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A metastable β-titanium Ti–Al–Mo–V–Cr–Nb alloy exhibits a synergistic increasing in strength and ductility via coupling thermomechanical processing and heat treatment in present work. Synergistic effect of the thermomechanical processing and heat treatment on microstructure evolution and mechanical properties of the metastable β-Ti alloy was systematically investigated. α+β rolled sample sheet consumes less time to finish recrystallization, and leads to more uniformly distribution and smaller size of β grains than β rolled sample after same solution treatment. Primary α phase in α+β rolled sample hinders the growth of β grains during solution treatment and the spacing distance of α lamella in α+β rolled sample is smaller than β rolled sample after aging treatment, enhancing the strength significantly. The small spacing distance of α lamella is due to an increase in precipitation density. The small size and homogeneous distribution of β grains provides moderate ductility. After solution treatment at 770 °C for 2 h and aging treatment at 525 °C for 6 h, α+β rolled samples exhibit higher ultimate tensile strength (1550 MPa) and better ductility (6.5 %) than β rolled sample. The relationship between the average grain size of β grains, the average spacing distance of α lamella, and the yield strength of the alloy is quantitatively calculated using mathematical methods. The mechanisms of synergistic enhancement of strength and ductility in the alloy is qualitatively analyzed. These findings facilitate manufacturing high-performance metastable β-Ti alloys at an industry scale using commercially available thermomechanical processing. [ABSTRACT FROM AUTHOR]

A long-standing conflict concerning metastable β -titanium (Ti) alloys is that high strength can be achieved through precipitation strengthening of α -phase while plasticity is largely compromised and even lost completely. Such strength-plasticity trade-off severely limits their structural applications. Here, an attempt to address the issue has been made in Ti–6Cr–5Mo–5V–4Al (Ti6554) high-strength metastable β -Ti alloy. Heterogeneous β -structure is architected by partial static recrystallization (SRX) through properly manipulating solution treatment in the hot-rolled bars. The resulting microstructure is composed of the recrystallized 'new' equiaxed β -grains (β e) and the remnant elongated β -fibers (β f). This β -morphology dramatically affects the subsequent α s -precipitation behavior during the aging process. α s -precipitation is rather uniform throughout β f -grains because the pre-existing dislocations left by hot rolling provide copious nucleation sites. However, α GB -films are preferentially formed along β e -grain boundaries, which leads to a boundary-affect zone (BAZ) and thereby renders α s -precipitation in a size gradient from β -grain boundaries to grain interiors. As a result, an ultra-high strength of ∼1700 MPa is achieved whilst the ductility can be up to be 5% in the β -hetero aged samples, which is superior to the β -homo aged siblings that have the same strength level but fully succumb to macroscopic brittleness. The enhanced plasticity in the β -hetero aged samples originates from much better deformation compatibility conferred by the aged β f -grains. However, the soft feature of BAZ around β e -grain boundaries in the β -homo aged samples causes strain localization, which eventually induces intergranular brittle fracture. These findings provide an effective strategy for enhancing plasticity of high-strength metastable β -Ti alloys by structural design, where homogeneous α s -precipitates can be easily constructed by combining conventional thermomechanical processing and heat treatment. • Strength-ductility trade-off is evaded by a new structural design in Ti6554 alloy. • Enhanced ductility of ∼5% together with high strength of ∼1700 MPa is achieved. • Heterogeneous β -structure is architected by partial static recrystallization. • α -precipitation is rather uniform whilst α GB -film is effectively suppressed. • The improved ductility is conferred by superior deformation compatibility. [ABSTRACT FROM AUTHOR]

