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Aging and recrystallization behavior of precipitation strengthened Al-0.25Zr-0.03Y alloy
Journal of Alloys and Compounds 696 (2017) 1039e1045
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Aging and recrystallization behavior of precipitation strengthened Al0.25Zr-0.03Y alloy Haiyan Gao a, b, *, Wuqiang Feng a, Jing Gu a, Jun Wang a, b, **, Baode Sun a, b a b
State Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University, Shanghai 200240, China Shanghai Key Laboratory of Advanced High-temperature Materials and Precision Forming, Shanghai 200240, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 17 July 2016 Received in revised form 30 October 2016 Accepted 5 December 2016 Available online 8 December 2016
Aging and recrystallization behavior of Al-0.25Zr-0.03Y were investigated through hardness and conductivity/resistivity measurement, transmission electron microscopy (TEM), scanning electron microscope (SEM) and atom probe tomography (APT) analysis. The results show that hardness peak of Al0.25Zr-0.03Y is about 6HV higher than that of Al-0.25Zr with the incubation period shortened from 50 h to 5 h as well. However, compared with Al-0.25Zr-0.08Y, smaller precipitates with higher number density, higher coarsening and recrystallization resistance was found in Al-0.25Zr-0.03Y. Precipitate coarsening constant in Al-0.25Zr-0.03Y is one order of magnitude lower than that in Al-0.25Zr-0.08Y. At the same time the recrystallization temperature of Al-0.25Zr-0.03Y reached 500 C, which is about 75 C higher than Al-0.25Zr-0.08Y and 125 C higher than Al-0.25Zr. In the ternary alloy, the equilibrium solubility of Y in a-Al decreased by 1/4 of that in binary Al-Y at 600 C and became negligible at 400 C. Few primary Al3Y was found in as-cast Al-0.25Zr-0.03Y and APT analysis revealed that the amount of Y in solid solution was about 4 times higher than in that in Al-0.25Zr-0.08Y, resulting in larger nucleation driving force for Al3Y and much reduced critical nucleus size and nucleation energy. The combine contribution of diminished primary Al3Y and improved physical properties of precipitates make for the higher coarsening and recrystallization resistance of Al-0.25Zr-0.03Y. © 2016 Published by Elsevier B.V.
Keywords: Rare earth alloys and compounds Precipitation Mechanical properties Scanning electron microscopy Transmission electron microscopy Kinetics
1. Introduction Dilute Al-Zr alloys find wide applications at elevated temperatures, especially for high-voltage cables for the power grids because L12-Al3Zr precipitated during aging of Al-Zr alloy is coherent with a-Al matrix and kinetically stable up to ~475 C [1]. However, precipitation kinetics of L12-Al3Zr is sluggish due to low diffusivity of Zr in a-Al, which is about 5 magnitude order lower than the selfdiffusivity of Al [2], leading to longer incubation time for nucleation, lower precipitation number density and larger particle size [3]. In industrial practice, aging treatment of binary Al-Zr alloy is always time and energy consumable, which lasts 70 h or longer. Researches on combined additions of Rear Earth (RE) elements and transition metals (TM), such as Sc [4e12], Yb [13e19], Er
* Corresponding Shanghai Jiao Tong ** Corresponding Shanghai Jiao Tong E-mail address:
author. State Key Laboratory of Metal Matrix Composites, University, Shanghai 200240, China. author. State Key Laboratory of Metal Matrix Composites, University, Shanghai 200240, China. [email protected] (H. Gao).
http://dx.doi.org/10.1016/j.jallcom.2016.12.064 0925-8388/© 2016 Published by Elsevier B.V.
[20e24], Ti [1,25e27] and Hf [28] have been carried out to accelerate precipitation kinetics through formation of L12-Al3M (M ¼ RE or TM) in the early stage of decomposition. It was well accepted that L12-Al3M could act as the nucleus for L12-Al3Zr due to larger diffusion coefficient of RE and TM in a-Al matrix. Compared with binary Al-Zr alloy, attractive properties, such as larger number density, higher recrystallization temperature and reduced coarsening kinetics was found in ternary Al-Zr-M alloys due to formation of Al3(M1xZrx) precipitates. Ternary Al-Sc-Zr alloy was widely investigated because of strong strengthening effect of Sc and precipitation of Al3Sc at the interdenrites, which filled up effectively the precipitation free zone of Al3Zr resulting from the segregation of Zr during solidification. However, the high cost of Sc limited its commercial use. Effective elements with lower cost are welcomed in engineering applications. Y locates in the same group as Sc. However, Al3Y was used to be regarded impossible to be effective nuclei for L12-Al3Zr of conventionally cast Al-Zr-Y alloys [29], because the L12-Al3Y was accessible only during the decomposition of rapid solidified Al-Y alloys [30]. Few publications could be found focusing on the
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3. Results
increment about 7HV. Fig. 2 gives the precipitates observed in Al-Y aged at 250 C for 1 h. EDS analysis showed that the precipitate is Al3Y consisting of 75.87 at% Al and 24.13 at%Y. Our research based on atom probe tomography analysis also confirmed the precipitation of short-bar-shaped Al3Y in the early stage of decomposition of Al-0.25Zr-0.08Y alloy [33]. However, in Al-Zr-Y alloys, no strengthening effect was observed near 250 C but slight hardness loss instead during 100e375 C. As will be discussed later that in ternary Al-Zr-Y alloy, the equilibrium solubility of Y decreased dramatically to about one quarter of that in binary Al-Y alloy. APT analysis revealed that in the as-cast ingot, the amount of Y dissolved in a-Al matrix is only 0.007 at% for Al-0.25Zr-0.03Y and 0.0019 at% for Al-0.25Zr-0.08Y. Therefore, precipitation strengthening of Al3Y is unexpectable. Hardness peak at 475e500 C should result from the precipitation strengthening of L12-Al3Zr. It is obvious that combined addition of Y and Zr resulted in stronger strengthening effect, lower Zr precipitation temperature and accelerated aging kinetics. Conductivity evolution is similar to that of the hardness. However, difference in conductivity