From single phase to dual-phase TRIP-TWIP titanium alloys: Design approach and properties

Aiming at increasing the yield strength of transformation and twinning induced plasticity (TRIP and TWIP) titanium alloys while maintaining a good work-hardening, a dual-phase alpha/beta alloy is designed and studied. The composition Ti - 7 Cr - 1.5 Sn (wt.%) is proposed, based on an approach coupling Calphad calculations and classical Bo-Md design tool used in Ti-alloys. Its microstructure is made of 20% of alpha precipitates in a beta matrix, the matrix having optimal Bo and Md parameters for deformation twinning and martensitic transformation. The alloy indeed displays a yield strength of 760 MPa, about 200 MPa above that of a Ti - 8.5Cr - 1.5Sn (wt.%) single beta phase TRIP/TWIP alloy, combined with good work-hardening and ductility. In situ synchrotron X ray diffraction and post-mortem electron back-scattered analyses are performed to characterize the deformation mechanisms. They evidence that the TRIP and TWIP mechanisms are successfully obtained in the material, validating the design strategy. The interaction of the precipitates with the {332}< 113 > beta twins is analyzed, evidencing that the precipitates are sheared when hit by a twin, and therefore do not hinder the propagation of the twins. The nature of the interaction is discussed, as well as the impact of the precipitates on the mechanical properties.