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[9] M. Bram, A. Ahmad-Khanlou, A. Heckmann, B. Fuchs, H. P. Buchkremer, D. Stöver, Mater. Sci. Eng. A 2002, 337, 254. [10] B. Bertheville, J. E. Bidaux, J. Alloys Compd. 2005, 387, 211. [11] H. Kato, T. Koyari, S. Miura, K. Isonishi, M. Tokizane, Scripta Metall. Mater. 1990, 24, 2335. [12] H. Kato, T. Koyari, M. Tokizane, S. Miura, Acta Metall. Mater. 1994, 42, 1351. [13] K. Johansen, H. Voggenreiter, G. Eggeler, Mater. Sci. Eng. A 1999, 273, 410. [14] H. Kyogoku, A. Terayama, S. Komatsu, in Proc. of Wld. PM2004, Vol. 4, EPMA, Shrewsbury, UK 2004. [15] J. W. Newkirk, J. A. Sago, G. M. Brasel, in Proc. and Fabrication of Adv. Mater. VII , TMS, Warrendale, PA 1998, 213. [16] J. W. Newkirk, G. M. Brasel, J. A. Sago, in Adv. in P-M and Part. Mater. Vol. 3 , Metal Powder Industries Federation, Princeton, NJ 1997, 187. [17] L. Krone, J. Mentz, M. Bram, H. P. Buchkremer, D. Stöver, M. Wagner, G. Eggeler, D. Christ, S. Reese, D. Bogdanski, M. Köller, S. A. Esenwein, G. Muhr, O. Prymak, M. Epple, Adv. Eng. Mater. 2005, 7, 613. [18] W. Tang, B. Sundmann, R. Sandström, C. Qui, Acta Mater. 1999, 47, 3457. [19] J. Mentz, L. Krone, M. Bram, H. P. Buchkremer, D. Stöver, Influence of Heat Treatment on Properties of Hot Isostatic Pressed (HIP) NiTi, in Proc. of SMST 2004, October 2004, Baden-Baden, Germany. [20] L. Krone, M. Bram, J. Mentz, H. P. Buchkremer, D. Stöver, in Proc. of Wld. PM2004, Vol. 4, EPMA, Shrewsbury, UK 2004, 485. [21] L. Krone, J. Mentz, D. Stöver, M. Epple, NiTi Shape Memory Alloy Parts Produced by Metal Injection Molding, in Proc. of SMST 2004, October 2004, Baden-Baden, Germany. [22] E. Schüller, O. A. Hamed, M. Bram, D. Sebold, H. P. Buchkremer, D. Stöver, Adv. Eng. Mater. 2003, 5, 918.

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DOI: 10.1002/adem.200500197

High Strength Nanocrystalline Ni-Co Alloy with Enhanced Tensile Ductility** By Changdong Gu, Jianshe Lian,* and Zhonghao Jiang Nanocrystalline (nc) materials, with average grain size typically below 100 nm, are experiencing a rapid development in recent years due to their existing and potential applications in a wide variety of technological areas such as electronics, catalysis, ceramics, magnetic data storage, structural component et al.[1,2] The functionality and overall reliability of next-generation microelectromechanical systems (MEMS), nanoelectromechanical systems (NEMS), integrated circuits and micro- and nanoscale devices are closely tied to the mechanical properties of the nc metals which they are constructed.[3] Nc Ni, especially fabricated by electrodeposition has been widely investigated as model material for studying the mechanical behavior of nc metals.[4–10] However, numerous experiments showed that nc Ni, with grain size typically smaller than 30 nm, often showed high strength compared with that conventional microns grain sizes counterparts, while exhibiting low tensile ductility with typically less than a few per cent at room temperature (RT).[4,5] For structural materials, strength and ductility are two key material parameters. Therefore, it is imperative to improve tensile ductility of nc materials for their structural applications which often require both high strength and good ductility.[11,12] Koch et al reviewed the limitations to the ductility in nc materials and presented the results of some recent breakthroughs in optimization of mechanical properties of nanostructured metals and alloys.[13] Strength and ductility of nc materials may be optimized in this way by the control of grain size and grain size distribution. For example, high strength and good ductility was obtained in the nanostructued Zn[14] and Cu[15] with a broad grain size distribution or a bimodal grain size distribution. Recently, we also reported an electrdeposited nc Ni with a special microstructure exhibited a combination of high

– [*] Dr. C. Gu, Prof. J. Lian, Prof. Z. Jiang Key laboratory of Automobile Materials College of Materials Science and Engineering Jilin University, Nanling Campus Changchun, 130025, China E-mail: [email protected] (Jianshe Lian) [**] This work was surported by the Foundation of national key basic research and development program No. 2004CB619301 and the Project 985-Automotive Engineering of Jilin University.

