Martensitic Ti-Nb alloys & Ni-free Ti-based Shape Memory Alloys
Metastable Ti-Nb alloys receive constantly growing attention in the ongoing development of novel Ni-free Ti-based shape memory and/or superelastic alloys. Various manufacturing & processing approaches such as casting, powder metallurgy, severe plastic deformation (SPD), selective laser melting (SLM) and rapid solidification were explored. The phase constitution, microstructure, thermal stability and mechanical properties of bulk and porous Ti-Nb alloys were investigated by a combination of experimental and theoretical methods.
Factors influencing the elastic moduli, reversible strains and hysteresis loops in martensitic Ti-Nb alloys
Monotonic and cyclic (hysteretic) loading-unloading experiments were used to study the influence of Nb-content and pre-straining on the elastic moduli. Particular attention has been paid to the separation of nonlinear elastic from anelastic strains, which govern the stress and strain limits to which a material can be loaded without deforming it plastically. Slight pre-straining of martensitic Ti-Nb alloys can lead to considerable reductions in their elastic moduli as well as their total reversible strains.
M. Boenisch, M. Calin, J. van Humbeeck, W. Skrotzki, J. Eckert, Mater. Sci. Eng. C 48 (2015), 511-520 URL
Phase transformations and mechanical properties of biocompatible Ti-16.1Nb processed by severe plastic deformation
The impact of hydrostatic pressure and severe plastic deformation on the phase stability and mechanical properties of a biocompatible Ti–16.1 wt.% Nb alloy containing α”-martensite was studied. By processing samples through high pressure torsion (HPT) at room temperature, i.e. applying both hydrostatic pressure and shear deformation, a nanocrystalline structure was obtained. The Young´s modulus of the alloys processed by different thermomechanical treatments can be successfully modelled in terms of phase fractions applying a rule of mixture.
A. Panigrahi, M. Boenisch, T. Waitz, E. Schafler, M. Calin, J. Eckert, W. Skrotzki, M. Zehetbauer, Journal of Alloys and Compounds 628 (2015), 434-441 URL
Composition-dependent magnitude of atomic shuffles in Ti-Nb martensite
The orthorhombic martensite α’’ commonly observed in rapidly quenched β-stabilized Ti-based alloys represents an intermediate structure between the hexagonal martensite α’ found at low solute contents and the bcc β phase present at high solute contents. We used X-ray diffraction combined with Rietveld-based analyses to study the gradual structural changes of α’ and α’’ martensites effected by addition of Nb and determined their compositional boundaries.
M. Bönisch, M. Calin, L. Giebeler, A. Helth, A. Gebert, W. Skrotzki, J. Eckert, Journal of Applied Crystallography (2014) 47, 1374-1379, URL
Thermal stability and phase transformations of martensitic Ti-Nb alloys
In Ti-Nb alloys the superelastic and shape memory properties can arise due to the thermoelastic martensitic β↔α″ phase transformation. To achieve shape memory behavior it is vital that both martensitic (α″) and austenitic (β) phases remain stable against thermal decomposition phenomena up to temperatures of the reverse martensitic transformation. The influence of Nb additions on the transformation behavior in relation to the metastable and stable phase diagrams was systematically studied.
M. Boenisch, M. Calin, T. Waitz, A. Panigrahi, M. Zehetbauer, A. Gebert, W. Skrotzki, J. Eckert, Science and Technology of Advanced Materials 14 (2013), 55004/1-9 URL