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The Ti6242S Ti-alloy was processed through β-forging combined with a two-phase zone heat treatment to investigate the effects of deformation and heat-treatment conditions on the final lamellar α phase. It was found that by increasing the deformation degree, the thickness of lamellar α was increased, while its volume fraction first increased and then decreased. This is because a larger deformation will result in more distortion energy, and promote nucleation and growth of lamellar α, leading to an increase in its volume fraction. When the deformation degree reached 70%, coarsening and spheroidization occurred in some lamellar α via cylinderization and boundary splitting during subsequent heat treatment, and resulted in an increase in the thickness and a decrease in the volume fraction of the lamellar α. A larger deformation resulted in more low-angle grain boundaries and made the high-angle grain boundaries gradually become continuous, and promoted the precipitation of lamellar α with a rotation axis of [ 10 ¯ 5 5 3 ¯ ]/63.26°. Increasing the two-phase zone heat-treatment temperature promoted the α → β phase transformation and the lamellar α growth in length and thickness. Increasing the holding time, martensite decomposition, primary α dissolution, and lamellar α precipitation occurred, resulting in an increase in the lamellar α volume fraction. [ABSTRACT FROM AUTHOR]

During the thermoforming of TC18 titanium alloy multi-cavity components, different deformation modes always exist and change, such as uniaxial compression (UC), shear-compression deformation (SCD), uniaxial tension (UT) and shear-tension deformation (STD). Dynamic recrystallization (DRX) of β grains occurs during the single-phase field deformation and has a great influence on the performance of components. In this study, the types and mechanisms of DRX in TC18 titanium alloy as well as transition and correlation under different deformation modes are investigated. It is found that discontinuous dynamic recrystallization (DDRX) initiates through grain boundary bulging at a low strain and dominates in different modes of deformation. Continuous dynamic recrystallization (CDRX) initiates at different strains depending on deformation modes, and the mechanisms vary, subgrain rotation within grains and lattice rotation near GBs under UC and STD, while only subgrain rotation under UT, in addition to these two mechanisms, grain fragmentation is also involved under SCD. Secondary dynamic recrystallization (SDRX) only occurs at a high strain under SCD and STD. Deformation modes lead to differences in orientation, slip and rotation of grains, further result in different dislocation density, distribution and accumulation, which contribute to the occurrence and transition of different types of DRX. Meanwhile deformation modes result in differences in the difficulty of GB migration and lattice rotation, ultimately in the initiation and degree of DDRX and CDRX. The shear stress in SCD and STD promotes the occurrence of CDRX. The present results can provide a guidance for obtaining good performance of the titanium alloy components. [Display omitted] [ABSTRACT FROM AUTHOR]

The formation of α precipitate-free zones (PFZs) has great influence on fatigue strength, tensile ductility and fracture toughness of titanium alloys. In this paper, the formation process of PFZ and the relationship between PFZ, β grain boundary and α phase in TC18 titanium alloy were investigated through β phase field deformation and α + β phase field heat treatment. It was found that the formation of PFZ was affected by the misorientation of β grain boundaries and the morphology of grain boundary α phase (α GB). The thick and continuous α GB phase would precipitate at low angle β grain boundaries (LAGBs), and led to the depletion of α phase stable elements near the β grain boundaries, which further resulted in the formation of PFZ. At high angle β grain boundaries (HAGBs), both discontinuous α GB phase and grain boundary Widmanstätten α phase (α WGB) can precipitate to occupy β matrix, no obvious PFZ. The pre-deformation in β phase field can increase the content of HAGBs and promote the formation of discontinuous α GB phase. In this case, the area of PFZ can be effectively reduced. • The misorientation of β grain boundary affects the growth and morphologies of α GB and finally affects the PFZs. • More continuous α GB precipitated along LAGBs, deplete α stable elements, and promote PFZs. • Discontinuous α GB and α WGB tend to along HAGBs thus no obvious PFZs. • Pre-deformation results in more HAGBs and further inhibit PFZs. [ABSTRACT FROM AUTHOR]

• α GB bifurcates to reduce the system energy by interfacial instability nucleation. • Whether bi- or tri-direction bifurcation occurs depends on α GB nucleation positions. • The bifurcation of α GB at TJ is related to its original morphology. • Flat α GB can bifurcate both in two and three directions. Morphology of secondary grain boundary α phase (α GB) has great influence on grain boundary Widmanstätten α phase (α WGB) cluster and further fracture toughness and ductility of the titanium alloys. In this paper, the bifurcation behavior and morphology of secondary α GB in TC18 titanium alloy were investigated through high and low temperature two-step heat treat experiment (910 °C/30 min→800 °C/45 min/WQ) combined with a three-dimensional reconstruction experiment. It was found that α GB showed flat, equiaxed, zig-zag morphologies, and α GB bifurcated to reduce the energy of the system and maintained the stability of structure by interfacial instability nucleation. There exist two types of bifurcation behaviors, bi-direction bifurcation and tri-direction one. The significant difference between them is nucleation positions, β/β grain boundary or the triple junction (TJ) of β grain boundary. The bifurcation behavior of α phase at TJ is related to its original morphology. Bi-direction bifurcation occurs mostly in the flat α GB , but is not observed in the zig-zag α GB. The present results provide new information on α GB of TC18 titanium alloy. [ABSTRACT FROM AUTHOR]

The formation and characteristics of bilamellar microstructure in Ti6242S titanium alloy under a dual heat treatment (single-phase heat treatment + two-phase heat treatment) were investigated. Heated at single β region and cooled by the air, when the grain boundary α (α GB) kept BOR (Burgers orientation relationship) with the β matrix on both sides, more grain boundary Widmanst a ¨ tten α (α WGB) clusters were generated. The cooling modes after single-phase region heat treatment will change the source of the primary lamellar α, β → α phase transition or martensite decomposition. During two-phase heat treatment, the secondary lamellar α nucleates at the β grain boundaries or the wide interface of the primary lamellar α and the existing primary lamellar α facilitates the formation of the secondary lamellar α with same or similar orientation, showing a typical intragranular Widmanst a ¨ tten α(α WI) microstructure. The V-shaped lamellar α with an angle of about 60° is precipitated from the martensite and shows a V-cone shape in three-dimensions. When the holding time is prolonged to 60 min, three α variants grow simultaneously and keep a mirror relationship in pairs, forming a triangular structure which has a pyramid shape in three dimensions. Variant selection occurred in the lamellar microstructure during dual heat treatment. Held at subsequent two-phase region, the primary lamellar α with the misorientation angle of 90° preferentially dissolved, increasing the variant selection. The primary lamellar α provided nucleation sites for the secondary lamellar α, leading to a decrease of variant selection. • When α GB kept BOR with the β matrix on both sides, more α WGB clusters generated. • Simultaneous nucleation and growth of 3 α variants resulted in triangular structure. • Lamellar α p with misorientation angle of 90°dissolving increased variant selection. [ABSTRACT FROM AUTHOR]