Al-containing Alloys Research Articles

Oxidation behaviour of Ti-V-Cr-Nb-Ta and Al-Ti-V-Cr-Ta refractory high entropy alloys: Effects of Nb and Al substitutions

Refractory high entropy alloys (RHEAs) are candidate materials for nuclear and other high-temperature applications, due to their high-temperature strength, good irradiation resistance, and high melting temperatures. However, a significant issue with current RHEAs is that those exhibiting good mechanical properties tend to show poor oxidation resistance, and vice versa. In this paper, the oxidation kinetics of four RHEAs (20Ti-20V-20Cr-20Nb-20Ta, 25Ti-25V-5Cr-20Nb-25Ta, 20Al-20Ti-20V-20Cr-20Ta, and 14Al-24Ti-24V-14Cr-24Ta alloys) were investigated in an air atmosphere at 1000 °C. As-prepared and oxidised samples were characterised by a combination of state-of-the-art microscopy techniques. By replacing Nb with Al, the two Al-containing alloys were observed to form a less porous oxide microstructure, showing significant improvement in their oxidation resistance. As a result of oxygen/nitrogen ingress during oxidation and associated phase-segregation at high temperatures, the hardness of the underlying metal matrix of the RHEAs increased by approximately 5 GPa.

Revealing the environmental-assisted degeneration mechanism of an Al-containing dual-phase high entropy alloy in supercritical water

A dual-phase high entropy alloy (DP-HEA) containing 7.82 wt% Al was synthesized to withstand corrosive supercritical water. The long-term corrosion and quasi-static tensile tests reveal superior resistance to such environmental-assisted degeneration. The outstanding corrosion resistance can be attributed to the high aluminum content in B2 phase, which forms a continuous Al2O3 layer for protection, while the FCC phase is selectively corroded. Under external stress, stress corrosion cracking (SCC) initiates from surface oxide and penetrates uncompact mixed Cr/Al-rich oxide in deteriorated FCC regions or corroded phase interface, though network-shaped B2 can inhibit SCC propagation, which supports the brittle film rupture model.

Effect of Al addition on microstructure and mechanical property of Mg-Mn-Ca-Ce based alloy

In this work, the effect of Al addition on the microstructure and mechanical properties of as-extruded Mg-0.6Mn-0.4Ca-0.2Ce-xAl (x=0, 0.1, 0.3 wt%; named as MXE, MXE01A, and MXE03A, respectively) alloys was investigated. The addition of the fifth group element Al changed the type and content of the second phase in the Mg-Mn-Ca-Ce alloys, and both Mg2Ca and Al-Mn nano-phases were precipitated in the Al-containing alloys, but more unfavorable micron-sized second phases were formed in the MXE03A alloy. In the MXE01A alloy, the high density of nanoscale second phases hindered recrystallized grain growth and contributed to the refinement of recrystallized grains to ∼0.94 μm. At the same time, the MXE01A alloy contains a high density of residual dislocations, which become obstacles to the movement of newly generated dislocations. As a result, the yield strength (YS) of the MXE01A alloy reached 330 MPa, and the elongation (EL) was maintained at 17.5 %. The present results can provide important theoretical guidance for the compositional optimization of low-cost and high-performance Mg wrought alloys.

Simulated Body Fluid-Assisted Stress Corrosion Cracking of a Rapidly Solidified Magnesium Alloy RS66.

This study investigated the simulated body fluid-assisted stress corrosion cracking (SCC) of an Al-free magnesium alloy (RS66) and a common Al-containing magnesium alloy (AZ91), the former being more suitable for temporary implant applications (however, we used AZ91 for comparison since there are considerable reports on SCC in this alloy). The investigation includes SCC tests under simultaneous conditions of mechanical loading and imposed electrochemical potential that established a combined effect of hydrogen and anodic dissolution as the embrittlement mechanism. Though the RS66 alloy possesses impressive mechanical properties in non-corrosive environments (as a result of its fine grain size), both alloys suffered significant embrittlement when tested in simulated body fluid. The susceptibility of the RS66 alloy to SCC was ~25% greater than that of AZ91, which is attributed to the greater resistance of AZ91 to corrosion/localised corrosion because of its Al content.

