РОССИЙСКАЯ АКАДЕМИЯ НАУК УРАЛЬСКОЕ ОТДЕЛЕНИЕ ИНСТИТУТ ХИМИИ TBEPДОГО ТЕЛА |
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30.04.2006 | Карта сайта Language |
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2.2. Arc melting and suction castingThe high vacuum arc melting furnace was vacuumized until the vacuum degree was up to 5 × 10−3 Pa after the preform was put onto the copper crucibles in it, followed by filling 0.05 MPa argon, and then the preform was melted by the electric arc under the argon atmosphere for 5 min applying the current of about 250 A with the alternative face melted in the same way successionally for the sake of the uniformity of the compositions. Then the composite melt was suctioned into a cavity with Ø7 × 120 mm3 in the copper mould with Ø110 × 125 mm3 rapidly to produce the sample in vacuum. Fig. 1 illustrates the equipment schematic of the arc melting and suction casting.
2.3. Microhardness testVickers microhardness values of the matrix and TiC particulates in the samples as well as pure Ni were tested under a load of 50 g with an indentation time of 10 s on Buehler Omninet Vickers hardness tester. This experiment was conducted on every sample for 10 times, and then the average of respective sample could be calculated. 2.4. Wear testDry sliding wear experiments were conducted in air at room temperature on a pin-on-disk machine using 600 grit abrasive papers with the load levels of 20 and 35 N as well as the sliding distance of 9 m, and the pin was loaded against the disk by a dead-weight loading system. The weight losses during the sliding tests were calculated from the weight differences of pin specimens before and after the tests to the nearest 0.1 mg, which were taken for three times, and then the average weight loss data gained were diverted to the volume losses per unit length. 2.5. Metallographic observationsMetallographic samples were prepared in accordance with standard procedures used for metallographic preparation of metal samples, and etched with about 15 vol.% HNO3 alcoholic solution for 10–15 s at 25 °C. The microstructure analysis of the composites was investigated by scanning electron microscopy (SEM) (Model SHIMADZU SSX–500, Japan), and phase analysis was conducted by X-ray diffraction (XRD) using Cu Kα radiation (Model D/Max 2500PC Rigaku, Japan). 3. Results and discussion3.1. SHS reaction and solidificationAccording to the phenomena observed, the SHS reactions of the preform were ignited firstly by the electric arc and the ceramic particulates were formed primarily, and then followed immediately by the product remelted at the high temperature of the arc. The final composites were fabricated after suction casting successionally. Therefore, although the SHS reactions have played an important part in them, the ultimate microstructures of the products, such as morphology, size, distribution, type and so on, are mainly determined by the solidification process. And the microstructures formed in this experiment are different from the ones synthesized only by SHS [11], [12], [16] and [26], which will be discussed in the following parts. 3.2. XRD resultsFig. 2 shows the XRD patterns of the four samples as well as Ni fabricated by arc melting and suction casting. It can be observed that the phases displayed in Fig. 2(a) and (b) are mainly Ni and Ni + TiC, respectively, as expected. However the products fabricated from Ni–Ti–B system are mainly composed of Ni, Ni3Ti, Ti2Ni and Ni4Ti3 as well as a small amount of TiB2 shown in Fig. 2(c), which is different from designed compositions (Table 1). And the ones in Fig. 2(d) and (e) are mainly composed of Ni, Ni–Ti compounds (including Ni3Ti, Ti2Ni, and Ni4Ti3) and TiC as well as a relatively small amount of TiB2.
However, what is more important is that in Fig. 2(b) the peaks corresponding with Ni deviating a little to the left compared with the ones of pure Ni in Fig. 2(a), which reveals that the lattice constant of Ni matrix of sample 1 increased. According to Fig. 2(b) and literature [17], the stoichiometry of TiCx has been calculated, and the value of x is about 0.8. Therefore, some C should be residual in the final products. However, no C phase is found in the XRD result. In addition to the volatilization of C at the high temperature of arc melting (3773 K), some C may be dissolved into Ni matrix to form an interstitial solid solution, which leads to the increase of Ni lattice constant and correspondingly the peaks of Ni turning to the left a little. Furthermore, according to Fig. 2(c)–(e), the Ni content reduce dramatically replaced by Ni–Ti compounds, and the content of TiB2 is relatively small compared with the designed ratio in samples 2, 3 and 4 which may be caused by the volatilization of B phase at the high temperature. Therefore, the matrices of samples 2, 3 and 4 are mainly composed of Ni and Ni–Ti compounds, and the composition change of the matrix will result in the variation of the mechanical property and wear resistance of the materials. 3.3. SEM analysisFig. 3 shows the typical SEM of the four samples fabricated by arc melting and suction casting. The gray parts are the matrices and the dark ones are ceramic particulates.
It can be observed from Fig. 3(a) that TiC particulates fabricated include the coarse blocky or dentritic primary phases with an average size less than 10 μm and the Chinese script type eutectic phases. Furthermore, it is interesting to note that also some fine eutectic phases adhere to the primary TiC, which indicates the eutectic TiC is the pre-eutectic phase during the solidification process. While the typical morphologies of sample 2 are shown in Fig. 3(b), from which it can be seen that there is a very small amount of TiB2 distributing in the matrix. This result is consistent with the XRD analyses. Moreover, through a large number of observations it is found that the distribution of TiB2 particulates is agglomerated in some region. The sizes of TiB2 particulates are of some differences and less than 10 μm, while the shapes of them are irregular. Fig. 3(c) and (d) show the typical morphologies of samples 3 and 4. The sizes of particulates are almost the same with sample 1 and the distribution of them is relatively dispersed. However, it is difficult to distinguish TiC from TiB2 in the two SEM due to their sizes and nondescript shapes. It is worth noting that the shapes of TiC and TiB2 particulates formed by SHS reactions, arc melting and suction casting are different from the ones formed only by the SHS reactions. In the present study the mechanism of TiC formation is nucleation-growth, and therefore, the shapes of TiC particulates are irregular; while the ones formed only by SHS reactions are usually round with a dissolution–precipitation mechanism [16] and [26]. At the same time, TiB2 particulates synthesized from SHS reactions are usually hexagonal prisms [11] and [12] and the mechanism needs further investigation. Otherwise the SEM shows the absence of macropores and blowholes as well as micropores, which indicates the near-fully dense composites can be fabricated by SHS reactions, arc melting and suction casting. 3.4. Microhardness and densityMoreover, Vickers microhardness values of the matrix in the four samples were tested, so were the densities of them including both the theoretical and practical ones, which are listed in Table 2.
From Table 2 it can be noted that the matrix microhardness of the four samples increases to different extents relative to pure Ni (146 HV) with several reasons, among which is the sustaining effect taken by the TiC and/or TiB2. Furthermore, the phenomenon that the matrix microhardness of sample 1 is 478 HV may be mainly caused by the solid solution strengthening of some C dissolved into Ni matrix, which is consistent with the XRD result (Fig. 2(b)). At the same time, it can be clearly found that the matrix microhardness of sample 2 is the highest in the four ones; therefore, combined with the XRD results it is concluded that there are more Ni–Ti compounds in the matrix instead of Ni matrix with the probability of the volatilization of B phase. Moreover, the matrix microhardness values of samples 3 and 4 are also pretty high compared with sample 1 si
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