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Changdong Gu et al./High Strength Nanocrystalline Ni-Co Alloy with Enhanced Tensile Ductility Table 1. Chemical impurity contents of the electrodeposited Ni-1.7 %Co alloy.

Element

S

C

Mg

Ca

Ba

B

Mn

Pb

Content (mass ppm)

310

580

5.11

10.22

6.64

132.9

5.62

511.0

What is seen there resembles in all aspects Moire fringes from overlapping grains of different crystallographic orientation.[23] High resolution electron microscopy is required to further confirm these structures. The SAD patterns shown as an inset of the Figure 1 confirms the expected ring patterns for such nc material and analysis of the patterns verified a single-phase, f.c.c structure which is agree with the analysis of XRD. A statistical analysis of ∼ 500 grains indicate that the nc Ni-1.7 %Co alloy has an average grain size of about 25 nm with a narrow distribution of 10-55 nm. Tensile mechanical behavior: Figure 2 gives the nominal engineering stress-strain curves of nc Ni-1.7 %Co alloy performed at strain rates of 1.04 × 10–6 to 1.04 s–1 and RT. It is interesting that the nc Ni-1.7 %Co alloy exhibits improved elongation to failure of 8-9.6 % with high ultimate tensile strength (UTS) of about 1600 MPa over a wide strain rate region. Furthermore, it should be noted that the variation of the stress depended on the strain rate is found in this nc Ni-1.7 %Co alloy. Numerous experimental evidences have shown that nc materials exhibit high strain-rate sensitivity of flow stress.[4–5, 7–9] However, to date, only limited investigation has exercised on the strain-rate sensitivity of nc alloys.[24] The strain rate sensitivity m of flow stress is defined as m=∂lnrf/∂ln_e, where rf and e_ is flow stress and strain rate, respectively. Logarithm plot of flow stress r1 % (at 1 % plastic strain) versus strain rate e of Ni-1.7 %Co alloy specimen is shown as the inset of Figure 2. The strain rate sensitivities m of nc Ni-1.7 %Co alloy is 0.016, which is in the range of 0.01-0.03 for nc Ni with grain size of about 20 nm.[7,25] The activation volume (V) is widely used recently to determine the possible deformation mechanisms through the tensile test data of nc materials.[26–28] The experimental activation volume can be given by :[26] Vˆ

Fig. 1. Bright field TEM image together with the corresponding SAD patterns (upright inset) for nc Ni-1.7 %Co alloy. The left-down inset shows the typical bulk nc NiCo alloy fabricated by a direct-current electrodeposition in this study.

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p ∂ ln_e 3kT ∂r

…1†

where j is the Bolzmann constant, T is the absolute temperature and r and e_ are flow stress and strain rate, respectively. In this study, the experimental activation volume of the Ni1.7 %Co alloy under deformation at RT is about 18 b3, where b is the Burgers vector. The activation length is about 4.5 nm, which is reasonably agreement with the length of a GB dislocation source.[29] For nc grains in this region of about 30 nm, recent numerous experiments and MD simulations suggest that the dislocations are emitted from GB sources (triple points, facets, steps and jogs), and traverse the grain under the applied stress to be re-incorporated into the opposing GB. And this process of GB-defect-assisted dislocation generation,

