Unusual dynamic precipitation softening induced by dislocation glide in biomedical beta-titanium alloys

Abstract

Softening of metallic materials containing precipitates during cyclic deformation occurs through dissolution of the precipitates, because the to-and-fro motion of the dislocation causes dissolution of the precipitate particles by cutting them. Here, however, we found the completely opposite phenomenon for the first time; a “dynamic precipitation softening” phenomenon. In a Ti-35Nb-10Ta-5Zr body-centered cubic structured β-Ti alloy single crystal developed for biomedical implant, the to-and-fro motion of the dislocation “induced” the selective precipitation of the ω-phase whose c-axis is parallel to the Burgers vector of the moving dislocation, which led to the significant cyclic softening of the crystal. The formation of the ω-phase is generally believed to induce significant hardening of β-Ti alloys. However, the present results suggest that this is not always true, and control of the anisotropic features of the ω-phase via control of crystal orientation can induce unusual mechanical properties in β-Ti alloys. The unique anisotropic mechanical properties obtained by the cyclic-deformation-induced oriented ω-phase formation could be useful for the development of “single-crystalline β-Ti implant materials” with advanced mechanical performance.

Control of crystallographic orientation by metal additive manufacturing process of β-type Ti alloys based on the bone tissue anisotropy

“…Low elasticity often appears in the unstable β region; however, precipitates such as the ωphase frequently appears in the region. Recently, our group found a unique dynamic precipitation softening phenomenon in Ti-Nb-base β-type alloy single crystals [4]. In this study, therefore, for the fabrication of a β-type Ti alloy bone substitute, the shape as well as microstructure, especially the crystallographic orientation, were controlled simultaneously via metal AM [13] of the powder bed fusion system, as shown in Fig. 1 [14].…”

Origin of age softening in the refractory high-entropy alloys

“…While it is true that secondary phase precipitates have been found to enhance hardness in some Ti alloys, the extent of strengthening is largely dependent on both the morphology and volume fraction of the precipitates ( 34 , 35 ) and the precipitate/matrix interface structure ( 36 ). We theoretically calculated the hardening effects resulting from the precipitates that are observed in the TiVNbTa-4 sample.…”

Heat Treatment of Metastable Beta Titanium Alloys

“…However, the embrittlement effect of the ɷ phase could be efficiently compensated by processing to realize fine β grains [51]. Researchers also reported dynamic precipitation of ɷ phase under cyclic loading condition [55].…”