Deformation-free geometric recrystallisation in a metastable β-Ti alloy produced by selective laser melting

Abstract

Super-transus heat treatments conducted on an additively manufactured body-centred-cubic β-Ti alloy led to the formation of equiaxed β grains from an initially columnar structure. The grain structure was transformed thanks to a combination of Plateau-Rayleigh instability and grain growth driven by reduction in grain boundary surface area. This is a new type of geometric recrystallisation with no deformation involved. Other mechanisms proposed based on either deformation-induced recrystallisation or martensitic decomposition are dismissed owing to low dislocation density and absence of martensitic transformation in the alloy studied. IMPACT STATEMENTColumnar to equiaxed transformation of β by super-tranus heat treatment of SLM-fabricated Ti is due to Rayleigh instability and grain growth, dismissing the belief that usual deformation-driven recrystallisation is responsible.

Effect of Heat Treatment on the Microstructure and Mechanical Properties of Selective Laser-Melted Ti64 and Ti-5Al-5Mo-5V-1Cr-1Fe

“…Previous references have demonstrated that a heat treatment temperature lower than Tβ prompted the precipitation of α phase, which would impede the growth of β grain boundaries [19,20]. As for the processing temperature higher than Tβ, the columnar grains were driven to become equiaxed due to the combination of Plateau-Rayleigh instability and the surface tension reduction in grain boundary surface area [30]. After αβ-STA, the metastable β structure converted to a bimodal structure with equiaxed αp and acicular αs phases (see Figure 8c).…”

Impact Toughness of As‐Built and Heat‐Treated Near‐β Ti–5Al–5Mo–5V–3Cr–1Zr Alloys Fabricated by Selective Laser Melting and Electron Beam Melting

“…[3] In addition, EBM process is carried out in a vacuum chamber with a high temperature (600-700 C), which is favorable to residual stress minimization and equilibrium phase formation. Some traditional Ti alloys, including Ti-6Al-4V, [9][10][11] Ti-5Al-2.5Sn, [12] Ti-5Al-5Mo-5V-3Cr [13,14] and its close variants [15][16][17] etc., have been explore their suitability in SLM and EBM processes. However, according to these studies, both SLM and EBM show unique metallurgical properties that have significantly effect on microstructure and mechanical properties of as-fabricated Ti alloys.…”

“…According to previous investigations, scientists have confirmed the capability of heat treatment in decomposing nonequilibrium phase, [17][18][19][20] releasing residual stress, [12,21] and achieving columnar-to-equiaxed transition of β grains. [14,21] Therefore, heat treatment strategies successfully reduce the anisotropy and improve the strength-ductility trade-off of as-built Ti alloys. Furthermore, hot isostatic pressing (HIP) treatment is usually applied to eliminate the defects, thus the tensile property and fracture toughness of as-built samples can be improved.…”

“…Some traditional Ti alloys, including Ti–6Al–4V, [ 9–11 ] Ti–5Al–2.5Sn, [ 12 ] Ti–5Al–5Mo–5V–3Cr [ 13,14 ] and its close variants [ 15–17 ] etc., have been explore their suitability in SLM and EBM processes. However, according to these studies, both SLM and EBM show unique metallurgical properties that have significantly effect on microstructure and mechanical properties of as‐fabricated Ti alloys.…”

Microstructure and Mechanical Property of Ti–5Al–5Mo–5V–3Cr–1Zr Alloy Fabricated by Selective Laser Melting with a Preheated Substrate

“…[7,8] According to previous studies, [9][10][11][12][13][14] the microstructure of SLM-fabricated Ti alloys is nonequilibrium, owing to the aforementioned high cooling rates. For instance, α' martensite formed in some SLM-fabricated near α and α þ β Ti alloys, [11,14,15] because the solidification rate of SLM is much higher than the critical cooling rate required for martensite formation. The metastable α' martensite structure causes a high strength but a low ductility of as-SLMed Ti-alloy.…”