RESEARCH ON INCONEL 718 TYPE ALLOYS WITH IMPROVEMENT OF TEMPERATURE CAPABILITY

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1 RESEARCH ON INCONEL 718 TYPE ALLOYS WITH IMPROVEMENT OF TEMPERATURE CAPABILITY Shuhng Fu 1,2, Jianxin Dng 1, Maicang Zhang 1, Ning Wang 3, Xishan Xie 1 1 High Temperature Materials Research Labratries, University f Science & Technlgy Beijing; Beijing , China 2 Natinal Key Labratry f Science and Technlgy n Advanced High Temperature Structural Materials Beijing Institute f Aernautical Materials, Beijing , China 3 Basteel Grup Crpratin, N.1269 Shuichan Rad, Shanghai, , China Keywrds: 718 type allys, Temperature capability, δ phase Abstract Incnel 718 was develped by Internatinal Nickel Cmpany in 1962, has becme the mst widely used superally in the wrld. The ally s ppularity is due t its excellent cmbinatin f mechanical prperties, mderate price and gd prcessability. Hwever, its maximum use temperature is restricted t 650ºC because the main strengthening γ phase respnsible fr the ally s utstanding prperties rapidly verages. There has been a substantial amunt f wrk by numerus investigatins ver the years t increase the temperature capability f Incnel 718. Xie and Dng et al. has systematically studied the effect f variatin f allying elements including Al, Ti, Nb, P, B, S and Mg etc. cntent n structural stability and mechanical prperties by experiments and thermdynamic calculatins. This attempt was made t imprve the temperature capability f Incnel 718 via cntrlling the mrphlgy f γ and γ phase and increase their slvus temperature by variatin f Al, Ti and Nb cntent, and W additin fr slid slutin strengthening. The phases precipitatin behavirs, lng time micrstructure stability and mechanical prperties f mdified 718 type allys have been systematically studied. The results indicate that cmpact mrphlgy f γ and γ has been bserved in grains. In additin, a new stable glbular phase with higher slvus temperature mainly precipitated at grain bundaries, different t the δ phase in cnventinal 718 ally, has been bserved in mdified 718 type allys (this phase is temprarily named δ phase fr descriptin cnvenience). After 680ºC aging fr 1,000h, the cmbinatin f δ granular phase at grain bundaries and cmpact mrphlgy precipitatin f γ and γ phase in grains characterize with superir thermal stability. Preliminary mechanical prperties test results shw that the ally with this kind f micrstructure can prvide better stress rupture prperties and fatigue resistance than that f cnventinal Incnel 718. It appears t prvide a new apprach fr imprvement f temperature capability f Incnel 718 and the new designed 718 type ally can be used at 680ºC r higher temperatures. Intrductin Incnel 718 is a nickel-base superally strengthened mainly by Ni 3 Nb type γ and partially by Ni 3 Al type γ precipitatin. Since Incnel 718 entered prductin, it experienced a perid f 281