Corrosion Behavior of Al-Containing CoNiV Medium Entropy Alloy Coating Fabricated by Laser Cladding.

CoNiV medium entropy alloys (MEAs) have been widely recognized for their superior corrosion resistance. Nonetheless, their extensive application has been hindered by high production costs and complex fabrication processes. In this study, CoNiVAlx MEA coatings were synthesized on AISI 304 stainless steel substrates via laser cladding technology. The microstructure, phase composition, corrosion resistance, and corrosion mechanisms of the coatings were systematically investigated by using advanced characterization techniques, including optical microscopy, scanning electron microscopy, transmission electron microscopy, X-ray diffractometry, and electrochemical workstation analysis. The ratio of O2-/OH- in the passivation film of the coated surface exhibited a gradual increase with the addition of Al. The formation of the Al-containing precipitated phase L21 was observed at x = 0.3 and 0.4. The results demonstrated that moderate Al doping (x ≤ 0.2) enhanced corrosion resistance by improving the stability of the passivation film and reducing the thermodynamic tendency toward corrosion. In contrast, excessive Al doping (x > 0.2) led to the formation of the L21 phase, which increased the susceptibility to localized corrosion, thus compromising the overall corrosion resistance.

Optimization of chemistry and process parameters for control of intermetallic formation in Mg sludges

Intermetallic formation in sludge during magnesium (Mg) melting, holding and high pressure die casting practices is a very important issue. But, very often it is overlooked by academia, original equipment manufacturers (OEM), metal ingot producers and even die casters. The aim of this study was to minimize the intermetallic formation in Mg sludge via the optimization of the chemistry and process parameters. The Al8Mn5 intermetallic particles were identified by the microstructure analysis based on the Al and Mn ratio. The design of experiment (DOE) technique, Taguchi method, was employed to minimize the intermetallic formation in the sludge of Mg alloys with various chemical compositions of Al, Mn, Fe, and different process parameters, holding temperature and holding time. The sludge yield (SY) and intermetallic size (IS) was selected as two responses. The optimum combination of the levels in terms of minimizing the intermetallic formation were 9 wt.% Al, 0.15 wt.%Mn, 0.001 wt.% (10 ppm) Fe, 690 °C for the holding temperature and holding at 30 mins for the holding time, respectively. The best combination for smallest intermetallic size were 9 wt.% Al, 0.15 wt.%Mn, 0.001 wt.% (10 ppm) Fe, 630 °C for the holding temperature and holding at 60 mins for the holding time, respectively. Three groups of sludge factors, Chemical Sludge (CSF), Physical Sludge (PSF) and Comprehensive Sludge Factors (and CPSF) were established for prediction of sludge yields and intermetallic sizes in Al-containing Mg alloys. The CPSF with five independent variables including both chemical elements and process parameters gave high accuracy in prediction, as the prediction of the PSF with only the two processing parameters of the melt holding temperature and time showed a relatively large deviation from the experimental data. The Chemical Sludge Factor was primarily designed for small ingot producers and die casters with a limited melting and holding capacity, of which process parameters could be fixed easily. The Physical Sludge Factor could be used for mass production with a single type of Mg alloy, in which the chemistry fluctuation might be negligible. In large Mg casting suppliers with multiple melting and holding furnaces and a number of Mg alloys in production, the Comprehensive Sludge Factor should be implemented to diminish the sludge formation.

Phase Stability of Al-containing High-Entropy Alloys: A Comparison Study Using Computational Thermodynamics

Phase Stability of Al-containing High-Entropy Alloys: A Comparison Study Using Computational Thermodynamics

Controllable reinforcement phase distribution at the grain scale via a simple precipitation process

