Search

In the present paper, the high cycle fatigue (HCF) of a novel 211Z.X aluminum alloy with high strength was studied under hot-rolling and as-cast states at room temperature. The effects of microstructure and distribution of precipitated phases and impurities on the mechanical properties, HCF performances, fatigue microcrack initiation, and propagation behavior of the 211Z.X alloy were studied by transmission electron microscopy (TEM), scanning electron microscopy (SEM) and energy dispersive spectrometry (EDS). The HCF S–N curves, P–S–N curves and Goodman fatigue diagrams of 211Z.X alloy consisting of two microstructures were drawn. The results suggested that the fine and dispersive distribution of the second phases improved the strength of the alloy. The formation of short-bar and spherical precipitates promoted coordinated deformation of the alloy. This promoted higher microcrack initiation resistance of 211Z.X alloy with a hot rolling state than in the cast state. As a result, the HCF properties of the hot-rolling alloy were better than those of the cast alloy. In sum, these results look promising for future reliable design of engineering structures and application of new aluminum alloys. [ABSTRACT FROM AUTHOR]

This paper aims to unveil effects of α colony and α plate parameters on tensile properties and impact toughness of Ti-55531 alloy with multilevel lamellar microstructures. Microstructures with different α colony and α plate sizes were prepared using varied heat treatments and analyzed by X-ray diffraction, scanning electron microscopy, electron back-scattered diffraction, transmission electron microscopy. Relationships of microstructure and strength, plasticity and impact toughness were discussed. Results indicate that tensile strength increases, while plasticity and impact toughness of the alloy decline with decreasing effective sizes (width/length ratio) of α colony (dc) and α plate (dα) and increasing of volume fraction of α phase. Both dc and dα follow the Hall–Petch principle with strength, and dc was determined to be the critical controlling parameter of strength owing to higher interfacial strengthening effect induced by α colony. However, plasticity is primarily related to dα. Moreover, impact toughness for both dc and dα follows the linear relation, but the coefficient of dα is higher. Furthermore, impact toughness is more dependent on plasticity than strength. Larger α plates lead to more mechanical twins during impacting and result in higher plasticity and impact toughness. Therefore, the impact toughness should depend more on the size of α plates. In order to unveil the influence of α plate size on impact deformation and microcrack initiation behavior of Ti-55531 alloy, the microstructural deformation and crack nucleation characteristics in the impact crack nucleation region were detected by TEM in detail; then, the schematic illustration for microstructural deformation and crack nucleation characteristics was made according to experiment results. [ABSTRACT FROM AUTHOR]

A combination of transmission electron microscopy and scanning electron microscopy was used to study torsion deformation and fracture behaviors of Ti–5Al–5Mo–5V–3Cr–1Zr (Ti–55531) alloy with lamellar microstructure (LM) and bimodal microstructure (BM) at ambient temperature. The results indicate that torsion strength and ductility of BM are significantly lower than those of LM. Shear bands and dislocation tangles are mainly observed in secondary α (α s ) lamellae for both microstructures. Furthermore, straight slip lines only exist in equiaxed primary α (α p ) phase. The coarsening α s lamellae dominate the deformation of LM, while the deformation of BM is greatly controlled by equiaxed α p phase. Fractographs of LM and BM show a mixture fracture mechanism of microvoid coalescence and transgranular shear fracture. Besides, BM exhibits intergranular fracture mechanism. Thus, Ti–55531 alloy with BM is easier to be damaged than LM during torsion loading. [ABSTRACT FROM AUTHOR]

The main objective of the present study was to understand the oxygen ingress in titanium alloys at high temperatures. Investigations reveal that the oxygen diffusion layer (ODL) caused by oxygen ingress significantly affects the mechanical properties of titanium alloys. In the present study, the high-temperature oxygen ingress behavior of TC21 alloy with a lamellar microstructure was investigated. Microstructural characterizations were analyzed through optical microscopy (OM), scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), and transmission electron microscopy (TEM). Obtained results demonstrate that oxygen-induced phase transformation not only enhances the precipitation of secondary α-phase (αs) and forms more primary α phase (αp), but also promotes the recrystallization of the ODL. It was found that as the temperature of oxygen uptake increases, the thickness of the ODL initially increases and then decreases. The maximum depth of the ODL was obtained for the oxygen uptake temperature of 960 °C. In addition, a gradient microstructure (αp + β + βtrans)/(αp + βtrans)/(αp + β) was observed in the experiment. Meanwhile, it was also found that the hardness and dislocation density in the ODL is higher than that that of the matrix. [ABSTRACT FROM AUTHOR]