2 rapid grwth and applicatin int a brad range f high temperature industries due t its unique characteristics and affrdability. Incnel 718 is still tday s mst widely used superally in the wrld [1, 2]. Despite the enrmus success f Incnel 718, current applicatins f Incnel 718 are still limited because f the metastability f γ -strengthening system and it can nt be used at temperatures higher than 650ºC. Hwever, the rapid develpment f aer-engines has led t stringent requirements f the ally that can be used at 680~700ºC, and meanwhile characterizes with superir micrstructure stability after lng time service, gd frmability and weldability. Numerus attempts have been made ver the past several decades t imprve temperature capability f Incnel 718, typically thrugh chemistry mdificatins invlving adjustment the cntent f Al, Ti and Nb and minr elements such as P and B. Czar and Pineau firstly develped the cmpact mrphlgy f γ and γ by adjustment Nb, Ti and Al cntent and their ratis, and fund this special cmpact micrstructure characterized with superir structure stability than that f the separated precipitatin f γ and γ [3, 4]. Subsequently, Tien, Gu, Radavich and Chang et al. adjusted the Al/Ti rati and (Al+Ti)/Nb rati based n cnventinal Incnel 718 ally, and the experimental results indicate that higher Al/Ti rati and (Al+Ti)/Nb rati can remarkably imprve the structure stability f 718 ally [5-15]. Hwever, these experimental results mainly cncentrate n the imprtance f γ and γ stability during lng time aging and neglect ther phases precipitatin behavir. Ca and Kennedy develped Allvac 718- ER ally by increasing P and B cntent and expectatin f the mdified 718 ally t be used at 700ºC, but it appeared that P and B mdificatin did nt have a nticeable effect n micrstructural stability at 700ºC/1,000h aging and the imprvement in temperature capability is nt greater than 25ºC [16, 17]. Hu et al. develped a mdified 718 ally by just keeping higher cntent f P and B and the experimental results indicate that this ally can be used at 680ºC [18-19]. Xie and Dng et al. has systematically studied the effect f allying elements including Al, Ti, Nb, P, B, S, and Mg etc. in Incnel 718 ally n structure stability and mechanical prperties. The results reveal that increasing Al cntent can bviusly increase γ /γ slvus temperature and imprve structure stability f 718 ally. Higher Ti cntent will result in large amunt f η phase frmatin. Higher Nb cntent can remarkably imprve the strength f ally 718[20-22]. The cmbinatin f 0.012~0.015%P with 0.005~0.01%B can nticeably imprve the stress rupture life f ally 718, but the structure stability can nt be increased [23, 24]. Recently, Allvac has develped 718Plus with 10%C additin and adjustment f Al and Ti cntents, which appears t have achieving the gal f increasing the temperature capacity Incnel 718 by 50ºC [25-27]. Hwever, China is in shrtage f cbalt. Therefre, the ally withut C additin that can be used at 680ºC r higher temperatures shuld be develped in China fr dmestic aviatin and ther industries. Materials and Experimental Prcedure Previus Therm-Calc calculatin indicates that γ /γ slvus is 904 C fr cnventinal Incnel 718 ally. The effect f increasing Al cntent shws a strng increase in γ /γ slvus [28]. An increase f Ti cntent has a mre mild effect n γ /γ slvus than Al. Nibium cntent shuld be kept at higher level t guarantee high strength. T imprve the stress rupture prperties, P and B shuld be kept at higher cntent. W is a strng slid slutin element t strengthen the γ matrix. Based n ur experimental results, several mdified 718 type allys have been prepared fr this investigatin as 282

3 shwn in Table I. Ally 1 is the cnventinal 718 ally with 0.5%Al and Ally 2 is a mdified 718 type ally with higher Al cntent. Additin 1.5% W t Ally 2 can get Ally 3. Ally 4 has higher Al cntent in cmparisn with Ally 2. Additin 1.5% W t Ally 4 can get Ally 5. All these allys were vacuum inductin melted (VIM). Ingts were hmgenized after cnditining. Hmgenized ingts were then frged and heat treated. The heat treatment fr these 718 type allys was the standard rutine heat treatment fr cnventinal 718 ally (954ºC/1h, AC, 720ºC/8h, FC 50ºC/h t 620ºC and hld fr 8h at 620ºC, AC). After heat treatment, these 718 type allys were aging at 680 C fr 100h~1,000h t study the structure stability. Fr accelerated evaluatin n lng term structure stability, experimental thermal expsure was als cnducted at higher temperature 760 C fr 100h. Table I. Chemical cmpsitin f 718 type allys (wt %) Ally C Ni Fe Cr M Al Ti Nb W S P B Bal < Bal Bal Bal Bal Sample Slutin N.1 Slutin N.3 Slutin N.2 Fig.1 Prcedure fr phase extractin and separatin Table II Slutins and cnditins fr electrlytic extractin f phases in mdified 718 type allys Slutin N. γ +γ +δ +MC Residue <A> δ+mc Residue <B> Slutin N.4 δ+ MC Residue <C> MC Residue <D> Chemical cmpsitins f slutins and electrlytic extractin cnditins 1 1%(NH 4 )SO 4 +2%C 4 H 6 O 6 + H 2 O; T=10, i=0.025~0.03a/cm 2 2 5%H 2 SO 4 +2~3%C 4 H 6 O 6 + H 2 O; Biling water fr 3~3.5h 5%HCl+5%C 3 H 5 (OH) 3 +1~2%C 6 H 8 O 7 H 2 O+CH 3 OH; 3 T=-10~-5, i=0.08~0.1a/cm ~15%HCl+2%C 4 H 6 O 6 +CH 3 CH 2 OH; Distillatin γ +γ = Residue <A>- Residue<B> δ = Residue<C>- Residue <D> MC =Residue < D> In rder t identify the phases in these mdified 718 type allys, X-ray diffractin technique (XRD) and EDS were used. SEM was used t bserve the mrphlgy and distributin f the precipitated 283

