Deformation mechanisms of metastable β titanium alloys
Metastable β titanium alloys are subject to many deformation mechanisms: stress-induced martensitic transformation, mechanical twinning, dislocation glide. These deformation mechanisms give rise to unique mechanical properties such as superelasticity, shape memory effect or TRIP/TWIP effects.
The activation of these deformation mechanisms is investigated using in situ or post mortem synchrotron X-rays diffraction, EBSD and TEM and associated with crystallographic models based on Schmid factor analysis.
Keywords: Titanium alloys, EBSD, Scmid factor analysis
Superelastic metastable titanium alloys may be designed using highly biocompatible elements such as Mo, Nb, Zr, Hf, Ta only. Numerous new alloys were recently developed; their performances are now close to those of nickel-titanium alloys which contain a high amount of nickel which is not biocompatible.
A Ti-24Nb-4Zr-8Sn alloy is grown as a single crystal. Cyclic tensile tests show a high superelasticity (recoverable strain reaching 4%). The superelasticity is associated to a stress-induced martensitic transformation from β phase to α’’ phase which is evidenced by in situ synchrotron X-ray diffraction under cyclic tensile tests.
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| Figure 1 : Cyclic tensile curve of the Ti-24Nb-4Zr-8Sn single crystal (a). In situ synchrotron X-ray diffraction profiles during a cyclic tensile test on loading (b) and after unloading (c) |
This stress-induced martensitic transformation is followed by {112}<111>β mechanical twinning which is observed in β phase once the stress is released. Analysis of the deformation mechanisms sequence shows that twinning actually occurs in α’’ phase: {112}<111>β actually correspond to {110}<110>α’ which reverse into β phase when the stress is released as shown on Figure 2.
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| Figure 2 : Deformation sequence of Ti-24Nb-4Zr-8Sn single crystal under loading and after unloading |
Schmid factor analysis shows that observed twins are activated in the opposite direction to that in classical {112}<111> in body-centered cubic structures. Indeed, such an operation, if it occurred in β phase would require shear and a very high shuffle, resulting in a very high activation energy of the phenomenon. However, the previous stress-induced martensitic transformation lowers both shear and shuffle involved in the twinning operation. The activation energy is reduced, making that mechanical twinning system possible to activate.
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| Figure 3 : Dichromatic diagrams of {112}<111>β et {110}<110>α’’ twinning systems |
Collaborations
INSA Rennes ISCR-CM