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strength of about 1200 MPa and enhanced ductility of 7.5– 8.3 % at strain rates of 1.04 × 10–4 to 1.04 s–1 and RT.[16] In addition, nc materials with grain scale of less than 50 nm and relatively narrow grain size distribution would also exhibit the optimization of strength and ductility, such as nc Co,[17] Cu,[18] and Ni-15 %Fe alloy [6]. Ni-Co alloys have been investigated as important engineering materials for several decades due to their unique properties, such as high strength, good wear resistance, heatconductivity, electrocatalytic activity and specific magnetic properties.[19–21] However there are very limited studies focused on the mechanical properties of nc Ni-Co alloys. In this paper, nc Ni-Co alloy was prepared by a direct-current electrodeposition technique. The mechanical behavior of this nc Ni-Co alloy was investigated by uniaxial tensile tests at different strain rates and RT. Results and discussion. Chemical composition, texture and microstructure: The typical bulk nc Ni-Co alloy fabricated by a direct-current electrodeposition in this study is shown as the inset of Figure 1. The content of Co in the Ni-Co deposits was 1.7 wt.%. Table 1 gives the main impurities contents (mass ppm) of the electrodeposited Ni-1.7 %Co alloy. All impurities were introduced from the used additives and chemicals. Pure Ni has the face-centered cubic (f.c.c) structure. The studied Ni-1.7 %Co alloy has a preferential orientation along the {200} family planes and solid solution f.c.c. structure revealled by XRD. Figure 1 gives the TEM bright field images together with corresponding SAD patterns of Ni-1.7 %Co alloy. Grain boundary (GB) features are difficult to distinguish in this nc Ni-Co alloy, just like the reported electrodeposited nc Ni.[7] Twins with a similar spacing have indeed been observed in nc metals, such as Pb after large deformation,[22] but such defects exhibit strictly planar interfaces as opposed to the curved appearance of those marked by arrows in Figure 1.

1.04E-6 1.04E-4 1.04E-3 1.04E-2 4.17E-2 1.04E-1 1.04

1600 1400 1200 7.45

1000

7.40

ln (flow stress σ1%)

800 600 400

7.35 7.30

m=0.016

7.25 7.20

200

7.15 -14

-12

-10

-8

-6

-4

-2

0

2

ln (strain rate)

0 0

2

4

6

8

10 12 14 16 18 20 22 24 26 28 30

Nominal engineering strain (%) Fig. 2. Nominal engineering stress-strain curves of nc Ni-1.7 %Co alloy at different strain rates and RT. The inset is the logarithm plot of flow stress (at 1 % plastic strain) versus strain rate. The strain rate sensitivities m of the nc Ni-1.7 %Co alloy is estimated about 0.016.

which is easier than that of emanating a lattice dislocation out of a GB,[25] can well be a thermally activated process usually with an activation volume of ∼ 10 b3, such as in 30 nm Ni and Co, respectively.[25] Therefore, the dislocation motion should be responsible for the plastic deformation of the nc Ni-1.7 %Co alloy. A very interesting feature of the tensile mechanical behavior of nc Ni-1.7 %Co alloy is the serration character of the plastic flow, especially at a strain rate of 1.04 × 10–6 s–1, which is marked by arrow in Figure 2. Surprisingly, the elongation to failure of the nc Ni-1.7 %Co alloy reaches about 9.6 % at a strain rate of 1.04 × 10–6 s–1 and RT. The serration flow started after the yield and continued until failure. The stress drop, defined as the stress change from the highest to the lowest points of each serration step, seems to increase with time and hence with the nominal strain. The maximum amplitude of the stress drop reaches about 160 MPa. Furthermore, some primary serrations with large stress drops are composed of the secondary serrations with small stress drops. It can also be found the slope of the rising portions of the serrations is of the same order of magnitude as the material’s elastic modulus, indicating that there is a significant elastic component to the deformation. It is generally accepted that clouds of solute atoms restrict dislocation motion and are responsible to the strain rate dependency of serrated flow.[30–31] In particular, solute atoms diffusion towards dislocations that are pinned by forest obstacles. In our case, longer dislocation waiting times associated with lower nominal strain rates (1.04 × 10–6 s–1) would allow for more solute (Co atoms) diffusion. The waiting times should be therefore related to the amplitude of the stress drop that occurs when dislocations break free. The further work should be done to explain this phenomenon. Figure 3 summarizes the elongation to failure and yield stress for electrodeposited nc Ni and Ni alloys from litera-

