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Cold dwell fatigue of titanium alloys used in aero-engine components has threatened flight safety for over five decades. This phenomenon was encountered in Al-containing titanium alloys, occurring at relatively low temperatures, where load hold at applied peak stress result in reduction in number of cycles by an order of magnitude or more (the so-called dwell debit). The present paper reviews the recent advances in dwell-fatigue of titanium alloys with the primary motivation being to understand the dwell-fatigue damage mechanism relating microstructural configurations, dislocations and local strain rate sensitivity. Numerous studies have focused on the identification of fatigue-critical microstructural configurations, such as microtextured regions, rogue grain pairs and (0001) twist grain boundaries. As an example of dwell resistant alloy design, Timetal 575 is a candidate alloy for aero-engine fan disc applications with both superior static and cyclic strengths and low dwell sensitivity.

In the present work, the effect of solute atoms on the lattice parameters, atomic volume, stacking fault energies (γSF), bulk modulus, and bonding structures of HCP Ti is studied comprehensively by first-principles calculations. Here, the alloying effects on the growth fault (I1), deformation fault (I2) and extrinsic fault (EF) are considered, with the solute atoms (X = Al, Cr, Mo, Nb, and V) commonly utilized in the high-strength Ti-7333 and Ti-5553 alloys selected. It is found that the stacking fault energies of pure Ti increase in the order of γI1<γI2<γEF, which is proportional to their corresponding numbers of fault layers. The variation tendencies of γSF of the binary Ti–X alloys are in the order of Al>V>Cr>Mo>Nb for I1 and I2 and V>Al>Cr>Nb>Mo for EF, respectively. The bonding charge density is utilized to characterize the electronic redistributions caused by the fault layers and the lattice distortions. It is presented that the rod-type bonds of the non-fault layers change into the tetrahedral-shaped bonds of fault layers, displaying the local HCP–FCC-type phase transformation. With the addition of various solute atoms with different atomic size and valance electrons, the bond strengths of Ti–Al and Ti–Nb are weaker than those of Ti–Cr and Ti–Mo as there are fewer densities of bonding electrons. This work gains some insights into the atomic and electronic basis for the solid-solution strengthened bulk and stacking faults of HCP Ti, providing fundamental information to the development of advanced high-strength Ti alloys. [ABSTRACT FROM AUTHOR]

The intergranular crack propagation of the lamellar structure β titanium alloys is investigated by using a modified Gurson-type damage model. The representative microstructure of the lamellar alloy, which consists of the soft α phase layer surrounding the hard grain interiors, is generated based on an advanced Voronoi algorithm. Both the normal fracture due to void growth and the shear fracture associated with void shearing are considered for the grain boundary α layer. The individual phase properties are determined according to the experimental nanoindentation result and the macroscopic stress-strain curve from a uni-axial tensile test. The effects of the strain hardening exponent of the grain interiors and the void shearing mechanism of the grain boundary α layer on fracture toughness and the intergranular crack growth behavior are emphatically studied. The computational predictions indicate that fracture toughness can be increased with increasing the strain hardening ability of the grain interiors and void shearing can be deleterious to fracture toughness. Based on the current simulation technique, qualitative understanding of relationships between the individual phase features and the fracture toughness of the lamellar alloys can be obtained, which provides useful suggestions to the heat treatment process of the β titanium alloys. [ABSTRACT FROM AUTHOR]

Within the rate-independent crystal plasticity framework, a constitutive model was constructed to describe the uniaxial tension of polycrystalline titanium alloy at ambient temperature by considering crystallographic slip as the main deformation mechanism. The three-dimensional (3D) Voronoi tessellation (VT) geometric model was used to create the polycrystalline aggregates for initial microstructure. Based on the model, the effects of polycrystalline aggregate features on the macroscopic mechanical responses were discussed. Results show that the size differences of the three directions in polycrystalline aggregates will lead to the anisotropies of the stress-strain responses in the three directions, wherein the necking phenomenon appears in the long length direction. Besides, it is found that the grain morphology and orientation have significant influences on stress-strain responses as well. The proposed model for single phase titanium alloy provides important informations for the microstructure controlling and the optimization of process parameters. [ABSTRACT FROM AUTHOR]

Double glow plasma surface alloying (DGPSA) process is an effective approach to providing satisfactory surface protection and enhancement of metallic materials. DGPSA can accomplish surface alloying by both non-metallic elements and solid metallic elements as alloying elements to create a strong/gradient metallurgical bonding between the coating and the substrate. This review begins with the technological principle and processing research of DGPSA. The studies and applications of the DGPSA for improving tribological performance of TC4 titanium alloy in China are reviewed in the sight of alloying element selections. [ABSTRACT FROM AUTHOR]

Abstract: In order to improve the biomedical properties of a titanium alloy surface, electro-spark surface alloying was carried out using a graphite electrode in air, in a nitrogen gas atmosphere and in silicone oil. The morphology and microstructure of the strengthened layers were analyzed using scanning electron microscopy (SEM) and X-ray diffraction (XRD). The hardness distributions as a function of depth were measured by a micro-hardness tester. Corrosion resistance capacities of the modified layers were evaluated using potentiodynamic polarization measurements and electrochemical impedance spectroscopy (EIS). In addition, wear resistance and corrosive wear properties in a simulated body fluid (SBF) were studied with a pin-on-disk tribometer. Alloyed layers, completely covering the substrate surface and about 40μm thick mainly composed of the TiC phase and with strong metallurgical bonding and adhesion to the substrate, were obtained. This can markedly improve hardness and wear resistance of the surface layer of the substrate. In comparison to coatings prepared in air and nitrogen gas atmospheres, the coating produced in silicone oil media exhibits a denser and more perfect surface structure. The wear resistance in air and corrosive wear resistance in SBF solution is the best for the coating produced in silicone oil. For instance, the wear rate in air with a GCr15 steel ball counterpart is reduced by a factor of 29 compared with the original titanium alloy and the corrosive wear rate in SBF solution with a corundum ball can decrease by a factor of 13.8. Simultaneously, the effect of electron-spark surface alloying of the titanium alloy surface on biocompatibility and biological activity was also investigated. The electron-spark surface strengthened layer treated in silicone oil shows good biocompatibility and biological activity, and can help cell attachment to the substrate surface. [Copyright &y& Elsevier]

The effect of solution treatment on the microstructure and mechanical properties of Ti-7333, a newly developed near β titanium alloy, was investigated. Compared to Ti-5553 and Ti-1023, Ti-7333 possesses the slowest α to β dissolution rate, allowing a wider temperature window for processing. The rate of β grain growth decreases with the increase of soaking time and increases with the increase of solution temperature. The β grain growth exponents (n) are 0.30, 0.31, 0.32 and 0.33 for solution treatment temperature of 860 °C, 910 °C, 960 °C and 1010 °C, respectively. The activation energy (Qg) for β grain growth is 395.6 kJ/mol. Water cooling or air cooling after solution treatment have no significant influence on microstructure, which offers large heat treatment cooling window. However, under furnace cooling, the fraction of α phase increases sharply. α phase maintains strictly the Burgers orientation relation with β phase ({0 0 0 1}α//{1 1 0}β and 〈1 1 −2 0〉α//〈1 1 1〉β), except the αp particles formed during forging. The tensile strength decreases with the increase of the solution temperature when only solution treatment is applied, whereas the ductility increases gradually. When aging is applied subsequently, the tensile strength increases with the increase of the solution temperature and the ductility decreases gradually. [ABSTRACT FROM AUTHOR]