Abstract
The paper examined annealing behavior of ultrafine-grained Ti Grade 4. The ultrafine-grained microstructure was produced by equal-channel angular pressing (ECAP) technique by using a Conform scheme and was characterized by a mean grain size of d=0.3 μm and non-equilibrium grain boundaries. The ultrafine-grained structure was found to be stable up to 400°C. The excellent thermal stability was attributed to a strain-ageing, i.e., the enhanced diffusion of interstitial solutes resulting in a formation of solute atmospheres at/near grain boundaries and dislocations. At 450–500°C, a rapid growth of strain-free grains was observed to occur. This process eliminated severely-deformed microstructure and gave rise to abrupt material softening. A further increase of the annealing temperature above 600°С resulted in precipitation of lenticular dispersoids as well as iron-rich globular β-particles. This surprising phenomenon promoted a subtle hardening effect.
Highlights
Due to excellent biocompatibility characteristics, commercial-purity titanium (CP Ti) is often considered as a promising material for biomedical applications [1]
To comprehend the hardening effect, it is necessary to realize that interstitial solutes in CP Ti become sufficiently mobile at low annealing temperatures and can form atmospheres near dislocations [5]; solute segregation has been observed at grain boundaries [18]
A further increase in annealing temperature to 600°C (0.44 Tm) resulted in a gradual softening followed by a slight strength increase at higher temperatures
Summary
Due to excellent biocompatibility characteristics, commercial-purity titanium (CP Ti) is often considered as a promising material for biomedical applications [1]. A considerable strengthening of CP Ti achieved by a formation of ultrafine-grained (UFG) structure had recently provided a good perspective to essentially widen its practical use. In this context, examination of thermal stability of the UFG Ti is becoming of interest. The recrystallization is usually followed by a grain growth. This process has been reported to be governed by the normal mechanism and leads to minor changes in texture [12]. The present work attempted to shed some light to this unusual phenomenon
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