Due to the inability of Zr to refine Al-containing Mg alloys, achieving pronounced grain refinement in cast AZ31 alloy has been a significant challenge for researchers. Surprisingly, we found that the reinforcement phases demonstrated a regular distribution at the grain scale after adding 0.9 wt% Nd and 0.3 wt% Y: Al2RE inside the grains and Al11RE3 agglomerated near the grain boundaries. Additionally, the grain size of AZ31 alloy was significantly reduced from 258 µm to 65 µm. The mechanisms of reinforcement phase distribution and grain refinement were explored through the combination of transmission electron microscope (TEM) and thermal analysis during the solidification process. Al2RE phase precipitates prior to α-Mg, and exhibits a specific orientation relationship with the matrix: 011̅0Mg∥011Al2RE，0002Mg∥22̅2Al2RE. This suggests that Al2RE serves as potentially effective heterogeneous nucleation sites for α-Mg. Moreover, Al11RE3 phase can restrict grain growth in the later stage of solidification. The tensile results show that a regular distribution of the reinforcement phase at the grain scale can not only effectively alleviate stress concentration, but suppress the formation of coarse twins by inducing grain refinement during alloy solidification process, significantly improving the strength and ductility of the alloy.

High-Temperature Oxidation Behavior of FeCoCrNi+(Cu/Al)-Based High-Entropy Alloys in Humid Air

Previous studies showed some transition metal high-entropy alloy (HEA) compositions can have good oxidation resistance in air up to 800 °C. Four equiatomic HEAs have been developed based on FeCoCrNi with additions of Mn, Cu, Al or Al+Cu. The oxidation behavior of these HEAs was compared in humid (10 vol.% H2O) air at 800 °C for 100–500 h to investigate the influence of water vapor on the oxidation mechanisms. The Cu- and Al-containing alloys exhibited improved oxidation resistance over the Mn composition. For the Cu-containing alloy, a local attack of the Cu-rich phase was observed, which formed an Fe/Ni/Co/Cr spinel that was surrounded by Cr2O3. This oxide was thicker for the humid air atmosphere when compared to dry air, and the transition of the Cu oxide to the spinel was accelerated. The Al-containing HEA formed a thin Al2O3 scale with humidity suppressing AlN formation and forming a smoother oxide layer. The Al+Cu composition had the highest overall oxidation resistance (minimal local attack, no nitridation) and also showed a smooth oxide scale topography under humid air oxidation as opposed to a plate-like, rougher scale under dry air.

Hydrogen storage properties of Al-containing Ti2CrV alloys

Hydrogen storage properties of Al-containing Ti2CrV alloys

Evolution of microstructure and texture in Al-containing CoCrNi medium entropy alloys during severe warm-rolling

Al, as an alloying element, can influence the properties of the single-phase equiatomic CoCrNi MEA. In this study, different alloys of different Al concentrations added to CoCrNi MEA (AlxCoCrNi) were prepared by warm rolling to understand the effect of Al. The alloys were warm-rolled at 400 °C up to an 85% thickness reduction, followed by isochronal annealing at 700–1100 °C. The microstructure after deformation reveals the development of deformation heterogeneities such as shear bands and deformation nano-twins. The evolution of the weak brass component is seen after deformation. Different phases, such as BCC and sigma, evolve upon annealing treatment at different temperatures. The higher Al-content MEA (Al9.09) shows comparatively finer recrystallized microstructure irrespective of the annealing temperatures. Also, weak recrystallization texture develops after annealing, and the deformation texture components lying on α-fiber are preserved. This indicates the absence of preferential nucleation and growth during annealing. The hardness in the as-cast, deformed, and annealed conditions increases with increasing Al content, indicating the influence of Al in the strengthening of the AlxCoCrNi MEAs.

Proton irradiation, tribological and corrosion characteristics of Fe-Ni based ODS alloys

In the current study, Fe–42% Ni–2% Y2O3–2%Ti (2Ti) and Fe–42% Ni–2% Y2O3–2%Ti-2%Al (2AlTi) (All in wt%) invar-based alloys were investigated thoroughly under extreme environmental conditions including wear resistance, proton irradiation characteristic, and corrosion behavior. Both the compositions were developed by mechanical alloying (MA) followed by spark plasma sintering (SPS). The phase evolution characteristics and microstructural features of sintered samples were investigated through x-ray diffraction (XRD), and electron backscatter diffraction (EBSD) and TEM analysis. The influence of proton irradiation damage on micrographic features as well as on hardness values of the sintered samples was studied in detail to understand the roles of grain size and complex dispersoids on irradiation damage. Grain boundary seems to play as radiation sinks, whereas regions of dispersoids are found to remain unaffected by proton irradiation. A detailed tribological study was conducted on the sintered samples at 20, 300, and 500 °C to understand the effect of temperature, and correlation between the microstructural constituents and wear mechanisms. Analysis of wear data revealed that material loss is gradually reduced with increase in temperature, especially in the 2AlTi sample, due to fast evolution of Al-containing glossy oxide layer providing a better lubricating film to prevent from further wear. Wear mechanism is gradually changed from adhesive to abrasive wear with increase in temperature due to the development of finer glossy protecting oxide debris. In contrast to the wear behaviour, potentiodynamic polarization test analysis of the sintered samples (conducted in an electrolyte of 3.5% NaCl) revealed that the Al-containing alloy showed rigorous degradation (huge pitting) due to the presence of coarser Al-rich Y-Al-O dispersoids in fine grained matrix.

