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Abstract: Ti–5Al–5Mo–5V–1Cr–1Fe near β titanium alloy was thermally deformed at 700–800°C and strain rates of 10−1–10−3 s−1. The relationship between lamellar α evolution and flow softening behavior of this alloy was investigated. The evolution of lamellar α includes the buckling, fragmentation and spheroidization sequentially. By increasing the temperature and decreasing the strain rate, both the peak stress and flow softening extent in the true stress/strain curves were reduced, and the buckling of lamellar α up to fragmentation was enhanced. The transition of lamellar α evolution from buckling to fragmentation contributes to the flow softening greatly, leading to the sharp reduction of flow softening extent. After obvious occurrence of the fragmentation of lamellar α, the reduction of flow softening extent decreases gradually. [Copyright &y& Elsevier]

Hot compression experiments of a Ti55511 titanium alloy are conducted to study the phase transformation characteristics and dynamic recrystallization (DRX) behavior. It is found that the fraction of α phase decreases with increasing the strain rate or deformation amount. Meanwhile, the original lamellar α phases easily transform into the spheroidal and bulk α phases at larger deformation amounts or higher strain rates. Most of α phases disappear and the final microstructures are mainly β phases when the temperature is over 800 °C. The DRX degree rises with raising the deformation amount or decreasing the strain rate. However, the DRX behavior becomes weaken with raising the deformation temperature. α phases are evenly distributed around β phases during hot compressive deformation, which not only restrains the growth of β phases but also promotes the DRX process of β phases. In addition, the geometric DRX takes place within α phases, while the continuous DRX occurs within β phases during the hot compression. Image 1 • Phase transformation and dynamic recrystallization (DRX) behavior of a Ti55511 titanium alloy are studied. • The volume fraction of α phase decreases with increasing the strain rate or deformation degree. • Initial lamellar α phases easily transforms into bulk/spheroidal α phases at the larger strains or higher strain rates. • The α phases not only restrains the growth of β phase, but also promotes the DRX process of β phase. • The geometric DRX takes place within α phases, while the continuous DRX occurs within β phases. [ABSTRACT FROM AUTHOR]

In this paper, a 16 µm CrN/CrAlN multilayer coating is prepared on the titanium alloy surface via arc ion plating. Since titanium alloys are susceptible to wear in a high-temperature environment, high-temperature wear performance and mechanism after coating are investigated. The results show that the friction coefficient of the CrN/CrAlN multilayer coating decreases with an increase in temperature during wear tests at room temperature, 300 ℃, 500 ℃, and 700 ℃. Moreover, the friction coefficient decreases from 0.9 at room temperature to 0.4 at 700 ℃. The wear rate increases with the temperature, more specifically from 1.32 × 10−5 mm3/(N·m) at room temperature to 6.45 × 10−5 mm3/(N·m) at 700 ℃. [ABSTRACT FROM AUTHOR]

The α ↔ β phase transformation in dual-phase Ti–55531 alloy during the continuous heating was investigated by combining dilatometer test with microstructure characterization and local composition mapping. It was found that the α ↔ β phase transformation in individual temperature regions mainly involved either of two types of α phases, i.e. the lamellar α phase which forms during the continuous heating or the slab α phase raised in preliminary isothermal reaction. The corresponding transformation sequence was verified as β → α lamellae , α lamellae → β, and α slab → β. The pile-ups of alloying elements at the α/β interface were understood by either local equilibrium condition or differences among net diffusion fluxes of alloying elements. More important was that the growth and dissolution kinetics of both α phases were modeled and compared with experiments. With the calculations, the dissolution of slab α phase was demonstrated to follow the kinetics of planar interface without the Gibbs-Thomson effect. In contrast, the Gibbs-Thomson effect was proven to do contribution to lamellar α phase. It not only decreased the stability of α tip, such that the re-dissolution of lamellar α tips took place at 838 K below the equilibrium temperature, but also introduced a competition between the shrink of tips and thickening of broad face in α lamellae at 838–911 K. Moreover, the lengthening of lamellar α tips was proven to follow a kinetics controlled by a mixture of bulk and boundary diffusion, in comparison with the pure bulk diffusion of fastest Al governed the re-dissolution of both tips and broad faces. [ABSTRACT FROM AUTHOR]

