In-situ studies of strain rate effects on phase transformation and microstructural evolution in metastable β titanium alloys

Abstract

Titanium (Ti) alloys are heavily utilized in the aerospace and defense industries for their high specific strength. Ti alloys are promising candidates for lightweight crash and blast resistant structural applications. Deformation mechanisms like TRansformation Induced Plasticity (TRIP) and TWinning Induced Plasticity (TWIP) simultaneously provide increased strength and ductility by engineering the work hardening response. Although TRIP and TWIP Ti alloys promise improvements in ductility and damage tolerance, fundamental understanding is still lacking, specifically with respect to dynamic mechanical response. In this work, we use a combination of in-situ/ex-situ experimental techniques to understand strain rate effects on TRIP, TRIP/TWIP, and TWIP Ti alloys as a function of alloying, initial microstructure, and processing. A novel strengthening strategy for TRIP Ti-10V-2Fe-3Al (wt.\%) (Ti-1023) is explored by low-temperature aging to coarsen athermal ω-phase precipitates, which allows for precise tuning of strength/ductility combinations in concert with TRIP. We find that artificial aging at 423 K causes a 4x increase in yield stress with no concurrent loss in ductility. Aging up to 7200 s at 423 K was found to inhibit TRIP activity completely. Instead, heterogeneous slip dominates, resulting in the formation of concentrated slip bands and reduced uniform elongation. In-situ synchrotron x-ray diffraction was also used to study the high strain rate deformation behavior of metastable Ti alloys. Combined TRIP/TWIP Ti-12Mo (wt.%) outperforms TWIP Ti-15Mo (wt.%) at high strain rates, due to the added contribution of local strain-relaxation from TRIP. Fine scale, post-mortem microstructure characterization revealed nanometer scale transformation product in Ti-12Mo caused by TRIP within the primary twins. This contrasts strongly with Ti-15Mo, where coarser microstructures contain heavily dislocated primary twins. Ti-15Mo was also found to exhibit reduced twinning activity at high strain rates compared to microstructures produced by quasi-static and intermediate strain rate testing. Coarser microstructures and reduced twinning activity ultimately caused reduced total elongation at high strain rates in Ti-15Mo compared to TRIP enabled Ti-12Mo. The role of low temperature aging on ω-phase precipitates and TRIP and high strain rate deformation were also studied in Ti-1023. Increasing aging times were found to provide strengthening at strain rates up to 2000 /s, but also result in reduced in elongations. Post-mortem electron backscatter diffraction (EBSD) revealed increasing aging time causes a reduction in the area fraction of martensite formed, which directly correlates to reduced elongation. Lastly, the groundwork has been performed to understand the physical parameters controlling dynamic lattice stability in BCC Ti. Density Functional Theory (DFT) modelling was performed to study the effects of local structural distortions caused by solute atoms in BCC β Ti. The dispersion relation in BCC Ti was found to be highly sensitive to local distortions in the lattice. These initial results provide insights into future modelling of β Ti alloys and the design of phase stability and deformation mechanisms.