Effect of microstructure on tensile properties of Ti–5Al–5Mo–5V–3Cr–1Zr alloy

Correlation between alpha phase morphology and tensile properties of a new beta titanium alloy

Influence of phase transformation kinetics on the formation of α in a β-quenched Ti–5Al–5Mo–5V–3Cr–1Zr alloy

As a crucial branch for titanium industry, high-strength titanium alloys (HS-TAs, with UTS ≥ 1100 MPa) are indispensable structural materials for advanced engineering applications such as aerospace and marine fields. Along with the expansion of HS-TAs’ market, achieving satisfying synergies of high strength, high ductility (elongation ≥ 6%) and high toughness (KIC ≥ 50 MPa⋅m1/2) has been identified as the uppermost technical bottleneck for their research and development. To overcome the challenge, two primary strategies have been initiated by the titanium community, developing novel alloys and innovating processing technologies. For the former, a dozen of newly-developed alloys were reported to exhibit excellent strength-ductility-toughness combinations, including Ti-5553, BT22, TC21 and Ti-1300, for which the ideal mechanical performances were based on specific microstructures realized by low impurity rate (e.g. oxygen content ≤ 0.15 wt%), complicated processing and complex heat treatment. For the latter, several innovatory forging and heat treatment technologies were originated for the mature alloys to optimize their balanced property by extraordinary microstructural characteristics. In this review, we provide a comprehensive overview over the research status, processing and heat treatment technologies, phase transformation, processing-microstructure-property correlation and strengthening-toughening mechanism of HS-TAs for aerospace engineering applications manufactured via melting-forging process. Finally, the prospects and recommendations for further investigation and development are proposed based on this review. 

High-strength titanium alloys for aerospace engineering applications: A review on melting-forging process

Elastic modulus of phases in Ti–Mo alloys

A combination of transmission electron microscopy, scanning electron microscopy and x-ray diffractometer is used to analyse the deformation mechanism and the fracture behavior of bi-model structure Ti–5Al–5V–5Mo–3Cr–1.5Zr–0.5Fe alloy during uniaxial tensile test. In the uniform deformation region, the strain of the primary α particles is dominated by the planar slipping and the dislocation tangling, and the deformation of the β matrix is accompanied by the dislocation tangling. In the necking region, the additional deformation twinning is found in the primary α particle, and the β phase is still strained by the dislocation tangling. The local shearing occurs in the necking region and results in the micro-twinning dominated deformation in secondary α precipitation and the 〈110〉β fiber texture. The room temperature tensile fracture of Ti-55531 alloy is extremely sensitive to the formation of the voids, and the critical dimension of the crack is the diameter of the primary α particles.

Tensile deformation and fracture of Ti–5Al–5V–5Mo–3Cr–1.5Zr–0.5Fe alloy at room temperature

β titanium alloys with full lamellar structure often exhibit the low elongation, and the brittle fracture mechanism of Ti–5Al–5V–5Mo–3Cr β titanium alloy with full lamellar microstructure is investigated in this paper. After tensile test, the fracture surface characterization and the cross sectional observation show that the transgranular dimple fracture, the intergranular fracture and the transgranular shearing fracture are the fracture mode of the alloy. It is discovered that the cracks form by the coalescence of the microvoid at the α/β interfaces, and the shearing is triggered by the gradual increment of the crack dimension inside the prior β grain at a high stress level. The first two fracture progresses contribute to the plastic deformation of the alloy, and the transgranular shearing occurs in the large scale β grain and is the main brittle fracture mechanism of Ti-5553 alloy.

Transgranular shearing introduced brittlement of Ti–5Al–5V–5Mo–3Cr alloy with full lamellar structure at room temperature

In this paper, the ageing response and the mechanical properties of Ti-5553 (Ti–5Al–5V–5Mo–3Cr) and Ti-5553s (Ti–5Al–5V–5Mo–3Cr–0.4Si) alloys are comparatively investigated. During ageing at 650 °C, the hardness of the alloys first increases in 30 min and remains constant from 1 h to 8 h, and then decreases with further increase the ageing time to 29 h. This variation is related to the chemical composition and the volume fraction of α phase. The mechanical properties of the 5 h aged Ti-5553 and Ti-5553s alloys are investigated, and the result shows that the ultimate tensile strength of the Ti-5553s alloy is higher than that of Ti-5553 alloy while the ductility of the Ti-5553s alloy slightly decreases. The tensile fracture of the Ti-5553s alloy shows a mixed model of intergranular fracture and cleavage fracture.

Effects of Si addition on mechanical properties of Ti–5Al–5V–5Mo–3Cr alloy

The role of the forging temperature, forging strain and solution temperature on the preparation of bimodal Ti-5553s alloy has been studied in this paper. The initial full lamellar Ti-5553s alloy could be forged within a wide temperature scope of 600–800 °C by a high compression strain (0.88), demonstrating the excellent thermal processing ability in the α+β field. The sub-transus forging is essential to the preparation of the bimodal alloy, and the recommended forging temperature and the critical forging strain for the lamellar Ti-5553 alloy are 780 °C and 0.3, respectively. To prevent the coarsening of the β grain, the recommended solution temperature for bimodal alloy lies in 790–820 °C. The decrease in the forging temperature or the increase in the solution temperature may lead to the increase in the Vickers hardness of the bimodal alloy. The relationship between the forging factors, the microstructure and the mechanical performance of Ti-5553s alloy has been developed, and these laboratory results provide a guideline for the preparation of the large-scale Ti-5553s alloy component with the homogeneous bimodal microstructure.

Yafeng Lu

Papers

Tensile deformation and fracture of Ti–5Al–5V–5Mo–3Cr–1.5Zr–0.5Fe alloy at room temperature

Transgranular shearing introduced brittlement of Ti–5Al–5V–5Mo–3Cr alloy with full lamellar structure at room temperature

Effects of Si addition on mechanical properties of Ti–5Al–5V–5Mo–3Cr alloy

On preparation of bimodal Ti–5Al–5V–5Mo–3Cr–0.4Si (Ti-5553s) alloy: α+β forging and heat treatment

X-ray photoelectron spectroscopy characterization of the ω phase in water quenched Ti-5553 alloy