The microstructural evolution and alloying element partitioning in the α + β ↔ β phase transformation of Ti-17 alloy were explored under continuous heating and cooling conditions using the dilatometric method. Scanning electron microscopy and transmission electron microscopy were used to evaluate microstructural characteristics and trace alloying element partitioning behaviors occurring at different temperatures during heating and cooling. Results showed that the finer needle-like α phase first dissolved into the β phase in the matrix with increasing temperature, while the grain boundary α phase first coarsened and then transformed gradually into β phase during continuous heating. The dissolution of α phase of the alloy with the alloying element partitioning during continuous heating was observed. On the contrary, αGB formed at the prior β grain of the alloy during continuous cooling, which might be the nuclei of α colony, thus resulting in the formation of α colony in the matrix. As the temperature decreased, the elements' concentrations in the α and β phases became increasingly varied due to element partition. Moreover, Al and Cr, which had higher diffusion coefficients than Mo, easily reached the concentration equilibrium of alloying elements in the α and β phases, respectively. The shrinkage of dilatometric curves during heating in the Ti-17 alloy are mainly attributed to the change of α-HCP (hexagonal close-packed) lattice to β-BCC (body-centered cubic) lattice; while the element partitioning during the β → α + β transformation plays an important role in the shrinkage of the dilatometric curves of the Ti-17 alloy during cooling. [ABSTRACT FROM AUTHOR]

[Display omitted] • A gradient microstructure was successfully fabricated by high-frequency induction quenching treatment (HFIQT). • Such a gradient microstructure is a result of the temperature gradient developed during induction heating. • The Ti-6Al-4 V alloy with gradient microstructure produces an attractive strength-ductility synergy. • The mechanical properties are noticeably better than those obtained with conventional treatments. A gradient microstructure was successfully fabricated by high-frequency induction quenching treatment (HFIQT) to improve the mechanical behavior of Ti-6Al-4V alloy in this work. Microstructural evolution was systematically characterized by electron backscattered diffraction and transmission electron microscopy. The results showed that the gradient microstructure of the alloy after HFIQT and aging varied from a fine α s lamellae decomposed from α′-martensite at the surface layer to a bimodal microstructure at the center. Moreover, the alloy with gradient microstructure presents an optimal strength-ductility synergy. The tensile test results showed that a best strength plastic product U T of 12433.1 MPa·%, an ultimate strength of 1231 MPa and an elongation of 10.1 % were obtained after HFIQT for 5.2 s and aging at 400 °C for 6 h. The tensile strength of the alloy aged at 400 °C markedly increased with the increase in soaking time from 5.1 to 5.3 s during HFIQT. Meanwhile, the tensile strength of the alloy slightly increased with the decrease in aging temperature from 550 to 400 °C during HFIQT. Surprisingly, the elongation of the alloy also increased with decreasing aging temperature. The effects of the gradient microstructure characteristic on strength and ductility of the Ti-6Al-4V alloy were discussed. [ABSTRACT FROM AUTHOR]

• A kind of heterogeneous martensitic microstructure was successfully fabricated in Ti-6Al-4V. • The alloy shows an excellent ductility and a considerable tensile strength. • It's the first time to achieve excellent ductility in Ti-6Al-4V by two types of phase transformations under tensile load. • Dislocation hardening and FCC phase transformations lead to excellent ductility. In this work, a kind of heterogeneous martensitic microstructure was successfully fabricated within the fine β grains of Ti-6Al-4V through a rapid heating and cooling processing (RHCP) method, resulting in excellent ductility during tensile deformation. A total elongation was reached at 25.1%, while a considerable tensile strength of 1118 MPa was maintained. The main reasons for achieving this good ductility were the occurrence of dislocation hardening and deformation-induced phase transformation during tensile deformation. We used transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) to characterize and analyze the dislocation configurations and new face-centered cubic (FCC) phase during tensile deformation. We found that, to accommodate plastic deformation between the microcrystalline soft lamellae and ultrafine hard lamellae, many geometrically necessary dislocations (GNDs) were generated, which inevitably led to dislocation hardening, thereby greatly improving the ductility of the alloy. As a result of dislocation hardening, the improved ductility induced two types of FCC phase transformations, and the FCC phase quantity increased significantly during further tensile deformation. Plastic deformation can be effectively accommodated by the formation of a new FCC phase so that the ductility of the material can be further improved. These accommodating plastic deformation mechanisms reveal the nature of the good ductility of the material under a tensile load and provide a new strategy for fabricating strong and ductile titanium alloys. [ABSTRACT FROM AUTHOR]

The main objective of this study was to optimize mechanical properties of TC21 alloys by combining hot rolling with single ageing. The microstructure was characterized by optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and image analysis software. Results showed that hot-rolled and 520 °C aged samples exhibited more excellent comprehensive properties. It was observed that the rolling process not only refined primary α phase (α p), but also resulted in the precipitation of fine secondary α phases (α s) in the β transformed microstructure (β trans). The ageing treatment enhanced the precipitation of more α s , but had little effect on the α p size. Hot-rolled sample exhibited the higher strength than the as-received sample, which was attributed to an increase in the number of α p /β trans interfaces and the refinement of α s precipitations. The aged samples had the higher strength than the hot-rolled sample due to an increase in the number of α s /β interfaces. The as-received and hot-rolled samples exhibited higher ductility than the aged sample, because the β trans underwent a larger deformation and had a better coordinated deformation between α p and β trans of the as-received and hot-rolled samples. Both strength and ductility of TC21 alloy decreased with increasing ageing temperature, the decrease in the strength was attributed to the reduction of the number density of α s precipitates, whereas the decrease in the ductility was attributed to the reduction in the α p volume fraction and the coarsening of α s. [ABSTRACT FROM AUTHOR]