Fabrication of Tic-20Mass%Ni Cermet Using MA-PCS Process

Materials Transactions, Vol. 47, No. 10 (2006) pp. 2561 to 2565 #2006 The Japan Institute of Metals

Fabrication of TiC-20 mass%Ni Cermet Using MA-PCS Process

Keizo Kobayashi and Kimihiro Ozaki

Materials Research Institute for Sustainable Development, National Institute of Advanced Industrial Science and Technology, Nagoya 463-8560, Japan

Mechanical alloying and pulsed-current sintering were used to synthesize TiC-20 mass%Ni cermet with ultra-ﬁne TiC particles dispersed. The mixture, in which Ti and Ni elements were uniformly distributed, was prepared by mechanical alloying of Ti, Ni and graphite powders for 18 ks. TiC particles were produced in the mixture milled for longer than 18 ks through a combustion synthesis reaction during heating. Mechanically alloyed Ti-C-Ni composite powder milled for 18 ks was consolidated using pulsed-current sintering at 1223 K under 70 MPa pressure. The sintered compact consisted of TiC and Ni. The average grain size of TiC particles in the sintered body was 0.80 mm. The relative density of the sintered compact reached 92% of the theoretical one; its Vickers hardness was 2260 Hv. [doi:10.2320/matertrans.47.2561]

(Received April 17, 2006; Accepted August 25, 2006; Published October 15, 2006) Keywords: mechanical alloying, pulsed-current sintering, cermet alloy, TiC-20 mass%Ni, combustion synthesis reaction, ultra-ﬁne particle

1. Introduction 2. Experimental Procedure

TiC-based cermet alloys are utilized for high-performance Using planetary ball milling for 18 and 36 ks, TiC-20 cutting tools because of their high hardness and chemical mass%Ni powders were synthesized from starting materials stability. It is important to enhance their mechanical proper- of commercial Ti powder (99.5 mass%Ti), Ni powder (99.9 ties, because the strength and toughness at room temperature mass%Ni), and graphite powder. The Ti powder particles for TiC-based cermet alloys are inferior to those of WC-Co were irregular in shape, with the approximate diameter of hard metals. For that reason, new binders and effective 30 mm. The nickel powder particles were spherical, with additives for TiC-based cermet alloys have been required. approximate 4–7 mm diameter. The vessel (5 104 m3) and Especially, miniaturization of TiC particles is one of the most the balls (0.01 m in diameter) for mechanical alloying (MA) effective techniques to improve the mechanical properties of were made of chromium steel. The ball-to-powder weight TiC-based cermet alloys. Therefore, both synthesis of fine ratio was 20 : 1. The rotational speed of the planetary ball TiC particles and sintering of cermet alloys at low temper- mill was 2.83 s1. The MA was performed under an argon gas atures are indispensable for miniaturization of TiC particles atmosphere of reduced pressure 66 kPa to prevent the milled in TiC-based cermet alloys. powder from oxidizing and nitriding. Microstructure and In order to prepare TiC-Ni with fine TiC particles, phase changes of the powder mixture with milling time were mechanical alloying of elementary powders has been analyzed using X-ray diffractometry (XRD) and scanning reported.1–3) However, TiC particles grow to be coarse electron microscopy (SEM). Characterization of these ob- during sintering even if the mechanically alloyed powder tained mixtures was carried out using differential scanning contains fine TiC particles. It has been reported that spark calorimetry (DSC) at the heating rate of 0.33 K/s under an plasma sintering4,5) was a potential process to prepare cermet argon gas flow in an alumina pan. alloys at a lower temperature.The process can consolidate The powder after milling for 18 ks was poured into a 30- various powders in a short time under a pressure application mm-deep graphite die of 10 mm internal diameter and 30 mm at a relatively low temperature, leading to fine crystal grains. external diameter. The powder was consolidated using PCS On the other hand, when a conductive material was sintered, at 1223 K under the pressure of 70 MPa. The sintered plasma generation could not be confirmed. Then, this temperature (‘die temperature’) was measured using an sintering process is called ‘pulsed current sintering (PCS)’ alumel-chromel type thermocouple with 3 mm inserted into in this paper. the graphite die. The sintered compact was characterized by In this study, we investigated the synthetic behavior of using XRD, SEM, Vickers hardness tester, and densimetry by TiC-20 mass%Ni cermet powder by mechanical alloying of Archimedes’ method. elemental Ti, Ni and graphite powders. Subsequently, a TiC- The XRD data were obtained with a diffractometer using 20 mass%Ni cermet alloy containing ultra-fine TiC particles Cu K radiation ( ¼ 0:15418 nm) with a monochromator. was fabricated by pulsed-current sintering of mechanically The Vickers hardness was measured on the polished surface alloyed powder through TiC formation by a combustion of the sintered compact with a load of 9.8 N and a loading synthesis reaction. Moreover, the microstructure of the time of 15 s. obtained cermet alloy was investigated. The combustion synthesis reaction is an operative technique to fabricate fine- 3. Results and Discussion grained ceramics, composite materials,6,7) and intermetallic compounds. 3.1 Synthesis of TiC-20 mass%Ni powders by mechanical alloying and the characterization Respective microstructures of TiC-20 mass%Ni powders 2562 K. Kobayashi and K. Ozaki