The microstructure evolution, mechanical property and deformation behavior of α lamella structure in a novel metastable β alloy Ti–5Al–4Zr–8Mo–7V were studied in this paper. Via different heat treatments, large differences in size, morphology and volume fraction of α lamella were obtained. Tensile tests show that high strength with low ductility is represented after low temperature aging. Tensile brittleness at low temperature aging is originated from precipitation of high fraction nano-scaled α lamella, which induced intensive stress concentration along α/β interface where local shear and plastic instability takes place. The alloy exhibits improved strength-ductility combination after BASCA (β annealing slow cooling plus aging) treatment and high temperature aging. High resolution-TEM results suggest that highly defected substructure and local distortion of (10 1 ‾ 0) planes are exhibited in α lamella. Both the highly defected substructure and finely dispersed α lamella could effectively strengthen the alloy. Improvement of ductility is on account of the increase of free slip length in β matrix where a large amount of dislocation accumulated and compatible deformation between coarse α lamella and β matrix. In addition, deformation twining is generated in coarse α lamella which will increase the ductility as well. • The α lamella spacing length determines the slip in β phase which in turn dominant the mechanical properties of the alloy. • The tensile brittleness at low temperature aging (490–550 °C) is attributed to the intense strain localization in shear bands of β grains. • Improved ductility of 680°C aging and BASCA samples is attributed to compatible deformation between β matrix and α lamella. [ABSTRACT FROM AUTHOR]

A novel metastable β-Ti alloy based on Ti–Al–Zr–Mo–Cr system was successfully designed and fabricated. Superior combination of high strength (UTS = 1534-1198 MPa) and ductility (El = 7.0%–20.9%) is achieved in this alloy. After high reduction β transus forging and subsequent heat treatment, a multi-scale distribution of α phase is displayed which contains micro-scale elongated primary α (α p), sub-micro α rod (α r) and nano-sized α platelets (α s). Detailed selected area electron diffraction (SAED) pattern and high resolution-TEM analysis show the production of FCC substructure inside α p and α r with an orientation relationship of <0001> hcp //<001> fcc , {11 2 ‾ 0} hcp //{ 2 ‾ 20} fcc , {10 1 ‾ 0} hcp //{220} fcc. Two variants of FCC lamella are discovered which is related by a 120° rotation around <001> axis. The effect of microstructure on the improved strength-ductility combination is briefly discussed. Both FCC lamella and multi-scale distribution of α phase could create an effective strain partition during plastic deformation and increase the ductility. High strength originates from the distribution of high fraction nano-scaled α s which could effectively hinder dislocation slip in β matrix. [ABSTRACT FROM AUTHOR]

Abstract A novel high-strength metastable β -Ti alloy Ti-5Al-4Zr-8Mo-7V has been successfully designed utilizing the " d -electron theory" combined with the semi-empirical element-hardening criteria. The excellent combination of strength σ b ~ 1460 MPa and ductility ɛ f ~ 10% is achieved by tailoring hierarchical distribution of α -phase in β -matrix. This hierarchical microstructure consisting of elongated primary α -phase (α p), sub-micro α -rods (α r) and nano-scale α -platelets (α s) was produced by β -transus forging together with heat treatment. Detailed TEM analysis shows that both the α p and α r could deform plastically, resulting in dense tangled dislocations. Furthermore, dislocation features of α p show that the α p /β interface is effectively hardened whilst the entire α p presents prominent deformation capability. Meanwhile, the plastic strain is partitioned among α p , α r and β -matrix compatibly, which is beneficial to improve the ductility. As for the α s , HRTEM observation shows highly defected substructure and local lattice rotation. This highly defected substructure and nano-scale α -distribution effectively block dislocation motion and ultimately strengthen the alloy. The approach presented in this study provides a guideline for the design and fabrication of other β -Ti alloys with hierarchical structure and superior mechanical properties. Graphical abstract Unlabelled Image Highlights • A novel β -titanium alloy, Ti-5Al-4Zr-8Mo-7V, was successfully designed using the 'd-electron theory'. • Hierarchical distribution α-phase in β-matrix was developed by proper β -transus forging and heat treatment. • The novel alloy possesses superior combination of strength and ductility. [ABSTRACT FROM AUTHOR]