Phase equilibria, diffusion growth and diffusivities in Ni-Al-Pt system using Pt/β-NiAl diffusion couples

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1 Phase equlbra, dffuson growth and dffusvtes n N-Al-Pt system usng Pt/-NAl dffuson couples We-yan GONG 1, L-jun ZHANG 2, Ha-xa WEI 1, Chun-gen ZHOU 1 1. Key Laboratory of Aerospace Materals and Performance, Mnstry of Educaton, School of Materals Scence and Engneerng, Behang Unversty, Bejng , Chna; 2. Interdscplnary Centre for Advanced Materals Smulaton, Ruhr-Unverstät Bochum, Bochum, Germany Receved 12 February 2011; accepted 8 May 2011 Abstract: The phase equlbra, dffuson growth and dffusvtes n the N-Al-Pt system at 1 150, and ºC were studed usng Pt/-NAl dffuson couples. Based on the measured concentraton profles coupled wth the local equlbrum hypothess, the te-lnes between neghborng phases were determned. Two ntermedate phases, Pt Al and α-npt(al), are found to develop between the Pt and -NAl couples. The thcknesses of Pt Al and α-npt(al) layers vares lnearly wth the square of annealng tme, ndcatng that the growth of Pt Al and α-npt(al) are dffuson-controlled. The calculated Arrhenus equatons of the parabolc growth constants, k, for the Pt Al and α-npt(al) phases are obtaned, respectvely. The average ternary nter-dffusvtes of the Pt Al and -NAl phases at dfferent temperatures are determned from an ntegraton of nter-dffuson fluxes calculated drectly from the expermental concentraton profles. Key words: platnum group; nckel alumndes; phase equlbra; thermodynamc; dffuson 1 Introducton Thermal barrer coatngs (TBCs) are wdely used n engne blades to provde nsulaton for hot sectons and ncrease the component durablty. TBCs typcally consst of the bond coat and the yttra-stablsed zrcona (YSZ) ceramc coat [1 2]. Currently, the Pt-modfed -NAl alloys are most wdely used materals as the bond coat [] due to ther excellent cyclc oxdaton resstance. In order to mprove the performance of the current bond coats, or even desgn new bond coats, full nterpretaton of the oxdaton behavor of the Pt-modfed alumnde coatngs s necessary. However, the major obstacles le n the lack of suffcent phase equlbra and dffusvtes n the N-Al-Pt system despte numbers of expermental nvestgatons on the Pt-modfed alumnde coatngs durng the past decades. As early as n 1977, JACKSON and RAIRDEN [4] presented a schematc sothermal secton at ºC, but no ternary phase was reported. Untl 1994, KAMM and MILLIGAN [5], and later MEININGER and ELLNER [6] nvestgated the same N Al-Pt Al jon, and observed a ternary phase wth L1 0, AuCu-type tetragonal structure. The very recent work from GLEESON s group [7 8] proposed a relatvely complete sothermal secton at ºC of the N-Al-Pt system n Al-poor regon by employng dffuson couple technque. Besdes, the ternary L1 0 phase was also confrmed n ther work [7], and denoted as α-npt(al). As for the dffusvtes n N-Al-Pt alloys, only two major contrbutons exst n the lterature. By means of the dffuson couple method, MINAMINO et al [9] determned the mpurty dffuson coeffcents of Pt n -NAl phase wth compostons of 4.7% 51.9% (mole fracton) Al at temperature rangng n ºC. BOUCHET and MEVREL [10] reported a seres of composton-dependent nter- dffuson coeffcents of -(N, Pt)Al phase at ºC by employng snglephase dffuson couples coupled wth a numercal nverse analyss [11]. Thus, there s an urgent need to remedy ths stuaton. Moreover, the phase equlbrum and dffusvty nformaton are the bass of the subsequent CALPHAD (CALculaton of PHAse Dagram) thermodynamc and atomc moblty databases [12 15], whch are ndspensable for quanttatve smulaton of the Foundaton tem: Project ( ) supported by the Natonal Natural Scence Foundaton of Chna; Project supported by the Program for New Century Excellent Talents n Unversty (NCET) Correspondng author: Chun-gen ZHOU; Tel: ; Fax: ; E-mal: cgzhou@buaa.edu.cn

