In situ synchrotron X-ray diffraction study of the martensitic transformation in superelastic Ti-24Nb-0.5N and Ti-24Nb-0.5O alloys

Abstract

Mechanisms of superelasticity were investigated by in situ cyclic tensile tests performed under synchrotron X-ray radiation on Ti-24Nb-0.5N and Ti-24Nb-0.5O compositions of metastable b titanium alloys. Analyses of diffraction patterns acquired under load and after unloading for each cycle were used to determine the characteristics of the potential mechanisms of deformation in both alloys. The Ti-24Nb-0.5N alloy exhibits the conventional behavior of superelastic b titanium alloys. Synchrotron X-ray diffraction (SXRD) experiments proved that superelasticity is exclusively due to the occurrence of a stress-induced martensitic (SIM) transformation from the b phase to the a 00 phase. The evolution of volume fraction of a 00 martensite corresponds exactly to the variation of the recovery strain of the cyclic tensile curve. Conversely, the Ti-24Nb-0.5O alloy displays a nonconventional behavior. SXRD experiments showed a huge ability of the b phase to deform elastically until 2.1%. Surprisingly, a reversible SIM transformation also occurs in this alloy but starts after 1% of applied strain that corresponds to the yield point of the stress-strain curve. Although the SIM transformation occurs, the b phase simultaneously continues to deform elastically. The superelasticity of this alloy is unexpectedly due to a combination of a high elastic deformability of the b phase and a reversible SIM transformation. In both alloys, the lattice parameters of the a 00 martensite evolve similarly in accordance with the initial texture of the b phase and the crystallography of the transformation.

In situ synchrotron X-ray diffraction study of stress-induced martensitic transformation in a metastable β-type Ti-33Nb-4Sn alloy

“…The tendency in the evolution of the lattice parameters of β and α" phases (within the strain range of 1.5-4.7% in Fig. 4a~b) is similar with that in a fully β-phase Ti-24Nb-0.5O alloy where the parameter of bα" increases and the parameter of cα" decreases during the variant selection of stress-induced martensitic transformation [22]. A possible reason for this similarity is that there is no significant difference in the crystallographic orientation relationship between β and α" phases for the (β+α")-phase Ti-33Nb-4Sn alloy and β-phase Ti-24Nb-0.5O alloy.…”

Design and tensile properties of a bcc Ti-rich high-entropy alloy with transformation-induced plasticity

“…[16,18,23,24] As expected from the chosen alloying design, cyclic stress-strain curves display a double yielding behavior (detailed in Figure 1(b) of the Supplementary material), typically observed in metastable β-alloys. [25,26] This behavior is reflected in the evolution of the normalized work-hardening rate (θ/G, θ = dσ T /dε p ), with the shear modulus G estimated from G/E = 3/8 as experimentally found for polycrystalline metallic materials. [27] Three stages as visible in Figure 2 (green curve): first, a classical decrease in θ/G until ε p = 0.018 (stage 1), then an increase in θ /G in stage 2 up to a maximum of about θ /G = 0.103 and finally a decrease from ε p = 0.07 to the end.…”

A Review of Metastable Beta Titanium Alloys

“…%) and Ti-24Nb-0.5O (wt. %) alloys [166]. The element O is shown to more strongly influence the transformation behavior than the element N. Besse et al [159] points out that in the case of the TNTZ and TNTZ-O alloys other mechanisms have been observed including dislocations, stress-induced {112}<111> or {332}<113> twins, and ω-phase plates and their role in superelastic behavior is not fully understood.…”