Microstructure evolution of Ti17 titanium with ultrahigh strength and moderate plasticity fabricated by laser powder bed fusion

Abstract

This study utilized laser powder bed fusion (LPBF) to fabricate Ti–5Al–2Sn–2Zr–4Mo–4Cr (Ti17) specimens, exploring their ultrahigh strength microstructural strengthening mechanisms and examining heat treatment effects on α lamellae and related mechanical property trends. In the as-deposited LPBF-Ti17, α lamellae features extremely fine dimensions, with an average lamellar thickness of 45 nm, and comprises approximately 52.2% of total volume. This finely dispersed α lamellar microstructure provides numerous α/β phase interfaces, enabling LPBF-Ti17 mechanical properties to reach ultra-high strengths close to 1500 MPa. During heat treatment, the Ostwald ripening mechanism primarily drives growth in α-phase lamellar thickness. The rate of α lamellar thickness growth reaches its peak between 650 °C and 700 °C. The main dislocation distribution of LPBF-Ti17 lies at α lamellae edges, adjacent to β phase, and dislocation density diminishes with rising heat treatment temperature. Concurrently, the Termination migration mechanism causes α lamellae ends to become progressively rounded from sharp features with increasing heat treatment temperature. The increase in α lamellar thickness and reduction in dislocation density collectively contribute to the effective enhancement of Ti17 plasticity.