Effects of Grain Size, Thickness and Tensile Direction on Ductility of Pure Titanium Sheet

Abstract

The effects of the thickness (t), grain size (d), ratio t/d, and tensile direction on tensile properties were carefully examined using ASTM grade 1 pure titanium having a strong B texture with the controlled thickness (0.2–0.4 mm), grain size (20–175 µm), and t/d (2.3–19.4). 0.2%-proof stress followed a Hall-Petch relationship in each tensile direction regardless of thickness, and this indicates that 0.2%-proof stress does not depend on t/d. On the other hand, tensile strength and uniform elongation were confirmed to depend on t/d and significantly decreased in some thin sheets, compared to thick sheets with the same grain size, except for the case tested in the transverse direction. Namely, both tensile strength and uniform elongation exhibited a drastic decrease when t/d becomes smaller than the critical values. The critical value of t/d depended on tensile direction and decreased with an increasing tensile angle in the rolling direction. Local elongation increased with decreasing grain size except for the case tested in the transverse direction. In addition, local elongation was not related to t/d. Tensile properties that depend on t/d are inferred to be affected by work-hardening behavior, enhanced by twinning deformation in hcp titanium. However, the formation of deformation twinning in the surface regions was found to be suppressed, presumably due to the relaxed, constrained conditions. Consequently, the work-hardening rate decreased with decreasing t/d because of the attribution from the surface region where twinning deformation is retarded. This Paper was Originally Published in Japanese in J. Japan Inst. Met. Mater. 84 (2020) 227–236.