Enhanced strength in pure Ti via design of alternating coarse- and fine-grain layers

The yield strength of metals is typically governed by the average grain size through the Hall-Petch relation, and it usually follows the rule of mixture (ROM) for layered composite materials. In the present study, an extraordinarily high yield strength, far beyond the predicted values from the Hall-Petch relation and the ROM, is achieved in a specially designed layered titanium that is characterized by alternating coarse- and fine-grain layers (C/F-Ti) where the grain sizes match the layer thicknesses in both the coarse- and fine-grained layers.

The strengthening mechanism of such a layered C/F-Ti is investigated based on detailed experimental characterizations, including nanoindentation tests of local hardness, in-situ synchrotron Laue X-ray microdiffraction (μXRD) measurement of the lattice strain distribution during tensile testing, and transmission electron microscopy (TEM) analysis of the dislocation structure after yielding.

In the coarse-grain layers, significantly higher hardness values are observed next to the layer interfaces compared to the layer center regions, and dislocations and densely populated pile-ups of dislocations are exclusively observed in the interface regions after yielding. These experimental results point to an enhanced interface constraint effect on the deformation mechanism in hexagonal close-packed (hcp) materials with a large difference in the critical resolved shear stress between slip and slip.

The C/F-Ti combines the strength of fine grains and the ductility of coarse grains, demonstrating a new structural design strategy for property optimization of single-phase hcp materials.