Microstructural Considerations of a Multi-Pass Rolled Ti-Nb-Ta-Zr Alloy

The microstructural characteristic evolution was investigated during thermomechanical processing of Ti-29Nb-9Ta-10Zr (wt %) alloy, which consisted of, in a first stage, in a Multi-Pass Rolling with increasing thickness reduction of 20%, 40%, 60%, 80%, and 90%; in step two, the multi-pass rolled sample with the highest thickness reduction (90%) was subjected to a series of three variants of static short recrystallization and then to a final similar aging. The objective was to evaluate the microstructural features evolution during thermomechanical processing (phase's nature, morphology, dimensions, and crystallographic characteristics) and to find the optimal heat treatment variant for refinement of the alloy granulation until ultrafine/nanometric level for a promising combination of mechanical properties. The microstructural features were investigated by X-ray diffraction and SEM techniques through which the presence of two phases was recorded: the beta-Ti phase and the alpha ''-Ti martensitic phase. The corresponding cell parameters, dimensions of the coherent crystallite and the micro-deformations at the crystalline network level for both recorded phases were determined. The majority beta-Ti phase underwent a strong refinement during the Multi-Pass Rolling process until ultrafine/nano grain dimension (about 9.8 nm), with subsequent slow growing during recrystallization and aging treatments, hindered by the presence of sub-micron alpha ''-Ti phase dispersed inside beta-Ti grains. An analysis concerning the possible deformation mechanisms was performed.

Automated summary

The microstructural characteristic evolution during thermomechanical processing of Ti-29Nb-9Ta-10Zr alloy was investigated. The alloy underwent multiple rolling processes, and the microstructural evolution occurred with increasing thickness reduction. The experimental results showed that finer grain size could be achieved by adjusting the heat treatment conditions. Two phases were observed through X-ray diffraction and SEM techniques, and the parameters and dimensions of the crystallites, as well as the micro-deformations at the crystalline network level, were determined. The β-Ti phase underwent refinement during the rolling processes, resulting in ultrafine/nano grain size, but its growth rate was slow during recrystallization and aging treatments.