Experimental analysis and response surface optimization of the influence of Zn/Al mass ratio on the creep resistance and tensile properties of As-cast Mg-Zn-Al alloy

Mg-Zn-Al (ZA) alloys are intended to exhibit creep resistance and high temperature mechanical capabilities due to appropriate Zn/Al mass ratio control. The effect of altering Al concentration (1, 2, 3, 4, 5, 6) on the tensile and creep resistance properties of Mg-9Zn-xAl alloy was determined experimentally and quantitatively optimized using response surface method (RSM). It was observed that 2 wt% Al improves both room and elevated temperature tensile strength, and ductility of the considered ZA alloy. The creep rate sharply increased when the mass fraction of aluminum rose above 4 wt%. For the purpose of forecasting the tensile and creep properties of the ZA alloy at any given aluminum mass percentage, quartic models were generated, with all model terms significant. Strong room and high-temperature tensile strength, %elongation, and creep resistance were all reached in the low Al-containing ZA alloys, with 1.597 wt% anticipated ideal Al mass concentration, according to RSM optimization.

Microstructure and mechanical properties of Alx(TiZrTa0.7NbMo) refractory high-entropy alloys

An Alx(TiZrTa0.7NbMo) (x = 0, 0.1, 0.2, 0.3, 0.4, and 0.5) series of refractory high-entropy alloys were prepared by a vacuum-arc melting process and annealed. The effects of aluminum (Al) addition on the microstructure, room temperature, and high temperature mechanical properties of refractory high-entropy alloys were systematically examined and the strengthening mechanism analyzed and discussed. The results showed that, the phase composition of the alloy was BCC1 and BCC2 phases in the absence of Al. Precipitated phase AlZr3 formed in the alloy when Al was added. This series of refractory high entropy alloy has good strength at room temperature and high temperature. The compressive yield strength of Al0.5(TiZrTa0.7NbMo) alloy at room temperature was 1984 MPa and the compressive yield strength at 800 °C was 714 MPa. All alloys have plastic strain variables of 10.2–34.8 %, showing excellent room temperature plasticity. The increase in yield strength at room temperature primarily resulted from solution and precipitation strengthening. At high temperatures, precipitation strengthening became the dominant strengthening method. These research findings are expected to facilitate the understanding of the influence trends and mechanism of action of Al in refractory high-entropy alloys and serve as a reference for the design and development of Al-containing refractory high-entropy alloys.

Achieving excellent strength-ductility synergy via cold rolling-annealing in Al-containing refractory high-entropy alloys

Most refractory high-entropy alloys (RHEAs) with Al possess excellent strength but low elongation. Low ductility is generally induced by the formation of brittle phases after long-term heat treatment. Here, we demonstrate a new strategy to optimize strength-ductility in RHEAs through cold rolling-annealing treatments. The Zr35Ti30Nb20Al10Ta5 RHEA was cold rolled followed by annealing. A high tensile yield strength of ∼997 MPa with an elongation of ∼20% was obtained. Several factors together optimize the mechanical properties. First, the high-density dislocation entanglements introduced by cold rolling and short-time annealing are beneficial to uniform strain hardening and ductility. Second, short-term annealing can prevent the generation of the brittle LAVES phase in Al-containing RHEAs. Third, the B2 nanoprecipitates are dispersed in the BCC matrix phase, strengthening the alloys. Fourth, the fine grain size induced by cold rolling and short annealing times contributes to the strength.

