Microstructure and mechanical properties of nanocrystalline WC-10Co cemented carbides

Microstructure and mechanical properties of nanocrystalline WC-10Co cemented carbides

Scripta mater. 44 (2001) 1535–1539 www.elsevier.com/locate/scriptamat MICROSTRUCTURE AND MECHANICAL PROPERTIES OF NANOCRYSTALLINE WC-10Co CEMENTED CA...

365KB Sizes 4 Downloads 86 Views

Scripta mater. 44 (2001) 1535–1539 www.elsevier.com/locate/scriptamat

MICROSTRUCTURE AND MECHANICAL PROPERTIES OF NANOCRYSTALLINE WC-10Co CEMENTED CARBIDES Seung I. Cha, Soon H. Hong, Gook H. Ha* and Byung K. Kim* Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology, 373-1 Kusung-dong, Yusung-gu, Taejon 305-701, Korea *Korea Institute of Machinery and Materials, 66 Sangnam-dong, Changwon, Kyungnam, Korea (Received August 21, 2000) (Accepted in revised form December 18, 2000) Keywords: Cemented carbides; Nanocrystalline; Transverse rupture strength; Hardness I. Introduction The cemented carbides, consisting of WC grains bound with Co phase, have been used for cutting tools during the last several decades. The effects of the variations of Co composition and the addition of other type of cubic carbides have been investigated to improve the properties of WC-Co cemented carbide [1]. In recent days, nanocrystalline WC-Co cemented carbides have been fabricated by thermochemical and thermomechanical process named as spay conversion process (SCP) [2]. The WC particle sizes of powders fabricated by spray conversion process are generally below 100 nm. When the grain growth inhibitors were added in WC-Co, the WC grain size after liquid phase sintering could be reduced below 400 nm. The mechanical properties, such as hardness and transverse rupture strength, are strongly related with the microstructural parameters such as WC grain size, Co mean free path and WC/WC contiguity and also other chemical factors such as solute concentration within Co phase, addition of grain growth inhibitors and composition of carbide. Recent studies on mechanical properties of nanocrystalline WC-Co cemented carbide proposed different mechanisms for fracture and deformation of nanocrystalline WC-Co cemented carbide according to the microstructure and chemical composition [2–5]. However, the deformation mechanisms of nanocrystalline or even conventional WC-Co cemented carbide have not been clearly understood yet [4,5]. In this study, the mechanical properties of nanocrystalline and conventional WC-Co cemented carbides were investigated. At the same time, the microstructure and solute content within the Co phase of nanocrystalline WC-Co cemented carbide with various amounts of the grain growth inhibitors were investigated. The relationships among the microstructures, mechanical properties and chemical compositions were discussed II. Experimental Procedures The initial WC-Co powders were prepared by spray conversion process from aqueous solution containing AMT(ammonium metatungstate) and Co(NO3)2, and followed by oxidation, reduction and carbonization process [2]. The average size of WC particles in initial powders was about 100nm and the WC particles were homogeneously mixed with Co phase. The initial WC-10Co powders were ball1359-6462/01/$–see front matter. © 2001 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. PII: S1359-6462(01)00835-1

1536

NANOCRYSTALLINE WC-10Co CEMENTED CARBIDES

Vol. 44, Nos. 8/9

Figure 1. Cumulative probabilities of WC size distribution of WC-10Co-X cemented carbides sintered at 1375°C under vacuum. (a) Nanocrystalline cemented carbides containing inhibitors, (b) nanocrystalline and conventional cemented carbides without inhibitors.

milled in Hexane for mixing with paraffin for 24 hours. The ball-milled powders were dried in an oven for 24 hours at 800°C and compacted under a pressure of 20MPa at room temperature. The compacted powders were liquid phase sintered at 1375°C for 1–5 hours under a pressure of 1mtorr. The microstructure of sintered WC-Co cemented carbide was analyzed by scanning electron microscope and X-ray diffractometer. The microstructural parameters such as WC grain size, Co mean free path and WC/WC contiguity were analyzed by image analyzer using planar sectional SEM micrographs. The saturated magnetic moment of sintered WC-10Co cemented carbides was measured in order to estimate the solute concentration within Co binder phase. Hardness of sintered WC-10Co cemented carbides was measured by Vicker’s hardness tester under a constant load of 1kg. Transverse rupture strength (TRS) of sintered WC-10Co was measured by three point bending test based on ASTM B528. III. Results and Discussion The grain size and grain size distribution of WC in WC-10Co-X cemented carbides were illustrated in Fig. 1. In Fig. 1, the WC grain size was quite dependent on the size of initial powders and the addition of grain growth inhibitors. When 0.7wt% TaC/VC was added as inhibitor, the WC grain size was measured about 350nm after sintering at 1375°C for 1hour, which is finer compared to 650nm for that containing 0.7wt%Cr3C2/VC as inhibitor. When the inhibitor was not added, the WC grain size considerably increased to 762nm due to a grain growth during sintering. The WC grain size distribution was also dependent on the addition of inhibitors. A narrower distribution of WC grain size was observed in WC-10Co-0.7TaC/VC, while a broader distribution of WC grain size was observed in WC-10Co0.7Cr3C2/VC. The TEM micrographs of nanocrystalline WC-10Co-0.7TaC/VC cemented carbide were quite different from that of conventional WC-10Co cemented carbide as shown in Fig. 2. In case of conventional WC-10Co cemented carbide, a high densities of dislocations and stacking faults were observed within WC grains. However, in case of nanocrystalline WC-10Co-0.7TaC/VC cemented carbide, there were only few dislocations and stacking faults within the WC grains. The binder phase was consisted of nanocrystalline Co alloy containing solute atoms of W and C as shown in Fig. 2(c). The saturated magnetic moment per unit volume of Co binder phase was sensitively dependent on the powder preparation process. As shown in Fig. 3, the conventional WC-10Co cemented carbides shows higher saturated magnetic moment compared to that of nanocrystalline WC-10Co cemented

