Microstructures and Mechanical Properties of (Ti 0:8 Mo 0:2 )C-30 mass% Ni without Core-Rim Structure

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1 Materials Transactions, Vol. 51, No. 8 (2010) pp to 1432 #2010 The Japan Institute of Metals Microstructures and Mechanical Properties of (Ti 0:8 Mo 0:2 )C-30 mass% Ni without Core-Rim Structure Hiroyuki Hosokawa, Kiyotaka Kato, Koji Shimojima and Akihiro Matsumoto Materials Research Institute for Sustainable Development, National Institute of Advanced Industrial Science and Technology, Nagoya , Japan Mechanically alloyed powders having the composition (Ti 0:8 Mo 0:2 )C-30 mass% Ni were sintered at 1723 K for 2, 3, and 6 h. After sintering, a TiC phase without a core-rim structure and a Ni phase appeared. In addition, the X-ray diffraction spectrum of 6 h sintered showed a Mo peak. With an increase in the sintering time, the hard phase grain size, mean free path of the binder phase, and the binder phase volume fraction increased. The transverse rupture strength and Vickers hardness of the sintered s were measured. The maximum average transverse rupture strength was 1.51 GPa for. The relationship between the hardness and the microstructure could be explained by the composite law including the structural factors. had the highest hardness because of the relatively short mean free path of binder phase. had the largest grain size, lowest volume fraction of the hard phase, and the longest mean free path of binder phase. However, this was harder than because of the significantly high hard phase contiguity. [doi: /matertrans.m ] (Received April 2, 2010; Accepted June 2, 2010; Published July 14, 2010) Keywords: cermet, (Ti,Mo)C-Ni, mechanical alloying, mechanical property, strength, hardness 1. Introduction Cutting tools made of TiC(N)-based cermets have high hardness, wear resistance and oxidation resistance, and hence, they are widely used for finishing and semi-finishing of metals. The mechanical properties of TiC(N)-based cermets have been improved by adding various elements. 1 17) Mo is one of the main additive elements for cermets. It enhances the wettability of hard phase with binder phase and refines crystal grain of the hard phase, and thereby improves fracture toughness of the. 1 5) WC is added to improve the wetting, densification, and fracture toughness of the cermet and for grain refinement of the hard phase. 6 8) Other carbides are added to modify the physical and mechanical properties of the cermet according to use conditions ) The additive elements for cermets cause the hard phase of the TiC(N)-based cermets to become corerim structures consisting of a TiC(N) core and a (Ti,X,Y)C rim (X and Y denote the additive elements). It is well known that microstructural inhomogeneity leads to deterioration of the mechanical properties of the cermet. Recently, it has been reported that (Ti,Mo)C-Ni cermets without a core-rim structure can be synthesized by mechanical alloying and subsequent sintering; 18,19) this method brings about an increase in the microstructural homogeneity and thus may be help improve the mechanical properties of the cermet. However, there is limited information about the mechanical properties of (Ti,Mo)C-Ni cermets. In the present study, (Ti 0:8 Mo 0:2 )C-30 mass% Ni s without core-rim structure were produced by mechanical alloying and subsequent sintering at 1723 K for 2, 3, and 6 h. The effect of the microstructures on the mechanical properties was investigated for the sintered s. 2. Experimental Procedures Ti (99.9%, 45 mm), C (99.9%, 20 mm), Ni (>99:9%, 5 mm), and Mo (>99:9%, 3 mm) powders were used. A powder mixture with a chemical composition of (Ti 0:8 Mo 0:2 )C- 30 mass% Ni was prepared. The powders were sealed in a 500 ml hardened steel pot together with WC/Co balls and then were dry-mixed for 200 h in an Ar atmosphere (667 kpa). A planetary ball mill (Fritsch Pulverisette 4) was used for milling and mixing. The ball-to-powder weight ratio (BPR) was fixed at 20 : 1. The as-synthesized powders obtained after 200 h milling were pressed into bars under a pressure of 100 MPa and then sintered at 1723 K for 2, 3, and 6 h under vacuum (hereinafter called, and 6-h sintered, respectively). The powders and sintered s were analyzed by X- ray diffraction using Cu K radiation. The microstructures of the sintered s were analyzed by field-emission scanning electron microscopy (FE-SEM) with energy-dispersive X-ray spectroscopy (EDS). The hard phase grain size, mean free path of the binder phase, the hard phase volume fraction, and hard phase contiguity were measured from the FE-SEM photographs. The hard phase grain size, d hard, the hard phase volume fraction, f hard and the mean free path of binder phase, binder, are given by the following equations: d hard ¼ 4 N L N S f hard ¼ 8 3 N2 L N S binder ¼ 1 f where N L is the number of hard phases intercepted per unit length, and N S is the number of hard phases included per unit square. 20) In addition, the hard phase contiguity, C TiC,is given by 2N hard/hard C hard ¼ ; ð4þ N hard/binder þ 2N hard/hard where N hard/hard is the number of interfaces between the hardphase grains intercepted per unit length, and N hard/binder is the N L ð1þ ð2þ ð3þ

