Deformation characteristics of δ phase in the delta-processed Inconel 718 alloy

Transcription

1 MATERIALS CHARACTERIZATION 6 (200) available at Deformation characteristics of δ phase in the delta-processed Inconel 78 alloy H.Y. Zhang a,, S.H. Zhang a,, M. Cheng a, Z.X. Li b a Institute of Metal Research, Chinese Academy of Sciences, Shenyang 006, China b Beijing Institute of Aeronautica Materials, Beijing 00095, China ARTICLE DATA Article history: Received 6 July 2009 Received in revised form 0 October 2009 Accepted 3 October 2009 Keywords: Inconel 78 Delta-processed δ phase Deformation Spheroidization ABSTRACT The hot working characteristics of δ phase in the delta-processed Inconel 78 alloy during isothermal compression deformation at temperature of 950 C and strain rate of s,were studied by using optical microscope, scanning electron microscope and quantitative X-ray diffraction technique. The results showed that the dissolution of plate-like δ phase and the precipitation of spherical δ phase particles coexisted during the deformation, and the content of δ phase decreased from 7.05 wt.% to 5.4 wt.%. As a result of deformation breakage and dissolution breakage, the plate-like δ phase was spheroidized and transferred to spherical δ phase particles. In the center with largest strain, the plate-like δ phase disappeared and spherical δ phase appeared in the interior of grains and grain boundaries Elsevier Inc. All rights reserved.. Introduction Ni-based Superalloy Inconel 78 is a precipitation strengthened alloy. The metastable body centered tetragonal coherent precipitate γ (Ni 3 Nb) phase and the face-centered cubic coherent precipitate γ (Ni 3 Al) phase are strengthening phases, and the γ phase is the major strengthening phase. The equilibrium phase corresponding to the γ phase is the orthorhombic incoherent δ (Ni 3 Nb) phase [,2]. Inconel 78 is an important material used for aero-engine turbine disks. The mechanical properties of Inconel 78 are sensitive to microstructure. As turbine disks are operated in an environment of high temperature and high stresses, to ensure the high temperature strength and high resistance to low-cycle fatigue, it is crucial to obtain a uniform and fine microstructure. Generally, turbine disks are manufactured by multi-stage hot deformation processes, and it is difficult to obtain a fine microstructure by well recrystallization and less grain growth. As the δ phase in Inconel 78 can control grain size through the strong pinning effect, the Delta Process has been applied to the forging of Inconel 78, which uses an intentional δ phase precipitation cycle and subsequent thermomechanical processing to produce uniform fine grain billet and bar stock [3]. In the subsequent thermomechanical processing, to prevent the dissolution of δ phase and ensure the occurrence of dynamic recrystallization, the deformation temperature must be controlled between the dissolution temperature of δ phase and the dynamic recrystallization temperature, and the deformation rate must be low enough. Ruiz [3] obtained large size billet of Inconel 78 with the average grain size of ASTM 8 by Delta Process. Dix [4] and Bhowal [5] attained Inconel 78 turbine disk with grain size of ASTM 3 and ASTM by Delta Process and closed die forging, respectively. Lv [6] obtained Inconel 78 sheet with grain size of ASTM 2 by Delta Process and rolling. Yuan [7] and Wang [8] investigated the effect of δ phase on the deformation behaviors of delta-processed Inconel 78, and their results showed that the δ phase can stimulate the occurrence of Corresponding authors. Tel.: ; fax: addresses: haiyanzhang@imr.ac.cn (H.Y. Zhang), shzhang@imr.ac.cn (S.H. Zhang) /$ see front matter 2009 Elsevier Inc. All rights reserved. doi:0.06/j.matchar

2 50 MATERIALS CHARACTERIZATION 6 (200) dynamic recrystallization. However, there are few reports about the deformation behaviors of δ phase during hot working. Cone [9] observed the development of δ phase during different δ phase precipitation cycle and rolling. In the present work, for the Delta Process of Inconel 78 alloy, isothermal compression test was carried out to study the deformation characteristics of δ phase during hot working. 2. Experimental Procedures The material used in this study is cut from a commercially available Inconel 78 wrought bar with a diameter of 250 mm. The chemical compositions (in wt.%) of the alloy are as follows: C, 0.027; Ni, 53.74; Cr, 7.58; Nb, 5.35; Mo, 3.0; Ti, 0.98; Al, 0.52; Mn, 0.07; Si, 0.009; B, ; Fe, balance. There is a little difference in microstructure at the center, mid-radius and outer profile. To minimize the scatter of the initial microstructure, the samples were cut from an outer diameter circle. The samples were first solution treated at 040 C for 45 min, followed by water quenching. The peak temperature of δ phase precipitation in Inconel 78 is about 900 C [0]. When the ratio of (Al+Ti) to Nb (in at.