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Rare Metals2019年第9期

Microstructure and properties after deformation and aging process of A286 superalloy

Si-Cheng Liu Yun Gao Zhong-Liang Lin Shuang-Shuang Guo Xiao-Bin Zhang Xiao-Jian Yin

Research Institute of Aerospace Precision Products Co.,Ltd

作者简介：Si-Cheng Liu e-mail:yang_haut@163.com;

收稿日期：14 September 2017

基金：financially supported by the National Natural Science Foundation of China(No. 11502285);

Microstructure and properties after deformation and aging process of A286 superalloy

Si-Cheng Liu Yun Gao Zhong-Liang Lin Shuang-Shuang Guo Xiao-Bin Zhang Xiao-Jian Yin

Research Institute of Aerospace Precision Products Co.,Ltd

Abstract：

This study focuses on the effects of different cold-drawing deformations and aging treatments of solid solution A286 superalloy.The grain configuration,texture,precipitates and tensile strength of A286 superalloy after different deformations and aging treatments were investigated by optical microscopy(OM),field emission scanning electron microscopy(FESEM),electron back-scattered diffraction(EBSD)and mechanical testing machine.The grain size and configuration of A286 alloy can hardly be changed during aging process.The initially equiaxial and twinned crystals are obvious when deformation is less than30%,while the grain boundaries become blurry and slip bands appear after 35% deformation or more.γ' phase and Cr-rich carbide are the precipitates of A286 alloy.For each deformation,γ' phase plays a major role during aging and its amount increases gradually when the aging temperature changes from 650 to 680℃,and a maximum tensile strength appears when following two-stage aging.With deformation increasing,the amounts of γ' phase and Crrich carbide increase in varying degrees.Meanwhile,the<111>wire textures become more obvious,the tensile strength is enhanced and the kernel average misorientation(KAM)increases gradually;the higher KAM of crystal lattices diffuses from the grain boundary to the matrix gradually.

Keyword：

A286 alloy; Deformation; Aging; Precipitate; Texture; Tensile strength;

Received： 14 September 2017

1 Introduction

A286 superalloy is a kind of precipitation-hardenable austenitic stainless steel [ 1] ,It is strengthened by the fccγ'phase which is coherent with the austenite matrix and formed during aging process.This alloy exhibits good mechanical properties,i.e.,fatigue property,at service eonditions,and also wonderful corrosion resistance due to the elevated Cr element content.Because of its excellent yield strength,permanence strength and wonderful forming property below 650℃,it is widely used to produce manufacture supporting parts in engine area and 1100 MPa strength-level fastener products,such as blind rivets,bolts and nuts [ 2, 3, 4, 5, 6] .

In the previous study,the effects of heat treatment on mechanical properties and microstructure were researched.The result indicates that adjusting the type,amount,size and distribution of precipitates through heat treatment(650-720℃in general) can meet different manufacturing requirements [ 7, 8, 9, 10, 11] .As one of the mainly used wrought superalloys,the deformation and heat treatment of A286alloy may have great influences on the mierostructural evolution and properties promotion [ 12, 13] .However,farther details or explanations are lacking due to the limited amount of experiments and data.Therefore,this study is aimed to predict the phase transformation of A286 alloy associated with different deformations and the evolution behaviors of microstructure and mechanical properties in continuing isothermal aging treatment.The paper has mainly focused on the changing of grain configuration,texture,precipitates and tensile strength after different deformations and aging processes to evaluate their influence on microstructure and mechanical properties.

2 Experimental

2.1 Material

The material of the study is a lubricant coiled wire in solution heat-treated condition.It is 4.02 mm in diameter with chemical composition listed in Table 1.The A286alloy was cold drawn by different deformation degrees of0%,15%,20%,30%,35%,40%and 50%,respectively.The deformation value is calculated by the equation,as follows:

where K is the deformation value,d₁ is the original diameter of the alloy and d₂ is the new diameter of the alloy after cold drawing.