In this study, CrN and CrAlN coatings were deposited on the surface of aerospace titanium alloys using the arc ion plating technology to investigate the effect of the microstructure of the coatings on the mechanical and erosion resistance properties. The cross-sections of the coatings were observed by Scanning Electron Microscope(SEM), the microstructures were characterized by Transmission Electron Microscope(TEM), and the crystalline structures were tested by X-ray diffraction (XRD); furthermore, the hardness of the coatings was determined by a micro-Vickers hardness tester, the film-base bond strength was measured by a scratch tester, the change in curvature was measured by laser scanning and the residual stress was calculated. The fracture mechanism of the coating was elucidated by simulating the fracture pattern of the cross-section of the erosion crater. The results indicated that the thicknesses of the CrN and CrAlN coatings were almost the same, but the number of large particles on the surface of the CrAlN coating was higher. Both sets of coatings exhibited a nanocrystalline structure with a grain size of around 10 nm. Due to the doping of the Al element, the CrAlN coatings exhibited a hardness of 2846 HV and a higher residual stress of −1.831 GPa. Both the CrN and CrAlN coatings showed lower erosion rates than the titanium substrate at 30° and 90° angles of attack. The results of finite element simulations indicated that the more ductile CrN coating exhibited a better stress absorption capacity, and a typical brittle fracture failure mechanism was observed for both sets of coatings. • CrN and CrAlN coatings with good erosion resistance were obtained. Compared with the titanium alloy substrate, the erosion rate of the coated sample is only 1/8 of the titanium alloy substrate at 30°, and only 1/3 of the titanium alloy at 90°. • The erosion fracture process of the hard coating was explored, in which the cracks were mainly in the direction perpendicular to the film-base interface, and were easily deflected by stress concentration at the microparticles and the interface. • Both CrN and CrAlN coatings are typical brittle fracture failure mechanisms, but the tougher CrN coating has better stress absorption capacity. [ABSTRACT FROM AUTHOR]

Abstract: Ti-5Al-5Mo-5V-1Cr-1Fe near β titanium alloys containing a thick and thin lamellar α phase were hot compressed at 700–800°C and strain rates of 10−3–10−1s−1. The influence of lamellar α on alloy deformation and microstructure evolution was investigated. The great impediment of thick lamellar α to plastic flow led to high deformation resistance, even though lamellar rotation caused a stress reduction at the beginning of continuous flow softening. In contrast, the integral and translational motion of thin lamellar α without apparent lamellar rotation presented a lower impediment to plastic flow. Moreover, the corresponding alloy exhibited low deformation resistance and gentle continuous flow softening. Furthermore, ultra-fine grained Ti-55511 with an α/β grain size<0.5 - 1.0 μm was prepared by introducing the thin lamellar α at a low deformation temperature to prevent the substantial dissolution of α. The thin lamellar α structures were easily fragmented into fine α grains. The ease with which dislocations were cut across the thin lamellar α enhanced the dislocation density in the small β regions among thin, disperse lamellar α regions, which further led to the formation of β subgrains by the slipping-climbing of dislocations. Subsequently, fine β grains formed through the subgrain rotation mechanism. [Copyright &y& Elsevier]

