Strain rate effects on the mechanical behavior of porous titanium with different pore sizes

Highlights: • Two stages of rate sensitivity reported firstly for porous Ti. • Yield strength decreases with increasing pore size. • XDIC reveals rate- and pore-size-dependent deformation heterogeneity. • Increasingly homogeneous deformation leads to strain rate effect. • Stress concentration contributes to pore size effect. High strain-rate (up to 6600 s⁻¹) and quasi-static compression tests are conducted on a powder-sintered porous titanium with different pore sizes (mean: $30\mu m$ and $120\mu m$). In situ X-ray imaging is implemented to characterize the pores-scale deformation dynamics. The yield strength as a function of strain rate exhibits two stages of rate sensitivity, and the transition occurs at $\sim$1600 s⁻¹. X-ray images show that pore compaction and strain localizations occur preferentially at pores oriented perpendicular to the loading direction under quasi-static loading, but become more random under high strain-rate loading as a result of higher driving force and plastic deformation nucleation rate. The more homogeneous nucleation of plastic deformation contributes to the increased rate sensitivity beyond $\sim$1600 s⁻¹. At the same strain rate, the yield strength of porous Ti as well as strain field homogeneity decreases significantly with increasing pore size. The small pore spacing in fine-pored Ti reduces the degree of stress concentrations around pores. Therefore, the higher stress concentrations in coarse-pored Ti lead to an earlier yield of matrix around pores and thus a lower bulk yield strength.