Investigation of the Strength–Ductility Balance in an Industrial-Grade TC18 Titanium Alloy: The Pivotal Role of β Grain Size

TL;DR

This study explores how β grain size affects the strength and ductility of TC18 titanium alloy, showing that even coarse grains can offer good mechanical performance.

Contribution

The study reveals a practical method to achieve a strong-ductile balance in industrial-grade titanium alloys using coarse β grains.

Findings

A strong inverse correlation exists between β grain size and both strength and ductility in TC18 titanium alloy.

Fine grains enhance ductility by promoting stress redistribution and forming denser kink bands.

Strain-induced martensite evolution improves plasticity and stress relief in fine-grained specimens.

Abstract

The β grain size in titanium alloys during industrial forging is critical for balancing toughness, cost-effectiveness, and processability. To address the industrial challenge of high cost and difficulty in refining β grains to the tens of micrometers scale, this study investigates the feasibility of achieving a superior strength–ductility balance in TC18 alloy with near-industrial coarse β grains (296~857 μm) under room temperature tension. A pronounced inverse correlation is observed between β grain size and both strength and ductility. The yield strength–grain size relationship follows the Hall–Petch effect, while the anomalous increase in ductility for fine-grained specimens is attributed to three factors. First, smaller grains provide a higher grain boundary density, promoting stress redistribution and mitigating stress concentrations. Second, more uniform stress distribution induces thinner, denser kink bands that enhance plasticity. Third, strain-induced martensite evolves from discrete nanoscale particles to discontinuous lines and ultimately coalesces into continuous planar bands along the (112)β and (110)β planes. This phase transformation, which initiates below a critical grain size of ~500 μm, further alleviates stress concentrations towards slip bands and contributes to dynamic work hardening. The findings demonstrate that coordinated deformation mechanisms enable excellent mechanical performance even in coarse-grained microstructures, providing a practical pathway for optimizing industrial-grade titanium alloys.

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