In this work, the microstructure and reinforcement mechanism of two Ti-based in situ eutectic composites-Ti60Cu14Ni12Sn4Nb10 and Ti65Fe35 alloys, both having high strength and high ductility simultaneously, are studied by using TEM, SEM and other related microstructure characterization and test methods.
It’s found that the Ti60Cu14Ni12Sn4Nb10 alloy has a non-equilibrium hypoeutectic structure of pseudo-binary Ti(Nb, Sn)-Cu(Ni) system, that is micrometer-sized dendriticbeta-Ti solid solution dispersed in a nano/ultrafine-structured eutectic matrix. The eutectic matrix is a rod-like beta-Ti solid solution embedded in a gamma-CuTi intermetallic compound, with lots of defects in the beta-Ti eutectic phase. Both the dendritic beta-Ti phase and eutectic beta-Ti phase are supersaturation.
The high strength of the alloy mainly comes from the nano/ultrafine-structured eutectic matrix due to grain refinement and defects strengthening, while the globular plasticity is attributed to the strong work hardening resulted from the confined deformation of ductile beta-Ti dendrites imposed by high strength matrix. The deformation mechanism is not shear bands but dislocation sliding. The in situ composite structure of the micrometer-sized ductile dendrites plus nano/ultrafine eutectic matrix leads to the high strength and high ductility simultaneously.
The Ti65Fe35 alloy has a non-equilibrium hypereutectic structure. Micrometer-sized FeTi dendrites disperse uniformly in FeTi/beta-Ti eutectic matrix. In eutectic, FeTi rods embed in supersaturated beta-Ti with high density of dislocations.
During the deformation, the FeTi/beta-Ti eutectic matrix underwent extensive deformation while the FeTi dendrite deformed little. There is no slip transfer between FeTi dendrite and beta-Ti, slip transfer occurs between the FeTi rods and beta-Ti. The different deformation behavior between FeTi dendrite and FeTi eutectic rods is attributed to the difference of orientational relationship and ordering energy. The deformation and micro-cracks in eutectic matrix was confined by the FeTi dendrites, leading to pronounced work hardening, hence to high ductility. The high strength of the alloy mainly comes from eutectic matrix due to dislocation strengthening and composite strengthening.
It is found that the multi-component recipe and grain refinement is just one of the methods, but not necessary, to achieve high strength and high ductility. The composite structure with a supersaturated matrix decorated with high density of defects is the key to obtain such property. A strategy to produce in situ eutectic composite structure with potential property through casting is suggested. That is: (1) Design the alloy composition to be at or near deep eutectics and the phases in eutectic should be solid solution phase or intermetallic with a range of compostion, while the “ line intermetallic ” should be avoided by adding more components. (2) Select the hypoeutectic or the hypereutectic compositions, based on the ductile or brittle nature of the eutectic matrix, to achieve an in situ eutectic composite with a proper primary phase. It’s very meaningful to select eutectic system with potential property easily from so many alloy systems. By using this strategy, a basic in situ eutectic composite can be provided for further development.
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