Intermetallics最新文献

Nanoindentation test, high-temperature compression test, SEM, EBSD, and TEM observations are performed to investigate the effects of Hf additions on the microstructure and properties of NiTi alloy. The high-temperature deformation behavior of NiTi-8/12Hf alloys is discussed. The constitutive model and hot processing map are constructed, and the optimal processing interval of the two alloys is predicted. The results show that the content and size of precipitates increase with the addition of Hf. The nanoindentation hardness of the matrix with different Hf content increased from 3.36 ± 0.11 GPa to 5.47 ± 0.13 GPa. The main strengthening mechanism of the addition of the Hf to improve the strength is the solid solution strengthening effect and the load transfer effect. With the addition of 8 wt% and 12 wt% Hf, the instability areas are expanded. The optimal processing area of 8/12Hf alloys are 900 °C, 0.01s⁻¹. Under this condition, the microstructure of the alloy has a high DRX volume fraction and low average ρ_GND. As hot deformation progresses, the accumulated energy inside the alloy increases, leading to the activation of dislocations and the formation of more DDRX at grain boundaries. Dislocations within the grains aggregate to form LAGBs. The softening mechanism of 56Ni-Ti-8/12Hf alloy are DDRX and dynamic recover.

This paper comprehensively explores the causes of intermediate-temperature brittleness of a fourth-generation Nickel-based single-crystal superalloy through a systematic study of tensile properties and deformation mechanisms at intermediate-temperatures. The research was performed at temperature of 700 °C, 800 °C, 850 °C and 900 °C. The experimental alloy demonstrated the highest yield strength of 914 MPa and worst plasticity at 800 °C. It was found that pure-shearing fracture occurred at 700 °C and 800 °C, while the fracture characteristics of shear fracture and ductile fracture were found at 850 °C and 900 °C. Then, the slip bands extended in the same direction at 700 °C and 800 °C. The phenomenon was different at 850 °C and 900 °C. These deformation traces extended in the different directions. At 700 °C and 800 °C, the partial dislocation with Burgers Vector of a/3 <112> shearing into the γ′ phases was the predominate deformation mechanism, while both the partial dislocation with Burgers Vector of a/3 <112> and the super-dislocation with Burgers Vector of a <110> and a <010> shearing into the γ′ phases were present at 850 °C and 900 °C. Nevertheless, the mechanism of the super-dislocation with Burgers Vector of a <110> and a <010> shearing γ′ phases pervaded in the alloy at 900 °C. In general, it was concluded that the alloy underwent intermediate-temperature brittleness at 800 °C in terms of the changes of fracture features, slip bands and dislocation configurations. The results of this study provided an experimental reference and guidance for improving the safe serviceability of the fourth-generation single crystal superalloy.

Titanium aluminides (TiAl) are distinguished by their exceptional strength-to-weight ratio, making them ideal for aerospace and medical applications. Notably, TiAl alloys offer a unique combination of high-temperature resistance and corrosion resilience, contributing to their growing prominence in advanced engineering and biomedical fields. Although initially developed for aerospace applications, TiAl alloys have demonstrated promising potential as implant materials over time. Hence, this research focuses on producing γ-TiAl alloy through electron beam powder bed fusion (EB-PBF) technology, utilising a powder with a composition of Ti-48Al-2Cr-2Nb. For comparative purposes, the corrosion characteristics of Ti6Al4V produced via EB-PBF were also evaluated under identical conditions. The findings indicate that the EB-PBF γ-TiAl exhibits exceptional resistance to corrosion. This is supported by the significantly high polarisation resistance and corrosion potential values, as well as the notably low corrosion current value. However, based on the analysis of the polarisation and impedance curves, it can be observed that the γ-TiAl sample displayed a less protective passive film formation. This occurrence can be attributed to the presence of aluminium ions within the passive layer, resulting in the formation of unstable oxides. As a consequence, it can be inferred that γ-TiAl exhibits inferior resistance to pitting corrosion when compared to Ti6Al4V alloy. The point defect model and Mott-Schottky test further revealed that the γ-TiAl alloy exhibited increased oxygen vacancies. Additionally, the presence of aluminium ions as impurities or dopants led to their substitution for titanium ions, creating cationic vacancies within the passive film. The accumulation of excessive cation vacancies ultimately led to the initiation of pitting corrosion.

