Apr 27, 2012 - of AlxCoCrFeNi high-entropy alloys. Woei-Ren Wang a,b, Wei-Lin Wang b, Shang-Chih Wang b, Yi-Chia Tsai b, Chun-Hui Lai b, Jien-Wei Yeh ...
Intermetallics 26 (2012) 44e51
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Effects of Al addition on the microstructure and mechanical property of AlxCoCrFeNi high-entropy alloys Woei-Ren Wang a, b, Wei-Lin Wang b, Shang-Chih Wang b, Yi-Chia Tsai b, Chun-Hui Lai b, Jien-Wei Yeh a, * a b
Department of Materials Science and Engineering, National Tsing-Hua University, Hsinchu 30013, Taiwan Nanopowder and Thin Film Technology Center, ITRI South, Industrial Technology Research Institute, Tainan 70955, Taiwan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 27 October 2011 Received in revised form 29 February 2012 Accepted 6 March 2012 Available online 27 April 2012
A five-component AlxCoCrFeNi high-entropy alloy (HEA) system with finely-divided Al contents (x in molar ratio, x ¼ 0e2.0) was prepared by vacuum arc melting and casting method. The effects of Al addition on the crystal structure, microstructure and mechanical property were investigated using Xray diffraction (XRD), scanning electron microscopy (SEM), and Vickers hardness tester. The as-cast AlxCoCrFeNi alloys can possess face-centered cubic (FCC), body-centered cubic (BCC) or mixed crystal structure, depending on the aluminum content. The increase of aluminum content results in the formation of BCC structure which is a dominant factor of hardening. All the BCC phases in the as-cast alloys have a nano-scale two-phase structure formed by spinodal decomposition mechanism. The Al0.9CoCrFeNi alloy exhibits a finest spinodal structure consisting of alternating interconnected twophase microstructure which explains its maximum hardness of Hv 527 among the alloys. The chemical composition analysis of FCC and BCC crystal structures, their lattice constants, overall hardness demonstrate that the formation of a single FCC solid solution should have Al addition 11 at.% of Al alloying). These suggest that the maximum amount of Al atoms which can be solved in the FCC solid solution of the AlxCoCrFeNi alloy system is 11.0 at.% (near x ¼ 0.5). When the Al content is at above 11.0 at.%, redundant Al atoms are rejected to the interdendritic regions to form BCC phase with spinodal structure. From the Al content of BCC phase, that without the formation of FCC phase requires at least 18.4 at.% Al. When the nominal Al content is above 18.4 at.% (e.g., Al0.9eAl2.0), a complete BCC crystal structure forms and the actual Al content of the BCC phase is nearly the same as the designed Al content. This conclusion on compositional range for different crystal structure from Fig. 8 also matches with the analysis from the variation of lattice constant and overall hardness as mentioned in Section 3.1.
3.3. Range of composition for different crystal structures From the above, the variation of Al contents in the AlxCoCrFeNi alloys plays an important role in determining the final microstructures and mechanical properties. Fig. 8 shows the variation of crystal structure, the actual Al content of each crystal structure (solid line), and the designed Al content (dotted line) against x
Fig. 8. Actual Al content of the as-cast alloys as a function of designed x value in the AlxCoCrFeNi system. The dotted line represents the nominal content of Al in the AlxCoCrFeNi alloys.
W.-R. Wang et al. / Intermetallics 26 (2012) 44e51
4. Conclusions The crystal structure, microstructure and mechanical property of as-cast AlxCoCrFeNi high-entropy alloy have been investigated on fourteen alloys with finely divided Al contents. Low Al content alloys form an FCC structure and continuous increase of Al content induces the formation of BCC phase which further spinodally decompose into modulated spinodal structure. The range of x value for FCC plus BCC mixture is from near 0.5 (11.0 at.%Al) to 0.9 (18.4 at.%Al). The solidified microstructure varies from columnar cellular structure (Al0e0.3) to columnar dendrite structure (Al0.4e0.6), then to equiaxed nondendritic grain (Al0.7e0.8), then to equiaxed dendritic grain (Al0.9e1.5), and finally to non-equiaxed dendritic grain (Al1.8e2.0) structures. The Widmanstätten side plates of FCC phase form in the equiaxed nondendritic grain structure of Al0.7 and A0.8 alloys. Complete spinodal structure can be obtained in Al0.9eAl2.0 alloys. Spinodal structure transits from interconnected and modulated plate structure in Al0.5e1.0 alloys to uniform structure of dispersed spherical particles in Al1.2e2.0 alloys. The as-cast Al0.9 alloy has a maximum hardness, HV 527, among the Al0eAl2.0 alloys due to its finest spinodally-decomposed and interconnected structure. Acknowledgment The authors gratefully acknowledge the financial support for this research by the ITRI South and the Ministry of Economic Affairs of Taiwan under Grant No.7327HB3110. References [1] ASM handbook committee. Metals handbook. 10th ed., vol. 1. Meterials Park, OH: ASM International; 1990. [2] Totten GE, Xie L, Funatani K. Hand book of mechanical alloy design. New York: Marcel Dekker Inc; 2004. [3] Ranganathan S. Alloyed pleasures: multimetallic cocktails. Curr Sci 2003;8: 1404e6. [4] Yeh JW, Chen SK, Lin SJ, Gan JY, Chin TS, Shun TT, et al. Nanostructured highentropy alloys with multiple principal elements: novel alloy design concepts and outcomes. Adv Eng Mater 2004;6:299e303. [5] Yeh JW, Chen SK, Chen YL. Novel alloy concept, challenges and opportunities of high-entropy alloys. In: Raj B, editor. Frontiers in the design of materials. CRC Press; 2007. p. 31e47. [6] Tong CJ, Chen YL, Chen SK, Yeh JW, Shun TT, Tsau CH, et al. Microstructural characterization of AlxCoCrCuFeNi high-entropy alloy system with multiprincipal elements. Metall Mater Trans 2005;36A:881e93. [7] Tong CJ, Chen MR, Chen SK, Yeh JW, Shun TT, Lin SJ, et al. Mechanical performance of the AlxCoCrCuFeNi high-entropy alloy system with multiprincipal elements. Metall Mater Trans 2005;36A:1263e71. [8] Wu JM, Lin SJ, Yeh JW, Chen SK, Huang YS, Chen HC. Adhesive wear behavior of AlxCoCrCuFeNi high-entropy alloys as a function of aluminum content. Wear 2006;261:513e9. [9] Tung CC, Yeh JW, Shun TT, Chen SK, Huang YS, Chen HC. On the elemental effect of AlCoCrCuFeNi high-entropy alloy system. Mater Lett 2007;61:1e5. [10] Zhang KB, Fu ZY, Zhang JY, Shi J, Wang WM, Wang H, et al. Nanocrystalline CoCrFeNiCuAl high-entropy solid solution synthesized by mechanical alloying. J Alloys Comp 2009;485:L31e4. [11] Zhang KB, Fu ZY, Zhang JY, Wang WM, Wang H, Zhang QJ, et al. Microstructure and mechanical properties of CoCrFeNiTiAlx high-entropy alloys. Mater Sci Eng A 2009;508:214e9. [12] Wang XF, Zhang Y, Qiao Y, Chen GL. Novel microstructure of multicomponent CoCrCuFeNiTix alloys. Intermetallics 2007;15:357e62. [13] Cantor B, Chang ITH, Knight P, Vincent AJB. Microstructural development in equiatomic multicomponent alloys. Mater Sci Eng A 2004;375e377:213e8.
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