2 222 We-yan GONG, et al/progress n Natural Scence: Materals Internatonal 21(2011) mcrostructure evoluton n N-Al-Pt alloys or coatngs usng the phase-feld method [16 18]. Furthermore, the dffuson couple experments are well known to be very effectve n nvestgatng the thermodynamcs and knetcs of alloy, n one experment a large area of the phase space can be screened, and the dffuson coeffcents can be extracted. Consequently, the Pt/-NAl dffuson couples are employed n the present work to measure the correspondng phase equlbra and dffuson propertes n the N-Al-Pt system. 2 Expermental One sngle-phase -NAl alloy was prepared by arc-meltng Al (purty of 99.98%, mass fracton) and N (purty of 99.9%) rods n an vacuum arc furnace. The button was re-melted fve tmes to mprove ts homogenety. Ths alloy was then annealed at C for 120 h n a vacuum furnace at 1 Pa, followed by X-ray dffractometry (XRD) and scannng electron mcroscopy (SEM) wth energy-dspersve X-ray analyss (EDX) to check the phase structure and mcrostructure homogenous. Then, ths -NAl alloy was cut nto blocks wth approxmate dmenson of mm mm mm. After annealng at C for 120 h, pure Pt peces (purty of 99.95%) were cut nto blocks wth dmenson of mm mm 1.5 mm. After groundng and polshng, the -NAl and the Pt blocks were bound together n a vacuum furnace at C for 0.5 h to make 12 sem-nfnte Pt/-NAl dffuson couples. All the Pt/-NAl dffuson couples were sealed n dfferent quartz capsules, and annealed at (1 150±1) C, (1 200±1) C and (1 250 ±1) C for 0, 60, 90 and 110 h, respectvely, n tube furnaces (GSL 1600X MTI, produced by Hefe Kejng Materals Technology Co., Ltd., Chna) under flowng argon. The dffuson couples were removed from the furnace and quenched nto water. The annealed Pt/-NAl dffuson couples were analyzed by SEM/EDX for morphology observaton, and electron probe mcroanalyss (EPMA, JXA 8100, JEOL, Japan) for accurate measurements of concentraton profles. Theory-determnaton of ternary nterdffusvtes The nter-dffuson flux ( J ~ ) at any secton x for each component can be drectly calculated from the concentraton profle from the relaton as [19 20] ~ 1 C ( x) J = ( x x )dc C C 2t or 0 (= 1, 2,, n) (1) where t s the tme, C and C are the termnal compostons, and x 0 s the locaton of the Matano plane, whch s usually determned by averagng the calculated Matano plane postons of components for a ternary system. Ths fact can result n addtonal errors for nter-dffusvty calculaton. In order to avod such errors, DAYANANDA and SOHN [21] proposed a new method to compute J ~ wthout calculatng the Matano plane postons, ~ C C J ( x) = 2t x [ ] Y ( x) 1 Y ( x) dx [1 Y ( x)] Y x ( x) dx (2) where Y (x) s the relatve concentraton varable, denoted by C C Y = (= 1, 2,, n) () C C The nter-dffuson flux J ~ determned from Eq. (2) can be ntegrated over a selected regon, x 1 to x 2, to yeld an expresson as [21] ~ = C1 ( ) ~ C2( ) ~ J dx D 1 dc 1 D 2 dc2 = x1 C1 ( x1 ) C2( x1 ) x 1 2 D [ C 1 ( x 1 ) C 1 ( x 2 )] D [ C2( x1) C2( 2)] (=1, 2) (4) where D 11 and D 22 refer to the average man nter-dffuson coeffcents, and D 12 and D 21 are average cross nter-dffuson coeffcents n a fcttous ternary 1-2- system. Takng D 1 and D 2 as constants, the Fck s law n ternary system can be rewrtten as ~ J C1 D x C x 2 = D 1 2 (=1, 2) (5) If both sdes of the Eq. (5) are multpled by ( x x0 ) and ntegrated over the dffuson zone between x 1 and x, the followng expresson can be obtaned [21]: 2 ~ ~ ~ J ( x x0) = 2 t{ D1[ J1( x1) J1( )] x 1 ~ ~ D J ( x ) J ( )]} (=1, 2) (6) 2[ Eqs. (4) and (6) provde four equatons to solve for D 11, D 21, D 21 and D 22. Ths method has been successfully appled n varous alloy systems [22 24] to determne average ternary nter-dffuson coeffcents. 4 Results and dscusson Fgure 1(a) presents a typcal SEM mcrograph of the nter-dffuson zone of the Pt/-NAl dffuson couple annealed at C for 0 h, and the correspondng concentraton profles of the elements N, Al and Pt are shown n Fg. 1(b). It s obvous that two new ntermedate phases, Pt Al and α-npt(al), formed between the Pt and -NAl couples, wth the resultng three nterfaces dentfed n Fg. 1 as 1, 2 and. The ntegers 1, 2 and

