Microstructural characterization of a V2C and V8C7 ceramic-reinforced Fe substrate surface compound layer by EBSD and TEM
Graphical abstract
Introduction
Hard compound coatings are a key material by which the surface of substrate materials can be strengthened for applications in the automotive industry, mining and metallurgy [[1], [2], [3]]. Generally, transition metal carbides are applied as reinforcements in compound coatings. In particular, vanadium carbides as the hard phase are usually applied to produce many structural materials, such as coatings [[4], [5], [6], [7]], hard alloys [[8], [9], [10]] and metal matrix composites [[11], [12], [13], [14], [15]], due to their high melting point and hardness as well as good resistance to wear and corrosion [1,3,4]. Additionally, according to the V-C binary phase diagram [16], the presence of metastable phases and polymorphism, i.e., V2C, V4C3, V6C5, V8C7 and VC, is a complex scientific phenomenon [17,18]. The crystallographic data of these carbides are displayed in Table 1. It can be seen that vanadium carbides exhibit multiple phases and various crystalline structures, which has attracted the interest of a number of researchers. In recent years, a wide range of processes have been utilized to produce vanadium carbides with the different phase types, and then, the formation mechanisms, physical and chemical compositions and mechanical behaviours of these vanadium carbides have been intensively investigated [[18], [19], [20], [21], [22], [23], [24], [25]]. Although a number of researchers made significant contributions to the development of the aforementioned carbides, several questions still remain, and further studies are needed.
Variations in the phase type, chemical composition, size and morphology of vanadium carbides are always related to the mechanical properties. The previously reported works [17,19,26] revealed that the strength (hardness) and stiffness (modulus) were gradually enhanced when the ratio of carbon to vanadium (C/V) was increased. Among the vanadium carbides, V2C possesses a minimum C/V ratio of 0.5 and exhibits the poorest mechanical behaviours. In contrast, VC has a high C/V ratio of 1.0 and exhibits relatively good mechanical properties [17,26]. Furthermore, V8C7 not only has a relatively high C/V ratio of 0.875 but also is more thermodynamically stable than the VC phase [1]. Therefore, V8C7 is expected to be obtained as a reinforcement phase for application in structural materials [3,20,27].
In a previous study conducted by X.S. Fan et al. [4], vanadium carbide coatings that consisted of V6C5 and a disordered state of VCx phases (x = 0.83–0.88) on AISI H13 steel were produced by a thermo-reactive deposition/diffusion (TRD) process, and the microstructure and growth kinetics were investigated. P. Stoyanov et al. [7] reported the influence of the C/V ratio on the microstructure, mechanical properties, and tribological behaviour of three different vanadium carbide/amorphous carbon nanocomposite coatings in the evaluation of the reliability and limitations of the coatings. E. Ghasali et al. [11] successfully prepared a VC-reinforced aluminium matrix composite by spark plasma sintering (SPS) and reported the effect of the sintering method on the structure and mechanical properties. Additionally, the morphology and formation mechanism of V8C7 powders synthesized by different processes were evaluated in several investigations [2,3,20,24,27]. In our previous studies [12,28], vanadium carbide ceramic particle-reinforced iron matrix composites were successfully produced through an inexpensive and promising in situ reaction (ISR), and their microstructures and tribological properties were investigated by traditional scanning electron microscopy (SEM). Unfortunately, the phase distribution, growth orientation and three-dimensional morphology of the vanadium carbide particles have not been clearly revealed.
In the present study, a surface vanadium carbide compound layer containing V2C and V8C7 phases with a thickness of tens of microns was formed by the ISR technique at a relatively lower temperature than that of our previous study. To clearly characterize the microstructure of the compound layer, such as the chemical composition, phase composition, phase distribution and grain morphology, a combination of SEM, transmission electron microscopy (TEM) and electron backscatter diffraction (EBSD) techniques was implemented. In the previously works [24,[29], [30], [31]], it was proven that the advanced EBSD technique as a useful tool can be used to investigate the polycrystalline materials, especially V-C system materials. Here, the EBSD technique was applied to the analysis of the distribution, size and orientation of the V2C and V8C7 grains in the compound layer. The acquired results provide a deeper understanding of the phase distribution of vanadium carbide in the compound layer and the growth orientation of V2C and V8C7 grains.
Section snippets
Preparation of the vanadium carbide compound layer
The vanadium carbide compound layer was produced on a cast iron (Fe) substrate by a controlled-diffusion reaction during an isothermal annealing process. The production procedure of the sample was similar to that introduced in a previous work [32]. A pure vanadium plate (99.9%) and the Fe substrate were used for preparation of the materials. The chemical composition of the substrate was 0.045 S, 0.077 P, 3.210 C, 0.014 Cu, 0.240 Cr, 1.050 Mn, 1.320 Si, and 94.044 Fe, in w (%). The aim of the
Microstructure and phase composition of the vanadium carbide compound layer
The phase composition of the cemented vanadium carbide layer produced during isothermal annealing at 1100 °C for 60 min was determined by XRD and EBSD. Fig. 1a shows that the XRD pattern of the specimen cross-section mainly includes V, α-Fe, Fe3C, V2C and V8C7 phases. A scanning region of XRD consists of the entire cemented carbide layer and a portion of the vanadium plate and Fe substrate. Hence, the diffraction peaks of the vanadium carbides at 2, corresponding to the different reflections,
Conclusions
A vanadium carbide ceramic-reinforced Fe substrate surface compound layer was produced via a general annealing process at 1100 °C for 60 min, and its microstructure was characterized. The microstructure of the compound layer consisted of four distinct zones with considerable differences in the phase distribution and chemical composition. Although the phase composition of the compound layer contained V2C, V8C7, Fe3C, V and α-Fe phases, the V2C and V8C7 phases were dominant.
The V2C grains with a
Acknowledgements
The authors are grateful for the support of the Doctoral Innovation Fund (No. 310-252071703), the Projects of Shaanxi Provincial Key Research (Nos. 2017ZDXM-GY-032 and 2017ZDXM-GY-043), and the Key Laboratory of Shaanxi Provincial Education Department of Scientific Research Project (No. 15JS055).
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