4 phases. Physic-chemistry phase analyses were used t cnduct the quantitative determinatin and chemical cmpsitin f all phases in these allys. The precipitated phases fr X-ray diffractin studies were extracted electrlytically frm the samples in different slutins fr different phases. Fig.1 and Table II shws the methd f phase extractin and separatin used fr XRD and micrchemical phase analyses. Experimental Results Micrstructure f 718 Type Allys at As-heat Treated Cnditin Fig.1 shws the micrstructures f experimental allys after fully heat treatment. Needle-like δ phase precipitatin is distributed mainly at grain bundaries in cnventinal 718 Ally 1 (Fig.1a). Hwever, the mrphlgy and distributin f phases in ther allys have bvius differences t thse in Ally 1. Firstly, the grain bundary phase that can be reslved in ther allys is in shrt rd r granular mrphlgy, which is different t needle-like δ phase in cnventinal 718 Ally 1. Secndly, δ phase in Ally 1 is mainly distributed at grain bundaries while shrt rd r granular phase in Ally 3 is distributed nt nly at grain bundaries but als in the grains (Fig.2a, b). Mrever, except the granular phase, there are still sme shrt needle-like phases distributed at grain bundaries in Ally 3 and Ally 4(Fig.2b, c), while there seems t be nly shrt rd r granular phase in Ally 5(Fig.2d). (b) (c) (d) Fig. 2 SEM micrstructures f Ally 1 (b)ally 3 (c)ally 4 (d)ally 5 at as-heat treated cnditin 284

5 Lng Time Structure Stability f 718 Type Allys Fig.3 reveals that during thermal expsure at 680 C fr 1,000h, the main strengthening phase γ in Ally 1 carsen rapidly and has transfrmed t a cnsiderable cntent f δ phase(fig.3a). Hwever, nly a small amunt f δ phase precipitate at grain bundaries in mdified 718 type allys (Fig.3c, e). The granular phase at grain bundaries is very stable after lng time aging (this phase is temprarily named δ phase fr descriptin cnvenience). (b) (c) (d) (e) (f) Fig. 3 SEM and TEM micrstructures f Ally 1: (b); Ally 3: (c)(d); Ally 4: (e)(f) aging at 680ºC fr 1000h 285