between Al-Y and Zr containing AlZr and Al-Zr-Y is obvious. Few data could be found in the published literature reporting the additional resistivity caused by dissolved Y atoms. Therefore, resistivity measurement was performed on Al0.25Zr-Y alloys with various amount of Y after being soaked at 600 C for 50 h, as shown in Fig. 3, where the data points are experimental measurements and the line are linear fit of the data. Slope of the two diagonals are close, 1.68 mU mm/wt%, which reflected the resistivity increment coming from the dissolved Y atoms. While the effect of dissolved Zr is about 17.4 mU mm/wt%, which is about an order of magnitude larger than that of Y. Therefore, conductivity difference between Al-0.02Y and Al-0.16Y is negligible and it is about 8% IACS higher than that of Zr containing alloys before precipitation of Zr. It is noticeable that both hardness and conductivity of Al-0.25Zr0.03Y are a bit higher than that of Al-0.25Zr-0.08Y. The average grain size of the as cast Al-0.25Zr, Al-0.25Zr-0.03Y and Al-0.25Zr0.08Ywas measured as 591.8 ± 2.4, 485.6 ± 1.3 and 396.2 ± 1.6 mm respectively. Due to greatly decreased equilibrium solubility of Y in Al-Zr-Y alloy, as will be discussed in detail later and shown in Fig. 9, excessive Y in Al-0.25Zr-0.08Y transformed into primary Al3Y which was reported acting as grain finer for a-Al during solidification of the alloy [32]. In addition, grain growth is obvious during aging, the average grain size of Al-0.25Zr-0.03Y increased by 50% and reached 745.7 ± 2.4 mm after being aged at 375 C for 1 h. Therefore, difference in grain size and grain grow during aging may be responsible for the observed hardness and conductivity difference and hardness loss below 400 C. More details for the aging behavior could be found in the hardness and conductivity evolution curves recorded through the isothermal aging, as shown in Fig. 4. Compared with Al-0.25Zr, accelerated precipitation kinetics was found in Al-0.25Zr-0.03Y, incubation period of Al-0.25Zr-0.03Y/0.08Y shortened from 50 h to 5 h and the time for hardness peak shortened from 300 h to 50 h.
3.1. Aging behavior
3.2. Coarsening kinetics and recrystallization
Fig. 1 shows the Vickers hardness and electrical conductivity evolution of Al-Zr-Y during isochronal aging. Binary Al-Y and Al-Zr alloys was also given for comparison purpose. Difference in hardness and conductivity evolution among alloys is obvious. Binary Al0.16Y and Al-0.25Zr witnessed a smaller hardness peak of about 2HV increment at 250 C and 500 C respectively, while hardness of Al-0.25Zr-0.03Y/0.08Y alloys decreased slightly till 375 C and then increased quickly and reached a peak at 475e500 C with
Fig. 5 shows the TEM images of Al-0.25Zr and Al-0.25Zr-0.03Y/ 0.08Y aged at 400 C for 50 h and 200 h. Table 1 gives the size and number density of the precipitates in detail. Compared with Al0.25Zr, precipitate in Al-0.25Zr-0.03Y and Al-0.25Zr-0.08Y is smaller in size with higher number density. After 50 h aging, the average radii of the precipitate in ternary alloys is only half of that in the binary with doubled number density. However, data in the table also revealed that the coarsening rate of Al-0.25Zr and Al-
precipitation evolution of Al-Zr-Y alloys. Our previous research showed that, compared with binary Al-Zr alloy, the number density of the precipitates in Al-Zr-Y is about one order magnitude higher and recrystallization temperature increased from 350 C to 400 C; however, when Y content increased to 0.16 wt%, the recrystallization temperature decreased to 325 C [31,32]. In present work, effect of Y and its content on the aging behavior, precipitation coarsening kinetics and recrystallization resistance of Al-Zr-Y alloy was further investigated for the development of high performance and low cost Al-Zr alloy. 2. Experimental Al-Zr-Y ingots were cast from 99.99 wt% pure Al with Al-Y and Al-Zr master alloys using alumina crucibles at 750 C in the open air. The melt was poured into a graphite mold after mechanical stirring. Composition of alloys was measured by inductively coupled plasma-atomic emission spectroscopy (ICP-AES). Al-Y and Al-0.25Zr-Y alloys with various Y content were prepared. Isochronal aging were carried out from 100 to 550 C with increments of 25 C, each lasting 1 h. Isothermal treatment was performed at 400 C up to 750 h. Some ingots were cold-rolled to 82% reduction after being aged at 400 C for 50 h and then annealed at 150e600 C with increments of 25 C for recrystallization temperature measurements. Samples were water quenched after aging. The microstructural evolution was investigated using JEM 2100F transmission electron microscopy (TEM) operated at 200 kV. TEM specimens were prepared through mechanical grinding and twinjet electro-polishing at 12 V in a 30% nitric acid and 70% methanol solution cooled to 30 C. FEI NOVA NanoSEM 230 fieldemission scanning electron microscope (FESEM) was employed to observe the distribution of the precipitates at 10 kV. The maximum depth below the specimen surface where precipitates can be detected was calculated to be 0.66 mm using KanayaeOkayama equation. Precipitates size and number density were analyzed using Image pro software. At least 100 precipitates were measured for each mean radius of precipitates and at least 6 SEM images were analyzed for each number density. Atom probe tomography (ATP) analyses with 200 kHz pulse repetition rate, 20% pulse fraction, 20 K specimen temperature and a residual pressure of 108 Pa was employed to measure the Y content dissolved in the a-Al matrix before aging. Specimen with tip radii less than 100 nm were prepared through electro-chemical polishing. Grain size of the specimen was determined using optical microscopy under polarized light. Vickers hardness and electrical conductivity of the aged specimen were measured at ambient temperature. The Vickers hardness measurements were performed on polished samples using a load of 200 g and a dwell time of 10 s. A minimum of 10 measurements were taken as the experimental data. Five conductivity measurements were recorded for each reading using a FD-102 eddy current conductivity tester.