Ni TiC Ti

(a) X-ray intensity (a.u.) (b)

20° 30° 40° 50° 60° 70° 80° 90° 2 θ

Fig. 2 X-ray diﬀraction patterns of TiC-20 mass%Ni powders prepared using mechanical alloying for (a) 18 ks and (b) 36 ks.

the early stage for MA of Ti, Ni and graphite powders, the peaks arising from graphite have disappeared. This suggests graphite would be amorphous phase and forced to diffuse in Ni matrix by MA. In other words, TiC have been already synthesized in the powder milled for 36 ks with carbon remained in Ni matrix. On the other hand, the generation of amorphous Ti-Ni phase in the mechanical alloying of TiC-Ni Fig. 1 SEM micrographs of mechanically alloyed TiC-20 mass%Ni 8) powders milled for (a)18 ks and (b) 36 ks. has been reported. It can be concluded that the peak shift of Ni is caused by the existence of Ni-C solid solution and Ti-Ni amorphous phase. milled for 18 and 36 ks are shown in Figs. 1(a) and (b). In The DSC curves of TiC-20 mass%Ni powders prepared by the powder milled for 18 ks, coarse flat powders and fine milling for 18 and 36 ks, and mixing by mortar, are shown in powders are found. On the other hand, large agglomerates are Fig. 3. The curves of the two milled powders reveal an observed in the powder milled for 36 ks. The diameter of the exothermic reaction below 750 K. In the powder milled for largest agglutinate reached about 1 mm. The XRD patterns of 18 ks, the exothermic reaction includes two peaks: one at TiC-20 mass%Ni powders milled for 18 and 36 ks are shown 573 K and another at 728 K. Because the peak of 573 K is not Fig. 2. The XRD pattern of the powder milled for 18 ks observed in the powder milled for 36 ks, in which TiC has (Fig. 2(a)) shows Ti and Ni peaks, without graphite peaks. already been synthesized, this peak is probably due to the Peaks of compounds such as TiC and TiNi are invisible. exothermic reaction of TiC synthesis. For the mixture of Ti, Therefore, the flat powder shown in Fig. 1(a) suggests the Ni, and graphite powders by using a mortar, a higher deformed solid metal of Ti and/or Ni. On the other hand, temperature is necessary to synthesize TiC because the Fig. 2(b) shows the powder milled for 36 ks consists of TiC exothermic reaction is not completed below 900 K as shown and Ni phases. It is suggested that TiC was formed through a in Fig. 3(c). The synthetic reaction of TiC probably pro- combustion synthesis reaction in the vessel during mechan- gresses slowly at the low temperature in mechanically ical alloying in the time range between 18 and 36 ks. The alloyed powders, even though the combustion synthesis cross-sectional observation of the large agglomerate in the reaction of Ti þ C ! TiC spreads instantaneously along powder milled for 36 ks by SEM revealed fine TiC particles with the great generation of heat. The peaks of graphite were with the average particle size of 1.54 mm were dispersed in Ni not observed in the XRD pattern of the powder milled for matrix. According to the fluorescent X-ray analysis, the 18 ks. It has been reported that graphite becomes amorphous amount of Fe in the powder milled for 36 ks was 1.0 mass%, at early stages of MA, and has partly dissolved in Ni powder. which corresponded to be more than three times as much as Therefore, TiC would be synthesized in the powder milled that in the powder milled for 18 ks. The detected Fe suggests for 18 ks at the lower temperature than 573 K. The exother- the contamination from the vessel and balls during the mic reaction of 728 K is thought to reflect stabilization of the mechanical alloying. nonequilibrium phase, such as Ni-based amorphous phase. It The peaks of Ni in the XRD pattern of the powder milled is inferred that the nonequilibrium phase was generated at the for 36 ks are shifted to the low angle side and broadened. At interface of Ti and Ni through mechanical alloying for 18 ks. Fabrication of TiC-20 mass%Ni Cermet Using MA-PCS Process 2563