6 In cmparisn with δ phase, γ and γ in these mdified type allys are very fine therefre they are difficult t reslve via SEM. TEM analyses indicate that the mrphlgy f strengthening phases after aging at 680ºC fr 1000h are greatly different. It is clear that there are nt disk-like precipitates f γ phase in mdified 718 type allys. Figure 3 shws that the carsening f separately precipitated γ and γ in cnventinal 718 Ally 1 is mre significant than that f assciated precipitatin r cmpact mrphlgy f γ +γ in ther 718 type allys (Fig.3b, d and f). Therefre, the carsening rate f assciated r cmpact mrphlgy precipitated phases in ther 718 type allys is much lwer than that f γ phase in ally 1 with prlnging aging time. Fig.4 is the carsening rate fr γ and γ phase 680ºC aging at different times (b) d,nm Ally1-γ Ally1- γ Ally2 Ally3 Ally4 Ally5 Cmpact r assciated γ and γ d,nm γ Ally1- Ally2 Ally3 Ally4 Ally5 γ Cmpact r assciated γ and γ T 1/3, h Fig. 4 The carsening rate cmparisn f γ and γ phases in cnventinal 718 Ally 1 with ther mdified 718 type allys at 680ºC aging fr different times Fr accelerated evaluatin n lng term structure stability f these 718 type allys, experimental thermal expsure was cnducted at a higher temperature f 760 C fr 100h after fully heat treatment. Frm Fig.5 it can be seen that, fr Ally 1, large amunts f δ phase and the separate precipitatin f γ phase carsen significantly in cmparisn with the micrstructure at as-heat treated cnditin (Fig.5 a and Fig.1 a). Fig.5 shws that a cnsiderable fractin f γ phase in ally 1 has evlved int lng needle δ phase and further expsure will accelerate it evlving int mre severe Widmanstatten structure. Hwever, there are significant differences in micrstructures between cnventinal 718 Ally 1 and the ther mdified 718 type allys. After aging at 760ºC fr 100h, nly a few needle-like phase precipitated at grain bundaries, which indicates great imprvement n thermal structure stability because nly a few f needle-like Ni 3 M phase frmed at grain bundaries and in grains. The δ phase can still keep discntinuus mrphlgy after lng time expsure demnstrating gd thermal stability as it cmpares with the micrstructure f 718Plus with 10%C additin (Fig.5g, h). Therefre, frm these experimental results it can be cncluded that the increase f Al cntent and W additin leads t imprve structure stability after thermal expsure at higher temperatures. Phases Identificatin at Grain Bundaries fr 718 Type Allys T 1/3, h Fig.1, Fig.2 and Fig.5 shw that in 718 type allys the granular phase at grain bundary is different t needle-like δ phase in cnventinal 718 Ally 1 and it characterizes with gd structure stability at

7 lng time thermal expsure. The assciated precipitatin r cmpact mrphlgy f γ +γ has been studied by many researchers, but little infrmatin fr the granular phase at grain bundaries is reprted. The fllwing wrk is t identify this granular phase in 718 type allys. (b) (c) (d) (e) (f) (g) (h) 287

8 Fig.5 SEM Micrstructures f Ally 1: (b); Ally 3: (c)(d); Ally 4: (e)(f); 718Plus: (g)(h) aging at 760ºC 1000h (1) EDS Identificatin f Grain Bundary Granular Phase in Mdified 718 Type Allys EDS analyses reveal that the granular phase at grain bundaries is rich in Nb and M fr allys 2 and 4, hwever, except fr Nb and M, it is als rich in W fr allys 3 and 5. Fig. 6 is the EDS mapping f allying elements in grain bundary granular phase fr Ally 4. (2) XRD Identificatin and Chemical Cmpsitin Determinatin Just as mentined in SEM structure f 718 type allys, there are granular phase distributed at grain bundaries. In rder t identify this phase, XRD was cnducted. Figure 7 shws the XRD patterns and phase identificatin f 718 type allys at as heat-treated cnditin, which indicates that there is a grup f diffractin pattern f a new phase, different t the familiar phases such as δ, γ, γ and MC in cnventinal 718 Ally 1. Physic-chemistry analyses were cnducted t determine the chemical cmpsitin f δ phase. The experimental results reveal that the chemical cmpsitin f δ phase fr Ally 2 and Ally 4 is at the same level and fr Ally 3 and Ally 5. Table Ш lists the chemical cmpsitin f δ phase in different mdified 718 type allys. The analysis indicates that δ phase is rich in Nb and M fr Allys 2 and 4 and W rich in δ phase fr Allys 3 and 5, the W cntaining allys. These experimental results are cnsistent with EDS analyses. 288

