Elsevier

Journal of Alloys and Compounds

Volume 788, 5 June 2019, Pages 873-880
Journal of Alloys and Compounds

In situ formed TiCx in high chromium white iron composites: Formation mechanism and influencing factors

https://doi.org/10.1016/j.jallcom.2019.02.292Get rights and content

Highlights

  • Influencing factors on the microstructure of high Cr white iron were investigated.

  • Ti3AlC2 precursor size has a minor influence on the grain size of in situ TiCx.

  • Fine TiCx particles in composites can be obtained from even coarse Ti3AlC2 powder.

  • Control of heating rate and dwelling time contributes to the distribution of TiCx.

  • This work offers a possibility to refine in situ high Cr white iron composites.

Abstract

Influencing factors on the preparation of high chromium white iron composites reinforced with in situ formed TiCx have been investigated. Various Ti3AlC2 precursor powders with sizes from 6.5 to 75 μm were used to examine their influence on the dimension of in situ formed TiCx. Different heating rates and dwelling times were also adopted to study their effects on the microstructure of the composites. The initial Ti3AlC2 precursor size has a minor influence on the grain size of in situ TiCx. However, the heating rate and the dwelling time have an impact on the distribution of TiCx in the matrix. The microstructure and phase composition of the prepared materials were characterized by scanning electron microscopy, transmission electron microscopy and X-ray diffraction. The nucleation and growth mechanism of TiCx in the composites has been discussed.

Introduction

TiC has been successfully used as an effective reinforcing phase to enhance the performance of metal and ceramic matrix composites due to its high melting temperature, high hardness, high chemical resistance, thermal stability and relatively low density. Such composites can be fabricated through traditional ex situ approach in which the TiC reinforcement is directly added into matrix materials and in situ approach in which TiC is formed by controlled reactions. However, the traditionally ex situ processed composites often accompany residual porosity, large grain size, inhomogeneous distribution of reinforcement, poor wetting of the reinforcement, weak reinforcement/matrix bonding interface, and undesirable interfacial reaction product. Compared to the traditional ex situ process, the in situ process exhibits considerable advantages [[1], [2], [3], [4], [5]]. For example, the in situ formed TiC reinforcement is finer in size; the in situ formed TiC has good wettability with matrix, contributing to its homogeneous distribution; the in situ formed TiC has a clean interface with matrix, resulting in a strong interfacial bonding. The above mentioned advantages endow the in situ composites with improved properties both at room temperature and high temperatures.

A series of approaches have been adopted for the in situ formation of TiC in the composites. These approaches include solid-gas reactions between Ti and CH4, Ti and C3H6 [6,7], solid-solid reactions between Ti and C [[8], [9], [10], [11], [12], [13]], TiO2 and C [14,15], B4C and Ti [16], solid-liquid salt reactions between C and K2TiF6 [17,18], decomposition reactions of MAX phases (M refers to a transition metal; A is a group IIIA or IVA element; X is C) such as Ti3AlC2, Ti3SiC2, Ti2AlC [[19], [20], [21], [22], [23], [24], [25]].

Among the above in situ approaches, the decomposition of MAX phases yields a C-deficient (or non-stoichiometric) TiCx (the theoretical values of x are 0.5, 0.67, and 0.75 from the decomposition of Ti2AlC, Ti3AlC2, and Ti4SiC3, respectively). The in situ formed C-deficient TiCx has better wettability with metal matrix than the in situ formed stoichiometric TiC, yielding a more uniform distribution microstructure and stronger bonding interfaces. It has been proved that the wettability between TiCx and copper increases as the value of x in TiCx deceases [[26], [27], [28]].