%) is higher than 0.66, the γ and γ phases can be precipitated between 700 and 900 C in short aging time [].But with the increasing of aging time, the γ and γ phases will transfer to δ phase. As the aging time is more than 24 h, there are no γ and γ phases existing [7]. The ratio of (Al+Ti) to Nb (in at.%) in the alloy used in this work is 0.69 (higher than 0.66). To ensure only δ phase precipitating in the alloy, the solution treated samples were aging treated at 900 C for 32 h, followed by water quenching. The delta-processed samples were machined into cylindrical specimens with a diameter of 8 mm and a height of 2 mm for the hot compression tests. The dissolution temperature of δ phase in Inconel 78 rises with the increasing of the content of Nb [], and as the temperature is higher than 980 C, the δ phase dissolves largely [2]. The results in Ref. [3] showed that the dynamic recrystallization temperature of Inconel 78 is higher than 850 C. Therefore, the isothermal compression test was conducted at temperature of 950 C and strain rate of s to study the deformation characteristics of δ phase during hot working. The compression test was carried out on a MMS-300 thermomechanical simulator. The specimens were heated to the test temperature at a rate of 5 C/s and held for 80 s, and deformed to a true strain of.. To retain the deformation microstructure, the deformed specimens were quenched by water as soon as the compression tests were completed. The deformed specimens were sectioned parallel to the compression axis for microstructure analysis. The microstructure was observed by an optical microscope (OM), and the morphology of δ phase was observed by a scanning electron microscope (SEM). The content of δ phase was measured by the quantitative X-ray diffraction (XRD) method. To improve the measuring precision due to the small δ phase content, the X-ray diffraction samples were prepared by mechanical polishing followed by chemical etching. For there are just the NbC, δ and γ phases in the samples, the contents of the NbC, δ and γ phases can be determined by Eqs. () (3). W γ = ρ γ d W δ ρ δ W NbC W δ n n i m m i = ρ NbC ρ δ ði γ i =Rγ i Þ d ði δ i =Rδ i Þ k k i m m i W NbC + W γ + W δ = ði NbC i =R NbC i ði δ i =Rδ i Þ Þ where W NbC, W δ and W γ are the content of the NbC, δ and γ phases in wt.%, respectively, ρ NbC, ρ δ and ρ γ are the volume density of the NbC, δ and γ phases, respectively, I i NbC, I i δ and I i γ are the integrated intensities from the NbC, δ and γ phases, respectively, k, m and n are the number of selected diffraction peaks, and the value of R i are given in Ref. [4]. 3. Results and Discussion Fig. a and b shows the microstructure and morphology of δ phase after holding at 950 C for 80 s before deformation, Fig. Optical microstructure (a), and SEM morphology and distribution of δ phase (b) after holding at 950 C for 80 s before deformation. ðþ ð2þ ð3þ

3 MATERIALS CHARACTERIZATION 6 (200) respectively. It can be clearly seen that the plate-like δ phase distributes throughout the grain structure. The initial microstructure before deformation is Widmanstätten δ, and the content of δ phase is 7.05 wt.%. Due to the effect of friction between the ends of specimen and dies, the strain in different zones of deformed specimen is different. The strain in the center is largest, and there is a small dead zone in each end. The content of δ phase after deformed is 5.4 wt.%, which indicates the dissolution of δ phase during the deformation. The microstructure and morphology of δ phase in the zone between the center and the end are shown in Fig. 2a andb, respectively. In this zone, there are a lot of plate-like δ phase and a little of spherical δ phase particles. The plate-like δ phase, of which the orientation is not perpendicular to compression direction, is distorted obviously during hot working, and the deformation breakage occurs in the severely deformed parts, which has also been observed in Ref. [9]. For the size of plate-like δ phase is large, the moving dislocations are piled in the vicinity of plate-like δ phase during the deformation, which can be confirmed by Ref. [8]. Thus, there is stress concentration in the vicinity of plate-like δ phase, and as the stress reaches the break limit of δ phase, the breakage occurs. Furthermore, the dissolution breakage takes place in some parts of plate-like δ Fig. 2 Optical microstructure (a), and SEM morphology and distribution of δ phase (b) in the area between the center and the end. phase during hot working. The crystallographic orientation relationship between the δ phase and the matrix γ phase is expressed as {} γ (00) δ, < 0 > γ jj½00š δ []. There are twelve orientation variants of plate-like δ phase precipitating in the matrix. In Fig. 2a, the plate-like δ phase is distorted during the deformation. Thus, most of the plate-like δ phase loses the orientation relationship with the matrix, and become unstable, which supplies driving force for the dissolution of plate-like δ phase. According to Ref. [5,6], subgrain boundaries and high density dislocation zones