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Mechanical Properties Optimization via Microstructural Control of a Metastable β-type Ti-Nb based Gum Metal

  • Author(s): Shin, Sumin
  • Advisor(s): Vecchio, Kenneth S
  • et al.
Abstract

Metastable β titanium alloys are essential materials for biomaterials and aerospace applications. It is well known that their properties can be manipulated by tailoring the β phase stability and/or microstructural design, which not only results in the activation of multiple deformation mechanisms but also leads to non-homogeneous plastic deformation. Aiming at an improved understanding of the deformation mechanisms associated with the β phase stability, especially the effects of elemental distributions on the deformation mechanisms, three groups of Ti–23Nb–0.8Ta–2Zr–O (at%) alloys with varied β phase stability were produced, which correspond to the occurrence of stress induced α″ martensite transformation, mechanical twinning and slip dislocation, respectively. The β-phase stability dependence of deformation features can explain the possibility of manipulating the mechanical properties without the evident elastic properties.

In twin-dominated metal, the prevailed deformation mechanism is found to be modified into multiple twinning systems based on the dependence of crystallographic orientation. Twinned structure mainly composed of {332}<112> twin system is proposed to account for the Twin-Induced Plasticity (TWIP) effect. Thermomechanical-cycling processes are systemically applied to control a volume fraction of mechanical twins. The dissociation residual stress and passage of {332}<113>β twin can render multiple mechanical twins form into the β-phase matrix and thus induce a softening effect for enhancing uniform ductility of Ti-Nb Gum metal, attributed to well-distributed twins throughout the entire microstructure based on the ‘composite effect’.

Lastly, this paper reports on a heterogeneous-structured β-Ti alloy with an exceptional combination of high strength and ductility, resulting from optimized hierarchical features in a lamellar microstructure. The microstructure is achieved by controlling a fraction of coarse/fine domains, spatial grain-size distribution, and different types of grain boundaries. The large degree of microstructural heterogeneity leads to obvious mechanical incompatibility and strain partitioning during plastic deformation. These experimental results demonstrate that the microstructure design can be undertaken to open new possibilities to expand the material property window between the relatively low Young’s modulus related to chemical composition optimization and better formability linked to improved ductility and enhanced strain hardening.

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