Impact of hierarchical martensitic structure on deformation twinning in titanium alloys during additive manufacturing

Abstract

A fully hierarchical martensitic microstructure is the common feature of additively manufactured α-type Ti alloys due to the rapid cooling rate inherent to the fabrication process. However, how this hierarchy governs defect formation within α laths under cyclic thermal loadings remains poorly understood. Here, we present a systematic electron microscopy investigation of a Ti-6Al-4V fabricated by laser powder-bed fusion. The as-fabricated microstructure consists of fine α′ martensite organised into multilevel lath hierarchies inherited from prior β grains. Deformation twinning is found to be strongly dependent on the martensitic hierarchy. Two types of twins are identified based on their thickness and interactions with martensitic laths. Twins that link to the endpoints of fine martensitic plates laths, i.e., linked twins, are consistently thicker than non-linked twins within lath interiors. Such twin structures enhance the room temperature tensile performance, enabling a simultaneous improvement in tensile strength, ductility, and work-hardening capability. These results demonstrate that hierarchical martensite actively governs twin formation in additively manufactured Ti alloys, elucidating the microstructural origin of their superior mechanical properties and providing guidance for microstructural optimization.

Keywords

Additive manufacturing / Ti alloys / deformation twinning / electron microscopy

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