.4a,as-deposited NiCrAlY coating displays a lamellar structure with dark gray Al-richβ-NiAl plate and light grayγ-Ni phase matrix [ 14] .It can be observed from Fig.4b that Ni,Ti and A1 lightly interdiffuse to form an element interdiffusion zone.After vacuum heat treatment at 750℃,it is obvious that Ti diffuses from substrate to coating as shown in Fig.5c.Ni and A1 diffuse from coating to substrate at 750℃according to Fig.5b,e.As shown in Fig.5d,there is no obvious diffusion of Cr,which still distributes in coating after vacuum heat treatment at 750℃.With the increase in vacuum heat treatment temperature to 950℃,Ni,Ti and A1 obviously interdiffuse for a long distance and tend to evenly distribute in coating and substrate as shown in Fig.6b,c,e,respectively.As shown in Fig.6d,although Cr still aggregates in coating and/or interface,it obviously diffuses from coating toβ-Ti stabilized layer for a certain distance.According to Ti-Cr binary system,Cr and Ti form only TiCr₂ intermetallic compound at room temperature when Cr content reaches the range of 63 at%-66 at% [ 20] .With the consumption of coating elements Ni,Cr and A1 by interface reactions or dissolution inβ-Ti stabilized layer,the thickness of NiCrAlY coating becomes lower than 10μm,as shown in Fig.6a.

Fig.4 Cross-sectional BSE image a and element line distributions by EDS b of as-deposited NiCrAlY/TC4 interface

Fig.5 Cross-sectional SEM image a and element planar distributions of Ni b,Ti c,Cr d and Al e of NiCrAlY/TC4 interface after vacuum heat treatment at 750℃for 3 h

According to the discussion above,it is obvious that the most elements participating in interdiffusion processes are Ni,Ti and Al during vacuum heat treatments below the allotropic transformation temperature of TC4 alloy for NiCrAlY coating/titanium alloy system.Cr accumulates in coating because the diffusion coefficient of Cr is smaller than those of A1 and Ni at this temperature [ 10] .Moreover,Al may reduce the diffusion coefficient of Cr in Ni-Cr-Al system [ 17] .With the increase in temperature,the element interdiffusion becomes more violent.Above the allotropic transformation temperature of TC4 alloy,Cr obviously diffuses toward the substrate and dissolve inβ-Ti to formβ-Ti stabilized layer due to its much higher solid solubility inβ-Ti.At the same time,Cr takes part in the interface reactions.Similarly,above the allotropic transformation temperature of TC4 alloy,Ni and A1 not only take part in interface reactions but also diffuse to substrate and dissolves inβ-Ti to formβ-Ti stabilized layer due to its higher solid solubility inβ-Ti.Finally,with the depletion of coating elements,the coating phases and interface products decompose to dissolve inβ-Ti substrate.Then,NiCrAlY coating totally degrades,and the coating elements tend to evenly distribute in substrate.Maybe adding proper diffusion barriers between NiCrAlY coating and TC4 alloy is an effective way to improve the element stability of NiCrAlY coating/TC4 alloy system [ 21] .

Fig.6 Cross-sectional SEM image a and element planar distributions of Ni b,Ti c,Cr d and A1 e of NiCrAlY/TC4 interface after vacuum heat treatment at 950℃for 3 h

Fig.7 Schematic representations for coating,interface and element stability of NiCrAlY coating/TC4 substrate system with temperature increasing during pure thermal exposure

Hence,the ideal long-time working temperature for NiCrAlY coating on TC4 alloy should be below the allotropic transformation temperature of this alloy.And the stability of NiCrAlY coating/TC4 substrate system with the increase in temperature during pure thermal exposure can be described as Fig.7.As shown in Fig.7,after vacuum heat treatment below the allotropic transformation temperature of TC4 alloy,β→γ'transformation happens in the coating and an interfacial reaction layer forms.Above the allotropic transformation temperature of TC4 alloy,γ'→γtransformation takes place in the coating and aβ-Ti stabilized layer with cracks forms under the interfacial reaction layer.Finally,only a thickβ-Ti stabilized layer forms with pores,indicating the total degradation of the coating.

4 Conclusion

During the pure thermal exposure,β→γ'→γphase transformation takes place in the NiCrAlY coating.The fine and dispersingγ'phase in theγgrains is coherent withγsubstrate.The NiCrAlY coating phases totally decompose above the allotropic transformation temperature of TC4.The allotropic transformation ofα-Ti toβ-Ti of the substrate significantly influences the stability of NiCrAlY coating/TC4 substrate system.With the interdiffusion of Ni,A1 and Ti between the coating and substrate,NiCrAlY coating elements are mainly consumed by interfacial reactions below the allotropic transformation temperature of TC4.Above the allotropic transformation temperature of TC4,NiCrAlY coating elements are mainly consumed by the element dissolution inβ-Ti with the interdiffusion of Cr,Ni,Al and Ti between the coating and substrate.NiCrAlY coating elements dissolve in the substrate to form thickβ-Ti stabilized layer above the allotropic transformation temperature of TC4,which leads to the depletion of coating elements and the following degradation of the coating.

Acknowledgments

This work was financially supported by the National Natural Science Foundation of China (No.51101054),the Foundation of State Key Laboratory of Advanced Metals and Materials (No.2012-Z03) and Hunan Provincial Natural Science Foundation of China (No.14JJ3132).

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