Search Results (9)

The effects of different processes for heat treatment on microstructures and mechanical properties of a Ti-Al-V-Cr-Fe-based alloy (TLC002) were investigated based on the Ti-6411 alloy designed by Northwest Institute for Nonferrous Metals Research. The results show that the TLC002 alloy treated with solid solution and aging has high strength and low impact toughness. For the annealed specimens, both strength and impact toughness are high. With the rising annealing temperature from 800 °C to 880 °C, the tensile strength (UTS), yield strength (YS), and impact toughness (α_u2) increase, especially for the α_u2 from 48.7 J/cm² to 86.0 J/cm². The tensile and impact specimens treated with both solid solution and aging and annealing are all typical ductile fractures. Both the size dimension and depth of the dimples for the equiaxed structures are greater than those of the bimodal structures, indicating that the plasticity of the equiaxed structures is superior to that of the bimodal structures. The heat treatment that annealing at 880 °C for 1.5 h and then air cooling leads to qualified mechanical properties and a good match of the strength and plasticity of the TLC002 alloy. Full article

The resistance to near-threshold fatigue crack growth and its correlation with the microstructure of the Ti-5Al-3Mo-3V-2Zr-2Cr-1Nb-1Fe alloy were investigated. K-decreasing fatigue crack propagation rate tests were conducted on compact tension samples (ASTM standard) with a stress ratio R of 0.1 and a frequency of 15 HZ in a laboratory atmosphere. At a similar strength level of 1200 MPa, the sample with a fine basket-weave microstructure (F-BW) displayed the slowest near-threshold fatigue crack propagation rate compared with the samples with equiaxed (EM) and basket-weave (BW) microstructures. The fatigue threshold value (ΔK_th) was 4.4 MPa·m^1/2 for F-BW, 3.6 for BW, and 3.2 for EM. The fracture surfaces and crack profiles were observed by scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD) to elucidate the mechanism of fatigue crack propagation in the near-threshold regime. The results revealed that the near-threshold crack growth in the three samples was primarily transgranular. The crack always propagated parallel to the crystal plane, with a high Schmid factor. In addition, the near-threshold fatigue crack growth behavior was synergistically affected by the crack tip plastic zone and crack bifurcation. The increased fatigue crack propagation resistance in F-BW was attributed to the better stress/strain compatibility and greater number of interface obstacles in the crack tip plastic zone. Full article

High-temperature titanium alloys are one of the most important research directions in the field of high-temperature aerospace alloys. They are mainly used in high-temperature-resistant components, such as blade disks, blades, and casings of aero-engines, and are key materials in a new generation of high thrust-to-weight ratio aero-engines. In the service environment of engineering applications, the creep resistance of high-temperature titanium alloys is one of the most important characteristic indicators. This paper reviews and analyzes the research status and progress on the creep properties of typical high-temperature titanium alloys in service in recent years. The effects of the creep parameters, alloy composition, and microstructure on the creep behavior of high-temperature titanium alloys are discussed, and various possible mechanisms for increasing the creep resistance of high-temperature titanium alloys are summarized. Full article

The microstructure and room temperature tensile properties of heat-treated TC25G alloy after thermal exposure were investigated. The results show that the α₂ phase dispersed in the α phase, and silicide precipitated firstly at the α/β phase boundary and then at the dislocation of the α_p phase and on the β phase. When thermal exposure was 0–10 h at 550 °C and 600 °C, the decrease of alloy strength was mainly due to the dominant effect of dislocations recovery. With the rise and extension of thermal exposure temperature and time, the increasing quantity and size of precipitates played an important role in the improvement of alloy strength. When thermal exposure temperature rose to 650 °C, the strength was always lower than that of heat-treated alloy. However, since the decreasing rate of solid solution strengthening was smaller than the increasing rate of dispersion strengthening, alloy still showed an increasing trend in the range of 5–100 h. When thermal exposure time was 100–500 h, the size of the α₂ phase increased from the critical value of 3 nm to 6 nm, and the interaction between the moving dislocations and the α₂ phase changed from the cutting mechanism to the by-pass mechanism (Orowan mechanism), and thus alloy strength decreased rapidly. Full article