2.2 Experimental procedure

Samples of the material were prepared using standard mechanical polishing procedures and etched with 20 g CuSO_4·5H₂O,5 ml H₂SO₄,50 ml HCl and 100 ml H₂O.Optical microscope (OM,Leica DMI 5000 M) was used to observe the microstructure,and field emission scanning electron microscope (FESEM,ZEISS Sigma-12) observations were performed at 15 kV.The composition of both matrix and precipitates was analyzed by energy-dispersive X-ray spectroscopy (EDS) using Bruker Nano Detector 6system interfaced to SEM.The change of texture and kernel average misorientation (KAM) was analyzed by the Bruker nano-electron back-scattered diffraction (EBSD)equipment.The standard mechanical tensile specimens were prepared following ASTM E8 specification,and CMT305-30T testing machine was employed to perform tensile strength tests.Aging treatments after different deformations were carried out by a vacuum temper furnace and cooled by Ar,and the aging projects are presented in Fig.1.

3 Results and discussion

3.1 Micros true ture characteristic

Figure 2 shows OM images of solid solution A286 alloy following different deformations.The grain configuration which is initially equiaxial becomes fibrous with deformation growing.More equiaxial and twinned crystals appear when deformation is less than 30%,while the grain boundaries become blurry and slip bands appear after 35%deformation or more.However,the grain size and configuration can hardly be changed during aging process,as shown in Fig.3.

Fig.1 Aging projects of A286 alloy

The theoretical equation of recrystallization temperature for alloy is defined by the following equation:

where T_r is the recrystallization temperature (K) and T_m is the melting point (K).The melting point of A286 is about1637-1697 K,so the recrystallization temperature is727℃when calculated by the equation using the median value of the coefficient and T_m.The highest aging temperature of this study is 680℃,which is below T_r,so there is no change in the microstructure before and after aging process.This result is consistent with that in Refs. [ 14, 15] in which grain size of A286 had no change after aging blow790℃.

Figure 4 shows SEM images ofγ'phase after different aging heat treatments with deformations of 20%,35%and40%.As seen in Fig.4a,there are no precipitates after solid solution.The sphericalγ'phase with an average size of 20-40 nm distributed in austenitic (γ) grains can be observed after aging [ 16, 17, 18, 19] .Relatively higher aging temperature makes the precipitation of new phases,so moreγ'phases appear obviously after 680 than 650℃

during aging process,and the mostγ'phase precipitates after two-stage aging (680+650℃) [ 20, 21] .The amount ofγ'phase increases slightly with deformation,and the size ofγ'particles becomes small with the deformation of 40%,as seen in Fig.4c,e and f.

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Table 1 Composition of A286 alloy (wt%)

Fig.2 OM images of A286 alloy before aging following different deformations:a 15%,b 20%,c 30%,d 35%,e 40%and f 50%

Fig.3 OM images of A286 alloy with 20%deformation after different heat treatments:a solid solution,b 650℃aging,c 680℃aging and d two-stage aging

Table 2 lists EDS composition of two kinds of precipitates,and the corresponding EDS values obtained from these particles are shown separately in Fig.5,indicating that these two kinds of particles are Cr-rich carbide and titanium compounds.Figure 5 shows the micro structure and distribution of Precipitate 1 which is identified as Crrich carbide after different aging processes with deformation of 20%,35%and 40%.Cr-rich carbide is smaller than200 nm and distributes on grain boundaries,slip bands and twin boundaries where the nucleation potential barrier is lower.As aging process changes from Project A to Project C,the amount of Cr-rich carbide increases slightly on grain boundaries (Fig.5a-d).While the Cr-rich carbide increases clearly with deformation changing from 20%to 40%,the distortion energy raises and more twin boundaries and slip bands obtain after large deformation (Fig.5c,e,f).Bothγ'phase and Cr-rich carbide can strengthen A286 alloy together because of the pinning of dislocation and coherency strain with matrix [ 22, 23, 24] .Figure 5d also shows the microstructure of Precipitate 2,which is considered as titanium compound.These compounds consisting of carbide (TiC) and nitride (TiN) exhibit an angular morphology [ 25] .And they are formed during solidification process [ 26] .

