1. Introduction

Adding refractory particles to the Fe matrix as dispersoid improves mechanical properties of the matrix and increases wear resistance. Among various ceramic particulates, TiB₂ is considered to be one of the best reinforcements for steel matrix. Fe-TiB₂ composite has attracted much attention due to its excellent mechanical properties. Various methods for the synthesis of TiB₂ phase have been reported and can be summarized as follows: laser cladding [1,2], plasma transferred arc (PTA) [3,4], aluminothermic reduction [5], spark plasma sintering (SPS) [6], and selfpropagating high temperature synthesis (SHS) [7-10]. These reports indicate that TiB₂ particles can be formed in situ via different routes from various initial materials.

The technique of in situ synthesis is widely used for fabricating composites. In general the in situ processes involve synthesizing the reinforcing phase within a matrix during composite fabrication by a chemical reaction between elements or their compounds. In situ formation process normally leads to the clean particle-matrix interfaces with higher interfacial strength (improved wettability), fine reinforcement size and homogeneous particlesize distribution (improved mechanical properties) [11]. We also reported already a result on in situ fabrication of Fe-TiB₂ nanocomposite powder having TiB particulates smaller than 5 nm by planetary ball-milling and subsequent heat-treatment of (FeB+TiH₂) powder mixture [12]. For a further application of the Fe-TiB₂ nanocomposite powder, a sintering is inevitable. Pressureless sintering is a simple and easy consolidation method that can produce a wide range of sintered geometries. However, for nanocomposite powders it should be performed that the grain growth or coarsening of reinforcement particulates will not occur or can be minimized.

In this work the feasibility of pressureless-sintering for the fabrication of Fe-TiB₂ composites is investigated from the experimental results of sintered density, microstructure and hardness.

2. Experimental Procedures

A composite powder with a composition of Fe-40 wt.% TiB₂ (55 vol.%) was fabricated from commercial iron boride (FeB) powder with 19 wt.% of boron and TiH₂ starting powders. FeB and TiH₂ powders were mixed for 2h in a turbular mixer and then high-energy milled in a planetary ball mill (AGO-2) at 700 rpm for 3 hours. The detailed high-energy milling process is given in our previous work [12].

The as-milled powder mixture was compacted by double- acting compaction using 10 mm cylindrical tool-steel die at pressure of 100 MPa. The green compacts were pressureless-sintered in a tube furnace (ThermVac., South Korea) under flowing Ar gas. Sintering conditions of temperature and time were chosen from a series of preliminary experiments: 1300°C, 1350°C and 1400°C for 0-2 hours. The sintered density was measured by ASTM B962.

Phase analysis for the sintered body was performed by X-ray diffractometer (XRD) using CuKα radiation. The microstructure was observed and analyzed by using field emission scanning electron microscope (FE-SEM) and energy-dispersive X-ray spectroscope (EDS). Hardness of all the sintered composites was measured by Vickers method using 2 kg load (19.6N, HV₂) and the values were converted to Rockwell Hardness (HRC).

3. Results and Discussion

3.1. Sintered density vs. sintering temperature and time

Fig. 1 shows the change of sintered density as a function of sintering temperature and time. The powder mixture of (FeB+TiH₂) was easily compacted at 100 MPa to give the green density of 75% theoretical. At 1400°C, the sintered density reaches to 95% even for short holding time of 0.25 h (15min). Further increase of holding time brings only a slight increase in sintered density (96.0%). At 1300°C, the effect of holding time on densification is evident. The density increases with holding time from 80.1% to 91.2%. At 1350⁰C, the sintered density increases steadily from 87.0% to 93.1% up to the holding time of 1 hour. A further increase in holding time to 2 hours leads to the sintered density of 95.5%. From these results it can be concluded that the optimal sintering temperature is in a range of 1350°C and 1400°C. For most ferrous materials, the pressureless-sintering is carried out in the temperature range of 1100°C and 1300°C or to achieve the higher density the materials should sinter at higher temperature, up to 1350°C [13]. The remarkably high sintering temperature in this study seems to be due to a high amount of TiB₂ (55 vol.%). As reported already in our previous work [12], the in situ synthesis reaction of nanoscale TiB₂ between Ti from TiH₂ and B in FeB occurs at 749°C under the following equation.

2FeB(s) + TiH2(s) → 2Fe(s) + TiB2(s) + H2(g) ↑

Fig. 1

Change in sintered density of (FeB+TiH₂) powder compacts as a function of sintering temperature and time.

The nanoscale TiB₂ particulates homogeneously dispersed in the Fe-matrix seem to retard the densification of Fe-TiB₂ composite powder, even though it is expected that the exothermic reaction between Ti and B to form TiB₂ phase during the sintering process may enhance the densification by a thermal contribution.

3.2. XRD phase analysis

XRD patterns of selected sintered bodies are shown in Fig. 2. It is evident that as expected, two main phases of Fe and TiB₂ are present in all patterns. Similarly to our previous results from the in situ fabrication of Fe-TiB₂ nanocomposite powder from (FeB+TiH₂) powder mixture [12], it seems that both of two phases keep unchanged after the formation at 749°C through the whole remained sintering process.

Fig. 2

XRD pattern of (FeB+TiH₂) powder compacts sintered at 1300°C for 0.25 h (bottom), at 1350°C for 2 hours (middle) and at 1400°C for 2 hours (top).

