Research On Microstructure And Mechanical Properties Of TiC/Ti-6Al-4V Composites Fabricated By Melting-casting Process

TiC/Ti-6Al-4V composites were prepared by melting-casting process in this study. TiC/Ti-6Al-4V composites are reinforced with 5%, 10% and 15% volume fraction of TiC respectively. The morphology formation and growth mechanisms of TiC in the composites were investigated. The control and improving methods for TiC morphology were studied and discussed. The effect of heat treatment on microstructure and properties of the composites was analyzed. The tensile properties at both room and elevated temperature were investigated on the TiC/Ti-6Al-4V composites fabricated by induction melting, and then the strengthening and fracture mechanisms of TiC/Ti-6Al-4V composites were studied.Thermodynamic analyses and XRD results showed that TiC phase is stable in melted TiC/Ti-6Al-4V composites. The solidified TiC is mainly eutectic TiC in the composites when the fraction of TiC is small, while the proportion of primary dendritic TiC increases obviously with the fraction of TiC increasing. The composition for TiC showed that the solidified TiC is of carbon deficiency, of which the atomic ratio of C/Ti is close to 0.5. This leads to a decrease for the density of solidified TiC.The research on the growth morphology of TiC indicated that the solid-liquid interface of TiC on solidification is a kind of transition interface in melting-casting TiC/Ti-6Al-4V composites. Thus, the solidified TiC presents both faceted and non-faceted growth morphologies. There are TiC crystal nucleuses with large size in the melt, which could promote the two-dimensional nucleation growth by depositing on the solid-liquid interface of TiC on solidification. Therefore, some TiC dendrites exhibit the layer growth morphology due to the two-dimensional nucleation growth of TiC.TiC powder, graphite powder and carbon black powder with different size and status were used as the C sources of TiC in the TiC/Ti-6Al-4V composites. The studies of the effect of C sources on TiC morphology showed that the TiC crystal nucleus is smaller and more dispersed in the melt when fine carbon black powder was used as the C source. Moreover, the size of solidified TiC is comparatively smaller and the primary TiC is not easy to growth to developed dendrite with the C source of carbon black.The melting-casting TiC/Ti-6Al-4V composites were heat treated at different temperature, and the results showed that the eutectic and primary TiC could be of granulation by heat treatment. The C content in TiC increases and the layer growth morphology of TiC becomes unobvious after heat treatment. The granulation effect of TiC was found better when heat treating atβ-Ti+TiC phase region as the diffusion coefficient of C element is comparatively higher. There are Ti precipitations within TiC after different heat treatments, and the Ti precipitations exhibit long strip morphology with inner lamellar structure. The orientation relationship betweenα-Ti precipitation and TiC goes to [112₀]Ti//[01₁]TiC,(0001₎Ti//(111)TiC.The hardness tests for TiC/Ti-6Al-4V composites showed that the hardness and brittleness of TiC particles decrease after heat treatment. Meanwhile, the cracks are not easy to be generated in heat treated TiC particles. The compression results indicated that the compression strength and compression ratio of composites have been improved because of the weakening of TiC growth layer, Ti precipitation toughening on TiC and granulation of TiC after heat treatment.TiC/Ti-6Al-4V composites with small size of TiC were prepared by induction melting process. The investigation of room and elevated temperature tensile tests showed the strengths of TiC/Ti-6Al-4V composites reinforced different fraction of TiC are higher than the matrix alloy, but the elongations of the composites decrease as the additions of TiC. The tensile strengths of the composites at elevated temperature were increased obviously compared the matrix alloy because of the promotion of load transfer strengthening. The ultimate tensile strengths at 500oC of TiC/Ti-6Al-4V composites with 5%, 10% and 15% volume fraction of TiC were increased by 20.4%, 27.0% and 33.9%,respectively. The TiC/Ti-6Al-4V composites prepared by induction melting were heat treated for 8 hours at 1050oC and then air cooled. The tensile results of the heat treated composites showed that the strengths were increased slightly and the elongations were improved obviously. The elongations of TiC/Ti-6Al-4V composites with 5%, 10% and 15% volume fraction of TiC were increased by 16.3%, 19.7% and 38.9% respectively compared with the as-cast composites.The tensile facture surfaces at room temperature of induction melting TiC/Ti-6Al-4V composites are quasi-cleavage fracture surfaces which are composed of brittle cleavage surfaces of TiC and dimples of matrix. The generation of cracks in the composites during tensile deformation is closely related to the TiC particles. When eutectic TiC is the most of TiC phase in the composites, the cracks are mostly generated inside the fine particles gathering region because eutectic TiC particles are usually fine. When primary TiC is the most of TiC phase in the composites, the cracks are caused either by fracture of defective TiC particles, or by the voids at the interface between TiC particle and matrix.