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Fig. 1. Vickers hardness (a) and electrical conductivity (b) of Al-Zr, Al-Y and Al-Zr-Y alloys during isochronal aging from 100 to 550 C.
follows equation:
rtn r0n ¼ Kt
Fig. 2. SEM image of precipitates in Al-0.16Y aged at 250 C for 1 h.
0.25Zr-0.08Y is similar, the precipitate size increased by 60% as the time extended from 50 to 200 h, while that for Al-0.25Zr -0.03Y is only 30%. Number density in Al-0.25Zr -0.03Y almost kept unchanged, indicating a quasi-static coarsening, while that in Al0.25Zr and Al-0.25Zr-0.08Y decreased by 33% and 17% respectively. Fig. 6 shows the time dependence of the mean size of the precipitates in the logarithmic scale. According to the KV model [34], relationship lies between precipitate size r and aging time t
(1)
where rt is the precipitate size at time t, r0 the initial size, K the coarsening constant. Slope of the fitting line in Fig. 6 gives the reciprocal of the time exponent n. The value of 1/n for Al-0.25Zr is 0.34, which is in agreement with the theoretical value of 0.33, and that for Al-0.21Zr-0.03Y and Al-0.21Zr-0.08Y is 0.14 and 0.27 respectively. Fig. 7 gives the hardness vs annealing temperature curves for the cold-rolled Al-0.25Zr-0.03Y alloys. The recrystallization temperature was defined as the temperature at which 50% hardness loss occurred. For comparison purpose, curves for Al-0.25Zr, Al-0.25Zr0.08Y and Al-0.25Zr-0.16Y obtained in our previous research is also given [32]. The recrystallization temperature of Al-0.25Zr-0.03Y is about 500 C, which is 75 C or 125 C higher than that of Al-0.25Zr0.08Y or Al-0.25Zr. Inserts in Fig. 7 shows the optical microstructure of Al-0.25Zr-0.03Y and Al-0.25Zr-0.08Y annealed to 475 C. Deformation structure was still found in Al-0.25Zr-0.03Y while Al0.25Zr-0.08Y had experienced recrystallization. 4. Discussion Based on the results mentioned above, alloying element Y and its content plays an important role on the aging behavior, physical properties of precipitates and recrystallization temperature of AlZr-Y alloy. Compared with Al-0.25Zr-0.08Y, although with lower Y content, precipitates in Al-0.25Zr-0.03Y have smaller size, higher number density and enhanced coarsening resistance, leading to higher recrystallization temperature of the alloy. According to the
Fig. 3. The resistivity-concentration profiles for Al-Y and Al-Zr-Y alloys with various Y content.
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Fig. 4. Vickers hardness and electrical conductivity of Al-0.25Zr, Al-0.25Zr-0.03Y/0.08Y during isothermally aging at 400 C.
Fig. 5. TEM images of the precipitates aged at 400 C for 50 h (a, b) and 200 h (c, d). (a) for Al-0.30Zr, (b, c) for Al-0.30Zr-0.03Y and (d) for Al-0.30Zr-0.08Y.
KV model, liner relationship lies between precipitates r3 and aging time t, as shown in Fig. 8, where the coarsening constant K for Al0.25Zr, Al-0.25Zr-0.03Y and Al-0.25Zr-0.08Y is 4.38 1026, 1.18 1026 and 3.67 1027m3 s1 respectively. Coarsening constant K could be expressed as [34]
8gVm
K¼ 9RT
CYa ð1kY Þ DY
2
þ
a ð1k Þ CZr Zr DZr
2
. ; ki ¼ Cib Cia
(2)
where, g is the interface energy between precipitate b and a-Al
matrix, Vm the cell volume, C the concentration, ki the partition coefficient of solute i, R the gas constant and T temperature. Obviously, the Y related left part of the nominator in Eq. (2) is responsible for the larger value of K for binary Al-Zr, and difference between Al-0.25Zr-0.03Y and Al-0.25Zr-0.08Y should result from the concentration in the a-Al matrix CYa . Interactions between alloying elements have effect on the element solubility. Solubility of Sc in Al decreased by 50% in ternary Al-Zr-Sc and Er decreased by 50% in ternary Al-Zr-Er [35,36]. Intersections of the two line in Fig. 2 predicated the equilibrium solubility of Y in the alloy at given temperature. Therefore, the equilibrium solubility of Y at 600 C in
H. Gao et al. / Journal of Alloys and Compounds 696 (2017) 1039e1045
Fig. 6. Logarithmic plot of mean precipitates radius vs. aging time at 400 C for Al0.25Zr, Al-0.25Zr-0.03Y and Al-0.25Zr-0.08Y, with time exponent 0.34, 0.14 and 0.27 respectively.
Al-0.25Zr is about 0.03 wt%, which is only 1/4 of the Al-Y binary alloy. Fig. 9 gives the SEM image of the as cast Al-0.25Zr-0.03Y and Al-0.25Zr-0.08Y, where eutectic Al3Y on the grain boundary and primary Al3Y precipitates inside the grain could be found in the as cast Al-0.25Zr-0.08Y, which confirmed the decrement in solubility of Y in the ternary alloy, while only few eutectic Al3Y was observed in the as cast Al-0.25Zr-0.03Y. Primary Al3Y is favorable nucleation sites for recrystallization, as shown in Fig. 10, which should partly account for the lower recrystallization temperature of Al-0.25Zr0.08Y. According to classic nucleation theory [37], the precipitation driving force DG is related to solute concentration: DG f ln(Ce/C), where C is the solute concentration and Ce the equilibrium solubility. The critical radii for nucleation r* decrease with increment in DG, while nucleation current and the incubation time increase with
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Fig. 8. Relationship between precipitate radii r and aging time t at 400 C for the experiment alloys. Slope of the line indicated the coarsening constant K with value of 4.38 1026, 3.67 1027, 1.18 1026m3s1 for Al-0.25Zr, Al-0.25Zr-0.03Y and Al0.25Zr-0.08Y respectively.
