Mechanical behavior of microstructure engineered multi-length-scale titanium over a wide range of strain rates

Abstract

In this work, we studied the mechanical behavior of commercial purity Ti powder consolidates with an engineered microstructure containing multiple-length-scale features, over a wide range of loading rates. The microstructural length scales were engineered by mixing powders of different sizes, followed by either hot quasi-isostatic forging (QIF) or spark plasma sintering (SPS). We used electron backscatter diffraction and transmission electron microscopy to examine the microstructure of the Ti materials. A bimodal grain size distribution has been achieved for the majority of the QIFed samples, while those consolidated via SPS exhibit a near-equiaxed morphology. All samples synthesized with powders milled in liquid argon show considerable uniform plastic deformation under quasi-static compression, with no failure, and their strength values are considerably high when compared to those of commercial purity Ti. Moreover, the materials consolidated from milled powders exhibit adiabatic shear banding under high rate uniaxial compression via the Kolsky bar technique. Samples prepared from a preselected proportion of powders milled in liquid nitrogen showed quasi-static strength as high as 2000MPa, and dynamic peak stress as high as 2700MPa, comparable to the strength of high-strength steels. However, these super-strong Ti samples are brittle under both quasi-static and dynamic compression. The strengthening of these Ti materials with an engineered microstructure is primarily attributed to the presence of interstitials. Twins were observed in nanometer-sized grains in the strongest and brittle samples, along with evidence of the TiN phase, which was attributed to exposure to a high level of nitrogen introduced during milling in liquid nitrogen. The high rate behavior can be rationalized on the basis of an adiabatic shear band model that takes into account strain and strain rate hardening.