Growth Mechanism of Primary and Eutectic TiB 2 Particles in a Hypereutectic Steel Matrix Composite
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AN efficient way to reduce automobile emissions is weight reduction which has traditionally been achieved by using advanced high strength steels (AHSS), as higher strength enables the use of thinner components. However, thickness reduction can significantly reduce the stiffness of components.[1–3] For some components such as chassis parts, thickness reduction has now reached its limits due to the demands of high stiffness. Therefore, there is a strong need to develop new AHSS with a higher Young’s modulus, which can be achieved by embedding ceramic particles into the steel matrix, forming a steel matrix composite (SMC). It has been demonstrated recently that the SMC reinforced by TiB2 particles can be produced by eutectic solidification using conventional steel technology, which can ensure low cost and high productivity and therefore enables potential automotive applications.[1,4] Furthermore, the SMC has a lower density compared to conventional AHSS due to the addition of a high volume fraction of low-density TiB2. Compared with other metal matrix composites (MMC) produced by the powder metallurgy method, the SMC manufactured by eutectic solidification exhibits a much better ductility as the interfaces between the matrix and TiB2 particles are semi-coherent[5] and can undergo interfacial plasticity prior to interface debonding.[6] The strength of the SMC depends on the volume fraction and the size of particles.[7,8] However, increasing the volume fraction of TiB2 will inevitably result in a higher fraction of large primary particles, which leads
Z.C. LUOIS, B.B. HE, Y.Z. LI, and M.X. HUANG are with the Shenzhen Institute of Research and Innovation, the University of Hong Kong, Shenzhen, 518000, China, and also with the Department of Mechanical Engineering, The University of Hong Kong, Hong Kong. Contact e-mail: [email protected] Manuscript submitted September 15, 2016. METALLURGICAL AND MATERIALS TRANSACTIONS A
to significant stress concentration and facilitates nucleation of cracks and hence could be detrimental to the ductility and toughness.[9] Particle refinement, therefore, becomes a promising method to improve the strength of the composite. For such eutectic SMC, the particle size can be refined by controlling the solidification rates.[10] It is noted that an amorphous matrix material with supersaturated solute was obtained from fast solidification (~107 K/s). The fine dispersed nano-scaled precipitation of TiB2 particles was expected to form during the subsequent annealing. However, such fast solidification technique remains a challenge to existing steel industry and is not suitable for mass production at present. Another way to control the particle size is using alloying elements. Some elements, such as Mo, Mn, Al, and Ta, have demonstrated their capability to refine eutectic particles.[11] However, the mechanism for such refinement is still not clear. Normally, the anisotropic growth nature of ceramic phase in metal/ceramic eutectic solidification systems will make the phase diagram diverge greatly from equilibrium
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