exist in the plate-like δ phase, which can result in the forming of grooves due to the effect of the tension at the phase interface. Compared with planes, the curvature of grooves is less. The dissolution of the δ phase occurs preferentially at the grooves, which breaks the balance of tension at the phase interface. To keep the balance, the platelike δ phase dissolves more at the grooves. As a result, the platelike δ phase dissolves gradually at the grooves, then breaks into pieces. During the deformation, the amount of subgrain boundaries and high density dislocation zones in the platelike δ phase increase, which can promote the occurrence of dissolution breakage. The dissolution breakage of the plate-like δ phase during the deformation has also been found in the tensile test of Inconel 78 [7]. Fig. 3a and b shows the microstructure and morphology of δ phase in the center, respectively. There is no plate-like δ phase in the center, and spherical δ phase particles appear in the interior of grains and grain boundaries. Figs. 2 and 3 indicates that the spheroidization of plate-like δ phase occurs during the deformation due to the effect of deformation breakage and dissolution breakage. Fig. 4 shows the schematic for the spheroidization of plate-like δ phase. For comparison purpose, Fig. 5a and b shows the microstructure and morphology of δ phase with undeformed after holding at 950 C for 400 s, corresponding to the deformation time, respectively. After holding, the content of δ phase is 4.37 wt.%. It indicates that the dissolution of plate-like δ phase occurs during the holding. In Fig. 5, the plate-like δ phase is broken into several spherical δ phase particles, which agrees with the reports in Ref. [5,6]. The saturation of δ phase precipitation in Inconel 78 depends on the aging temperature and the alloy compositions [7,8]. The driving force of δ phase precipitation increases with the increasing of the content of Nb, and decreases with the increasing of the content of (Al+Ti) and the ratio of Al to Ti (in wt.%) [2,7]. The saturation of δ phase in Inconel 78 is about 7.37 wt.% at 950 C, in which the contents of Nb, Al and Ti (in wt.%) are 5.373, and 0.9, respectively [8]. The content of Nb, Al and Ti in the alloy used in this work is close to them in Ref. [8]. Thus, the saturation of δ phase in the used alloy at 950 C can be considered to be 7.37 wt.%. The stable morphology of δ phase precipitation depends on the temperature [9]. The stable morphology of δ phase is plate-like at the temperature below 930 C, and as the temperature above 00 C, the stable morphology of δ phase is spherical. At the temperature between 930 and 00 C, the stable morphology is plate-like as well as spherical. The initial content of δ phase before deformation is 7.05 wt.%, which is a little less than the saturation of 7.37 wt.% at 950 C, and the morphology of δ phase is mostly plate-like. For the δ phase is an incoherent phase to the matrix γ [,2,7], the phase interface energy

4 52 MATERIALS CHARACTERIZATION 6 (200) Fig. 3 Optical microstructure (a), and SEM morphology and distribution of δ phase (b) in the center. between the δ phase and the matrix γ is high. When held at 950 C, the plate-like δ phase is less energetically stable than the spherical δ phase. This implies a trend of the dissolution of plate-like δ phase during the holding. For subgrain boundaries and high density dislocation zones exist in the plate-like δ phase, where the dissolution occurs preferentially. Thus, the dissolution breakage of plate-like δ phase was observed in both the deformed alloy and the undeformed one after holding, and the corresponding content of δ phase decreases. The δ phase content of 5.4 wt.% in the deformed alloy is more than that of 4.37 wt.% in the alloy with undeformed after holding for the corresponding deformation time. The precipitation and dissolution of δ phase are diffusion processes, in which the diffusion of Nb is crucial. Because the moving dislocations Fig. 5 Optical microstructure (a), and SEM morphology and distribution of δ phase (b) with undeformed after holding at 950 C for 400 s. are piled in the vicinity of plate-like δ phase during hot working, the element Nb can be sucked to the surrounding of dislocation line under the stress filed generated by dislocations, which can offer a fast path for the diffusion of Nb. The relationship between δ phase and matrix γ is incoherent [,2,7]. It is difficult for the δ phase precipitating from the grain interiors of matrix γ directly. During hot working, there are more microdefects in the alloy, e.g. dislocation, subgrain and twin, which can decrease the energy of δ phase nucleating from the matrix γ.theδ phase content in the alloy is lower than the saturation of δ phase at 950 C. Thus, it is possible that spherical δ phase particles precipitate from the matrix during the deformation. This can be supported by the reported in Ref. [7], in which there are Fig. 4 Schematic for the spheroidization of plate-like δ phase.