9 Fig.6 EDS mapping f allying elements distributin at grain bundary granular phase in Ally 4 Ally 1 Cnventinal 718 γ -(112) γ -(111) γ δ δ-(012) δ-(211) γ' δ-(131) γ -(200) γ -(200) γ -(204) γ -(220) γ -(311) (d) (b) Ally 3 Mdified 718 δ -(110) δ -(103) γ -(112) γ -(111) δ -(201) δ -(112) δ-(211) γ -(200) γ -(200) 2θ δ -(123) γ -(204) δ-(131) γ -(220) γ δ γ δ γ -(311) (f) (c) Ally 4 Mdified 718 NbC -(111) δ -(103) γ -(112) γ -(111) δ-(012) δ -(112) δ -(201) δ-(211) γ -(200) 2θ γ δ γ δ MC-(Nb,Ti)(C,N) δ-(400) γ -(311) (e) (d) Ally 4 Mdified 718 NbC -(111) δ -(110) δ -(103) δ -(112) δ-(012) δ -(201) δ-(211) 2θ δ δ δ -(123) δ-(131) MC-(Nb,Ti)(C,N) θ 289

10 Fig.7 XRD identificatins f Ally 1; (b) Ally 3; (c)(d)ally 4 Table Ш The chemical cmpsitin f δ phase determined by physic-chemistry analyses (at%) Ally Ni Fe Cr M W Nb Ti Mechanical Prperties f 718 Type allys at Heat Treated Cnditin and After Lng Time Aging (1) Mechanical Prperties f 718 Type Allys at Heat Treated Cnditin The tensile prperties f 718 type allys determined at 20, 650 and 680 C are shwn in Fig C and 650 C the ultimate tensile strength and yield strength fr the 718 type allys are cmparable with cnventinal 718 ally. Mdified 718 type allys, with higher Al cntent, exhibit higher ultimate tensile strength and yield strength at test temperature 680 C than the cnventinal cmpsitin. Stress rupture tests were cnducted at 680 C and 690MPa. The results, shwn in fig 9, indicate that stress rupture life fr Ally 2 and Ally 4 is almst twice as cmpared t cnventinal 718 Ally 1. Ally 3 and Ally 5, with W additin, is als superir t Ally 1. Fatigue crack grwth rate testing f 718 type allys were cnducted and cmpared with cnventinal 718 Ally 1. The test specimens were carefully prepared t prduce the same grain sizes. Cyclic stress testing was perfrmed at 650 C with initial K=20MPa m 1/2 and 90s dwelling time. The experimental results are pltted in Fig.10. Under these test cnditins the fatigue crack prpagatin rates f the mdified 718 type allys (Ally 2, Ally 3 and Ally 4) are all remarkable lwer than that f cnventinal 718, Ally 1. It is believed that δ phase frmatin at grain bundaries and the assciated r cmpact mrphlgy precipitatin f γ +γ phases plays a key rle fr decreasing fatigue crack grwth rate. (2) Mechanical Prperties f 718 Type allys After Lng Time Aging Our experimental results reveal that allying element mdificatins have a nticeable effect n structure stability. This is demnstrated by the imprved elevated temperature mechanical prperties, Fig.11 and Fig

11 Tensile Strength, MPa UTS YS UTS Ally 1 Ally 2 Ally 3 Ally 4 Ally 5 UTS YS YS Elngatin(δ) and Ductility(Ψ), % δ Ally 1 Ally 2 Ally 3 Ally 4 Ally 5 ψ δ ψ δ ψ (b) Test Temperature, C Test Temperature, C Fig.8 Tensile prperties f 718 type allys Tensile strength (b) Elngatin and ductility Stress Rupture Life, Elngatin and Ductility C /690MPa Ally 1 Stress Rupture Life,h Elngatin,% Ductility,% Ally 2 Ally 3 Ally 4 Ally 5 Fig.9 Stress rupture prperties f 718 type allys 1 da/dn, (mm/cycle) s 90s 15s 15s 15s 15s Ally 1 Ally 2 Ally 3 Ally 4 1E-3 T: 650 C Initial K= 20MPa m 1/ K, MPa m 1/2 Fig.10 Fatigue crack prpagatin behavirs f 718 type allys at 650 C cyclic stress cnditin in cmparisn with cnventinal 718 Ally 1 291