increasing DG and diffusion coefficient. To further study the relationship between Ce and the temperature T, resistivity of Al-0.25Zr0.3Y soaked at various temperature was further measured and then converted into ln(Ce)~1/T plot, as shown in Fig. 11. Therefore, equilibrium solubility of Y in Al-0.25Zr is negligible at 400 C through linear extrapolation. Chemical composition analysis carried out on the matrix through atom probe tomography showed that the Y content is 0.007 at% for Al-0.25Zr-0.03Y and 0.0019 at% for Al-0.25Zr-0.08Y. Our previous investigation showed that Al3Y precipitate in the early stage of decomposition of Al-Zr-Y and then act as nucleus for Al3Zr. Therefore, difference in aging behavior between Al-Zr and Al-Zr-Y should result from nucleation process, the former is Al3Zr and the latter Al3Y. diffusion coefficient of Y in a-Al is several order of magnitude higher than Zr, which may be responsible for the larger number density of precipitates. As mentioned above, the equilibrium concentration of Y in Al-0.25Zr-Y is negligible, that is, the super-saturation of Y in Al-0.25Zr-0.03Y is about 3.6 times higher than that in Al-0.25Zr-0.08Y. Therefore, the nucleation driving force in Al-0.25Zr-0.03Y is about 1.3 times larger and the critical nucleus radius and energy are expected to cut by 50% and 75% respectively. In addition, compared with Al-Y, the larger number density and smaller size of the precipitates of Al-Zr-Y should also result from the greatly decreased equilibrium concentration of Y.
5. Conclusion
Fig. 7. Hardness of Al-0.25Zr-0.03Y and Al-0.25Zr, Al-0.25Zr-0.08Y, Al-0.25Zr-0.16Y (reproduced from Ref. [32]) annealed at various temperature for 1 h.
Effect of Y and its content on the aging behavior, precipitation coarsening kinetics and recrystallization temperature of Al-Zr-Y alloy was investigated. Accelerated precipitation kinetics and higher coarsening resistance was found in Al-0.25Zr-0.03Y with the coarsening time exponent n ¼ 0.14 and coarsening constant K ¼ 3.67 1027m3 s1 due to higher number density and smaller precipitate radius. Equilibrium solubility of Y in a-Al in the ternary Al-0.25Zr-Y decreased by 1/7 (to about 0.03 wt%) at 600 C and became negligible at 400 C, which lead to formation of primary Al3Y during solidification and sharp decline down to 1/4 in
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Fig. 9. SEM images of the as-cast alloy, (a) Al-0.25Zr-0.03Y and (b) Al-0.25Zr-0.08Y.
Table 1 Mean radius (r) and number density (N) of participates in the alloys aged at 400 C. Alloy
50 h r/nm
Al-0.25Zr Al-0.25Zr-0.03Y Al-0.25Zr-0.08Y
20.2 ± 3.2 12.4 ± 4.6 11.9 ± 2.3
200 h 3
N/m
18
1.51 10 3.01 1018 2.11 1018
r/nm
N/m3
31.6 ± 1.4 15.5 ± 2.7 20.6 ± 5.1
1 1018 2.91 1018 1.75 1018
effective Y content for the followed aging, with nucleation driving force in Al-0.25Zr-0.03Y being 36 times larger than that in Al0.25Zr-0.08Y according to the classical nucleation theory. Owing to free of primary Al3Y and improved thermal stability of precipitates, recrystallization temperature of Al-0.25Zr-0.03Y reached 500 C, which is 75 C/125 C higher than that of Al-0.25Zr-0.08Y/ Al-0.25Zr. Fig. 11. Relationship between equilibrium solubility Ce and the temperature T of Al0.25Zr-0.3Y.
Acknowledgment The project was supported by National Natural Science Foundation of China (Grant No. 51671131 and 51274141). References
Fig. 10. Recrystallization occurred in the vicinity of primary Al3Y formed during the solidification in Al-0.25Zr-0.08Y.