High chromium white irons (HCWIs), as engineering materials, have been used in the mining and minerals industries due to their good wear resistance. However, the good wear resistance of these materials is accompanied with poor toughness which restricts their use. Hence, HCWI composites reinforced with in situ formed ceramic particles have been prepared to alter the microstructure and, hence, toughness. Our previous work demonstrated that the in situ formed TiCx from the decomposition of Ti3AlC2 not only effectively refined the microstructure, but also greatly improved the toughness and wear resistance of HCWI composites [24,25]. In the TiCx/HCWI in situ composites, the good wettability of TiCx with matrix yields the homogeneous distribution of TiCx and the strong TiCx/matrix interfacial bonding. In addition, the in situ formed TiCx is round in shape and smaller in size, about 1–2 μm. Generally, the finer reinforcing particles provide better wear resistance.

During fabrication of in situ composites reinforced with TiC from the above mentioned solid-gas, solid-solid and solid-liquid reactions, factors including sintering temperature, dwelling time, heating rate, and the size of reactants exert a significant effect on the in situ particle size and morphology. For example, the mean particle size of in situ TiC increased with increasing sintering temperature and holding time [[29], [30], [31]]. The fine and coarse Ti precursor powders influenced the distribution and size of TiC in Cu matrix during self-propagating high temperature synthesis (SHS) of Cu-Ti-C system [32].

However, there is limited information on the influencing factors on the size and morphology of in situ TiCx from the decomposition of Ti3AlC2. From the industrial point of view, it is necessary to confirm the above factors for the easy fabrication and the low-cost of TiCx/HCWI composites with superior performance.

The main purpose of this work is to investigate the influence of the particle size of Ti3AlC2 precursor, heating rate and dwelling time on the formation of in situ TiCx and the microstructure of composite, and to discuss the nucleation and growth mechanism of TiCx.

Section snippets

Experimental procedures

The fabrication of Ti3AlC2 precursor powder and the high chromium white iron composites reinforced with in situ formed TiCx has been described in detail elsewhere [25]. Briefly, Ti3AlC2 was prepared by pressureless sintering of Ti, Al and C powders with a molar ratio of Ti:Al:C = 3:1.1:2 at 1450 °C for 1 h in Ar. The sintered Ti3AlC2 green bodies were pulverized and then sifted with sieves. The pulverized Ti3AlC2 powder was separated into four groups with various particle sizes of 38–75 μm

Precursor powder characterization

SEM micrographs of Ti3AlC2 precursor powders with various sizes are shown in Fig. 1. Fig. 1a shows that the large particles with a size of 38–75 μm (Group 1) are composed of aggregated Ti3AlC2 grains. These Ti3AlC2 grains are less than 30 μm in size. In contrast, the Ti3AlC2 precursor powders in Groups 2–4 are similar and plate-like in shape. Their sizes decreased and the amount of aggregated particles was significantly reduced after passing through the finer meshes. (Fig. 1b–d).

Particle size of Ti3AlC2 powder

After sintering

Discussion

Fig. 9 illustrates the in situ formation mechanism of TiCx. Our previous work showed that the decomposition of Ti3AlC2 into TiCx in HCWI occurred at above 1000 °C [24]. At high sintering temperatures above 1000 °C, the out-diffusion of Al from the layered Ti3AlC2 (Fig. 9a) leads to the formation of a Ti3C2 slab structure (Fig. 9a). After severe depletion of Al, the Ti3C2 slabs will relax, detwin and TiCx (x = 0.67) forms by C redistribution [33]. Hence, the formation TiCx from the decomposition

Conclusion

In situ formation of TiCx as a reinforcement is a promising route to fabricate high chromium white iron (HCWI) composites with high performance. Influencing factors including precursor powder size, heating rate and dwelling time on the formation of reinforcing particle and the microstructure of the composites have been determined. The in situ formed TiCx in the HCWI composite is about 1 μm. The initial Ti3AlC2 precursor size has a minor influence on the grain size of in situ TiCx. This result

Acknowledgement

This work was supported by the National Natural Science Foundation of China under Grant nos. 51772020 and 51372015, the Beijing Natural Science Foundation (2182058), and Beijing Government Funds for the Constructive Project of Central Universities.

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