The addition of rare earth elements (REEs) can effectively modify the microstructure of alloy and improve the properties. The effects of doping REEs in equiatomic CrMnFeCoNi high entropy alloys were investigated. Doping with REEs led to the formation of a few new phases enriched in the interdendritic regions and reduced the grain size. Doping REEs significantly improved the hardness and wear resistance of the alloy, and the Sm-doped alloy showed the most notable enhancement with the hardness value, the average friction coefficient and wear rate of HV 347.9, 0.28 and 0.91 × 10⁻⁶ mm³ N⁻¹ m⁻¹, respectively. The mechanical properties of the alloy were improved by the second phase strengthening and grain refinement and the wear mechanism was typical abrasive wear. However, the electrochemical properties indicated that doping with REEs weakened the corrosion resistance of the alloys; Pr- and La-doped alloys altered the corrosion behaviors on the alloy surfaces. The self-corrosion current density and potential of CrMnFeCoNiSm_0.2 alloy were 2.94 × 10⁻⁶ A cm⁻² and -0.46 V, respectively. This research plays a guiding role to study in REEs doping in HEA, meanwhile is of great significance in promoting the industrial application of HEA.

Low-cycle fatigue behavior of high-entropy alloy (HEAs) and medium-entropy alloys (MEAs) have rarely been reported in literature though it is critical for industrial applications. In our previous work, a non-equiatomic Ni₂Co₁Fe₁V_0.5Mo_0.2 MEA was designed by adding V and Mo elements with bigger atomic size to heighten solution strengthening effect. Due to larger atomic size mismatch and more severe lattice distortion, the Ni₂Co₁Fe₁V_0.5Mo_0.2 MEA exhibits stronger strain hardening effect than that in CoCrFeMnNi HEA. In present work, the low-cycle fatigue behavior of the non-equiatomic Ni₂Co₁Fe₁V_0.5Mo_0.2 MEA with heterogeneous grain structures was further investigated. The heterogeneous Ni₂Co₁Fe₁V_0.5Mo_0.2 MEA exhibits high fatigue resistance at 0.25 % strain amplitude (51285 N), attributed to the pronounced dislocation planar slip and formation of stacking faults. At 0.3 % and 0.5 % strain amplitudes, dislocation interactions (including tangles and microbands) induced by extensive dislocation cross-slip result in obvious cyclic hardening but reduced lifetime. The findings assess the effect of solid solution strengthening on the fatigue behavior of MEAs. The fatigue cracks form either along slip bands in large grains or grain boundaries of small grains.

A complex concentrated alloy (CCA) with a nominal composition of Al_0.35CoCrFeNi (mol.%) was prepared by vacuum induction melting and tilt casting. The microstructure of the alloy in the as-cast state consists of columnar dendritic grains. The ingots were solution annealed, rotary swaged, and heat treated to obtain a uniform fine-grain structure. To study the behavior of recrystallization and grain growth, heat treatment was carried out at temperatures from 1150 °C to 1300 °C and holding times up to 480 min. The resulting microstructures were analyzed by LM, SEM, TEM, EBSD, and XRD methods followed by a comparison with the results of hardness measurements. The alloy has a thermally stable single-phase face-centered cubic (FCC) structure in the studied temperature range. The grain growth kinetics were analyzed using classical models, and the activation energy was estimated to be ∼458 kJ mol⁻¹ using an Arrhenius-type equation. The greatest resistance to grain growth was observed at a temperature of 1150 °C. Hardness tests demonstrated an almost double increase in hardness after swaging and a sharp drop during the following heat treatment due to the onset of recrystallization. The Hall-Petch hardening coefficient was calculated to be ∼277.5 HV μm^−1/2.