2 Microstructures and Mechanical Properties of (Ti 0:8 Mo 0:2 )C-30 mass% Ni without Core-Rim Structure 1429 number of interfaces between the hard phase grains and the binder phase intercepted per unit length. 21) The hard phase composition of the sintered s were analyzed from the center of a grain to its edge by transmission electron microscopy (TEM) with EDS to confirm non core-rim TiC structure. Transverse rupture strength tests were conducted by using the three point bending method (span length: 10 mm, crosshead speed: 0.5 mm min 1 ). Vickers hardness was measured with a macro load range of 294 N. 3. Results and Discussion The XRD patterns for the milled powder and the sintered s are shown in Fig. 1. For the milled powder, wide peaks exist only for the TiC phase. It is observed that these peaks tend to shift toward higher angles compared to those of the TiC reference peaks, as well as those mentioned in previous studies. 18,19) It may be suggested that Mo has been dissolved in the TiC phase. The sintered s show peaks due to the TiC phase at angles higher than the TiC Intensity (a.u.) Milled powder : TiC : Mo : Ni θ (degree) Fig. 1 XRD patterns for the milled powder and sintered s. reference angles, as well as in the milled powder. However, the TiC peaks for the sintered s indicate lower angles than those for the milled powders. It might be suggested that most of the TiC dissolved in the binder phase and restructured during sintering process with lower Mo content. Moreover, the peaks for Ni structures had lower angles compared to Ni reference peaks, indicating that Ni contains traces of Ti and Mo. TiC structure peaks and Ni structure peaks of were lower than those of the others. has the peaks for Mo in addition to these peaks. The SEM photographs of the sintered s are shown in Fig. 2. As seen from the images, the hard phase has no contrast, unlike the core-rim structure of ordinary cermets. 4 8) This probably indicates that this cermet consists of a single phase. The microstructures of 2-h and s are homogeneous. On the other hand, in the, agglomeration of each phase is observed, and the hard phase contains some abnormally grown grains. The SEM photographs and EDS mapping images of 3-h and s are shown in Fig. 3. Each element is distributed uniformly in the case of. On the other hand, the corresponding images of show regions of definitely high Mo concentration. This result is in agreement with the results of XRD analysis and demonstrates that Mo is eccentrically located in 6-h sintered. The hard phase grain size, hard phase volume fraction, mean free path of the binder phase, and hard phase contiguity for the sintered s are shown in Table 1. The average hard phase grain sizes are 230, 233, and 335 nm for 2-h, 3-h, and s, respectively, indicating that hard phase grain growth occurred only in the. The hard phase volume fractions in the case of 2-h, 3-h, and s are 72.3%, 66.1%, and 60.9%, respectively. From these values, it is apparent that the hard phase volume fraction decreases with an increase (a) (b) 2µm 2µm (c) 5µm Fig. 2 SEM photographs of (a) 2-h, (b) 3-h and (c) s.

3 1430 H. Hosokawa, K. Kato, K. Shimojima and A. Matsumoto SEM Ti Mo C Ni 3-h sintered 10 µm 6-h sintered 10 µm Fig. 3 SEM photographs and EDS mapping images of 3-h and s. Table 1 Hard phase grain size, hard phase volume fraction, mean free path of the binder phase, and hard phase contiguity for the sintered s. Hard phase grain size, d hard /nm Hard phase volume fraction (%) Free path of binder phase binder /nm TiC structure contiguity (%) Table 2 Hard phase composition analyzed from the center of a grain and its edge by EDS. (at%) 1 (Center) (Edge) Ti Mo Ti Mo Ti Mo Ti Mo nm Fig. 4 TEM photograph of the sintered. Circles with numbers indicate areas of EDS analysis. in the sintering time. The mean free paths of the binder phase are 60, 87, and 150 nm for 2-h, 3-h, and s, respectively, indicating that the mean free path decreases with an increase in the sintering time. With an increase in the sintering time, the mean free path of the binder phase increases owing to grain growth and decrease in the hard phase volume fraction. The hard phase contiguities of 2-h, 3-h and s are 0.43, 0.46, and 0.71, respectively. This indicates that the number of boundaries between the hard phase and the binder phase is considerably lesser in than in the other two sintered s. The TEM photograph of the sintered is shown in Fig. 4. EDS spot analyses are carried out at four points between the center and the edge of the hard phase grains. The results for each sintered are shown in Table 2. In each sintered, the hard phase grains are made of TiMoC without any core-rim structure. The Mo content of the TiC structure is in the range at% between the center of a grain and its edge, except for. It is about 25 to 30, and the Ti content is approximately at% for. The average transverse rupture strength and average Vickers hardness of each sintered are shown in Fig. 5. The average transverse rupture strengths of the 2-h, 3-h, and s are 1450, 1510, and 520 MPa, respectively, indicating that the is the strongest. From Table 1, the hard phase contiguity of the 3-hsintered is almost equal to that of the and lower than that of. Because of the large binder phase volume fraction, the mean free path of the binder phase for is longer than that for. Therefore, it is reasonable to state that has the highest strength among all the samples. On the other hand, 6-h sintered has the shorter mean free path because of its inhomogeneous microstructure, although this has the largest volume fraction of the binder phase.

