The XRD patterns of the samples sintered at 700, 900, 1100,1300 ℃ for 40 min in vacuum were shown in Fig. 1. It can be revealed from the XRD patterns that the raw materials were stable up to 700 ℃. The boron element was not detected in the patterns because it existed in amorphous state. When the temperature increased to 900 ℃, orthorhombic Mo2NiB2 phase was formed, and the content of Mo decreased. Some unknown phases were also detected, and they were not the alloy of Mo and Ni, neither the boride of Mo or Ni. Like the method using metal diboride as raw materials, the Ni peaks shifted towards lower 2h angle due to the solution of Mo in Ni during the formation of the matrix phase Mo2NiB2 with the increasing of sintering temperature. At higher temperatures than 1100 ℃, the peaks of Mo weakened and eventually disappeared.
Fig. 1. XRD patterns of the samples sintered at 700, 900, 1100, 1300℃.
Based on the above analysis, however, it was not sure that any intermediate product was formed during the sintering process or the Mo2NiB2 phase was formed directly from the elementary substances, Mo, Ni, and B. Accordingly, more detail analysis in the temperatures range from 700 to 900℃ should be necessary.Fig. 2 showed the XRD patterns of the samples sintered between 700 and 900 ℃. Except the raw materials, only the peaks of MoB were identified in the sample sintered at 800℃. At 850 ℃, the peak intensity of MoB decreased and the Mo2NiB2 phase was formed. More Mo2NiB2 phase was formed and intermediate prod- uct of MoB disappeared at 900 ℃. So, the formation process below 900 ℃ could be inferred as follows:
when the temperature increased above 900 ℃, the peak of MoB was not identified in the XRD patterns any more. So, there are two different mechanisms for the formation of Mo2NiB2 phase at above 900 ℃. One is that the newly formed Mo2NiB2 directly grow on those which have formed already (can be looked as seeds). The other is that Mo2NiB2 is still formed based on reactions (1) and (2). However, once the MoB was formed according to reaction (1), it immediately reacted with Ni to form Mo2NiB2. No matter by which mechanism the Mo2NiB2 was formed, MoB was formed prior to the formation of Mo2NiB2. So, the formation of orthorhombic phase was influenced by the same structure of MoB at high temperatures, which was the same conclusion with the literature.
Fig. 2. XRD patterns of the samples sintered at 700, 800, 850, 900℃.
To ascertain by which mechanism Mo2NiB2 was formed at higher temperatures, 30 wt.% of powders sintered at 1100 ℃ (E, mainly composed of Mo2NiB2) and 70 wt.% of starting mixed powders were mixed again. The new mixtures were sintered at 800 ℃. The phase compositions of the samples both before and after sintering were also identified by XRD. Not any peak for MoB was detected in the XRD pattern, as shown in Fig.3. The peak intensity of Mo2NiB2 was strengthened evidently after sintering at 800 ℃, while that of Mo was weakened. It means that the already formed Mo2NiB2 particles could be as seeds for the new formation of Mo2NiB2 directly from the three elementary substances. The reaction could be expressed as follow:
The raw materials were heated to 1300 ℃ using the same heating rate and with the protection of Ar atmosphere to get the TG-DTA curve, as shown in Fig. 4. It can be found that the mass of the system changed little and the DTA curve was flat and the endo-thermal or exothermic processes were not evident below 700 ℃.
Fig. 3. XRD patterns of the samples sintered from the new mixture (30 wt.% sintered powder at 1100 ℃ and 70 wt.% of starting mixed powders) at 800 ℃ and its corresponding green body.
Fig. 4. TG-DTA curve of raw materials heating to 1300 ℃ at a heating rate of 10℃/min and with the protection of Ar atmosphere.
Liquid phases appeared at the temperature above 1100 ℃, because there was a wide endothermal peak at the corresponding temperature range. The appearance of liquid phases can be further confirmed by the surface images at different temperatures in Fig. 6a and c and the phase diagram of Ni–B. The liquid phases were beneficial for the densification of samples from 1100 to 1200 ℃.
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