3 We-yan GONG, et al/progress n Natural Scence: Materals Internatonal 21(2011) denote the nterfaces of Pt/Pt Al, Pt Al/α-NPt(Al) and α-npt(al)/-nal, respectvely. The observaton of the α-npt(al) phase confrms the prevous work [5 8]. Moreover, a large solublty of Pt exsts n the -NAl phase, whch s consstent wth the prevous expermental work [7]. Accordng to the local equlbrum hypothess [25], the te-lnes between adjacent phases can be obtaned from the concentraton profles. The detaled procedure can be referred to HAYASHI et al [7]. The te-lnes at dfferent temperatures obtaned n the present work are as follows: for Pt/Pt Al, / at ºC, / at ºC, and / at ºC; for Pt Al/α-NPt(Al), / at ºC, / at ºC, and / at ºC; for α-npt(al)/-nal, / at ºC, / at ºC, and / at ºC (Al Pt, n mole fracton, the remanng amount s for N). The te-lnes should be mportant supplement to the sothermal sectons at the three temperatures. The thcknesses of the Pt Al and α-npt(al) phases formed between the Pt and -NAl couples were determned from the SEM mcrographs and the EPMA concentraton profles. The results are plotted n Fgs. 2(a) Fg. 1 Cross-sectonal mcrostructure (a) and concentraton profles (b) across nter-dffuson zone of Pt/-NAl dffuson couple annealed at ºC for 0 h

4 224 We-yan GONG, et al/progress n Natural Scence: Materals Internatonal 21(2011) Fg. 2 Parabolc relatonshp for growth of Pt Al (a) and α-npt(al) (b) ntermedate layers at 1 150, and ºC and Arrhenus plots (c) of parabolc rate constants for Pt Al and α-npt(al) phases and (b) n terms of the layer thckness versus the square root of tme. As can be seen that, the layer thcknesses for both the Pt Al and α-npt(al) vary lnearly wth t 1/2, ndcatng that the growth of the Pt Al and α-npt(al) phases are dffuson-controlled. Fgure 2(c) shows the Arrhenus plot of the parabolc growth constants k for the Pt Al and α-npt(al) phases wth k 1/2 beng slopes obtaned from Fgs. 2(a) and (b). The calculated Arrhenus equatons of the parabolc growth constants, k, for the Pt Al and α-npt(al) phases are k = exp[ /( RT )] and k = exp[ /( RT )], respectvely. Fgure shows the measured concentraton profles of the Pt/-NAl dffuson couples annealed at 1 150, and C for 90 h, and the correspondng nter-dffuson fluxes of ndvdual components by means of Eq. (2). As can be seen that, the varaton of Pt concentraton n the -NAl phase resembles a horzontal mrror reflecton of that of the N concentraton, whle the concentraton profle of the Al s almost flat. Indcatng that the Pt atoms prefer to occupy the N stes n -NAl lattce. Based on the concentraton profles and the nterdffuson fluxes n Fg., the four average ternary nterdffuson coeffcents of the Pt Al and -NAl phases were determned usng Eqs.(4) and (6), as lsted n Table 1.

5 We-yan GONG, et al/progress n Natural Scence: Materals Internatonal 21(2011) Fg. Concentraton profles and correspondng nter-dffuson fluxes of Pt, N and Al n Pt/-NAl dffuson couples annealed at dfferent temperatures for 90 h: (a), (d) ºC; (b), (e) ºC; (c). (f) ºC Table 1 Comparson between presently calculated average nter-dffusvtes of Pt Al and -NAl phases at dfferent temperatures and lterature data (m 2 /s) D 1) Present work 2) Case 1 Case ºC ºC ºC ºC ºC ºC Lterature ) (1 100 ºC) Pt Al PtPt D PtAl PtAl D PtAl D PtAl AlAl D D PtPt [10]

