Formation of ultrafine grain and mechanical properties in commercial pure titanium subjected to heavy-reduction thermomechanical processing around β transus temperature

Abstract

The purpose of this study was to investigate the flow stress behavior and formation of ultrafine grains (UFGs) by focusing on the effects of strain-induced dynamic transformation (SIDT) and dynamic recrystallization (DRX) under hot forming near the β transus temperature. Heavy-reduction thermomechanical controlled processing (TMCP) of commercial pure (CP) titanium was performed at deformation temperatures ranging from 700 to 1000 °C, a strain rate of 1 s−1, and a 70% height reduction, followed by water cooling immediately after compression. The flow stress at 0.2 strain decreased significantly from 96 to 25 MPa when the deformation temperature increased from 800 to 900 °C. Equiaxed UFGs with a grain size of 2–4 μm formed in this temperature range. The primary mechanisms of grain refinement in the CP titanium under heavy-reduction TMCP were DRX occurring at 800 °C and a combination of DRX and SIDT (β → α) at 900 °C. Of the 700, 900, and 1000 °C TMCP-treated specimens subjected to a 90° V-bending test, the yield force of the 700 °C TMCP-treated specimen was the highest. When the TMCP temperature was increased from 900 to 1000 °C, the yield force decreased significantly, whereas the springback amount increased. The α phase grain refinement achieved by the TMCP at 700 °C increased the force, and the retained β phases within the 1000 °C TMCP-treated specimen increased the springback amount.