Formation of nanodispersoids in Fe–Cr–Al/30%TiB2 composite system during mechanical alloying

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Abstract

Mechanical alloying is an effective processing technology which allows the synthesis of nanocrystalline composite materials. In this study, a nanocrystalline Fe–Cr–Al/30%TiB2 composite material is synthesized without process control agents by the mechanical alloying process using a planetary high-energy ball mill. Crystalline (or grain) size reduction and dispersion behavior of brittle TiB2 powder during ball milling are investigated together with mechanical alloying behavior of ductile metallic matrix powder for synthesis of the composite. Mechanical alloying between metallic elements is almost completed after approximately 8 h milling. The crystalline size of TiB2 decreases to 36 nm after 48 h of ball milling, while the average particle size of the composite powder increases in comparison with the initial size. Transmission electron microscopy reveals formation of TiB2 nanodispersoids of size around 50 nm after 48 h milling.

Introduction

Titanium diboride (TiB2), which is an advanced ceramic material, has shown peculiar physical and mechanical properties such as relatively high melting point, elastic modulus, hardness, strength-to-density ratio, wear resistance and oxidation resistance [1]. Furthermore, pure TiB2 would not be deformed plastically even at very high temperatures due to its intrinsically high Peierls barrier to the dislocation movement [2]. For this reason, TiB2 dispersoids have been used as a reinforcing material for metal matrix composites providing a desirable combination of high-temperature strength and hardness with adequate ductility and fracture toughness.

To synthesize metal matrix TiB2 nanocomposites, in situ reaction or self-propagating high-temperature synthesis processes have been used widely. For example, Tanaka et al. made a high-modulus steel reinforced with high volume (10–45 vol.%) of 200–800 nm TiB2 particles through an in situ reaction of Fe–Ti and Fe–B powders [3]. Finer (30–50 nm) TiB2 particles (up to 57 vol.%) in Cu matrix were fabricated by SHS incorporated with high-energy ball milling of Ti, B and Cu powders [4], [5], [6], [7]. Much finer (around 17 nm) TiB2 particles in Fe–Cr–Ni matrix were synthesized by mechanically activated self-sustaining reaction during high-energy ball milling of Ti/BN/Fe–Cr–Ni powder mixture [8]. For the synthesis of Cu/TiB2 composites containing low volume (below 3 wt.%) nanoparticles (around 20 nm), in situ reaction technique was used in a melt [9] or during rapid solidification [10]. Grain refinement of TiB2 to nano-scale during high-energy ball milling was reported for the TiB2/Al composite systems. Nanocrystalline TiB2 was synthesized in one case by mechanically activated reaction between Ti, Al and B powders [11], and in another case between Al, TiO2 and B2O3 powders [12]. Fe–Cr–Al/TiB2 composite system will be investigated in the present study because of its promising character for high-temperature applications. Fe–Cr–Al alloy, which is a ferritic stainless steel, is being used in various applications up to temperature as high as 1300 °C due to their excellent oxidation resistance [13], and is selected as the metallic matrix or binder material in this study.

The purpose of the current study is to fabricate metal matrix nanocomposite powder containing high volume fraction of nanocrystalline TiB2 directly from coarse powders of raw materials i.e. Fe, Cr and Al powders, using only single process of high-energy ball milling without in situ reaction. Nanocrystallization and dispersion behavior of brittle ceramic powder during ball milling are investigated together with mechanical alloying behavior of ductile metallic powder for synthesis of a Fe–Cr–Al/30%TiB2 nanocomposite.

Section snippets

Experimental procedures

In the present study, TiB2, 99% Fe, 99.8% Cr and 99.9% Al with average particle size of 6, 3, 10 and 4 μm, respectively, were used as the starting powder. Fe, Cr and Al powders were physically mixed in a composition of Fe–20%Cr–5%Al (percentage by weight is used unless otherwise specified). This powder was further mixed with TiB2 to make a physical mixture of composition Fe–Cr–Al/30%TiB2. These powder mixtures were prepared by mixing for 20 min in a vibration mixer (Retsch MM-201). A mixture of

Results

Fig. 1 shows the SEM surface morphology of powder particles at different stages of ball milling for synthesis of the Fe–Cr–Al/30%TiB2 composite powder. The starting material, which is a mixture of TiB2, Fe, Cr and Al powder, exhibits almost globular particles of various size (Fig. 1a). After ball milling starts, some big particles or agglomerates are noted among small particles (Fig. 1b). The agglomerates have rough surface, and the shape of the small particles becomes somewhat irregular. With

Discussion

It has been known that a critical balance between cold welding and fracturing is necessary for successful mechanical alloying, which enables powder particles to be always in contact with each other with atomically clean surfaces, minimizing diffusion distance [14]. Soft metallic materials such as aluminum are hardly alloyed mechanically because the process is hindered by excessive cold welding of the powder particles, preventing them from fracturing [15]. Hence, surface or process control

Conclusions

Nanocrystalline Fe–Cr–Al/30%TiB2 composite powder has been synthesized without process control agents from a micron sized mixture of metallic (Fe, Cr and Al) and ceramic (TiB2) powders by a high-energy ball milling process. Fine TiB2 dispersoids of size around 50 nm or lower are observed in the metallic matrix of Fe–Cr–Al solid solution. In addition to it, the crystalline size of TiB2 particles decreases with longer ball milling duration, resulting in an average size of 36 nm after 48 h of ball

Acknowledgements

This research was supported by an Institutional R&D Program of KIST. We wish to thank Dr. J.H. Shim and Dr. Y.W. Cho for their kind discussion and the supply of sample preparation facilities. We are also very thankful to Mr. S.I. Baik and Dr. Y.W. Kim at Seoul National University for transmission electron microscopic analysis work.

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