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tures and this study. It has been shown that large size specimen gives low strength and ductility.[8] Considering the effects of specimen size on the tensile tests, data in Figure 3 are obtained from the specimen having considerable “large” size, and our specimen has the largest gauge section size among the compared experiments. Generally, the yield stress and ductility usually have opposing characteristics. As to most nc Ni specimens, the higher the yield stress, the lower the ductility, and vice verse. However, our Ni-1.7 %Co alloy in this study and Ni-15 %Fe alloy[6] are set in the tradeoff region between the strength and elongation to failure. The 40 nm Ni by electrodeposition also showed a combination of yield stress of 800-940 MPa and total elongation of 7-8 %,[16] which is also shown in Figure 3. Compared with the 40 nm Ni,[16] the yield stress and ductility of Ni-1.7 %Co alloy are both enhanced. The refined grain size and solid solution strengthening should be responsible for the enhanced strength of the Ni-1.7 %Co alloy. In addition, a recent molecular dynamics simulation shows that a reduction in the stacking fault energy (SFE) reduces the stress needed to nucleate partial dislocations from grain boundaries.[32] So the effect of SFE should be more pronounced at smaller grain sizes where the emission of partial dislocations are more prevalent. Ebrahimi et al investigated the effect of SFE on the tensile stress-strain behavior of nc face-centered cubic metals, where pronounced that decreasing the SFE of Ni by alloying with Cu and Fe increased the strain hardening rate.[33] Addition of alloying element Co into Ni is known to produce a reduction in the SEF.[34] So, Ni-alloyed Co in the case increases the strain hardness rate and improve the strength, compared the nc Ni [16]. Enhancing strain hardening could minimize the mechanical instabilities of nc metals, and hence improve the ductility.[13] Lastly, the low contents of impurities of our electrodeposits (see Tab. 1) should also be attributed to holding its intrinsic

10 9

Elongation to failure (%)

1800

Engineering stress (MPa)

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Changdong Gu et al./High Strength Nanocrystalline Ni-Co Alloy with Enhanced Tensile Ductility

8 Ni-Co alloy, present Ni-Fe alloy [6] Ni [4] Ni [6] Ni [8] Ni [9] Annealed Ni [10] Ni [16]

7 6 5 4 3 2 1 400

600

800

1000

1200

1400

1600

Yield stress (MPa) Fig. 3. Summary of elongation to failure vs. yield stress for electrodeposited nc Ni and Ni-based alloys. Nc Ni-1.7 %Co alloy in this study sets in the typical tradeoff region between yield stress and elongation to failure. The dashed curve serves as guide to the eye.

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Changdong Gu et al./High Strength Nanocrystalline Ni-Co Alloy with Enhanced Tensile Ductility

Experimental Bulk nc Ni-1.7 %Co alloy with a thickness of about 350 lm were electrodeposited from a electrolyte containing nickel sulfate, nickel chloride, boric acid and saccharin[37] with addition of cobalt sulfate. All reagents were analytical grad. The anode was a pure Ni plate and the cathode used was 15.0 cm × 6.0 cm × 0.1 cm steel sheets (C1008, AISI). The volume of plating bath was 30 L and the electrolytes were maintained in a pH level of 5.0 and a temperature of 50 °C during deposition, respectively. The current density was chosen 2.5 A/dm2. Sodium hydroxide solution and dilute sulfuric acid were used to adjust the pH value of the electrolyte. Crystallographic structure was analyzed by X-ray diffractometer (XRD, D/max 2500PC) with a Cu target and a monochronmator at 50 kV and 300 mA. Microstructure of the nc Ni-1.7 %Co alloy was observed by the transmission electron microscope (TEM, H-800). For determining the contents of Co and impurities in the deposits, chemical analysis by the inductively coupled plasma atomic emission spectrometry (ICP-AES, Plasma/1000) and Carbon/ Sulfur determinators were undertaken. The dog-bone shaped tensile specimens with a gauge cross-section of 2.0 mm × 0.25 ∼ 0.3 mm and a gauge length of 8.0 mm were cut from a as-deposited Ni-1.7 %Co alloy sheet by using a wire Electro-Discharging-Machine (EDM) and were then polished using 2500 SiC 3 L sandpapers and 0.5 lm diamond powder to a mirror-like finish surface. Tensile tests were carried out on MTS-810 system at strain rates of 1.04 × 10–6 to 1.04 s–1 and RT. Tensile ductility in this study was measured through the cross-head movement of the tensile machine. Tensile specimens were cut from the same material in order to avoid the effect of baths on the results of tensile tests.[8] The morphologies of fracture surface of specimens were observed by using Scanning Electron Microscope (SEM, JSM-5600).

Received: September 06, 2005 Final version: November 03, 2005

– [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]

[15] Fig. 4. Fracture morphology of nc Ni-1.7 %Co alloy deformed at a strain rate of 1.04 × 10–2 s–1 and RT.

