Cryogenic Mechanical Properties of Warm Multi-Pass Caliber-Rolled Fine-Grained Titanium Alloys: Ti-6Al-4V (Normal and ELI Grades) and VT14

The effect of microstructural refinement and the beta phase fraction, V (beta), on the mechanical properties at cryogenic temperatures (up to 20 K) of two commercially important aerospace titanium alloys: Ti-6Al-4V (normal as well as extra low interstitial grades) and VT14 was examined. Multi-pass caliber rolling in the temperature range of 973 K to 1223 K (700 A degrees C to 950 A degrees C) was employed to refine the microstructure, as V (beta) was found to increase nonlinearly with the rolling temperature. Detailed microstructural characterization of the alloys after caliber rolling was carried out using optical microscopy (OM), scanning electron microscopy (SEM), electron back-scatter diffraction (EBSD), and transmission electron microscopy (TEM). Complete spheroidization of the primary alpha laths along with formation of bimodal microstructure occurred when the alloys are rolled at temperatures above 1123 K (850 A degrees C). For rolling temperatures less than 1123 K (850 A degrees C), complete fragmentation of the beta phase with limited spheroidization of alpha laths was observed. The microstructural refinement due to caliber rolling was found to significantly enhance the strength with no penalty on ductility both at room and cryogenic temperatures. This was attributed to a complex interplay between microstructural refinement and reduced transformed beta phase fraction. TEM suggests that the serrated stress-strain responses observed in the post-yield deformation regime of specimens tested at 20 K were due to the activation of tensile twins. (C) The Minerals, Metals & Materials Society and ASM International 2017