AlCoCrFeNiTi high-entropy alloys (HEAs) have attracted much attention because of their excellent mechanical properties. Here, we systemically studied the effects of elements on the microstructure and mechanical properties of AlCoCrFeNiTi HEAs. The results showed that the phase composition and morphology are significantly affected by the elements. With increasing Ti addition, the lattice parameters of the solid solution phase increased slightly, and lattice distortion occurred. Al changes the crystal structure of FCC to BCC and reduces the lattice distortion energy of the alloy. The BCC phase obviously increases with increasing Al content. However, excessive Al, Ti and Cr promote the formation of intermetallic compound phases, while Ni, Fe and Co promote the transformation of the alloy into a solid solution. The properties of AlCoCrFeNiTi HEAs are closely related to their phase composition and morphology. When HEAs consist only of FCC and BCC phases, their ductility and strength are greatly improved. The presence of an intermetallic compound phase in the microstructure can significantly reduce the configurational entropy of adjacent solid solutions, thus reducing the strengthening effect of solid solutions. Additionally, the AlCoCrFeNiTi HEAs with different microstructures show different deformation mechanisms. The deformation of FCC + BCC HEAs with cellular structures is uniform and presents great plasticity and strength. When the cellular-structure HEAs contain equiaxed BCC, thick lamellar BCC/FCC or intermetallic compound phases, cracks tend to occur and propagate along the phase boundary due to the local nonuniform deformation. For AlCoCrFeNiTi HEAs with dendrite structures, after initiation at the phase boundary, the crack does not easily spread to the dendrite FCC phase but causes the interdendritic BCC phase to crack. Full article

Gradient structure metals have good comprehensive properties of strength and toughness and are expected to improve the dynamic mechanical properties of materials. However, there are few studies on the dynamic mechanical properties of gradient structured materials, especially titanium alloys. Therefore, in this study, ultrasonic surface rolling is used to prepare a gradient structure layer on the surface of Ti-6Al-4V, and the quasi-static and dynamic compressive properties of coarse-grained Ti-6Al-4V (CG Ti64) and gradient-structured Ti-6Al-4V (GS Ti64) are investigated. The results show that a GS with a thickness of 293 µm is formed. The quasi-static compressive strength of GS Ti64 is higher than that of CG Ti64. Both CG Ti64 and GS Ti64 exhibit weak strain hardening effects and strain rate insensitivity during dynamic compression, and the compressive strength is not significantly improved. The lateral expansion of CG Ti64 is more obvious, while the lateral side of GS Ti64 is relatively straight, indicating that uniform deformation occurs in GS Ti64. The α phase in the GS produces dislocation cells and local deformation bands, and the lamellar structure is transformed into ultrafine crystals after dynamic compression. Both of them produce an adiabatic shear band under 2700 s⁻¹, a large crack forms in CG Ti64, while GS Ti64 forms a small crack, indicating that GS Ti64 has better resistance to damage. The synergistic deformation of GS and CG promotes Ti-6Al-4V to obtain good dynamic mechanical properties. Full article

Ti₂AlNb alloys with few-layer graphene were fabricated by spark plasma sintering (SPS) to enhance the tribological properties (TP) of the composite materials. Microstructure characteristics of the original few-layer graphene (FLG), Ti₂AlNb powders, and the sintered composites were characterized by X-Ray Diffraction (XRD), scanning electron microscopy (SEM), and Raman spectroscopy. The experimental results indicated that FLGs were homogeneously distributed in the composites. Tribological results indicated that the coefficient of friction (COF) of the composites was reduced as the content of FLG increased. Compared with the pure Ti₂AlNb alloy, the average COF of the composite with 1.0 wt.% FLG was decreased by 9.4% and the wear rate was decreased by 36%. Meanwhile, the microstructures of the worn surface showed that TiC particles and friction layers formed by residual FLGs were present on the surface of the composites after tribological test. It is proposed that Ti₂AlNb alloys with FLGs presented the enhanced wear resistance. Full article

Ti-6Al-4V alloy (TC4) with different concentrations of graphene nanoplatelets (GNPs) were fabricated by ball milling and spark plasma sintering (SPS). Microstructure characteristics of the composites were characterized by X-Ray Diffraction (XRD), Scanning electron microscopy (SEM), and Raman. Microhardness and the compressive mechanical properties were also investigated. Experimental results showed that in the process of SPS, most of the GNPs were still retained at high pressure and temperature, and a new phase of TiC was presented due to the in-situ reaction between TiC and GNPs. Also, the strength of the composites was depended on the concentration of GNPs in TC4 matrix. Consequently, the composite with 0.8 wt. % GNPs was increased 18% in microhardness. The maximum yield strength and ductility of the composite were increased by 22.2% and 43.2%, respectively. The strengthening mechanism of the composites was further discussed, and the Orowan strengthening mechanism was the main strengthening factor. Full article

The hot deformation behavior of a new Al–Mn–Sc alloy was investigated by hot compression conducted at temperatures from 330 to 490 °C and strain rates from 0.01 to 10 s⁻¹. The hot deformation behavior and microstructure of the alloy were significantly affected by the deformation temperatures and strain rates. The peak flow stress decreased with increasing deformation temperatures and decreasing strain rates. According to the hot deformation behavior, the constitutive equation was established to describe the steady flow stress, and a hot processing map at 0.4 strain was obtained based on the dynamic material model and the Prasad instability standard, which can be used to evaluate the hot workability of the alloy. The developed hot processing diagram showed that the instability was more likely to occur in the higher Zener–Hollomon parameter region, and the optimal processing range was determined as 420–475 °C and 0.01–0.022 s⁻¹, in which a stable flow and a higher power dissipation were achieved. Full article