2.5

2 0.5mW/mg 1.5 /mm

(a) d 1

(b) 0.5 Shrinkage, (c) 0

-0.5

Endotherm. D S C Exotherm. -1 200 400 600 800 1000 1200 1400 200 300 400 500 600 700 800 900 1000 Die temperature, T /K Temperature, T /K Fig. 4 Shrinkage of TiC-20 mass%Ni cermet prepared by mechanical alloying for 18 ks plotted against die temperature. Fig. 3 DSC curves of TiC-20 mass%Ni powders: (a) mechanically alloyed for 18 ks, (b) mechanically alloyed for 36 ks, and (c) mixed using a mortar.

Therefore, this exothermic reaction at 728 K was observed in both mechanical alloyed powders. It is expected that TiC- 20 mass%Ni powder milled for 18 ks can be sintered completely at the low temperature after synthesizing TiC at the early stage of sintering.

3.2 Consolidation of TiC-20 mass%Ni by pulsed current Ni sintering and the characterization TiC Under 70 MPa pressure, the TiC-20 mass%Ni was consolidated using PCS. Figure 4 shows the relationship between the amount of shrinkage and the die temperature. After the compact expanded slightly to 400 K, it gradually shrank X-ray intensity (a.u.) with increasing the die temperature. The shrinkage is due to the combustion synthesis reaction of TiC, according to the DSC profile as shown in Fig. 3(a). It is thought that local heat generation was caused by the combustion synthesis reaction, 20° 30° 40° 50° 60° 70° 80° 90° and that the formation of necks among powder particles θ occurred at a low temperature. Considerable shrinkage at the 2 temperatures of more than 1100 K is attributed to be typical Fig. 5 X-ray diffraction pattern of TiC-20 mass%Ni cermet sintered at densification of TiC-20 mass%Ni cermet. TiC-20 mass%Ni 1223 K of mechanically alloyed powder for 18 ks. cermet fabricated by PCS at 1223 K had metallic luster and the relative density was 92% of the theoretical one of TiC- 20 mass%Ni. On the other hand, TiC-20 mass%Ni cermet The microstructure of TiC-20 mass%Ni cermet sintered prepared at 1223 K by PCS of the powder milled for 36 ks had by PCS at 1223 K is shown in Fig. 6. The dark region a metallic luster, but did not improve the compact density. As corresponds to TiC and the bright one Ni. Ni pools of about the powder milled for 36 ks has already generated TiC during 10 mm in length are observed. This is probably because liquid the MA, the necking among particles would not be formed at Ni appears at the high temperature and penetrates into the the low temperature. In addition, because the powder milled empty space. Figure 7 shows SEM images of the part where for 36 ks has a rough and large particle size relative to that TiC is rich and the Ni pool. Ultra-fine TiC particles are milled for 18 ks as shown in Fig. 1, larger energy would be exactly combined with Ni in the part where TiC is rich necessary for the densification of the compact. (Fig. 7(a)). This part would be formed during the combustion The XRD pattern of TiC-20 mass%Ni sintered by PCS synthesis reaction that occurs at the low temperature. Since process at 1223 K is shown in Fig. 5. The sintered body the Ni pool contains few TiC particles in Fig. 7(b), it was consists mainly of TiC and Ni. Compounds except for TiC probably formed without accompanying the combustion are not synthesized though the synthetic temperature of TiC synthesis reaction. A few pores are included in the Ni pool. in the powder milled for 18 ks is low as shown in Fig. 3(a). Therefore, it is presumed that Ni pool was formed at a higher 2564 K. Kobayashi and K. Ozaki

