Enhanced hydrogen storage properties of ZrTiVAl1-x Fe x high-entropy alloys by modifying the Fe content.

Lightweight ZrTiVAl high-entropy alloys have shown great potential as a hydrogen storage material due to their appreciable capacity, easy activation, and fast hydrogenation rates. In this study, transition metal Fe was used to improve the hydrogen storage properties of the equimolar ZrTiVAl alloy, and ZrTiVAl1-x Fe x (x = 0, 0.2, 0.4, 0.6, 0.8, 1) alloys were prepared to investigate the microstructure evolution and hydrogen storage properties. The results show that the ZrTiVAl1-x Fe x alloys are composed of a C14 Laves phase and Ti-rich HCP phase. With Fe substituting Al, the fraction of the C14 Laves phase increases and that of the HCP phase decreases. Besides, the interdendritic area fraction reaches the maximum when the Fe ratio is 0.2. The element V transferred to the C14 Laves phase from the HCP phase, which is caused by the strong affinity between V and Fe. The ZrTiVAl1-x Fe x alloys show enhanced hydrogenation kinetics and capacities. Notably, the ZrTiVFe alloy can reversely absorb 1.58 wt% hydrogen even at room temperature under 1 MPa H2. The reduced interdendritic phase is beneficial to shorten the H atom diffusion distance, thus improving the hydrogenation rates. Both the transfer of the hydrogen-absorbing element V to the C14 Laves phase and the increased fraction of the C14 Laves phase lead to the increase of hydrogen storage capacity with the addition of Fe. Moreover, the increased Fe content leads to an increase of average valence electron concentration (VEC), where a larger VEC destabilizes the hydrides, and the desorption temperature of ZrTiVAl1-x Fe x hydride decreases significantly.

The effects of the formation of a multi-scale reinforcing phase on the microstructure evolution and mechanical properties of a Ti2AlC/TiAl alloy.

In order to acquire TiAl composites with a multi-scale reinforcing phase, and to improve the microstructure and tensile properties at elevated temperatures, TiAl alloys have been prepared with different added carbon content levels via vacuum arc melting. The results show that when the carbon content is greater than or equal to 1.0 at%, then Ti2AlC forms and the microstructure changes from having a dendrite morphology to an equiaxed crystal morphology. The B2 phase disappears in the Ti2AlC-containing alloys. As the carbon content increases from 0 to 3.0 at%, the lamellar colony size decreases from 148.4 to 32.8 µm and the lamellar width decreases from 441.2 to 117.6 nm. More nanoscale Ti2AlC particles form in the α2 lamellae at a higher carbon content, and there are a lot of dislocations around them. As the carbon content, the Ti2AlC content increases from 0 to 16.8 vol% and the length-diameter ratio decreases from 9.2 to 1.8. The reason for the microstructure refinement is that carbon and carbide act as heterogeneous particles during solidification, and carbide dissolves some alloy elements, improving the microstructure uniformity. Compressive testing shows that the maximum compressive strength is 2324.3 MPa at a carbon content of 1.5%. At a carbon content of 2.5%, the compression strain is higher (28.1%). Tensile testing at elevated temperatures shows that upon increasing the temperature from 750 to 850 °C, the tensile strength increases from 398 to 541 MPa, and the strain increases from 6.1 to 12.2% with a temperature increase from 750 to 950 °C. The increase in the mechanical properties is attributed to the refined lamellar colonies and lamellar width, the solid solution of elements, and the formation of nanoprecipitates.

Effect of mechanical combined with electromagnetic stirring on the dispersity of carbon fibers in the aluminum matrix.