4 Microstructures and Mechanical Properties of (Ti 0:8 Mo 0:2 )C-30 mass% Ni without Core-Rim Structure 1431 Table 3 Hardness estimation of sintered. Hard phase Binder phase H H c V cc V c C H hard H m V m H Mo V Mo H binder H calculated H measured Vickers hardness, H /HV Transverse rupture strength,σ /MPa Vickers hardness Transverse rupture strength Fig. 5 Transverse rupture strength and Vickers hardness of sintered s. The average Vickers hardnesses of 2-h, 3-h, and 6-h sintered s are HV 1378, 1246 and 1257, respectively. This indicates that has the highest hardness among all the samples. The hardness of is slightly higher than that of, although the former has a larger hard phase grain size, longer mean free path of the binder phase, and lower hard phase volume fraction. D. Mari et al. 5) have reported that the hardness of a cermet is calculated according to the composite law including the structural factors, and the hardness values thus calculated were in agreement with the measured values. The equation for calculating the hardness is given by H ¼ H hard þ H binder ¼ H C V CC þ H m ð1 V CC Þ; ð5þ where H hard is the hardness assumed by the hard phase in total, H binder is the hardness assumed by the binder phase in total, V CC is the continuous volume of the hard phase (¼ V C C, where V C is the hard phase volume and C is the contiguity). The hard-phase hardness, H C, depends on the grain size d and is given by H C ¼ H 0C þ d ð1=2þ ; ð6þ where H m is the binder phase hardness, and it depends on the mean free path in the binder phase, m. H m ¼ H 0m þ ð1=2þ m ; ð7þ where and are adjustable parameters, and H 0c and H 0m represent the characteristic hardness of very large hard phase and metal single crystals. Equation (5) is combined with eqs. (6) and (7) and the resulting equation is used to fit the measured hardness of the cermets. When the hard phase and binder phase are TiC and Ni, respectively, eqs. (6) and (7) are as follows: H C ¼ 1689 þ 5:9 d ð1=2þ ð8þ H m ¼ 300 þ 8:5 ð1=2þ ð9þ The values of the terms in the above equations correspond to 2-h and s. In the case of, a Mo phase is observed in addition to the Ni based binder phase, and therefore, eq. (5) should be modified as H ¼ H C V CC þðh m V m þ H Mo V Mo Þð1 V CC Þ; ð10þ where V m is the volume fraction of the Ni-based binder phase in binder total, V Mo is the volume fraction of the Mo phase in binder total and H Mo is the hardness of Mo. V m and V Mo are estimated from the ratio between the value multiplying peak intensity by half-width for Ni and that for Mo. The strength,, of nanocrystalline Mo with grain size of from 97 to 180 nm is about 400 MPa. 22) Therefore, H Mo is estimated to be HV 1200 in this study, since H ¼ 3. The values of the terms for are obtained from eq. (10). The corresponding values obtained for each sintered are shown in Table 3. The calculated values are slightly lower than the measured values; however, the trend of the calculated H is in agreement with that of the measured H. The has the highest hardness because of its high H binder, which in turn is due to the short mean free path in the binder phase. H hard was the largest and H binder was the lowest for. Therefore, H hard, which is derived from the high hard phase contiguity, is responsible for the hardness of the being slightly higher than that of. 4. Conclusion The powder produced by mechanical alloying of (Ti 0:8 Mo 0:2 )C-30 mass% Ni was sintered at 1723 K for 2, 3, and 6 h. After sintering, the hard phases were found to comprise (Ti,Mo)C (Mo/Ti ratio: / ) without any core-rim structure. With an increase in the sintering time, the hard phase grain size and mean free path of the binder phase increased, while the binder phase volume fraction decreased. TiC and Ni phases were present in the sintered s. In addition, a Mo phase was present in and the microstructure was inhomogeneous. The highest transverse rupture strength was obtained for. This may be explained by the homogeneous microstructure of this and its relatively longer mean free path of the binder phase. The hardness of the s could be explained by the composite law, including the structural factors.

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