Fig.4 SEM images of A286 alloy after different treatments:a 20%deformation,b 20%deformation and 650℃aging,c 20%deformation and680℃aging,d 20%deformation and two-stage aging,e 35%deformation and 680℃aging and f 40%deformation and 680℃aging

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Table 2 Composition of precipitates of A286 alloy (wt%)

Fig.5 SEM images for distribution of precipitates:a 20%deformation,b 20%deformation and 650℃aging,c 20%deformation and 680℃aging,d 20%deformation and two-stage aging,e 35%deformation and 680℃aging and f 40%deformation and 680℃aging

3.2 Texture analysis

Figure 6 shows the orientation distribution function (ODF)images (ψ₂=45°) and inverse pole figures (IPF) maps in terms of the tensile direction (ND) of solid solution A286alloy after different deformations.With deformationincreasing,the preferred orientation of<111>appearsgradually,compared with the random texture of theundeformed sample.

Fig.6 ODF and ND-IPF maps of solid solution A286 alloy after different deformations:a 0%,b 20%,c 35%and d 40%

Fig.7 KAM maps of solid solution A286 alloy after different deformations:a 0%,b 20%,c 35%and d 40%

Fig.8 Evolution of KAM distribution after different deformations based on KAM maps

Figure 7 shows KAM maps overlapped with grain boundaries of solid solution A286 alloy after different deformations.The KAM value calculates the average misorientation between a pixel i and its neighbors,with the proviso that the misorientation does not exceed a predefined threshold value (usually 5°).The color of blue to red represents the KAM value in the range of 0-5.The tensile direction of samples in Fig.7 is horizontal.The undeformed sample exhibits low misorientation,since the color of the KAM map is almost blue in Fig.7a.With the deformation increasing up to 20%,a green color around grain boundaries can be observed obviously,but most of crystals in grains still keep free misorientation in Fig.7b.More green areas spread into grains and slip bands appear as the deformation increases to 35%in Fig.7c.Moreover,the KAM map of 40%deformation in Fig.7d even reveals yellow in grains and grain boundaries.This means that with deformation increasing,crystals lattice rotation also increases and is mainly localized at grain boundaries,which means that the geometrically necessary dislocations(GNDs) density increases near grain boundaries which is a sign for strain incompatibility in this area [ 27, 28, 29, 30, 31] .So,the dislocation generates and develops into slip bands.Thus,the amount of precipitates on grain boundaries increases obviously with deformation increasing,as shown in Fig.5.

Figure 8 shows the evolution of KAM distribution of solid solution A286 alloy for different deformations.The KAM value is about 0.8°for undeformed sample.With deformation increasing,the KAM distribution curve is more asymmetric and widespread and the average misorientation raises simultaneously.The KAM is up to 1.5°when the deformation is 40%.

3.3 Tensile strength analysis

Figure 9 shows the tensile strength of A286 alloy at room temperature after different deformations and heat treatments.For a certain heat treatment,the tensile strength increases with deformation increasing.For each deformation,alloy shows the lowest tensile strength after solid solution,while the tensile strength increases with aging temperature changing from 650 to 680℃and the maximum appears after two-stage aging.

Fig.9 Tensile strength of A286 alloy

According to the result of microstructural analysis that the deformation is beneficial for the precipitation of strengthening phase,the tensile strength increases with deformation.Compared with 650℃aging heat treatment,moreγ'phases appear following 680℃aging and twostage aging.The amount and size ofγ'phase influence the tensile strength directly,and the sample after two-stage aging shows the highest tensile strength.

4 Conclusion

The initially equiaxial and twinned crystals of A286 alloy appear when deformation is less than 30%,while the grain boundaries become blurry and slip bands appear after 35%deformation or more.But the grain configuration can be hardly changed during aging process.The deformation and aging heat treatment can change the quantity and size of the precipitates of A286 alloy,which consist ofγ'phase and Cr-rich carbide.

With deformation increasing,the<111>wire textures become more obvious,the tensile strength is enhanced,the kernel average misorientation (KAM) increases gradually and the higher KAM of crystal lattices diffuses from the grain boundary to the matrix gradually.For a certain heat treatment,the tensile strength increases with deformation growing.For each deformation,the maximum tensile strength appears after two-stage aging.

Acknowledgements This study was financially supported by the National Natural Science Foundation of China (No.11502285).