Strengthening Strategy for A Ductile Metastable B Titanium Alloy Using Low Temperature Aging

Abstract

Low temperature aging treatments, at around isothermal transition temperature (T iso ), were performed on a ductile metastable Ti-12Mo alloy displaying a combination of TRIP (Transformation induced Plasticity) and TWIP (Twinning induced Plasticity) effects. The main objective is to improve the yield strength of the material while keeping the advantage of combined stress induced phase transformation and mechanical twinning, as observed in the solution treated samples. Based on a resistometric investigation dedicated to low temperature phases transformations, a set of short-time LTA treatments were performed. It is shown that, for aging temperature situated around T iso , tensile strength of Ti-12Mo can be largely enhanced without losing ductility. A superior combination of high strength (close to 1200MPa in true stress) and a stable plasticity of =0.4 (in true strain) was achieved with a significant strain hardening effect. IntroductionOver the last few decades, titanium alloys have been attractive materials for aerospace, biomedical and sporting goods industries and automotive applications due to their high strengthto-density ratio [1] good hardenability [2] and good corrosion resistance [3]. However, the poor average ductility of titanium alloys remains as a drawback when compared to steels or Co-Cr family of alloys. Promising mechanical properties have been recently investigated by our group and co-workers [4, 5, 6] on a metastable alloy, namely Ti-12Mo (wt. %). The experimental results showed that this alloy possessed a combination of high ductility (with an engineering elongation of about 53 56%) and high strain hardening rate thanks to a complex combination of stress induced transformation and intense mechanical twinning (combined TRIP/TWIP effects). It is known that the mechanical performances of metastable alloys can be optimized through various thermo-mechanical strategies [7] involving grain size reduction or secondary phase precipitation. Our approach of the latter strengthening strategy is actually based on a low temperatures thermal treatment [8] around the onset of isothermal omega precipitation. In metastable Ti alloys, the level of chemical instability of matrix is sensitive in the activation of stress induced phase transformations and mechanical twinning [4]. Therefore, the double objective of the research is to promote precipitation strengthening without excessive modification of β matrix chemical composition and with local coherent elastic stress between the particles and matrix, in order to maintain the combined TRIP and TWIP effects. Nucleation rate of omega phase mainly depends on temperature, whereas growth depends on time: therefore the mechanical properties of titanium alloys change drastically by the phase precipitation after long time aging. In that case, the large increase in yielding stress is detrimentally accompanied by a drastic reduction in ductility. Consequently, in this work, the low temperature aging (LTA) treatments have been performed for very short durations (from 60s to 600...

Effect of thermal cycling on mechanical and microstructural properties of heat-treated Ti-6Al-4V alloy

“…Sun et al [11] reported that at low aging temperature (150 °C for 60 s,) the yield strength of the titanium alloys increased to 730 MPa from 480 MPa. The aging temperature above 200 °C, the alloy has a detrimental effect on the ductility.…”