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(2013) 2849e2856. [23] H. Li, J. Bin, J. Liu, et al., Precipitation evolution and coarsening resistance at 400 C of Al microalloyed with Zr and Er, Scr. Mater 67 (2012) 73e76. [24] S.P. Wen, K.Y. Gao, Y. Li, H. Huang, Z.R. Nie, Synergetic effect of Er and Zr on the precipitation hardening of Al-Er-Zr alloy, Scr. Mater. 65 (2011) 592e595. [25] K.E. Knipling, D.C. Dunand, D.N. Seidman, Atom probe tomographic studies of precipitation in Al-0.1Zr-0.1Ti (at.%) alloys, Microsc. Microanal. 13 (2007) 503e516. [26] K.E. Knipling, D.C. Dunand, Creep resistance of cast and aged Al-0.1Zr and Al0.1Zr-0.1Ti (at.%) alloys at 300-400 C, Scr. Mater. 59 (2008) 387e390. [27] K.E. Knipling, D.C. Dunand, D.N. Seidman, Precipitation evolution in AleZr and Al-Zr-Ti alloys during isothermal aging at 375-425 C, Acta Mater. 56 (2008) 114e127. [28] S. Srinivasan, P.B. Desch, R.B. Schwarz, Metastable phases in the Al3X (X ¼ Ti, Zr, and Hf) intermetallic system, Scr. Metal. Mater. 25 (1991) 2513e2516. [29] K.E. Knipling, D.C. Dunand, D.N. Seidman, Criteria for developing castable, creep-resistant aluminum-based alloys-a review, Z. Met. 97 (2006) 246e265. [30] J.C. Foley, J.H. Perepezko, D.J. Skinner, Formation of metastable L12-Al3Y through rapid solidification processing, Mater. Sci. Eng. A 179 (1994) 205e209. [31] Yongzhi Zhang, Haiyan Gao, Yuan Kuai, Yanfeng Han, Jun Wang, Baode Sun, Sunwang Gu, Weiren You, Effects of Y additions on the precipitation and recrystallization of Al-Zr alloys, Mater. Chara. 86 (2013) 1e8. [32] Y. Zhang, J. Gu, Y. Tian, H. Gao, J. Wang, B. Sun, Microstructural evolution and mechanical property of Al-Zr and Al-Zr-Y alloys, Mater. Sci. Eng. A 616 (2014) 132e140. [33] H. Gao, W. Feng, Y. Wang, Structural and compositional evolution of Al3(Zr,Y) precipitates in Al-Zr-Y alloy, Mater. Chara. 121 (2016) 195e198. [34] C.J. Kuehmann, P.W. Voorhees, Ostwald ripening in ternary alloys, Metall. Mater. Trans. A 27 (1996) 937e943. [35] Christopher Booth-Morrison, David C. Dunand, David N. Seidman, Coarsening resistance at 400 C of precipitation-strengthened Al-Zr-Sc-Er alloys, Acta Mater. 59 (2011) 7029e7042. [36] N.Q. Vo, D.C. Dunand, D.N. Seidman, Improving aging and creep resistance in a dilute Al-Sc alloy by microalloying with Si, Zr and Er, Acta Mater. 63 (2014) 73e85. [37] T.H. Courtney, Mechanical Behavior of Materials[M], Waveland Press, 2005.
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Aging and recrystallization behavior of precipitation strengthened Al0.25Zr-0.03Y alloy Haiyan Gao a, b, *, Wuqiang Feng a, Jing Gu a, Jun Wang a, b, **, Baode Sun a, b a b
State Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University, Shanghai 200240, China Shanghai Key Laboratory of Advanced High-temperature Materials and Precision Forming, Shanghai 200240, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 17 July 2016 Received in revised form 30 October 2016 Accepted 5 December 2016 Available online 8 December 2016
Aging and recrystallization behavior of Al-0.25Zr-0.03Y were investigated through hardness and conductivity/resistivity measurement, transmission electron microscopy (TEM), scanning electron microscope (SEM) and atom probe tomography (APT) analysis. The results show that hardness peak of Al0.25Zr-0.03Y is about 6HV higher than that of Al-0.25Zr with the incubation period shortened from 50 h to 5 h as well. However, compared with Al-0.25Zr-0.08Y, smaller precipitates with higher number density, higher coarsening and recrystallization resistance was found in Al-0.25Zr-0.03Y. Precipitate coarsening constant in Al-0.25Zr-0.03Y is one order of magnitude lower than that in Al-0.25Zr-0.08Y. At the same time the recrystallization temperature of Al-0.25Zr-0.03Y reached 500 C, which is about 75 C higher than Al-0.25Zr-0.08Y and 125 C higher than Al-0.25Zr. In the ternary alloy, the equilibrium solubility of Y in a-Al decreased by 1/4 of that in binary Al-Y at 600 C and became negligible at 400 C. Few primary Al3Y was found in as-cast Al-0.25Zr-0.03Y and APT analysis revealed that the amount of Y in solid solution was about 4 times higher than in that in Al-0.25Zr-0.08Y, resulting in larger nucleation driving force for Al3Y and much reduced critical nucleus size and nucleation energy. The combine contribution of diminished primary Al3Y and improved physical properties of precipitates make for the higher coarsening and recrystallization resistance of Al-0.25Zr-0.03Y. © 2016 Published by Elsevier B.V.
Keywords: Rare earth alloys and compounds Precipitation Mechanical properties Scanning electron microscopy Transmission electron microscopy Kinetics
1. Introduction Dilute Al-Zr alloys find wide applications at elevated temperatures, especially for high-voltage cables for the power grids because L12-Al3Zr precipitated during aging of Al-Zr alloy is coherent with a-Al matrix and kinetically stable up to ~475 C [1]. However, precipitation kinetics of L12-Al3Zr is sluggish due to low diffusivity of Zr in a-Al, which is about 5 magnitude order lower than the selfdiffusivity of Al [2], leading to longer incubation time for nucleation, lower precipitation number density and larger particle size [3]. In industrial practice, aging treatment of binary Al-Zr alloy is always time and energy consumable, which lasts 70 h or longer. Researches on combined additions of Rear Earth (RE) elements and transition metals (TM), such as Sc [4e12], Yb [13e19], Er
* Corresponding Shanghai Jiao Tong ** Corresponding Shanghai Jiao Tong E-mail address:
author. State Key Laboratory of Metal Matrix Composites, University, Shanghai 200240, China. author. State Key Laboratory of Metal Matrix Composites, University, Shanghai 200240, China. [email protected] (H. Gao).
http://dx.doi.org/10.1016/j.jallcom.2016.12.064 0925-8388/© 2016 Published by Elsevier B.V.