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[16]

H. Gleiter, Acta Mater. 2000, 48, 1. S. C. Tjong, H. Chen, Mater. Sci. Eng. 2004, R45, 1. K. J. Hemker, Sci. 2004, 304, 221. N. Wang, Z. Wang, K. T. Aust, U. Erb, Mater. Sci. Eng. 1997, A237, 150. K. S. Kumar, S. Suresh, M. F. Chisholm, J. A. Horton, P. Wang, Acta Mater. 2003, 51, 387. H. Q. Li, F. Ebrahimi, Appl. Phys. Lett. 2004, 84, 4307. K. S. Kumar, H. Van. Swygenhoven, S. Suresh, Acta Mater. 2003, 51, 5743. F. Dalla Torre, H. Van. Swygenhoven, M. Victoria, Acta Mater. 2002, 50, 3957. R. Schwaiger, B. Moser, M. Dao, N. Chollacoop, S. Suresh, Acta Mater. 2003, 51, 5159. Y. M. Wang, S. Cheng, Q. M. Wei, E. Ma, T. G. Nieh, A. Hamza, Scripta Mater. 2004, 51, 1023. Y. Zhu, X. Liao, Nat. Mater. 2004, 3, 351. Y. M. Wang, E. Ma, Acta Mater. 2004, 52, 1699. C. C. Koch, K. M. Youssef, R. O. Scattergood, K. L. Murty, Adv. Eng. Mater. 2005, 7, 787. X. K. Zhu, X. Zhang, H. Wang, A. V. Sergueeva, A. K. Mukerjee, R. O. Scattergood, J. Narayan, C. C. Koch, Scripta Mater. 2003, 49, 429. Y. M. Wang, M. W. Chen, F. Z. Zhou, E. Ma, Nature 2002, 419, 912. C. D. Gu, J. S. Lian, Z. H. Jiang, Q. Jiang, Scripta. Mater. 2006, 54, 579.

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ductility.[6,13] Both the Ni-Co alloy and Ni-Fe alloy[6] exhibit high strength and enhanced ductility, which may indicate that alloying of nc Ni is an effective path to strengthen the nc Ni without decreasing its ductility. Morphology of fracture surface: Sign of plasticity could be detected on the fracture surfaces of nc Ni-1.7 %Co alloy. Figure 4 shows typical morphology on fracture surface of nc Ni1.7 %Co alloy specimen. Significant necking of the cross-section can be seen clearly from Figure 4. Evident dimples morphologies are shown in the insert, which are the magnified region marked by the white rectangle in Figure 4. A relative uniform dimples structure with average size of about 400 nm is found on the fracture surface of nc Ni-1.7 %Co alloy. Similar dimple-like features with an average dimple size of several grain sizes have also been observed in other electrodeposited nc materials,[5,7,8,16] which indicates that the fracture mechanism operates involving collective grain activity. Guided by large-scale atomistic simulations,[35–36] because of the presence of GBs that are resistant to sliding in the nc structures, the cooperative grain activity leads to the formation of local shear planes concentrated around their neighboring planes that extend over several grain sizes, and therefore providing an explanation of the dimension of the dimple structures. Conclusion: In this study, bulk nc Ni-1.7 %Co alloy with an average grain size of about 25 nm was fabricated by a directcurrent electrodeposition technique. XRD revealed that the Ni-1.7 %Co alloy possessed the single f.c.c phase structure and strong {200} texture. Tensile tests at RT showed that nc Ni-1.7 %Co alloy has a combination of high UTS of about 1600 MPa and enhanced ductility of 8-9.6 % over a wide strain rate range of 1.04 × 10–6-1.04 s–1. Considering the typical tradeoff between yield stress and elongation to failure among electrodeposited nc Ni and Ni-based alloys, the nc Ni1.7 %Co alloy would have potential use in different applications in the future.