12 1400 T=20 C Ally 1 Ally T=680 C Ally 1 Ally 4 (b) 1300 Yield strength, MPa Yield strength, MPa Aging time, h Aging time, h Fig.11 The effect f thermal expsure at 680 C n tensile prperties 25 C and (b)680 C 50 Ally 1 Ally Ally 1 Ally 4 (b) C/690MPa C/ 825MPa/100h Stress rupture life, h Strain, % Ttalled strain Plastic strain Elastic strain Aging time, h Fig.12 The effect f thermal expsure n stress rupture and (b) creep prperties Discussins Our study shws that the chemical cmpsitin mdificatins f Al cntent and W additin change the phase precipitatin behavirs frm needle-like δ phase and separate precipitatin f disk-like γ and glbular γ phase in cnventinal 718 Ally 1 t granular δ phase with a small fractin f needlelike δ phase, and assciated r cmpact mrphlgy precipitatin f γ and γ phase in mdified 718 type allys (Allys 2, 3, 4, 5). The 718 type allys with granular δ phase at grain bundaries and cmpact mrphlgy precipitatin f γ and γ phase characterize with superir cmbinatin f structure stability and mechanical prperties during lng time thermal expsure at temperatures abve 650 C. Thermdynamic calculatin results indicate that with the increase f Al cntent, γ phase slvus temperature increases remarkably (1%W additin has minr effect n γ phase slvus temperature). The γ slvus temperature fr cnventinal 718 Ally 1 is 904 C, hwever, it is abut 963 C fr Ally 2 and 4, and it is 947 C and 974 C fr Ally 3 and 5 respectively. Physic-chemistry analyses results shw that δ phase is rich in allying element with higher slvus temperature such as Nb and M(W) (Table Ш). Our experimental results indicate that δ phase slvus 292

13 temperature is near t 1040 C, which is higher than that f δ phase in cnventinal 718 Ally 1(1010 C) [28]. Therefre, chemical cmpsitin adjustment nt nly n Nb and Ti but als n Al has a gd effect n the imprvement f phase slvus temperatures. Quantitative phase fractins results are listed in Table ІV. It can be achieved that increasing Al cntent can bviusly prmte the frmatin f γ +γ phase and remarkably restrain δ phase frmatin. The fractins f MC phase in these allys are almst equal t each ther. Anther fact shuld be nted is that the fractin f δ phase increases with the increment f Al cntent and it appears that greater fractin f δ phase is cnsistent with the absence f δ phase. Therefre, it seems that higher Al cntent can prmte the frmatin f γ +γ and granular δ phase, but restrain δ phase frmatin. The fractin f δ phase in Ally 2 and Ally 3 is 0.85% and 2.88% respectively. The main difference f chemical cmpsitin in these tw allys is that Ally 3 has 1.5% W additin. Therefre tungsten is anther allying element t prmte δ phase frmatin. This is in agreement with the results by thermdynamic calculatin [28]. Table ІV Different phase fractins in mdified 718 type allys after heat treatment via physic-chemistry analyses (wt%) Ally γ +γ δ δ" MC Cnclusins (1) It is remarkable that increasing Al cntent and W additin nt nly retards but als significantly restrains δ phase frmatin and prmtes mre stable δ phase frmatin. Mrever, separate precipitatin f γ and γ is substituted by assciated precipitatin f γ +γ in mdified 718 type allys and the carsening rate f the later is much lwer than that f the frmer. δ phase is rich in Nb and M(W) and characterizes with stable behavir during lng time thermal expsure. (2) The 718 type allys with higher Al cntent display better micrstructural stability and mechanical prperties than cnventinal 718 ally. (3) In rder t achieving mdified 718 type ally with higher strength, superir stress rupture life and structure stability at 680 C r higher temperatures, suggested chemical cmpsitin mdificatin shuld be cntrlled in the range f 0.8~1.3%Al, 5.3~5.5%Nb, 0.013~0.015%P, 0.005~0.01%B respectively. 293

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