In order to improve the uneven distribution of carbon fibers (CFs) in the matrix by traditional single mechanical stirring, mechanical combined with electromagnetic (M-E) stirring was used to prepare short carbon fibers reinforced aluminum matrix (Csf/Al) composites. The 3-D flow field of aluminum melt under mechanical/M-E stirring were calculated and compared. The calculation results show that the complexity of flow field under M-E stirring could be significantly enhanced relative to a single mechanical stirring, especially there was a strong melt flow near the crucible wall due to the skin effect. It was found that except the inertial force under mechanical stirring and the melt collision with the crucible walls, CFs were also subjected to the electromagnetic force and the oscillating flow between the eddy currents, which would promote the dispersity of the short CFs in the composites. The experimental results are consistent with the calculation results. The experimental results show that the distribution of CFs at each position in the composite samples prepared under M-E stirring was stable. The uniform distribution of CFs in the composites would play an important role in improving the overall performance of the Csf/Al composites.

An as-cast high-entropy alloy with remarkable mechanical properties strengthened by nanometer precipitates.

High-entropy alloys (HEAs) with good ductility and high strength are usually prepared by a combination of forging and heat-treatment processes. In comparison, the as-cast HEAs typically do not reach strengths similar to those of HEAs produced by the forging and heat-treatment processes. Here we report a novel equiatomic-ratio CoCrCuMnNi HEA prepared by vacuum arc melting. We observe that this HEA has excellent mechanical properties, i.e., a yield strength of 458 MPa, and an ultimate tensile strength of 742 MPa with an elongation of 40%. Many nanometer precipitates (5-50 nm in size) and domains (5-10 nm in size) are found in the inter-dendrite and dendrite zones of the produced HEA, which is the key factor for its excellent mechanical properties. The enthalpy of mixing between Cu and Mn, Cr, Co, or Ni is higher than those of mixing between any two of Cr, Co, Ni and Mn, which leads to the separation of Cu from the CoCrCuMnNi HEA. Furthermore, we reveal the nanoscale-precipitate-phase-forming mechanism in the proposed HEA.

An innovation for microstructural modification and mechanical improvement of TiAl alloy via electric current application.

In this article, microstructural evolution during the solidification of Ti-48Al-2Cr-2Nb with current density, as well as the formation mechanisms, are discussed, along with the impacts on microhardness and hot compression properties. The applied electric current promotes the solidification from the α primary phase to a largely ß solidification in Ti-48Al-2Cr-2Nb. With an increase in supercooling, the solidification process have a tendency to change from an α-led primary phase to (α + ß)-led primary phase. The primary dendrites, grain size, and lamellar spacing show a tendency to decrease first before increasing with increasing current density. Microhardness and high-temperature yield strength increase with a decrease in primary dendrite spacing, grain size, and lamellar spacing. Correlations between primary dendrite spacing, lamellar spacing, microhardness, yield strength, and current density are described by a fitting formula. An increase of α2 phase, due to the application of electric current, results in improved microhardness. The yield strength of Ti-48Al-2Cr-2Nb alloy increases linearly with microhardness. Yield stress increases with a decrease in microstructure parameters, in accordance with the Hall-Petch equation. The predominant modification mechanism with electric current application for TiAl solidification is the variation of supercooling and temperature gradients ahead of the mush zone due to Joule heating.

Effects of Tantalum on Microstructure Evolution and Mechanical Properties of High-Nb TiAl Alloys Reinforced by Ti2AlC.

Experiments have been carried out to study the relationship between the addition of tantalum and microstructure, especially the formation of the B2 phase in lamellar colonies. The mechanical properties, with different contents of Ta, were also measured. Ti46Al8Nb2.6CxTa alloys were prepared by casting with the content of Ta varying from zero to 1.0 at.%. Experimental results show that the B2 phase forms in lamellar colonies with the addition of Ta, and its content increases when the content of Ta increases. Meanwhile, the γ phase decreases and the lattice parameter of the α 2 phase increases. The size of the lamellar colony decreased from 29.9 to 21.6 µm. Ta dissolves into Ti2AlC by substitution, and its solubility is more than 1.1% tested by EDS. Nb, which is necessary for the formation of the B2 phase, comes from two aspects. The first is that Ta dissolves into the Ti2AlC and partly replaces the Nb atom and the second is the decrease in the γ phase because it has higher solid solubility for Nb. The increase in Nb in the liquid phase increases the composition supercooling and heteronucleation at the solidification front, which accounts for refining the lamellar colony. Room temperature compressive testing showed that the compressive strength and the strain increased when the Ta content increased up to 0.8% and then decreased. Improvement of the compressive properties resulted from the grain boundary strengthening and their decrease induced by more content of the B2 phase. Tensile properties, at elevated temperature, were improved with testing temperature increasing from 750 to 950°C, because solid solution strengthening is a major influence factor.

