Directional migration behavior of cerium during sintering process of mischmetal doped cemented carbide

Abstract

Three observation methods were used to investigate the existing form and the behavior of rare earth during the sintering process of high activity mischmetal (RE, with lanthanum and cerium) doped WC-8%Co-0.048%RE(mass fraction) alloy with low carbon-containing level by scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDXS), considering the fact that the addition amount of rare earth in the alloy is very minute. The directional migration process and mechanism of cerium were discussed. First, the sinter skin (surface) is observed. It is shown that there exists a dispersedly distributed cerium containing oxide on the sinter skin, and lanthanum in these cerium enrichment positions is very minute. Secondly, the polished section is observed, and lanthanum containing phase/micro-zone in the alloy is identified. Finally, based on the fact that the fracture of cemented carbide is resulted from the heterogeneous phase or other defects within the microstructure, the fracture surface is observed and cerium containing phase/micro-zone in the fracture source approximately 260 μm from the surface is identified. These combined observations reveal adequately the fact that lanthanum and cerium get separated and cerium predominantly migrates towards the surface during the sintering process.

Keywords

cemented carbide / rare earth / surface segregation / directional migration / liquid phase sintering

References

[1] ZhangL., SchubertW. D., HuangB.-y., WuE.-xi.. Surface observation of rare earth doped cemented carbide[J]. Journal of Central South University of Technology: Science and Technology, 2004, 35(5): 742-747

[2] ZhangL., SchubertW. D., ChenS., ZhangC.-f., HuangB.-yun.. Rare earth enrichment phenomenon during the sintering process of grainy hardmetal[J]. Materials Science and Engineering A, 2004, 384(1/2): 395-401

[3] ZhangL., ChenS., XiongX.-j., HuangB.-y., ZhangC.-fu.. Cerium enrichment on sinter skin of mischmetal doped WC-Co cemented carbide[J]. Journal of Central South University of Technology, 2005, 12(s1): 5-8

[4] ZhangL., HuangB.-y., WuE.-xi.. Research on dual phase cemented carbides[J]. Materials Review, 2000, 14(12): 17-19

[5] ZhangL., ChenS., XiongX.-j., HuangB.-yun.. Microstructure and Vickers hardness of functionally graded WC-Co cemented carbide with dual phase structure[J]. The Chinese Journal of Nonferrous Metals, 2005, 15(8): 1194-1199

[6] ZhangL., ChenS., XiongX.-j., HeY.-h., HuangB.-y., ZhangC.-fu.. Phase composition, phase transition and phase structure stability of functionally graded cemented carbide with dual phase structure[J]. Journal of Central South University of Technology, 2007, 14(2): 149-152

[7] LiuY., WangH.-b., YangJ.-g., HeY.-h., LongZ.-yi.. Relationship between structure and properties of graded cemented carbide[J]. Materials Science and Engineering of Powder Metallurgy, 2005, 10(6): 356-360

[8] ZhangL., ChenS., SchubertW. D., HuangB.-yun.. Thermodynamic analysis on the phenomenon of the enrichment of Ce on the sinter skin during the sintering of mischmetal doped cemented carbide[J]. Cemented Carbide, 2004, 21(2): 70-76

[9] JiZ.-s., XiaL.-f., WanH.-b., WuY.-q., ZhaoM., ChengX.-hui.. Phase composition of initial rare earth-B-Al permeation layer[J]. The Chinese Journal of Nonferrous Metals, 1998, 8(s1): 66-69

[10] JiJ.-w., CheY.-y., LiuA.-s., LuX.-l., YuoM.. Research and theory in internal friction of alloying of rear earth in iron and steel[J]. Journal of the Chinese Rare Earth Society, 1996, 14(4): 350-359

[11] KolaskaH., HirschfeldD., GreweH.Powder metallurgy of hardmetals[M], 1995, Shrewsbury, EPMA: 22

[12] SarinV. K.. Morphology of Eta phase in cemented WC-Co alloys[J]. Modern Development in Powder Metallurgy, 1977, 10: 553-565

[13] PetrovaL. A., EffenbergG.Red book-phase diagrams of Metallic systems[M], 1990, Stuttgart, Material Science International Services GmbH: 137

[14] BakerH., OkamotoH.ASM handbook: Alloy phase diagrams[M]. Vol.3, 1992, Ohio, American Society for Metals: 151-153

[15] XuC.-h., AiX., HuangC.-zhen.. Research and development of rare-earth cemented carbides[J]. International Journal of Refractory Metals and Hard Materials, 2001, 19(3): 159-168