100 100 Cumulative distribution (%) 80 80

60 60

40 40

20 Frequency distribution (%) 20

0 0 Fig. 6 SEM micrograph of TiC-20 mass%Ni cermet of mechanically 0 0.5 1.0 1.5 2 2.5 3 alloyed powder for 18 ks sintered at 1223 K using pulsed current sintering. particle size, d / µm

Fig. 8 Particle size distribution of TiC particles in TiC-20 mass%Ni cermet sintered at 1223 K by PCS of mechanically alloyed powder for 18 ks. The ﬁlled circles indicate the cumulative distribution.

Distribution of the TiC particle size of TiC-20 mass%Ni sintered at 1223 K by PCS is shown in Fig. 8: more than 70% of the TiC particles are 0.5–1.0 mm. In this process, the deviation particle size of TiC in the sintered compact has become small. The average TiC particle size in the sintered compact was very minute: 0.80 mm. The TiC-20 mass%Ni cermet alloy containing ultra-ﬁne TiC particles was prepared through the combustion synthesis of TiC particles during sintering. It was found that the TiC particle grew only slightly at the sintering temperature of 1223 K. The TiC- 20 mass%Ni cermet alloy sintered at 1223 K of the mechanically alloyed powder milled for 18 ks showed Vickers hardness of about 2260 Hv. In addition, the TiC-20 mass%Ni cermet alloy sintered at 1423 K of the mixture of TiC and Ni powders showed Vickers hardness of about 2000 Hv. The diﬀerence of Vickers hardness arises from TiC particle size in the sintered compact. The TiC-20 mass%Ni prepared using the mixture of TiC and Ni powders required a high temperature to obtain the same density as that prepared using the MA powder milled for 18 ks, but the hardness of the former was a little smaller.

4. Conclusion

TiC-20 mass%Ni powder was synthesized rapidly through mechanical alloying of elemental Ti, Ni and graphite powders. The mechanically alloyed powder without TiC Fig. 7 Greatly-enlarged detailed sectional view of TiC-20 mass%Ni synthesized was consolidated with pulsed current sintering at cermet sintered at 1223 K by PCS of mechanically alloyed powder for 18 ks: close-up of (a) composite material and (b) Ni pool in Fig. 6. a low temperature. The reaction path during pulsed current sintering and microstructures of TiC-20 mass%Ni were investigated. Results indicate the following: temperature than that at which Ni softened. The existence of (1) TiC was synthesized by mechanical alloying of ele- Ni pool makes the microstructure of TiC-Ni cermet inho- mental Ti, Ni, and graphite powders using planetary mogeneous, leading to the decrease of its strength. It is ball mill for 36 ks with a ball-to-powder weight ratio necessary to optimize PCS sintering conditions (compaction of 20 : 1. The mechanically alloyed powder milled for pressure or heating rate) to produce a homogeneous compact 18 ks consisted of Ti and Ni phases without reacted at lower sintering temperature. compounds. Graphite became amorphous at the early Fabrication of TiC-20 mass%Ni Cermet Using MA-PCS Process 2565

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