High-density deformation nanotwin induced significant improvement in the plasticity of polycrystalline γ-TiAl-based intermetallic alloys.

Intermetallic alloys with high melting point can mostly serve as promising high-temperature structural materials, but their intrinsic brittleness limits their further application. Herein, we developed a strategy to realize high strength and high plasticity simultaneously in Cr-rich γ-TiAl-based intermetallic alloys via introducing high-density deformation nanotwins. Non-equilibrium continuous casting followed by annealing in the (α + γ) phase region generated numerous Shockley partial dislocations and stacking faults as well as a number of α2 nanoparticles in the γ-TiAl phase. The substantial Shockley partial dislocations and stacking faults acting as effective heterogeneous nucleation sites favored the generation of high-density nanotwins in the as-annealed alloys during deformation, especially within the γ lamellae. This strategy can also be applied to other brittle alloys with a favorable twinning deformation mechanism and paves the way for the development of high-strength and high-ductility materials.

Effects and mechanism of ultrasonic irradiation on solidification microstructure and mechanical properties of binary TiAl alloys.

In spite of their high temperature and reactivity, the binary TiAl alloys are successfully imposed by the ultrasonic irradiation and the microstructure evolution, solidification behaviors and mechanical properties are elaborately investigated. After ultrasonic irradiation, a high quality ingot without shrinkage defects and element segregation is obtained and the coarse dendrite structure is well modified into fine non-dendrite globular grains. The coarse lamellar colony and lamellar space of Ti44Al alloy is refined from 685µm to 52µm and 1185nm to 312nm, respectively (similarly, 819µm to 102µm and 2085nm to 565nm for Ti48Al alloy). For Ti48Al alloy, the α peritectic phase is simultaneously precipitated from the melt as well as the ß primary phase before the peritectic reaction and the solidification is transformed into the mixed α-solidifying and ß-solidifying. Ultrasonic irradiation promotes the peritectic reaction and phase transformation completely and the phase constituent becomes more close to the equilibrium level. The compressive strength of Ti44Al and Ti48Al alloys are increased from 623MPa to 1250MPa and 980MPa to 1295MPa, respectively. The grain refinement and dendrite transformation enhance the grain boundary sliding improving the plastic deformation ability. Ultrasonic irradiation significantly accelerates the melt flow and solute redistribution and the main grain refinement mechanism is the cavitation-enhanced nucleation by inclusion activation and heightened supercooling.

Design of (Nb, Mo)40Ti30Ni30 alloy membranes for combined enhancement of hydrogen permeability and embrittlement resistance.

The effect of substitution of Nb by Mo in Nb40Ti30Ni30 was investigated with respect to microstructural features and hydrogen dissolution, diffusion and permeation. As-cast Nb40-xMoxTi30Ni30 (x = 0, 5, 10) alloys consist of primary bcc-Nb phase and binary eutectic (bcc-Nb + B2-TiNi). The substitution of Nb by Mo reduces the hydrogen solubility in alloys, but may increase (x = 5) or decrease (x = 10) the apparent hydrogen diffusivity and permeability. As-cast Nb35Mo5Ti30Ni30 exhibits a combined enhancement of hydrogen permeability and embrittlement resistance as compared to Nb40Ti30Ni30. This work confirms that Mo is a desirable alloying element in Nb that can contribute to a reduction in hydrogen absorption and an increase in intrinsic hydrogen diffusion, thus improving embrittlement resistance with minimal permeability penalty.