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采用电磁悬浮和自由落体两种实验技术对二元Co-50%Mo过共晶合金中初生Co7Mo6金属间化合物的生长机理和维氏硬度进行了系统研究.电磁悬浮实验中，合金熔体获得的最大过冷度为203 K（0.12TL），初生Co7Mo6枝晶生长速度与过冷度之间呈现幂函数关系.随着过冷度的增大，初生枝晶中Co元素含量单调递增，枝晶尺寸明显减小，并且其维氏硬度逐渐升高.在自由落体状态下，随着液滴直径的减小，合金熔体的过冷度和冷却速率均增大.当液滴直径减小到392 m以下时，初生Co7Mo6枝晶从小平面向非小平面形态进行转变.实验发现，深过冷条件下Co7Mo6化合物发生了显著的溶质截留效应，其维氏硬度与Co元素分布和形貌特征密切相关.The dendritic growth process and Vickers microhardness enhancement of primary Co7Mo6 phase in undercooled liquid Co-50%Mo hypereutectic alloy are systematically investigated by using electromagnetic levitation and drop tube. It is found that the rapid solidification microstructures are mainly characterized by primary Co7Mo6 dendrites plus interdendritic (Co7Mo6+Co) eutectic irrespective of experimental conditions. In electromagnetic levitation experiment, the obtained maximum undercooling reaches 203 K (0.12TL). With the rise in bulk undercooling, primary Co7Mo6 dendrite growth velocity monotonically increases according to a power function and reaches 22.5 mm-1 at the highest undercooling. The secondary dendrite spacing decreases from 45.8 to 13.6 m, while Co content in primary dendrites shows an increasing trend. This indicates that an evident grain refinement and solute trapping take place for primary Co7Mo6 dendrites during rapid solidification. The dependence of Vickers microhardness on Co content follows an exponential function. Moreover, the variation of Vickers microhardness with the grain size also satisfies an exponential relationship. In addition, Lipton-Kurz-Trivedi/Boettinger-Coriel-Trivedi model is used to analyze the growth kinetics of primary Co7Mo6 dendrites. In the experimental undercooling range, the growth process of primary Co7Mo6 dendrites is controlled mainly by solute diffusion and they grow sluggishly. Under free fall condition, liquid Co-50%Mo alloy is subdivided into many droplets inside a drop tube and their diameters range from 1379 to 139 m. With alloy droplet size decreasing, both droplet undercooling and cooling rate increase rapidly. In a large droplet-diameter regime above 392 m, primary Co7Mo6 phase displays faceted-growth characteristics. Furthermore, primary Co7Mo6 dendrites are refined greatly and their solute solubility is significantly extended as droplet size becomes smaller. Once the alloy droplet diameter decreases to a value below this threshold value, the faceted-growth characteristics start to disappear gradually, which is accompanied with a conspicuous grain refinement and a solute solubility extension. Both the solute solubility enhancement and grain size refinement contribute significantly to the exponential improvement in microhardness if primary Co7Mo6 phase grows in a faceted way. Otherwise, the solute solubility enhancement and grain size refinement result in the linear increase of Vickers microhardness. Theoretical analyses demonstrate that the primary phase microhardness is strongly dependent on its solute content and morphology characteristic.
[1] Silva L S, Mercena S G, Garcia D J, Bittar E M, Jesus C B R, Pagliuso P G, Lora-Serrano R, Meneses C T, Duque J G S 2017 Phys. Rev. B 95 134434
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[5] Wu Y H, Chang J, Wang W L, Hu L, Yang S J, Wei B 2017 Acta Mater. 129 366
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[8] Sato J, Omori T, Oikawa K, Ohnuma I, Kainuma R, Ishida K 2006 Science 312 90
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[11] Yao W J, Dai F P, Wei B 2007 Phil. Mag. Lett. 87 613
[12] Hu Z P, Zhang J B, Xu S F, Wu C J, Wang Z H, Yang K L, Wang W Q, Du X B, Su F 2012 Acta Phys. Sin. 61 207501 (in Chinese)[侯志鹏, 张金宝, 徐世峰, 吴春姬, 王子涵, 杨坤隆, 王文全, 杜晓波, 苏峰 2012 物理学报 61 207501]
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[14] Wei S L, Huang L J, Chang J, Yang S J, Geng L 2016 Acta Phys. Sin. 65 096101 (in Chinese)[魏绍楼, 黄陆军, 常健, 杨尚京, 耿林 2016 物理学报 65 096101]
