<p>With a significantly increasing demand for miniaturized titanium products, micro forming is a promising approach with high productivity to fabricate micro parts efficiently. When the sheet thickness is scaled down to a micro-scale, the fracture behavior in micro-scale plastic deformation is quite different from the conventional one due to the size effect. To develop the application of micro forming, it is thus critical to study the size effect on fracture behavior of titanium sheets in micro-scale plastic deformation. In this research, commercially-pure titanium (CP-Ti) sheet metals with a thickness of 0.1mm were first annealed at different temperatures and durations to obtain various grain sizes. The influence of grain size on the fracture behavior of CP-Ti sheets was investigated by the uniaxial tensile tests combined with a DIC measurement system as well as the scanning electron microscopic (SEM) observations. The ductile-to-brittle transformation of fracture behavior with the grain size increasing from 33.07 to 107.70μm was revealed. On the one hand, the macroscopic elongation and critical fracture stress of CP-Ti samples decrease with the increase of grain size. On the other hand, the number of dimples in the fracture surface decreases with the increase of grain size, while the cleavage planes and river patterns gradually become more dominating suggesting the transformation of fracture behavior from ductile to brittle. To explore the fracture mechanism, the microstructure of deformed samples is observed. The twinning activity decreases with the grain size decreasing, while the same twinning volume fraction around 0.38 at fracture is determined. The dislocation motion is affected by twinning in various grain size conditions leading to different fracture strains. Moreover, the dislocation of various grain sizes in deformed samples is further observed by a transmission electron microscope (TEM). The dislocation emission from crack-tips was revealed at different grain sizes. The dislocations obstructed by grain boundaries result in a high dislocation density at the crack-tip in fine-grain samples with a grain size of 33.07μm. Those intense dislocations blunt crack tips to reduce stress at the crack tip and prevent crack propagation. In contrast, the dislocations generated at the crack tip prefer to move away in the sample with coarse grains. Thus, the dislocation density decreases strongly with the increase of grain size leading to high crack-tip effective stress and less crack plasticity. Hence cleavage fracture was dominated in coarse grain CP-Ti sheets. Variation shielding effect strength leading to different crack plasticity is the reason for the ductile-to-brittle fracture of CP-Ti sheets with various grain sizes.</p>

Ductile-to-Brittle Fracture Size Effect of Titanium Sheets in Micro/Meso-Scale Plastic Deformation

Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2021/volume/85543

ASME 2021 International Mechanical Engineering Congress and Exposition (IMECE2021)

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