The internal stress in titanium deformed at low temperatures (T<0.33TM)

Microstructural Evolution in Ti-5111 Friction Stir Welds

“…The thermal conductivity of Ti-5111 is 7.5 W m À1 K À1 ; [57] which is much smaller than that of steel (~50 W m À1 K À1 ) or aluminum (~250 W m À1 K À1 ) alloys, resulting in a steep thermal gradient around the tool. The flow stress of a titanium alloys also decreases rapidly with increasing temperature, [58][59][60][61] with a significant decrease at the b transus temperature. [60,61] Coupled with the steep thermal gradient, this diminished flow stress limits the size of the TMAZ transitional zone between the b-transformed material near the tool and the undeformed a microstructure of the BP.…”

“…All of the new a laths in a prior b grain follow the Burgers OR with the parent b lattice orientation, and the a laths exhibit a gradual refinement as the coarser primary a is replaced by fine laths of secondary a. The steep thermal gradient produced by the poor thermal conductivity of titanium couples with the steep increase in yield stress with decreasing temperature [58][59][60][61] to limit the transfer of the shear deformation into the a phase, although sufficient strain does get transmitted across the b phase to effect some deformation in the partially transformed region. This deformation manifests as a gradual rotation of the primary a lattice orientations in the partially-transformed region.…”