  A new $\beta$-metastable Ti-alloy is designed with the aim to obtain a TWIP
alloy but positioned at the limit between the TRIP/TWIP and the TWIP dominated
regime. The designed alloy exhibits a large ductility combined with an elevated
and stable work-hardening rate. Deformation occurring by formation and
multiplication of {332}<113> twins is evidenced and followed by in-situ
electron microscopy, and no primary stress induced martensite is observed.
Since microstructural investigations of the deformation mechanisms show a
highly heterogeneous deformation, the reason of the large ductility is then
investigated. The spatial strain distribution is characterized by micro-scale
digital image correlation, and the regions highly deformed are found to stand
at the crossover between twins, or at the intersection between deformation
twins and grain boundaries. Detailed electron back-scattered imaging in such
regions of interest finally allowed to evidence the formation of thin needles
of stress induced martensite. The latter is thus interpreted as an
accommodation mechanism, relaxing the local high strain fields, which ensures a
large and stable plastic deformation of this newly designed Ti-alloy.
