Effect of microstructure on dynamic failure resistance of titanium diboride/alumina ceramics

The dynamic compressive strength and microscopic failure behavior of TiB2/Al2O3 ceramic composites with a range of microstructural morphologies and size scales are analyzed. A split Hopkinson pressure bar is used to achieve loading rates of the order of 400 s(-1). The time-resolved analysis of the mechanical response is conducted with a resolution of 500 us. The dynamic compressive strengths of the materials are 4.3-5.3 GPa, indicating a dependence of strength on microstructure. Microstructures with finer phases as measured by linear intercept length have higher strength levels. The dynamic strength levels are similar to27% higher than the values of 3-4 GPa measured at quasi-static loading rates for these materials. These strength levels are also higher than the strength levels of monolithic TiB2 and Al2O3 under similar dynamic conditions. A soft recovery mechanism in the experimental configuration allows the specimens to be subjected to loading under a single, well-defined stress pulse. Scanning electron microscopy and energy dispersive spectrometry indicate that failure associated with the Al2O3 phase is transgranular cleavage in all microstructures. On the other hand, failure associated with the TiB2 phase is a combination of transgranular cleavage and intergranular debonding and varies with the microstructures. Quantitative image analysis shows that the measured compressive strength of the materials directly correlates with the fraction of TiB2-rich areas on fracture surfaces. The size distribution of the fragments is quantified using digital image analysis. Comparisons of the measured distributions with the predictions of several theories suggest that the lack of accounting for microstructural characteristics contributes to the inaccuracies of the models.