[20e24], Ti [1,25e27] and Hf [28] have been carried out to accelerate precipitation kinetics through formation of L12-Al3M (M ¼ RE or TM) in the early stage of decomposition. It was well accepted that L12-Al3M could act as the nucleus for L12-Al3Zr due to larger diffusion coefficient of RE and TM in a-Al matrix. Compared with binary Al-Zr alloy, attractive properties, such as larger number density, higher recrystallization temperature and reduced coarsening kinetics was found in ternary Al-Zr-M alloys due to formation of Al3(M1xZrx) precipitates. Ternary Al-Sc-Zr alloy was widely investigated because of strong strengthening effect of Sc and precipitation of Al3Sc at the interdenrites, which filled up effectively the precipitation free zone of Al3Zr resulting from the segregation of Zr during solidification. However, the high cost of Sc limited its commercial use. Effective elements with lower cost are welcomed in engineering applications. Y locates in the same group as Sc. However, Al3Y was used to be regarded impossible to be effective nuclei for L12-Al3Zr of conventionally cast Al-Zr-Y alloys [29], because the L12-Al3Y was accessible only during the decomposition of rapid solidified Al-Y alloys [30]. Few publications could be found focusing on the
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3. Results
increment about 7HV. Fig. 2 gives the precipitates observed in Al-Y aged at 250 C for 1 h. EDS analysis showed that the precipitate is Al3Y consisting of 75.87 at% Al and 24.13 at%Y. Our research based on atom probe tomography analysis also confirmed the precipitation of short-bar-shaped Al3Y in the early stage of decomposition of Al-0.25Zr-0.08Y alloy [33]. However, in Al-Zr-Y alloys, no strengthening effect was observed near 250 C but slight hardness loss instead during 100e375 C. As will be discussed later that in ternary Al-Zr-Y alloy, the equilibrium solubility of Y decreased dramatically to about one quarter of that in binary Al-Y alloy. APT analysis revealed that in the as-cast ingot, the amount of Y dissolved in a-Al matrix is only 0.007 at% for Al-0.25Zr-0.03Y and 0.0019 at% for Al-0.25Zr-0.08Y. Therefore, precipitation strengthening of Al3Y is unexpectable. Hardness peak at 475e500 C should result from the precipitation strengthening of L12-Al3Zr. It is obvious that combined addition of Y and Zr resulted in stronger strengthening effect, lower Zr precipitation temperature and accelerated aging kinetics. Conductivity evolution is similar to that of the hardness. However, difference in conductivity between Al-Y and Zr containing AlZr and Al-Zr-Y is obvious. Few data could be found in the published literature reporting the additional resistivity caused by dissolved Y atoms. Therefore, resistivity measurement was performed on Al0.25Zr-Y alloys with various amount of Y after being soaked at 600 C for 50 h, as shown in Fig. 3, where the data points are experimental measurements and the line are linear fit of the data. Slope of the two diagonals are close, 1.68 mU mm/wt%, which reflected the resistivity increment coming from the dissolved Y atoms. While the effect of dissolved Zr is about 17.4 mU mm/wt%, which is about an order of magnitude larger than that of Y. Therefore, conductivity difference between Al-0.02Y and Al-0.16Y is negligible and it is about 8% IACS higher than that of Zr containing alloys before precipitation of Zr. It is noticeable that both hardness and conductivity of Al-0.25Zr0.03Y are a bit higher than that of Al-0.25Zr-0.08Y. The average grain size of the as cast Al-0.25Zr, Al-0.25Zr-0.03Y and Al-0.25Zr0.08Ywas measured as 591.8 ± 2.4, 485.6 ± 1.3 and 396.2 ± 1.6 mm respectively. Due to greatly decreased equilibrium solubility of Y in Al-Zr-Y alloy, as will be discussed in detail later and shown in Fig. 9, excessive Y in Al-0.25Zr-0.08Y transformed into primary Al3Y which was reported acting as grain finer for a-Al during solidification of the alloy [32]. In addition, grain growth is obvious during aging, the average grain size of Al-0.25Zr-0.03Y increased by 50% and reached 745.7 ± 2.4 mm after being aged at 375 C for 1 h. Therefore, difference in grain size and grain grow during aging may be responsible for the observed hardness and conductivity difference and hardness loss below 400 C. More details for the aging behavior could be found in the hardness and conductivity evolution curves recorded through the isothermal aging, as shown in Fig. 4. Compared with Al-0.25Zr, accelerated precipitation kinetics was found in Al-0.25Zr-0.03Y, incubation period of Al-0.25Zr-0.03Y/0.08Y shortened from 50 h to 5 h and the time for hardness peak shortened from 300 h to 50 h.
3.1. Aging behavior
3.2. Coarsening kinetics and recrystallization
Fig. 1 shows the Vickers hardness and electrical conductivity evolution of Al-Zr-Y during isochronal aging. Binary Al-Y and Al-Zr alloys was also given for comparison purpose. Difference in hardness and conductivity evolution among alloys is obvious. Binary Al0.16Y and Al-0.25Zr witnessed a smaller hardness peak of about 2HV increment at 250 C and 500 C respectively, while hardness of Al-0.25Zr-0.03Y/0.08Y alloys decreased slightly till 375 C and then increased quickly and reached a peak at 475e500 C with
Fig. 5 shows the TEM images of Al-0.25Zr and Al-0.25Zr-0.03Y/ 0.08Y aged at 400 C for 50 h and 200 h. Table 1 gives the size and number density of the precipitates in detail. Compared with Al0.25Zr, precipitate in Al-0.25Zr-0.03Y and Al-0.25Zr-0.08Y is smaller in size with higher number density. After 50 h aging, the average radii of the precipitate in ternary alloys is only half of that in the binary with doubled number density. However, data in the table also revealed that the coarsening rate of Al-0.25Zr and Al-
