The mechanical behavior dependence on the TiB whisker realignment during hot-working in titanium matrix composites

Abstract Low-cost TiB whiskers reinforced titanium matrix composite (TMCs) was fabricated with enhanced mechanical performances using in situ technologies and hot working. Morphologies observation indicates that needle-like TiB whiskers with a hexagonal transverse section grow along the [010] direction due to B27 crystal structure and its growth mechanism. Mechanical properties tests show that the mechanical behavior of the TiB whiskers reinforced TMCs is dependent on the deformation amplitudes applied in hot-working. The improvement in yield strength by hot-working is attributed to the TiB whiskers realignment and the refinement of microstructure. Models are constructed to evaluate the realignment of TiB whisker during deformation and the increase in yield strength of the composite at elevated temperatures. These models clarify the alignment effect of TiB whiskers under various deformation amplitudes applied in hot-workings and reveals the yield strength dependence on TiB whiskers orientation. Introduction Titanium matrix composites (TMCs) present high strength at room and moderately elevated temperature. Ceramic reinforcements in TMCs can improve the modulus, strength and creep resistance of the matrix significantly1,2,3. Many reinforcements have been used in TMCs, such as TiB, TiC, SiC, Al2O3 and TiB2. TiB ceramic is regarded as a powerful reinforcement because it presents excellent chemical stability and thermal stability in the Ti matrix4,5,6,7,8. Among the various designs of TMCs, the in situ synthesis techniques, in which reinforcements are formed in situ in matrix through reactions between reactants with matrix, have been paid great attention due to the low interface contamination and incompatibility between the matrix and the reinforcements9,10,11,12,13,14,15. The in situ processing techniques have evolved over last decades for the preparation of ceramic phase reinforced composites, including self-propagating high-temperature synthesis, mechanical alloying, reactive squeeze casting, exothermic dispersion technology, reactive hot pressing process, reactive spontaneous infiltration, direct reaction synthesis, combustion assisted cast and directed melt oxidation process16,17,18,19,20,21,22,23,24,25,26. But cost of most preparation methods of TMCs is high. On the other hand, mechanical properties of the TMCs are not improved significantly by the just addition of reinforcement simply. The facile processing is under challenge for the TMCs fabrication with high mechanical properties. The mechanical behavior of TMCs is different from those of matrix alloy, but it depends on the matrix, the reinforcement, reinforcement/matrix interfaces and processing technology. In recent years, numerous studies have been performed on hot-working to process TMCs based on the traditional hot-working technology of Ti alloys, such as rolling, extrusion and forging. These studies indicate that mechanical properties of the composite are great dependent on the microstructure of matrix, inclusions and second phases, and these factors can be controlled by hot-working27,28,29,30,31,32. Furthermore, Gorsse et al.33 found that the composite with aligned TiB whiskers along tensile direction presented higher strengths compared to that in which the orientation of TiB whiskers is random. Guo34 studied the evolution of TiB whiskers orientation in the composite during rolling suggesting a model to predict the orientation of TiB whiskers with deformation amplitude. However, the effect of TiB orientation on the mechanical behavior of TMCs is not clear enough, and the model between them has not been set up by present studies. And these studies may be very useful in the optimizing mechanical behavior of such composites. The aim of this work is to study the effect of deformation amplitude applied in hot-working on TiB whiskers orientation and the mechanical behavior dependent on the TiB whisker realignment. In this work, mechanical behaviors of the TMC with various deformation amplitudes applied in hot-working is characterized and analyzed. It was clarified that the improvement in yield strength by hot-working results from the TiB whiskers realignment and the refinement of microstructure. And a model is constructed to evaluate the yield strength of TMCs reinforced with whiskers based on the dependence of the inclinations of whiskers and grain size of matrix. Results and Discussion TiB whisker morphology and formation mechanism Figure 1(a) shows the TiB distribution in the cast composite and morphologies of TiB deep etched. The size of TiB needles diameter in this work are about 1 μm to 3 μm with an aspect ratio about 12.2 ± 4.3. Surface of TiB needles is clean and flat. TEM bright field image of TiB and corresponding selected area diffraction patterns are presented in Fig. 1(b), which indicates that needle-like TiB whisker is B27 orthorhombic structure and lattice parameters a = 0.612 nm, b = 0.306 nm and c = 0.456 nm. And the whis (...truncated)