[15] Leonhardt M, Lser W, Lindenkreuz H G 1999 Acta Mater. 47 2961
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[17] Masslaski T B, H Okamoto, P R Subramanian, L Kacprzak 1990 Binary Alloy Diagrams (2nd Ed.) (Geauga: ASM International) pp1208-1209
[18] Boettinger W J, Coriell S R, Trivedi R 1987 Proceedings of the Fourth International Conference on Rapid Solidification Processing: Principles and Technologies (Baton Rouge: Claitor's Publishing Division) pp13-20
[19] Trivedi R, Lipton J, Kurz W 1987 Acta Metall. 35 965
[20] Gale W, Totemeier T C 2004 Smithells Metals Reference Book (8th Ed.) (Amsterdam:Elsevier Butterworth-Heinemann Publications) p14-1
[21] Levi C G, Mehrabian R 1982 Metall. Trans. A 13 221
[22] Lee E S, Ahn S 1994 Acta Metall. Mater. 42 3231
[23] Kurz W, Fisher D J 1992 Fundamentals of Solidification (third edition) (Aedermannsdorf:Trans. Tech. Publications Ltd) pp34-59
[24] Aziz M J 1982 J. Appl. Phys. 53 1158
[25] Yang S J, Wang W L, Wei B B 2015 Acta Phys. Sin. 64 056401 (in Chinese)[杨尚京, 王伟丽, 魏炳波 2015 物理学报 64 056401]
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[1] Silva L S, Mercena S G, Garcia D J, Bittar E M, Jesus C B R, Pagliuso P G, Lora-Serrano R, Meneses C T, Duque J G S 2017 Phys. Rev. B 95 134434
[2] Verma S, Pandey O P, Paesano, A, Sharma P 2016 J. Alloy. Compd. 678 284
[3] Hu J Q, Xie M, Zhang J M, Liu M M, Yang Y C, Chen Y T 2013 Acta Phys. Sin. 62 247102 (in Chinese)[胡洁琼, 谢明, 张吉明, 刘满门, 杨有才, 陈永泰 2013 物理学报 62 247102]
[4] Evans M J, Wu Y, Kranak V F, Newman N, Reller A, Garciagarcia F J, Hussermann U 2009 Phys. Rev. B 80 064514
[5] Wu Y H, Chang J, Wang W L, Hu L, Yang S J, Wei B 2017 Acta Mater. 129 366
[6] Hernando A, Amils X, Nogus J, Suriach S, Bar M D, Ibarra M R 1998 Phys. Rev. B 58 11864
[7] Ahmad R, Cochrane R F, Mullis A M 2012 J. Mater. Sci. 47 2411
[8] Sato J, Omori T, Oikawa K, Ohnuma I, Kainuma R, Ishida K 2006 Science 312 90
[9] Lobiak E V, Shlyakhova E V, Bulusheva L G, Plyusnin P E, Shubin Y V, Okotrub A V 2015 J. Alloy. Compd. 621 351
[10] Oikawa K, Qin G W, Sato M, Kitakami O, Shimada Y, Sato J, Fukamichi K, Ishida K 2003 Appl. Phys. Lett. 83 966
[11] Yao W J, Dai F P, Wei B 2007 Phil. Mag. Lett. 87 613
[12] Hu Z P, Zhang J B, Xu S F, Wu C J, Wang Z H, Yang K L, Wang W Q, Du X B, Su F 2012 Acta Phys. Sin. 61 207501 (in Chinese)[侯志鹏, 张金宝, 徐世峰, 吴春姬, 王子涵, 杨坤隆, 王文全, 杜晓波, 苏峰 2012 物理学报 61 207501]
[13] Ohmori T, Go H, Nakayama A, Mametsuka H, Suzuki E 2001 Mater. Lett. 47 103
[14] Wei S L, Huang L J, Chang J, Yang S J, Geng L 2016 Acta Phys. Sin. 65 096101 (in Chinese)[魏绍楼, 黄陆军, 常健, 杨尚京, 耿林 2016 物理学报 65 096101]
[15] Leonhardt M, Lser W, Lindenkreuz H G 1999 Acta Mater. 47 2961
[16] Royer Z L, Tackes C, Lesar R, Napolitano R E 2013 J. Appl. Phys. 113 214901
[17] Masslaski T B, H Okamoto, P R Subramanian, L Kacprzak 1990 Binary Alloy Diagrams (2nd Ed.) (Geauga: ASM International) pp1208-1209
[18] Boettinger W J, Coriell S R, Trivedi R 1987 Proceedings of the Fourth International Conference on Rapid Solidification Processing: Principles and Technologies (Baton Rouge: Claitor's Publishing Division) pp13-20
[19] Trivedi R, Lipton J, Kurz W 1987 Acta Metall. 35 965
[20] Gale W, Totemeier T C 2004 Smithells Metals Reference Book (8th Ed.) (Amsterdam:Elsevier Butterworth-Heinemann Publications) p14-1
[21] Levi C G, Mehrabian R 1982 Metall. Trans. A 13 221
[22] Lee E S, Ahn S 1994 Acta Metall. Mater. 42 3231
[23] Kurz W, Fisher D J 1992 Fundamentals of Solidification (third edition) (Aedermannsdorf:Trans. Tech. Publications Ltd) pp34-59
[24] Aziz M J 1982 J. Appl. Phys. 53 1158
[25] Yang S J, Wang W L, Wei B B 2015 Acta Phys. Sin. 64 056401 (in Chinese)[杨尚京, 王伟丽, 魏炳波 2015 物理学报 64 056401]
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