The effect of ultra-fine grained structure on the phase transformations in metastable beta titanium alloysThursday (09.11.2017) 09:15 - 09:35 Part of:
Metastable beta titanium alloys are perspective materials for use in biomedicine but they are already used in aircraft and automobile industry due to its outstanding mechanical properties and biocompatibility. Severe plastic deformation (SPD) methods provide further opportunity to improve their properties. Nevertheless, the ultra-fine grained (UFG) microstructure achieved by SPD significantly affects the phase transformations occurring in metastable beta titanium alloys during heating.
Metastable beta titanium alloys are characterized by metastable body centered cubic (BCC) coarse grained (CG) β-phase after quenching the material to room temperature from temperatures above so-called beta-transus. Annealing of the material at higher temperatures (around 400-500 °C) leads to precipitation of the α-phase with hexagonal closed packed (HCP) symmetry. The α-phase particles preferentially form at grain boundaries. The SPD methods allow to achieve UFG microstructure by reducing grain size and inserting high amount of dislocations. Due to high density of grain boundaries (GB) in the UFG microstructure the α-phase particles are sub-micrometer range and equiaxed.
Two types of materials were used for this investigation: a simple binary Ti-15Mo alloy and Ti 15Mo and Ti-6.8Mo-4.5Fe-1.5Al (TIMETAL LCB) alloys. These materials were deformed by SPD methods: high pressure torsion (HPT) and equal-channel angular pressing (ECAP) after solution treatment. The difference in the phase transformation´s sequence of CG and deformed samples was revealed by in-situ differential scanning calorimetry and electrical resistance measurements. The microstructure of deformed and pre-annealed samples were investigated by ex situ scanning and transmission electron microscopy.