Effect of grain size on the ductile-brittle fracture behavior of commercially pure titanium sheet metals

Abstract

Fabrication of miniaturized titanium thin-walled products directly using sheet metals is proven to be a promising approach with high productivity. In this process, however, there are many unknowns in terms of size effect and its affected fracture behavior. This study is thus aimed at investigating the effect of grain size on the fracture behavior of commercially pure titanium (CP–Ti) sheets with a thickness of 0.1 mm. Tensile tests combined with a digital image correlation (DIC) measurement system were conducted to determine the critical fracture stress of the specimens with different grain sizes. The fracture morphologies of the stretched specimens were further characterized by a scanning electron microscope (SEM). According to the experimental observation, an evident transition from ductile to brittle fracture as the grain size increases from 33.1 to 107.7 μm is revealed. Macroscopically, the elongation and critical fracture stress decrease with the increase of grain size. While according to the microscopic observations, the number of dimples decreases with the increase of grain size. Meanwhile the cleavage planes and river patterns gradually appear in the coarse grain fracture surface. To explore the fracture mechanism, transmission electron microscope (TEM) observations were made utilizing the specimens with fine and coarse grains. Significant dislocation emission from crack-tips was revealed at different grain sizes. Moreover, evident dislocation pile-up at grain boundaries was observed in the specimen with a grain size of 33.1 μm. Those intense dislocations reduce the effective stress at the crack tip resulting in higher crack propagation resistance. Nevertheless, the dislocation density at crack-tip decreases strongly with the increase of grain size leading to high crack-tip effective stress and less crack plasticity. Thus cleavage fracture was dominated in coarse grain CP-Ti sheets. A dislocation shielding model was developed to capture the dislocation observation in various grain sizes. The critical stress onset of CP-Ti sheets fracture during the tensile test was also predicted for validation.