Vol. 44, Nos. 8/9

NANOCRYSTALLINE WC-10Co CEMENTED CARBIDES

1537

Figure 2. TEM micrographs of WC-10Co-X cemented carbides sintered at 1375°C for 1 hour under vacuum. (a) Conventional WC-10Co, (b) nanocrystalline WC-10Co-0.7TaC/VC, (c) Co phase in conventional WC-10Co cemented carbides and its diffraction pattern.

carbides prepared by spray conversion process. These results indicate that the nanocrystalline WC-10Co powders prepared by spray conversion process have higher solute concentration within Co phase compared to conventional WC-10Co. These difference in solute concentration results in a different

Figure 3. The saturated magnetic moment per unit volume of Co phase in WC-10Co cemented carbides with or without inhibitors sintered at 1375°C for 1 hour.

1538

NANOCRYSTALLINE WC-10Co CEMENTED CARBIDES

Vol. 44, Nos. 8/9

Figure 4. The x-ray diffraction results of WC-10Co cemented carbides before and after transverse rupture strength test. (a) conventional WC-10Co cemented carbide, (b) nanocrystalline WC-10Co-0.7TaC/VC cemented carbide.

hcp/fcc ratio of Co phase as shown in Fig. 4. The nanocrystalline WC-10Co cemented carbide shows lower hcp/fcc ratio than that of conventional WC-10Co cemented carbide in Fig. 4. The hardness of WC-10Co cemented carbide was sensitively dependent on the grain size of WC. The hardness of nanocrystalline WC-10Co-X cemented carbides was considerably higher than that of conventional WC-10Co cemented carbides. The effect of WC grain size on hardness of WC-10Co cemented carbide could be described by the Hall-Petch type relationship as shown in Fig. 5(a). A phenomenological grain size-hardness relationship for WC-10Co cemented carbides is formulated as Eq. (1), H V ⫽ 550 ⫹

23500

冑 d WC

(1)

where HV is Vicker’s hardness in unit of kg/mm2, dWC is grain size of WC in unit of nm. In case of WC-10Co cemented carbides, the volume fraction of hard WC skeleton was over 70%. Due to the higher volume fraction and elastic modulus of WC than those of Co phase, the major portion of load is applied in WC skeleton when external load is applied. Therefore, the hardness of WC-10Co cemented

Figure 5. The mechanical properties of WC-10Co cemented carbides. (a) Hardness representing the Hall-Petch type relationship, (b) Transverse rupture strength according to the saturated magnetic moment of Co alloy phase.

Vol. 44, Nos. 8/9

NANOCRYSTALLINE WC-10Co CEMENTED CARBIDES

1539

carbides, which represents the resistance against the deformation, was mainly determined by the grain size of WC. The hcp/fcc ratio within the Co phase is related to the transverse rupture strength of WC-10Co cemented carbides. As shown in Fig. 4, the structure of Co phase within the nanocrystalline WC-10Co0.35TaC-0.35VC cemented carbide did not changed and maintained fcc structure during transverse rupture strength test. However, in case of conventional WC-10Co cemented carbides, which have higher hcp/fcc ratio in Co phase, the hcp phase was transformed into fcc phase by the deformation during transverse rupture strength test. It is considered that the higher transverse rupture strengths in conventional WC-10Co cemented carbides compared to the nanocrystalline WC-10Co-X, as shown in Fig. 5, is related to the added toughening mechanism resulted from the hcp/fcc phase transformation of Co phase in cemented carbides. IV. Conclusions 1. The nanocrystalline WC-10Co powders with average WC size of about 100nm could be produced by the spray conversion process. The grain sizes of WC in WC-10Co cemented carbides sintered from the nanocrystalline powders could be controlled to 300⬃700nm by the addition of grain growth inhibitors. 2. Microstructures of WC-10Co-X cemented carbide were sensitively dependent on the addition of grain growth inhibitors. The grain growth of WC grain was more effectively retarded by the addition of 0.35wt%TaC and 0.35wt%VC compared to the addition of 0.35wt%Cr3C2 and 0.35wt%VC during the liquid phase sintering of nanocrystalline WC-Co powders prepared by spray conversion process. 3. The hardness of WC-10Co increased with decreasing the WC size following the Hall-Petch type relationship. The transverse rupture strengths of nanocrystalline WC-10Co fabricated by spray conversion process were lower than those of conventional WC-10Co. The hcp/fcc phase transformation of Co phases is considered as the major reason for higher transverse rupture strength in conventional WC-10Co cemented carbides. References 1. 2. 3. 4. 5.

J. Gurland, Int. Mater. Rev. 33, 151 (1988). B. K. Kim, G. H. Ha, and D. W. Lee, J. Mater. Process. Technol. 63, 317 (1997). B. Roebuck and E. A. Almond, Int. Mater. Rev. 33, 90 (1988). H. E. Exner, Int. Mater. Rev. 4, 149 (1979). K. Jia, T. E. Fischer, and B. Gallois, Nanostruct. Mater., 10, 875 (1998).