Ti₂AlNb alloy, as a highly promising superalloy in the aerospace field, is limited by inferior workability due to centimeter-grade coarse grains formed through casting. An in-depth understanding of the relationship between deformation heterogeneities and recrystallization kinetics of the matrix B2 phase is critical to refine and optimize its microstructure. Plane strain compression followed by heat treatment, microstructure characterizations, and full-field crystal plasticity simulations were conducted. The research found that uniform primary-slips existed in most regions of the alloy. These regions exhibited negligible deformation stored energy and misorientation, and therefore, recrystallization cannot occur after heating. The observed slip transfer at grain boundaries with good geometric alignment also indicates the difficulty in dislocation pileup as the potential recrystallization site. Three typical band-like structures, i.e., transition band, slip-interlacing band, and shear band, formed by intersection and localization of slips, possessed high deformation stored energy. Cell-like substructures readily developed in the first two regions with intersecting slips, rather than in the shear band with parallel slips. As a result, many subgrains and unclosed boundaries were formed in the first two types of bands within grains after heating due to the significant recovery effect. These multilevel deformation heterogeneities were found to be strongly associated with the dislocation structure of the alloy. TEM observations found the dissociation of dislocations with narrow widths, which enhances dislocation mobility. Consequently, the primary-slip characteristic can be maintained at a relatively large deformation, and slip transfer can occur at grain boundaries where a good geometric alignment exists.

In this study, Al₂O₃/TiAl composites were synthesized via powder metallurgy by incorporating TiO₂ nanoparticles and nanofibers as oxygen sources into Ti-45Al-8Nb pre-alloy powders, followed by vacuum hot-pressing sintering to form in-situ Al₂O₃ particles as reinforcements. The addition of TiO₂ nanofibers results in a better grain refinement effect and a more uniform distribution of Al₂O₃ particles within the composites. High-temperature tensile testing revealed that the composites prepared using TiO₂ nanofibers exhibited slightly higher strengths and significantly improved ductility compared to those synthesized with TiO₂ nanoparticles. This work not only introduces a novel additive for fabricating high-performance in-situ Al₂O₃/TiAl composites but also demonstrates a unique application of TiO₂ nanofibers.

The article describes how substitution of chromium by copper affects phase equilibria in Fe–Ni–Co–Cr high-entropy alloy. The alloys with copper content ranging from 0 to 20 % (at.) of Cu were prepared. The alloys were equilibrated at 900, 800, 700, and 650 °C. The samples were investigated by electron microscopy, EDX spectroscopy, and EBSD method. The face-centred cubic phases have only occurred in equilibrated alloys: austenite matrix, copper-rich FCC(Cu) phase, and regions containing these phases. The compositions of equilibrated samples at annealing temperatures are given. Fine microstructures including a semicoherent FCC phase rich in Cu and an FCC phase including the main magnetic elements (Fe, Co, Ni) were formed. The experimental results were compared with the calculated phase equilibria obtained by the CALPHAD method.

As cast (CrCoNi)₉₆V₄ medium entropy alloy（MEA）was prepared by vacuum arc melting. The MEA is deformed by rolling at room temperature with a total deformation of 67 %, The yield strength and tensile strength of the deformed MEA are 1297 MPa and 1410 MPa respectively, the elongation of the alloy is only 8 %.Then the deformed MEA was annealed for 30min at 700 °C, 800 °C and 900 °C respectively. The results show that the strength and ductility of the MEA after annealing were well balanced. The yield strength and tensile strength of the MEA after annealing at 900 °C/30 min are 678 MPa and 1024 MPa, and the elongation is maintained at 26 %. After annealing, a large number of annealing twins were formed in the MEA, which makes the strength and ductility of the MEA significantly improved. At the same time, Cr-rich particles were found in the MEA, which had positive effect on the improvement of the strength and ductility of the alloy.