5 MATERIALS CHARACTERIZATION 6 (200) spherical δ phase particles precipitating during the superplastic deformation of Inconel 78 alloy. 4. Conclusions () During the deformation, the dissolution of plate-like δ phase occurred. In addition, there were spherical δ phase particles precipitating. The content of δ phase decreased from 7.05 wt.% to 5.4 wt.%. (2) Due to the effect of deformation breakage and dissolution breakage, the spheroidization of plate-like δ phase took place during the deformation, and the plate-like δ phase was transferred to spherical δ phase particles. (3) In the center with largest strain, the plate-like δ phase was transferred to spherical δ phase particles completely. Acknowledgement This work has been supported by the National Nature Science Foundation of China with the Grant Number: REFERENCES [] Sundararaman M, Mukhopadhyay P, Banerjee S. Precipation of the δ-ni3nb phase in two nickel base superalloys. Metall Trans, A 988;9A: [2] Collier JP, Wong SH, Phillips JC, Tien JK. The effect of varying Al, Ti, and Nb content on the phase stability of Inconel 78. Metall Trans, A 988;9A: [3] Ruiz C, Obabueki A, Gillespie K. Evaluation of the microstructure and mechanical properties of Delta processed alloy 78. Superalloys 992. Warrendale, PA: TMS; 992. p [4] Dix AW, Hyzak JM, Singh RP. Application of ultra fine grain alloy 78 forging billet. Superalloys 992. Warrendale, PA: TMS; 992. p [5] Bhowal PR, Schirra JJ. Full scale Gatorizing TM of fine grain Inconel78. Superalloys 78, 625, 706 and various derivatives. Warrendale, PA: TMS; 200. p [6] Lv HJ, Yao CG, Zhang KF, Jia XC. Fine-grain forming process, mechanism and properties of GH469 alloy. Materials for Mechanical Engineering 2003;27(): (in Chinese). [7] Yuan H, Liu WC. Effect of the delta phase on the hot deformation behavior of Inconel 78. Mater Sci Eng, A 2005;408:28 9. [8] Wang Y, Zhen L, Shao WZ, Yang L, Zhang XM. Hot working characteristics and dynamic recrystallization of delta-processed superalloy 78. J Alloy Compd 2009;44:34 6. [9] Cone FP. Observations on the development of delta phase in IN78 alloy. Superalloys 78, 625, 706 and various derivatives. PA: TMS; 200. p [0] Thomas A, El-Wahabi M, Cabrera JM, Prado JM. High temperature deformation of Inconel 78. J Mater Process Technol 2006;77: [] Schafrik RE, Ward DD, Groh JR. Application of alloy 78 in GE aircraft engines: past, present and next five years. Superalloys 78, 625, 706 and various derivatives. Warrendale, PA: TMS; 200. p.. [2] Hu J.P. Numerical simulation of superalloy In78 and Gatorized Waspaloy during hot working. Ph.D Dissertation, Steel Research Institute, 999. [in Chinese]. [3] Zhao D, Chaudhury PK. Effect of starting grain size on as-deformed microstructure in high temperature deformation on alloy 78. Superalloys 78, 625, 706 and Various Derivatives. Warrendale, PA: TMS; 994. p [4] Liu WC, Xiao FR, Yao M. Quantitative phase analysis of Inconel 78 by X-ray diffraction. J Mater Let 997;6: [5] Cai DY, Zhang WH, Liu WC, Yao M. Dissolution behavior of δ phase in Inconel 78. Journal of Iron and Steel Research, 4; p (in Chinese). [6] Cai DY, Zhang WH, Nie PX, Liu WC, Yao M. Dissolution kinetics and behavior of phase in Inconel 78. Trans Nonferrous Met Soc China 2003;3(6): [7] Huang Y, Langdon TG. The evolution of delta-phase in a superplastic Inconel 78 alloy. J Mater Sci 2007;42:42 7. [8] Stockinger M, Kozeschnik E, Buchmayr B, Horvath W. Modeling of δ phase dissolution during preheating of Inconel78 turbine disks. Superalloys 78, 625, 706 and Various Derivatives. Warrendale, PA: TMS; 200. p [9] Desvallées Y, Bouzidi M, Bois F, Beaude N. Delta phase in Inconel78: mechanical properties and forging process requirements. Superalloys 78, 625, 706 and Various Derivatives. Warrendale, PA: TMS; 994. p