Microscopic plastic response in a bulk nano-structured TiAl intermetallic compound processed by high-pressure torsion

Intermetallic compounds of titanium aluminides (Ti-Al) have attracted much attention for their excellent properties including light-weight, high specific strength and high-temperature creep strength. Nevertheless, bulk Ti-Al require suitable processing routes for improving the mechanical properties, especially plasticity and ductility. Accordingly, the present study applies a strategy of grain refinement through high-pressure torsion (HPT) processing which is an effective processing technique for difficult-to-work alloys due to the large compressive hydrostatic stresses that are developed during processing. A γ-based Ti-45Al-7.5Nb intermetallic compound was processed by HPT for 5 and 10 turns under 6.0 GPa at room temperature. The duplex microstructure was successfully refined to have ultrafine laths with thicknesses of 40-100 nm after 10 HPT turns. Improved hardness was recorded by both Vickers microhardness and nanoindentation measurements. The X-ray diffraction profiles confirmed the occurrence of phase transformation from γ-TiAl to α2-Ti3Al after HPT. The macroscopic plastic flow was estimated through the nanoindentation technique by applying both Berkovich and spherical indenters. The results predict an improved plastic behavior by increasing the strain rate sensitivity and a significant increase in yield strength whereas a slight decrease is measured in the strain hardening capability after HPT for 10 turns. Furthermore, this nanoindentation study demonstrates an alternative and promising technique of small-scale testing for predicting the macroscopic plastic flow behavior of ultrafine-grained materials.