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[17] A. A. Karimpoor, U. Erb, K. T. Aust, G. Palumbo, Scripta Mater. 2003, 49, 651. [18] K. M. Youssef, R. O. Scattergood, K. L. Murty, J. A. Horton, C. C. Koch, Appl. Phys. Lett. 2005, 87, 091904. [19] L. Wang., Y. Gao., Q. Xue, H. Liu, T. Xu, Appl. Surf. Sci. 2005, 242, 326. [20] A. Masoeroa, B. Mortenb, G. L. Olcesec, M. Prudenziatib, F. Tangod, F. Vinai, Thin Solid Films. 1999, 350, 214. [21] A. N. Correia, S. A. S. Machado, Electrochim Acta. 2000, 45, 1733. [22] H Rosner, J Markmann, J Weissumüller, Philos Mag Lett. 2004, 84, 321. [23] C. Rentenberger, T. Waitz, H. P. Karnthaler, Scripta Mater. 2004, 51, 789. [24] T. Mukai, S. Suresh, K. Kita, H. Sasaki, N. Kobayashi, K. Higashi, A. Inoue, Acta Mater. 2003, 51, 4197. [25] Y. M. Wang, E. Ma, Appl. Phys. Lett. 2004, 85, 2750. [26] R. Asaro, S. Suresh, Acta Mater. 2005, 53, 3369. [27] L. Lu, R. Schwaiger, Z. W. Shan, M. Dao, K. Lu, S. Suresh, Acta Mater. 2005, 53, 2169. [28] Q. Wei, S. Cheng, K. T. Ramesh, E. Ma, Mater. Sci. Eng. 2004, A381, 71. [29] D. Wolf, V. Yamakov, S. R. Phillpot, A. K. Mukherjee, H. Gleiter, Acta Mater. 2005, 53, 1. [30] J. M. Robinson, M. P. Shaw, Int. Mater. Rev. 1994, 39, 113. [31] J. M. Reed, M. E. Walter, Mater. Sci. Eng. 2003, A359, 1 [32] V. Yamakov, D. Wolf, S. R. Phillpot, A. K. Mukherjee, H. Gleiter, Nat. Mater. 2004, 3, 43. [33] F. Ebrahimi, H. Q. Li, Appl. Phys. Lett. 2004, 5, 3749. [34] X. Nie, R. Wang, Y. Ye, Y. Zhou, D. Wang, Solid State Commun. 1995, 96, 729. [35] A. Hasnaoui, H. Van Swygenhoven, P. M. Derlet, Science 2003, 300, 1550. [36] H. Van Swygenhoven, P. M. Derlet, A. Hasnaoui, Adv. Eng. Mater. 2003, 5, 345. [37] C. D. Gu, J. S. Lian, J. G. He, Z. H. Jiang, Q. Jiang, Surf. Coat. Technol. 2006, in press, Doi: 10.1016/j.surfcoat.2005. 07.001.

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DOI: 10.1002/adem.200500230

Optimization of Mechanical Properties of NiAl-base Alloy by Suction Casting** By Kaiwen Huai, Jianting Guo, Hutian Li and Rui Yang NiAl has recently received much attention as a potential structural material because of its many superior high-temperature properties. The lack of high-temperature strength and fracture toughness and ductility at room temperature, however, limit the use of NiAl alloys.[1,2] Of the various attempts to enhance the ductility and high temperature strength, ductile refractory-metals, like Cr, Mo, W, Re and V, can successfully improve the mechanical properties of brittle NiAl by particle or dispersion strengthening and fiber reinforcement.[3–6] In particular, NiAl-28Cr-6Mo eutectic alloys are regarded as the most logical choice of the multicomponent system due to their relatively high melting point, low density, good thermal conductivity and high creep resistance[7–9] as well as higher fracture toughness compared to many NiAlbased alloys. Hafnium (Hf) was found to be very effective in improving the elevated temperature strength of NiAl-Cr(Mo) eutectic alloy. Unfortunately, Hf addition weakened the fracture toughness and ductility at room temperature severely.[10,11] For NiAl-Cr(Mo)/Hf lamellar eutectic alloy, the feasible way to improve the ductility and strength further is to reduce interlamellar spacing and microsegregation of solute element Hf. Solidification at high cooling rate is anticipated to attain such microstructure. Suction casting, as a popular method to fabricate bulk amorphous,[12,13] can obtain bulk NiAl alloys at a relatively high cooling rate of about 50 ∼ 102 K/s that is higher than 10–1 ∼ 10–2 K/s for the conventionally cast. Almost no work has been reported about the effect of suction casting technique on NiAl-Cr(Mo) eutectic alloys so far. Hence, the microstructural evolution and mechanical properties of NiAl-28Cr-5.5Mo-0.5Hf alloy prepared by conventionally cast and suction casting were investigated. The results show that suction casting technique markedly improves the

– [*] Dr. K. Huai, Porf. J. Guo, Dr. H. Li, Prof. R. Yang Institute of Metal Research Chinese Academy of Sciences 72 Wenhua Road, Shenyang, 110016, China E-mail: [email protected] [**] The authors would like to acknowledge the National Natural Science Foundation of China (contract No. 59895152) for financial supports.

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