Anisotropic surface stability of TiB 2 : A theoretical explanation for the easy grain coarsening
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Huimin Xiang and Fu-Zhi Dai Science and Technology on Advance Functional Composite Laboratory, Aerospace Research Institute of Materials & Processing Technology, Beijing 100076, China
Jiachen Liu Key Laboratory of Advanced Ceramics and Machining Technology of Ministry of Education, School of Materials Science and Engineering, Tianjin University, Tianjin 300072, China
Yanchun Zhoua) Science and Technology on Advance Functional Composite Laboratory, Aerospace Research Institute of Materials & Processing Technology, Beijing 100076, China (Received 15 February 2017; accepted 31 March 2017)
The exaggerated grain growth, anisotropic crystallite morphology, and thermal expansion are the main reasons for the microcracking of sintered TiB2, wherein grain coarsening and anisotropic crystallite morphology are believed to be controlled by the surface stabilities of TiB2. To deeply understand the grain growth mechanism, the anisotropic stability and bonding features of TiB2 surfaces, including ð1120Þ, two types of (0001), and three types of ð1010Þ, are investigated by first-principles calculations. By employing the two-region modeling method, surface energies are calculated and the ð1120Þ surface is found to be more stable than (0001) and ð1010Þ surfaces. Hexagonal plate-like grain morphology is predicted. The different bonding conditions of surface Ti and B atoms contribute to the difference of surface structure relaxation between surfaces with Ti- and B-termination, which lead the B-terminated ones to be more stable. It is also found that the surface energies of TiB2 are much higher than those of ZrB2 with a similar structure, which may be responsible for the easy coarsening of TiB2.
I. INTRODUCTION
In the last decades, transition metal diborides based ultrahigh temperature ceramics (UHTCs) have received wide attentions for the potential applications in extreme environments such as the nose tip and sharp leading edges of hypersonic vehicles and hot structure components for scramjet engines.1,2 They are also attractive for many other applications like cutting tools, impact resistant armor, wear resistant coating, and electrode materials owing to the combination of properties including high melting point, high hardness, high strength, electrical conductivity, absence of phase changes in the solid state, and exceptional corrosion and erosion resistance in many harsh environments.3–5 Among them, titanium diboride (TiB2) is particularly intriguing because of its low density, remarkable thermal, Contributing Editor: Sung-Yoon Chung a) Address all correspondence to this author. e-mail: [email protected], [email protected] DOI: 10.1557/jmr.2017.147
and chemical stability. Despite having these excellent properties, the broader applications of monolithic TiB2 are rather limited because of poor sinterability and grain coarsening at high temperatures.6 Due to the anisotropic thermal expansion of TiB2,7–9 localized residual stress arises primarily from the thermal expansion mismatch between individual grains during the cooling p
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