The microstructure was observed using backscattered electrons in the SEM (see Fig. 1). The gray and the dark phases were Mo2FeB2 hard phase and Fe binder phase, respectively. It can be seen the grain size is affected by the carbon content. The cermets without carbon addition exhibited the biggest grain size. The content of the main alloy elements in the binder phase was determined, the content of Mo element in binder phase increased with increasing carbon content.
Fig. 1. Microstructure of the cermets with carbon contents of: (a) 0 wt.%; (b) 0.5 wt.%; (c) 1.0 wt.%; and (d) 1.5 wt.%.
Fig. 2 shows the plots of the average relative density at different sintering temperatures for the cermets with and without carbon addition. It can be seen that carbon addition significantly decreased the liquid phase formation temperature. Thus, the carbon addition prolonged the time of existence of liquid phase under the same sintering process. It was reported that the growth of Mo2FeB2 grains was controlled by solution–reprecipitation mechanism. The longer the time of existence of liquid phase, the coarser the hard grains should be. Thereby, it can be deduced that carbon addition inhibited the growth of Mo2FeB2 grains.
Fig. 2. Plots of the average relative density at different sintering temperatures for the cermets with and without carbon addition.
As outlined above, the carbon addition facilitated reduction of oxides, and accordingly increased the wetting of the ceramic grains by liquid metal during sintering. Thereby, the grain size was decreased when 0.5 wt.% carbon addition was added. On the other hand, it was found that although the porosity increased for the cermets with a higher carbon addition, the grains were further refined. So, it is reasonable to suggest that in addition to the wetting, there were other factors that were responsible for the decrease of the Mo2FeB2 grain size at a higher carbon addition. Sadangi investigated the grain growth inhibition in sintered nanophase WC/Co alloys, who suggested that the formation of metal/non-metal clusters, e.g. W, V, Cr/C clusters, in the liquid Co was a decisive factor that impeded liquid phase transport of W and C atoms from one grain to the next adjacent one and restricted the WC coarsening rate. Furthermore, Wittmann studied the WC grain growth in the binder phase of Ni, Co and Fe. Their results indicated that the metal-to-carbon bond influenced the WC growth behavior, and iron had a higher stability to form metal– carbon bonds compared with Ni and Co, which impeded carbon transport and precipitation and accordingly decreased the WC grain growth in WC–Fe alloys. In the present study, superfluous carbon would dissolve in the Fe binder phase when the amount of carbon addition is greater than the amount for deoxidation, since the Fe binder phase has a high affinity to carbon, which is beneficial to form a stable Fe/C cluster. Consequently, a stable Fe/C cluster may restrict the Fe binder phase transport and result in a further decrease of Mo2FeB2 grain size for the cermets with 1.0 wt.% carbon addition at the present study. At a higher carbon addition, saturation concentration of the carbon in Fe binder phase was reached, and the grain size almost kept steady.
To elucidate the characteristics of the microstructure with different carbon additions in detail, TEM observations were also carried out, as shown in Figs. 3and 4. It was found that the binder phase was martensitic when carbon content was 1.0 wt.%. In addition, the small amount of martensite present for the cermets with a carbon content of 0.5 wt.%, indicated that 0.5 wt.% carbon addition was enough to degas the oxygen completely. On the other hand, the pores were increased at higher carbon contents, which suggests that the wetting of the ceramic grains by liquid metal during sintering instead decreased. Thereby, a further decrease of grain size at higher carbon additions cannot be attributed to a variation of wetting, but may due to the formation of a stable Fe/C cluster .
Fig. 3. TEM observations of the ferrite binder phase with carbon content of 0.5 wt.%: (a) TEM micrograph in bright field; (b) diffraction pattern of the selected area 1.
Fig. 4. TEM observations of the martensite binder phase with carbon content of 1.0 wt.%: (a) TEM micrograph in bright field; (b) diffraction pattern of the selected area 1.
本文由桑尧热喷涂网收集整理。本站文章未经允许不得转载；如欲转载请注明出处，北京桑尧科技开发有限公司网址：http://www.sunspraying.com/
|