The precipitation orientation of α phases in a Ti-55511 alloy during the heat treatment and the subsequent hot deformation is investigated. It is observed that the β grains prominently influence the precipitation process of α phases, i.e., for the neighbouring β grains without similar {110} pole, the precipitation orientation of grain boundary (GB) α phases (α GB) keep {0001}//{110}β1 or {0001}//{110}β2 to maintain the burgers orientation relationship (BOR) with the neighbouring β grains. For the neighbouring β grains with similar {110} pole, the {0001} poles of α GB phases precipitate parallel to this similar {110} pole and maintain BOR with the neighbouring β grains. For the neighbouring β grains with multiple similar {110} poles, the {0001} poles of α GB phases precipitate along these similar poles and maintain BOR with the neighbouring β grains. Besides, the interface instability nucleation and sympathetic nucleation are the main nucleation modes of the GB widmanstatten α phase (α WGB), which primarily precipitate within β grains with the similar orientation of α GB phases. The intracrystalline widmanstatten phase (α WI) precipitate in a single β grain with 12 precipitation orientations. The BOR between α WI phases and β grains is destroyed during the subsequent hot compressive deformation. • Precipitation orientation of α phases in Ti-55511 alloy during heat treatment plus hot deformation is studied. • Precipitation orientation of GB α phases maintains the BOR with neighbouring β grains along different poles. • Interface instability and sympathetic nucleations are the main nucleation modes of GB widmanstatten α phases. • The intracrystalline widmanstatten phases (α WI) precipitate in a single β grain with 12 precipitation orientations. • The BOR between α WI phases and β grains is destroyed during the subsequent hot compressive deformation. [ABSTRACT FROM AUTHOR]

Hot compression features of Ti-55511 alloy are investigated by high-temperature compression tests in α+β region. It is found that the flow stress and softening mechanisms are obviously influenced by deformation conditions. The true stress decreases with the reduced strain rate or the raised temperature. The spheroidization of α phase and dynamic recrystallization (DRX) of β phase easily occur at low temperatures such as 973, 1003 and 1033 K, while the dynamic recovery (DRV) of β phase mainly occurs at high temperatures such as 1063 K because of the transformation from α phase to β phase at relatively high temperatures A dislocation density-based constitutive model, which is associated with DRV, work hardening mechanisms and the spheroidization of α phases, is established and validated to describe flow behavior. The correlation coefficient (R) and average absolute relative error (AARE) of the established model are 0.9924 and 6.8%, respectively. 3D power dissipation efficiency maps and processing maps are established to determine the appropriate processing window, i.e., too low temperatures (lower than 973 K) or too high strain rates (higher than 1 s−1) easily induce flow instability. Therefore, the medium temperature (1003–1063 K) and the low strain rate (0.001–0.1 s−1) are applicable for thermal compression of the studied titanium alloy. • Hot compression features of Ti-55511 alloy is investigated by high-temperature compression tests in α+β region. • DRX easily occurs at low temperatures, while DRV mainly occurs at high temperatures. • A accurate dislocation density-based constitutive model considering the spheroidization of α phases is established. • The optimal forming temperature and strain rate are 1003–1063 K and 0.001–0.1 s−1. [ABSTRACT FROM AUTHOR]

Spheroidization and dynamic recrystallization (DRX) mechanisms of a Ti-55511 alloy during hot compression in α+β region is investigated. It is found that the spheroidization of lamellar α phases and DRX of β phases, which are obviously influenced by deformation conditions, are the main softening mechanisms. The spheroidization fraction of lamellar α phases first increases with the raised temperature or strain rate. However, the spheroidization fraction of lamellar α phases begins to drop at about 700 °C as the strain rate is higher than 0.01 s−1, and the maximum spheroidization fraction of lamellar α phase is about 2.3%. This phenomenon is attributed to the Implanting-Mechanism, which is induced by the common or close orientation relationships of the spheroidized and equiaxed α phases, as well as few dislocations between the spheroidized and equiaxed α phases. Contrarily, the lamellar α phases are obstructed from equiaxed α phases by a mass of dislocations. Additionally, the DRX degree of β phases increases with the reduced temperature or the raised strain rate. • Spheroidization and DRX mechanisms of a Ti-55511 alloy during hot compression in α+β region is investigated. • Spheroidization fraction of lamellar α phase first increases then drops with the raised strain rate. • The maximum spheroidization fraction of lamellar α phase is about 2.3% near 700 °C and 0.01 s−1. • Implanting-Mechanism, induced by the close orientation relationships of spheroidized and equiaxed α phases, is proposed. • The DRX extent of β phases increases with the reduced temperature and raised strain rate. [ABSTRACT FROM AUTHOR]