Microstructure Characterisation of cp-Ti and Metastable β Titanium Alloy Ti-15Mo Processed by Rotational Constrained Bending

The commercially pure Ti (grade 4) and Ti-15Mo alloy in a metastable β solution treated condition were successfully processed by a novel severe plastic deformation technique referred to as rotational constrained bending (RCB) at the temperature of 300 °C. This technique introduces a bending deformation to the sample in the form of a bar, by its passing through the special constrained bending die. The investigation of the influence of the increasing number of passes on the microstructure homogenisation, texture and dislocation density evolution was followed by precise microhardness mapping and XRD. Microstructural observations in the central part of the billet cross-section were performed by the combination of electron backscatter diffraction and a special technique of automated crystal orientation mapping in TEM. The repetitive RCB processing leads to the gradual microstructure homogenisation and grain refinement in both cp-Ti and Ti-15Mo samples. The heterogeneous distribution of microhardness observed after a single pass was gradually transformed to almost uniform one in the whole cross-section after 10 and 6 passes in cp-Ti and Ti-15Mo alloy, respectively. The initial grain size of both samples decreased below 1 μm. Moreover, RCB processing significantly increases both the microhardness and the strength of both materials. The contribution of the grain size, the dislocation density and the texture to the room temperature proof stress is discussed in detail.