precipitation evolution of Al-Zr-Y alloys. Our previous research showed that, compared with binary Al-Zr alloy, the number density of the precipitates in Al-Zr-Y is about one order magnitude higher and recrystallization temperature increased from 350 C to 400 C; however, when Y content increased to 0.16 wt%, the recrystallization temperature decreased to 325 C [31,32]. In present work, effect of Y and its content on the aging behavior, precipitation coarsening kinetics and recrystallization resistance of Al-Zr-Y alloy was further investigated for the development of high performance and low cost Al-Zr alloy. 2. Experimental Al-Zr-Y ingots were cast from 99.99 wt% pure Al with Al-Y and Al-Zr master alloys using alumina crucibles at 750 C in the open air. The melt was poured into a graphite mold after mechanical stirring. Composition of alloys was measured by inductively coupled plasma-atomic emission spectroscopy (ICP-AES). Al-Y and Al-0.25Zr-Y alloys with various Y content were prepared. Isochronal aging were carried out from 100 to 550 C with increments of 25 C, each lasting 1 h. Isothermal treatment was performed at 400 C up to 750 h. Some ingots were cold-rolled to 82% reduction after being aged at 400 C for 50 h and then annealed at 150e600 C with increments of 25 C for recrystallization temperature measurements. Samples were water quenched after aging. The microstructural evolution was investigated using JEM 2100F transmission electron microscopy (TEM) operated at 200 kV. TEM specimens were prepared through mechanical grinding and twinjet electro-polishing at 12 V in a 30% nitric acid and 70% methanol solution cooled to 30 C. FEI NOVA NanoSEM 230 fieldemission scanning electron microscope (FESEM) was employed to observe the distribution of the precipitates at 10 kV. The maximum depth below the specimen surface where precipitates can be detected was calculated to be 0.66 mm using KanayaeOkayama equation. Precipitates size and number density were analyzed using Image pro software. At least 100 precipitates were measured for each mean radius of precipitates and at least 6 SEM images were analyzed for each number density. Atom probe tomography (ATP) analyses with 200 kHz pulse repetition rate, 20% pulse fraction, 20 K specimen temperature and a residual pressure of 108 Pa was employed to measure the Y content dissolved in the a-Al matrix before aging. Specimen with tip radii less than 100 nm were prepared through electro-chemical polishing. Grain size of the specimen was determined using optical microscopy under polarized light. Vickers hardness and electrical conductivity of the aged specimen were measured at ambient temperature. The Vickers hardness measurements were performed on polished samples using a load of 200 g and a dwell time of 10 s. A minimum of 10 measurements were taken as the experimental data. Five conductivity measurements were recorded for each reading using a FD-102 eddy current conductivity tester.
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Fig. 1. Vickers hardness (a) and electrical conductivity (b) of Al-Zr, Al-Y and Al-Zr-Y alloys during isochronal aging from 100 to 550 C.
follows equation:
rtn r0n ¼ Kt
Fig. 2. SEM image of precipitates in Al-0.16Y aged at 250 C for 1 h.
0.25Zr-0.08Y is similar, the precipitate size increased by 60% as the time extended from 50 to 200 h, while that for Al-0.25Zr -0.03Y is only 30%. Number density in Al-0.25Zr -0.03Y almost kept unchanged, indicating a quasi-static coarsening, while that in Al0.25Zr and Al-0.25Zr-0.08Y decreased by 33% and 17% respectively. Fig. 6 shows the time dependence of the mean size of the precipitates in the logarithmic scale. According to the KV model [34], relationship lies between precipitate size r and aging time t
(1)
where rt is the precipitate size at time t, r0 the initial size, K the coarsening constant. Slope of the fitting line in Fig. 6 gives the reciprocal of the time exponent n. The value of 1/n for Al-0.25Zr is 0.34, which is in agreement with the theoretical value of 0.33, and that for Al-0.21Zr-0.03Y and Al-0.21Zr-0.08Y is 0.14 and 0.27 respectively. Fig. 7 gives the hardness vs annealing temperature curves for the cold-rolled Al-0.25Zr-0.03Y alloys. The recrystallization temperature was defined as the temperature at which 50% hardness loss occurred. For comparison purpose, curves for Al-0.25Zr, Al-0.25Zr0.08Y and Al-0.25Zr-0.16Y obtained in our previous research is also given [32]. The recrystallization temperature of Al-0.25Zr-0.03Y is about 500 C, which is 75 C or 125 C higher than that of Al-0.25Zr0.08Y or Al-0.25Zr. Inserts in Fig. 7 shows the optical microstructure of Al-0.25Zr-0.03Y and Al-0.25Zr-0.08Y annealed to 475 C. Deformation structure was still found in Al-0.25Zr-0.03Y while Al0.25Zr-0.08Y had experienced recrystallization. 4. Discussion Based on the results mentioned above, alloying element Y and its content plays an important role on the aging behavior, physical properties of precipitates and recrystallization temperature of AlZr-Y alloy. Compared with Al-0.25Zr-0.08Y, although with lower Y content, precipitates in Al-0.25Zr-0.03Y have smaller size, higher number density and enhanced coarsening resistance, leading to higher recrystallization temperature of the alloy. According to the
Fig. 3. The resistivity-concentration profiles for Al-Y and Al-Zr-Y alloys with various Y content.
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Fig. 4. Vickers hardness and electrical conductivity of Al-0.25Zr, Al-0.25Zr-0.03Y/0.08Y during isothermally aging at 400 C.
Fig. 5. TEM images of the precipitates aged at 400 C for 50 h (a, b) and 200 h (c, d). (a) for Al-0.30Zr, (b, c) for Al-0.30Zr-0.03Y and (d) for Al-0.30Zr-0.08Y.
KV model, liner relationship lies between precipitates r3 and aging time t, as shown in Fig. 8, where the coarsening constant K for Al0.25Zr, Al-0.25Zr-0.03Y and Al-0.25Zr-0.08Y is 4.38 1026, 1.18 1026 and 3.67 1027m3 s1 respectively. Coarsening constant K could be expressed as [34]
8gVm
K¼ 9RT
CYa ð1kY Þ DY
2
þ
a ð1k Þ CZr Zr DZr
2
. ; ki ¼ Cib Cia
(2)
where, g is the interface energy between precipitate b and a-Al
matrix, Vm the cell volume, C the concentration, ki the partition coefficient of solute i, R the gas constant and T temperature. Obviously, the Y related left part of the nominator in Eq. (2) is responsible for the larger value of K for binary Al-Zr, and difference between Al-0.25Zr-0.03Y and Al-0.25Zr-0.08Y should result from the concentration in the a-Al matrix CYa . Interactions between alloying elements have effect on the element solubility. Solubility of Sc in Al decreased by 50% in ternary Al-Zr-Sc and Er decreased by 50% in ternary Al-Zr-Er [35,36]. Intersections of the two line in Fig. 2 predicated the equilibrium solubility of Y in the alloy at given temperature. Therefore, the equilibrium solubility of Y at 600 C in
H. Gao et al. / Journal of Alloys and Compounds 696 (2017) 1039e1045
Fig. 6. Logarithmic plot of mean precipitates radius vs. aging time at 400 C for Al0.25Zr, Al-0.25Zr-0.03Y and Al-0.25Zr-0.08Y, with time exponent 0.34, 0.14 and 0.27 respectively.
