Elsevier

Materials Science and Engineering: A

Volume 624, 29 January 2015, Pages 213-219
Materials Science and Engineering: A

High temperature deformation of α-Ti

https://doi.org/10.1016/j.msea.2014.11.080Get rights and content

Abstract

Alpha titanium was hot deformed in compression in vacuum at temperatures ranging from 700 to 900 °C and strain rates ranging from 0.003 to 1 s−1. From the flow stress behavior the strain rate sensitivity was determined and plotted as an iso-strain rate sensitivity contour map. This map showed two high strain rate sensitivity domains, one around a temperature range of 850–900 °C and strain rate range of 0.01–0.1 s−1 and the other around a temperature of 700 °C and strain rate of 0.003 s−1. The microstructure observed using electron backscatter diffraction showed discontinuous dynamic recrystallization in the first domain and possible recovery or geometric dynamic recrystallization in the second lower temperature domain. The microtexture development was also different in the two domains.

Introduction

Titanium and its alloys are extensively used as structural and bio-implant materials due to their attractive properties of high specific strength, good corrosion resistance and high biocompatibility [1]. It is known that the deformation of hexagonal closed pack α-Ti at ambient to warm temperatures is restricted due to the limited available active slip systems. Thus it is essential that thermo-mechanical processing of these alloys be carried out at temperatures greater than about 700 °C (which is about half its melting temperature) where multiple slip systems can be activated. There have been numerous studies on the deformation behavior of α-Ti ranging from cryo-temperatures to high temperatures [2], [3], [4], [5], [6], [7], [8], [9], [10]. However studies related specifically to deformation at temperatures more than half the melting point are few [2], [8], [11], [12], [13]. One such work is by Prasad et al. [2] where pure Ti was deformed in compression in a temperature range of 600–850 °C and strain rate range of 10−3–100 s−1. They obtained a peak strain rate sensitivity of 0.27 at 775 °C and 10−3 s−1 which they correlated with the microstructure to conclude the occurrence of dynamic recrystallization. Chao [13] also carried out similar hot deformation studies at 850 °C and 0.1 s−1 and correlated the microstructural changes at these conditions to dynamic recrystallization. The low to warm temperature deformation behavior of α-Ti studied by other workers have shown that significant twinning, which occurs at lower temperatures (and higher strain rates), affects the flow stress behavior; at higher temperatures (and lower strain rates) twinning is reduced [3], [4]. At present the strain rate sensitivities of α-Ti in the hot deformation zone, its deformation mechanism, the corresponding activation parameters and the links with microstructural evolution is still not well understood.

The present work studies the hot deformation behavior of α-Ti from 700 to 900 °C and strain rates from 0.002 to 1 s−1. The aim is to determine the regime of optimum workability of α-Ti, to obtain the activation parameters and to correlate these with the resultant microstructure.

Section snippets

Experimental

Titanium of grade II with composition of (C—0.1 wt%, Fe—0.3 wt%, H—0.015 wt%, N—0.03 wt%, O—0.025 wt%, balance Ti) was used in this work. Uniaxial compression tests were carried out on solid cylindrical samples of length 10 mm and diameter 5 mm at temperatures ranging from 700 to 900 °C in steps of 50 °C and strain rates of 0.003, 0.01, 0.1 and 1 s−1. As Ti is prone to oxidation at high temperatures all the tests were carried out in vacuum of 10−4 mbar using a deformation dilatometer. S type thermocouple

Flow stress behavior

Fig. 1 shows the observed flow stress behavior of α-Ti at various temperatures and strain rates. At the lower tested temperatures of 700 and 750 °C and at strain rates of 0.1 and 1 s−1 Ti showed a significant work-hardening behavior. At the lower strain rates of 0.01 and 0.003 s−1 Ti showed a steady state behavior. At higher temperatures of 800 and 850 °C the flow stress at the higher strain rate of 1 s−1 showed a work-hardening behavior. At strains rates of 0.1 s−1 and below the flow stress showed a

Conclusion

α-Ti hot deformed in the temperature range of 700–900 °C and strain rate range of 0.003–1 s−1 showed a high strain rate sensitivity domain of 0.3 around a temperature range of 850–900 °C and strain rate range of 0.01–0.1 s−1. Within this domain the microstructure showed dynamically recrystallized grains, which were refined as compared to the starting condition. The activation energy of 224 kJ/mol, a stress exponent near 4 and the activation volume in the range of 10 to 100 b3 suggested cross slip to

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