Abstract
Fine-tuning of stress-induced martensitic (SIM) transformation was studied in Ti-Mo based β metastable alloys, showing combined Transformation Induced Plasticity (TRIP) and Twinning Induced Plasticity effects (TWIP) effects. The work aimed to clarify the transition and interaction between the two deformation mechanisms and their influences on the mechanical properties of Ti-Mo based alloys. Electron parameter design methods (Bo-Md and e/a ratio) were cross-used to increase the β phase stability from near-TRIP to near-TWIP by adding third alloying elements. SIM α″ transformation and mechanical twinning were traced by in-situ EBSD (Electron backscatter diffraction) mapping under cyclic tension. The deformation modes of β phase exhibited significant changes when shifting its metastability via chemical composition modifications. In near-TRIP conditions (dominated by the growth of SIM α″ in plastic regime), SIM α″ transformation and internal twinning of martensite were the main mechanisms to accommodate the local stress-strain conditions. In near-TWIP ones (dominated by the growth of {332}<113>β twinning in plastic regime), SIM α″ was observed only at twinning interface during strain process and disappeared after stress release.
Highlights
Since the last few years, a new family of metastable β titanium alloys with TRansformation Induced Plasticity and/or TWinning Induced Plasticity effects has been developed by design based on electron parameter design methods (Bo-Md and e/a ratio)
The Transformation Induced Plasticity (TRIP)/Twinning Induced Plasticity effects (TWIP) Ti alloys present an unprecedented combination of high strength, large ductility, and excellent work hardening rate when compared to the conventional Ti alloys [1,2,3,4]
A shift of deformation mechanism from combined TRIP/TWIP to TWIP-dominant was observed when Zr content is increased from 5% to 10%
Summary
Since the last few years, a new family of metastable β titanium alloys with TRansformation Induced Plasticity and/or TWinning Induced Plasticity effects (denoted as TRIP/TWIP hereafter) has been developed by design based on electron parameter design methods (Bo-Md and e/a ratio). Recent studies have reported complex deformation mechanisms mainly involving mechanical twinning ({332} mode, {112} mode in β BCC) and stress/strain-induced martensite (SIM α′′, BCC β® α′′ Orthorhombic) [5, 6] The simultaneous activation of dislocations slip with the various mechanisms results in dynamic Hall-Petch effect, heterogeneous composite effect, hierarchical twin/SIM microstructure, multimodal twinning network, etc [1,2,3,4,5,6,7] It is well-known that the activation of mechanical twinning and SIM closely depends on the stability of the β phase. The in-situ method is used to reveal the formation, development, and reversion of martensite/twin during the tensile loading and unloading processes
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