6 226 We-yan GONG, et al/progress n Natural Scence: Materals Internatonal 21(2011) D PtAl D [8] AlAl [8] D [10] Pt Al 1) j D (, j=pt, Al) and D j (, j=pt, Al) are the average nter-dffusvtes of Pt Al and -NAl phases, the N s the ndependent element; 2) All four nter-dffusvtes were consdered n Case 1, whle cross nter-dffusvtes were assumed to be neglgble n Case 2; ) Inter-dffusvtes reported by GLEESON et al [8] at composton of N-42.5% Al-2.% Pt (mole fracton) n sngle phase, and those from BOUCHET and MEVREL [10] are average values. It should be ponted that the wdth of the α-npt(al) phase s extremely narrow, and thus the concentraton ponts from EPMA are not enough to compute the reasonable nter-dffusvtes. In order to compare wth the lterature data, the data of ternary nter-dffusvtes reported by GLEESON et al [8] and BOUCHET and MEVREL [10] are cted n Table 1. The assumpton that the neglgble of the cross terms was also made n the present work, and named as Case 2 n order to be dfferent from Case 1, n whch all the man and cross terms were consdered. As can be seen from Table 1 that, the presently AlAl calculated D n both Case 1 and Case 2 agree well wth the expermental values at C from Refs. [8, 10], whle the D PtPt s a lttle bgger than that by BOUCHET and MEVREL [10]. The present value of the D agree reasonably wth that by GLEESON et al [8], and both are negatve, sgnfyng that Al-Pt nteracton parameter s negatve, whch n turn sgnfes that the Pt decreases the chemcal actvty of Al [9]. Consderng the fact that the nter-dffusvtes from the present work and Refs. [8, 10] are based on dfferent methods, but ther devatons are wth one order (a typcal error n dffusvty measurement), the concluson s that all the data n Table 1 are reasonable. 5 Conclusons 1) Based on the local equlbrum hypothess, the te-lnes between the respectve phases are determned from the measured concentraton profles. 2) The Pt Al and α-npt(al) layers developed between the Pt and -NAl couples, and ther growth was found to be dffuson-controlled. The calculated Arrhenus equatons of the parabolc growth constants, k, for the Pt Al and α-npt(al) phases are k = exp[ /( RT)] and k = exp[ /( RT )], respectvely. ) The average nter-dffusvtes of the Pt Al and -NAl phases at dfferent temperatures were calculated. The values of D are close to those reported n AlAl lterature. The value of D are negatve, sgnfyng that the Al-Pt nteracton parameter s negatve. Acknowledgements L-jun ZHANG would lke to thank the Alexander von Humboldt Foundaton of Germany for supportng and sponsorng the research work at ICAMS, Ruhr-Unverstät Bochum, Germany. References [1] PADTURE N P, GELL M, JORDAN E H. Thermal barrer coatngs for gas-turbne engne applcatons [J]. Scence, 2002, 296(12): [2] BALINT D S, XU T, HUTCHINSON J W, et al. Influence of bond coat thckness on the cyclc rumplng of thermally grown oxdes [J]. Acta Materala, 2006, 54 (7): [] KIM D E, MANGA V R, PRINS S N, et al. Frst-prncples calculatons and thermodynamc modelng of the Al-Pt bnary system [J]. CALPHAD, 2011, 5(1): [4] JACKSON M R, RAIRDEN M R. The alumnzaton of platnum and platnum-coated IN-78 [J]. Metallurgcal and Materals Transactons A, 1977, 8 (11): [5] KAMM J L, MILLIGAN W W. Phase stablty n (N,Pt) Al alloys [J]. Scrpta Metallurgcal et Materala, 1994, 1(11): [6] MEININGER H, ELLNER M. Phase transformaton and the type of lattce dstorton of some platnum-rch phases belongng to the Cu famly [J]. Journal of alloys and compounds, 200, 5(1): [7] HAYASHI S, FORD S I, YOUNG D J, et al. α-npt(al) and phase equlbra n the N-Al-Pt system at 1150 C [J]. Acta Materala, 2005, 5(11): [8] GLEESON B, WANG W, HAYASHI S, et al. Effects of platnum on the nterdffuson and oxdaton behavor of N-Al-based alloys [J]. Materals Scence Forum, 2004, 461/462/46/464: [9] MINAMINO Y, KOIZUMI Y, TSUJI N, et al. Pt dffuson n B2-type ordered NAl ntermetallc compound and ts dffuson mechansms [J]. Scence and Technology of Advanced Materals, 2000, 1(4): [10] BOUCHET R, MEVREL R. Calculatng the composton-dependent dffusvty matrx along a dffuson path n ternary systems: Applcaton to [beta]-(n,pt)al [J]. CALPHAD, 200, 27(): [11] BOUCHET R, MEVREL R. A numercal nverse method for calculatng the nterdffuson coeffcents along a dffuson path n ternary systems [J]. Acta Materala, 2002, 50(19): [12] ZHANG L, WANG J, DU Y, HU R, et al. Thermodynamc propertes of the Al-Fe-N system acqured va a hybrd approach combnng calormetry, frst-prncples and CALPHAD [J]. Acta Materala, 2009, 57(18):

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