Annealing behavior of severely-deformed titanium Grade 4

The static-annealing behavior and evolution of the microstructure-strength relationship of severely-deformed commercial-purity titanium Grade 4 over the temperature range of 50-850 degrees C (0.16-0.57 T-m , where T-m is the melting point) were established. The severely-deformed material was obtained via equal-channel angular pressing (ECAP) using the Conform (ECAP-C) technique at 200 degrees C to an effective accumulated true strain of 8.4. The resulting ultrafine structure was stable to 400 degrees C. The excellent thermal stability was concluded to be associated with a strain-aging effect, i.e., the enhanced diffusion of solutes within this temperature interval resulting in the formation of solute atmospheres at/near dislocations. At 450-500 'C, rapid growth of strain-free grains occurred, which eliminated the severely-deformed microstructure and promoted softening. This process was deduced to be controlled primarily by grain-boundary energy and therefore was interpreted primarily in terms of grain growth rather than discontinuous recrystallization expected in this temperature range. A further increase in annealing temperature to 600 degrees C led to normal grain growth. Analysis of the microstructure-strength relationship suggested a significant influence of mechanical twinning on yield strength of the fully-annealed material. At 600 degrees C and higher temperatures, dissolution of constituent iron-rich particles was observed. This promoted a partial a alpha -> beta transformation at the temperatures noticeably below the typical beta-transus of pure titanium (similar to 880 degrees C). This phenomenon resulted in the precipitation of nanoscale beta particles which imparted substantial strengthening. Water quenching of the material annealed at 850 degrees C gave rise to a beta -> alpha' martensitic transformation. The latter process was governed by exceptionally strong variant selection and thereby provided a nearly-ideal restoration of crystallographic orientations of parent alpha-grains.