| As ideal substitute materials for titanium alloys, titanium matrix composites(TMCs) have a good prospect. It is well known that there are several problems, such as high room temperature brittleness of the prepared TMCs and poor production efficiency, for preparing TMCs by powder metallurgy. Facing with these problems, the present study successfully prepared a TiBw/Ti-6Al-4V composite with a TiB columnar reinforced structure by a low energy ball milling + pre-sintering + canned hot extrusion process, and the results showed that not only the comprehensive mechanical properties of the TMCs were modified, but the production efficiency was greatly improved. Subsequently, the microstructure evolution of the composites during the fabrication process and the effect of TiB content on the microstructure, texture, room and high temperature mechanical properties of the composites were studied.The results showed that the densification of the composite and the effective synthesis of short rod-like TiB whiskers could not be achieved during the pre-sintering process due to the pressureless sintering condition and the concentrated distribution of TiB2 powders. Moreover, it was also found that only when the sintering temperature higher than 1150°C, the sintering necks as well as the short rod-like TiB whiskers began to form. During the hot extrusion process, not only the densification of the composite and the rapid growth of TiB whiskers were observed, but a fiber texture of <0001>α//ED and <10-11>α//ED was formed in the matrix alloy. Along with the increase of deformation degree, the TiB network reinforced structure was elongated and the TiB whiskers gradually aligned along the extrusion direction, leading to the formation of a TiB columnar reinforced structure. The inhomogeneous distribution feature of TiB and the refinement effect of TiB on the prior β grain leaded to a significant prior β grain size difference between the TiB-rich reigion and the TiB-poor reigion. Tensile test results showed that the 2.6vol.%TiBw/Ti-6Al-4V composite exhibited excellent room temperature comprehensive mechanical properties with an ultimate tensile strength of ~1274Mpa and an elongation to failure of ~9.4%.With the increasing of TiB content, not only the α colony was significantly refined, but the thick grain boundary α lath gradually disappeared and the α lath showed a spheroidization trend. The results revealed that it was the TiB whiskers contributed to the refinement of prior β grain and provided more nucleation sites for α phase that contributed to the refinement of the matrix. The oxygen element aggregation effect of TiB whiskers and the destruction of Burges orientation relationship between α and β grains by TiB whiskers leaded to the disappearing of thick grain boundary α lath and the spheroidization of α lath. Furthermore, it was found that the reduction of texture intensity with the increase of TiB content could weaken the strengthening effect of <0001>α//ED, <10-11>α//ED fiber texture on the composites in extrusion direction. Room temperature tensile test results showed that the yeild strength and ultimate strength of the composites increased linearly with the increasing of the TiB volume fraction, indicating that the TiB whiskers played an effective strengthening effect. The analysis results demonstrated that the improvement in strength of the composites not only owing to the shear-lag strengthening effect of TiB whiskers and the fine crystal strengthening effect of the matrix alloy, but the dispersion strengthening effect of TiB whiskers. The TiB columnar reinforced structure could make the most of the coordinate deformation ability of the matrix alloy and retard the propagation of cracks, which could contribute to an excellent plasticity of the composite. However, when the TiB content reach to a high value, a sharp decrease in plasticity of the composites could also be observed. Tensile test results at different temperatures revealed that the strengthening effect of TiB whiskers was weakened significantly with the increasing of temperature, and almost no strengthening effect of TiB whiskers was retained at 700°C. It was found that that the weakening of TiB whiskers strengthening effect was the result of the softening of the matrix alloy and the debonding of the interface between TiB and the matrix alloy. The increasing of TiB content could deteriorate the plasticity of the composites when the temperature lower than 600°C, however, an opposite phenomenon was observed. In view of the refinement and spheroidization of the grains in matrix alloy, it was believed that the strengthened grain boundary silde effect with the increasing of TiB content contributed to the improvement in plasticity of the composites at 700°C. |
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