Microhardness and microstructure evolution of ultra-fine grained Ti-15Mo and TIMETAL LCB alloys prepared by high pressure torsion

Abstract

Two metastable β-Ti alloys, Ti-15Mo and Ti-6.8Mo-4.5Fe-1.5Al (TIMETAL LCB) were solution treated and subjected to severe plastic deformation by high pressure torsion. The evolution of microhardness, microstructure and elastic constants with increasing strain imposed by high pressure torsion was investigated.Fragmentation of the microstructure with increasing strain was observed by scanning electron microscopy. Significant twinning in system {1 1 2} <111> after high pressure torsion was observed in both studied alloys by electron backscatter diffraction. Multiple twinning contributes significantly to the fragmentation of grains and consequently to the overall refinement of the microstructure.Microhardness significantly increases with increasing strain and was fitted using the Hollomon and Voce laws. Hollomon's hardenability exponent is much higher for both studied β-Ti alloys than for the commonly used Ti-6Al-4V alloy. It reflects high capability of strengthening β-Ti alloys by intensive plastic deformation.The measurement of elastic constants using resonant ultrasound spectroscopy showed that the deformation by high pressure torsion increases the Young's modulus as compared to solution treated material. On the other hand, further straining causes subsequent decrease of the Young's modulus.