Al-0.25Zr is about 0.03 wt%, which is only 1/4 of the Al-Y binary alloy. Fig. 9 gives the SEM image of the as cast Al-0.25Zr-0.03Y and Al-0.25Zr-0.08Y, where eutectic Al3Y on the grain boundary and primary Al3Y precipitates inside the grain could be found in the as cast Al-0.25Zr-0.08Y, which confirmed the decrement in solubility of Y in the ternary alloy, while only few eutectic Al3Y was observed in the as cast Al-0.25Zr-0.03Y. Primary Al3Y is favorable nucleation sites for recrystallization, as shown in Fig. 10, which should partly account for the lower recrystallization temperature of Al-0.25Zr0.08Y. According to classic nucleation theory [37], the precipitation driving force DG is related to solute concentration: DG f ln(Ce/C), where C is the solute concentration and Ce the equilibrium solubility. The critical radii for nucleation r* decrease with increment in DG, while nucleation current and the incubation time increase with
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Fig. 8. Relationship between precipitate radii r and aging time t at 400 C for the experiment alloys. Slope of the line indicated the coarsening constant K with value of 4.38 1026, 3.67 1027, 1.18 1026m3s1 for Al-0.25Zr, Al-0.25Zr-0.03Y and Al0.25Zr-0.08Y respectively.
increasing DG and diffusion coefficient. To further study the relationship between Ce and the temperature T, resistivity of Al-0.25Zr0.3Y soaked at various temperature was further measured and then converted into ln(Ce)~1/T plot, as shown in Fig. 11. Therefore, equilibrium solubility of Y in Al-0.25Zr is negligible at 400 C through linear extrapolation. Chemical composition analysis carried out on the matrix through atom probe tomography showed that the Y content is 0.007 at% for Al-0.25Zr-0.03Y and 0.0019 at% for Al-0.25Zr-0.08Y. Our previous investigation showed that Al3Y precipitate in the early stage of decomposition of Al-Zr-Y and then act as nucleus for Al3Zr. Therefore, difference in aging behavior between Al-Zr and Al-Zr-Y should result from nucleation process, the former is Al3Zr and the latter Al3Y. diffusion coefficient of Y in a-Al is several order of magnitude higher than Zr, which may be responsible for the larger number density of precipitates. As mentioned above, the equilibrium concentration of Y in Al-0.25Zr-Y is negligible, that is, the super-saturation of Y in Al-0.25Zr-0.03Y is about 3.6 times higher than that in Al-0.25Zr-0.08Y. Therefore, the nucleation driving force in Al-0.25Zr-0.03Y is about 1.3 times larger and the critical nucleus radius and energy are expected to cut by 50% and 75% respectively. In addition, compared with Al-Y, the larger number density and smaller size of the precipitates of Al-Zr-Y should also result from the greatly decreased equilibrium concentration of Y.
5. Conclusion
Fig. 7. Hardness of Al-0.25Zr-0.03Y and Al-0.25Zr, Al-0.25Zr-0.08Y, Al-0.25Zr-0.16Y (reproduced from Ref. [32]) annealed at various temperature for 1 h.
Effect of Y and its content on the aging behavior, precipitation coarsening kinetics and recrystallization temperature of Al-Zr-Y alloy was investigated. Accelerated precipitation kinetics and higher coarsening resistance was found in Al-0.25Zr-0.03Y with the coarsening time exponent n ¼ 0.14 and coarsening constant K ¼ 3.67 1027m3 s1 due to higher number density and smaller precipitate radius. Equilibrium solubility of Y in a-Al in the ternary Al-0.25Zr-Y decreased by 1/7 (to about 0.03 wt%) at 600 C and became negligible at 400 C, which lead to formation of primary Al3Y during solidification and sharp decline down to 1/4 in
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Fig. 9. SEM images of the as-cast alloy, (a) Al-0.25Zr-0.03Y and (b) Al-0.25Zr-0.08Y.
Table 1 Mean radius (r) and number density (N) of participates in the alloys aged at 400 C. Alloy
50 h r/nm
Al-0.25Zr Al-0.25Zr-0.03Y Al-0.25Zr-0.08Y
20.2 ± 3.2 12.4 ± 4.6 11.9 ± 2.3
200 h 3
N/m
18
1.51 10 3.01 1018 2.11 1018
r/nm
N/m3
31.6 ± 1.4 15.5 ± 2.7 20.6 ± 5.1
1 1018 2.91 1018 1.75 1018
effective Y content for the followed aging, with nucleation driving force in Al-0.25Zr-0.03Y being 36 times larger than that in Al0.25Zr-0.08Y according to the classical nucleation theory. Owing to free of primary Al3Y and improved thermal stability of precipitates, recrystallization temperature of Al-0.25Zr-0.03Y reached 500 C, which is 75 C/125 C higher than that of Al-0.25Zr-0.08Y/ Al-0.25Zr. Fig. 11. Relationship between equilibrium solubility Ce and the temperature T of Al0.25Zr-0.3Y.
Acknowledgment The project was supported by National Natural Science Foundation of China (Grant No. 51671131 and 51274141). References
Fig. 10. Recrystallization occurred in the vicinity of primary Al3Y formed during the solidification in Al-0.25Zr-0.08Y.
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