XRD pattern for the sintered compacts at 1350 and 1400°C for 2 hours reveal that there is TiB phase as a minor phase. The peak intensity of TiB increases on the compact sintered at 1400°C for 2h. According to other reports [7, 8], the following reaction can occur when the excess titanium exists:

Ti + TiB2 →2TiB    ΔG1300K = −53 kJ.mol-1

It seems that the excess Ti which was intentionally added for a complete reaction of TiB₂ formation reacts again at high temperature with the TiB₂ formed at lower temperature during heating stage.

3.3. Microstructure of sintered compacts

Fig. 3 shows the SEM images of (FeB+TiH₂) powder compacts sintered at 1350°C for 0.5, 1 and 2 hours. The polished cross-section of sintered compact reveals many pores and incompletely densified regions up to the holding time of 1 hour (Fig. 3(a-b)). The increase in holding time to 2 hours results in nearly fully dense microstructure with 95.5% relative density (Fig. 3(c)). It should be noted here that the dark-grey TiB₂ particulates can be discerned from the Fe-matrix. The high magnification compoimage (×10,000)(Fig. 3(d)) shows this more clearly. The size of the TiB₂ particulates is extremely scattered from several tens nanometer to micron. It is evident that a rapid growth of nanoscale TiB₂ particulates occurs between the holding time of 1 and 2 hours at 1350°C.

Fig. 3

SEM images of (FeB+TiH₂) powder compacts sintered at 1350°C for (a) 0.5h, (b) 1h, (c) 2h (x3,000), and (d) 2h (×10,000, compo-image).

Fig. 4 shows the SEM images of (FeB+TiH₂) powder compacts sintered at 1400°C for 0.5, 1 and 2 hours. All of them show a similar microstructure with Fig. 3(c) described just before. But they have higher sintered densities and reveal more homogeneous microstructure in the particulate size. The number of fine particulates with several tens nanoscale is drastically reduced. The average size of TiB₂ particulates seems to be shifted unimodal to submicron or micron. The overall microstructure is also very homogeneous. Fine TiB₂ particulates are evenly distributed in the Fe-matrix, which cannot be obtained from a conventional powder metallurgy process or other approaches. It is reported that the size of TiB₂ in the composites fabricated by these methods is in the range of several to tens micrometers.

Fig. 4

SEM images of (FeB+TiH₂) powder compacts sintered at 1400°c for (a) 0.5h, (b) 1h, (c) 2h (×3,000), and (d) 2h (×10,000, compo-image).

3. 4. Hardness

Fig. 5 shows the change of hardness as a function of sintering temperature and time. In case of the sintering temperature of 1350°C, it is evident that the increase in sintered density leads to the increase in hardness from 64.8 HRC to 69.1 HRC. The densification effect seems more dominant than the effect of microstructural defects like pores. For the sintering at 1400°C the longer holding time do not result in the increase in sintered density but only the growth of TiB₂ particulates. This affects the hardness negatively. Maximum hardness of 71.2 HRC for 0.5 h is slightly decreased to 70.0 HRC for 2 hours, which is nearly equivalent to the HRC of a conventional WC-Co Hardmetals containing 20 wt.% Co [14]. Even though the hardness of TiB₂ is higher than WC, it is very remarkable that the Fe-TiB₂ composite with 60 wt.% Fe exhibits such a high hardness value. It is because of the homogeneous distribution of TiB₂ particulates with relatively fine size of submicron and the sound interface between TiB₂ particulates and Fe matrix. If we use a novel sintering method for restraining from grain growth such as the spark-plasma sintering, the hardness will be surely increased also with the same composition here.

Fig. 5

Change in sintered density and hardness of (FeB+TiH₂) powder compacts as a function of sintering temperature and time.

4. Conclusion

Sintered body of the TiB₂-reinforced iron matrix composite (Fe-TiB₂) was fabricated by pressureless-sintering of the mixture of titanium hydride (TiH₂) and iron boride (FeB) powders. From the results of sintered density, X-ray phase analysis, observation and analysis of microstructure with use of FE-SEM and EDS, and hardness measurements the followings can be concluded:

• (1) The optimal sintering temperature for high density above 95% relative density is between 1350-1400°C, where the holding time can be varied from 0.25-2 hours. Maximum 96.0% relative density is obtained from the (FeB+TiH₂) powder compacts sintered at 1400 for 2 hours.

• (2) In case of sintering at 1300°C, the sintered density increases with holding time, while the sintering at 1400°C shows almost no change in sintered density after the holding time of 0.5 h.

• (3) Sintered compacts have two main phases of Fe and together with trace of TiB which seems to be formed through the reaction of TiB₂ formed at lower temperature during heating stage and the excess Ti which was intentionally added for a complete reaction of TiB₂ formation.

• (4) Nearly fully densified sintered compacts show homogeneous microstructure composed of fine TiB₂ particulates with submicron size and Fe-matrix.

• (5) Maximum hardness of 71.2 HRC is obtained from the specimen sintered at 1400 for 0.5 hour which is nearly equivalent to the HRC of a conventional WC-Co Hardmetals containing 20 wt.% Co.

Acknowledgements

Acknowledgements

This work was supported by the National Research Foundation Grant funded by the Korean Government (NRF-2012R1A1A2044930).

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References

Citations

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