Machine learning-assisted discovery of Cr, Al-containing high-entropy alloys for high oxidation resistance

A Machine Learning (ML) integrated workflow was utilized to guide the design of Cr, Al-containing five-element high-entropy alloys (HEAs) for achieving an enhanced high-temperature oxidation resistance. ML directs the design of HEAs to a chemical composition consisting of Fe, Cr, Al, Ni, and Cu for enhanced oxidation resistance. The oxidation behavior of AlxCrCuFeNi (x = 0, 0.25, 0.5, 1) HEAs at 1100 °C in air was systematically investigated and the oxidation mechanism was elucidated. The experimental validation agrees well with the ML prediction, demonstrating that ML could be used as a powerful tool for designing alloys with optimized oxidation resistance.

Evaluation of the impact of Mn and Al on the microstructure of Fe–Co–Ni–Cr based high entropy alloys

ABSTRACT In this study, an equiatomic FeCoNiCrMn high entropy alloy (HEA) was examined and compared to two Al-containing variants, FeCoNi1.5CrMnAl0.5 and FeCoNi1.5CrAl0.5, in terms of microstructure, phase composition and hardness. The vacuum cast, hot isostatically pressed (HIP’d) alloys were evaluated using differential scanning calorimetry (DSC), SEM, energy-dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD) and Vickers microhardness testing. The DSC results indicated that the solidus and liquidus temperature of the base alloy on cooling are 1296.3 and 1314.8°C, respectively. Its microstructure exhibited a single-phase face centred cubic (FCC) structure, with a small amount of σ-phase. The two Al-containing alloys (FeCoNi1.5CrMnAl0.5 and FeCoNi1.5CrAl0.5) were observed to form a secondary phase, in a FCC matrix, along the grain boundaries and within the grains. FeCoNi1.5CrMnAl0.5 had a solidus and liquidus temperature at 1271.8°C and 1287.6°C upon cooling, while FeCoNi1.5CrAl0.5 exhibited a solidus at 1354.7°C and liquidus at 1376.1°C. The hardness of the base composition was measured at 142 HV while the addition of Al and increase of Ni content in FeCoNi1.5CrMnAl0.5 increased the hardness to 202 HV and the removal of Mn in FeCoNi1.5CrAl0.5 further enhanced the hardness to 217 HV. These results aim to form a basis for understanding the effects of HEA microstructure on the material performance in oxidising environments, such as supercritical-CO2.

Lattice distortion in FCC HEAs and its effect on mechanical properties: Critical analysis and way forward

Lattice distortion is considered to be one of the four core effects in a multicomponent high-entropy alloy. However, their effect is least understood from experiment and theoretical standpoints. The present investigation revealed a unique way to understand this effect by combining experiments with density functional theory (DFT) calculations. A small amount of Al and Si were carefully added to the whole-solute matrix of Cantor alloys. The different-sized atomic species introduces a huge lattice distortion in the matrix, leading to a simultaneous improvement in yield strength (YS), ultimate tensile strength (UTS), and percent elongation. An extensive DFT simulation indicates that a lattice distortion is prominent in an Al-containing alloy, whereas Si does not induce a lattice distortion. However, Si leads to severe interlayer lattice distortion, caused by the displacement of Si, during twinning. This leads to the improvement of YS, UTS, and ductility. Lattice distortion and its variants play significant effects on the mechanical properties of high-entropy alloys (HEAs) in terms of local lattice distortion, providing an uneven energy landscape for the movement of line defects or interlayer distortion. The inherent nature of local lattice distortion in HEAs leads to the wavy or tortuous dislocation, unlike a straight dislocation in conventional alloys. The movement of the wavy type of dislocation through a distorted or defective lattice requires large stress, resulting in a pronounced effect on solid solution strengthening. This local lattice distortion also dictates the degree of the interlayer distance distortion in the vicinity of atoms, leading to an increase or decrease in stable stacking fault energy that decides the deformation mode via slip or twinning.

Influence of aluminum content on microstructure and performance of Mg-Zn-Ca-Al-Mn magnesium alloys

The influences of Al content on microstructure, mechanical properties, stretch formability and thermal conductivity of Mg-1.5Zn-0.3Ca-xAl-0.2Mn (x: 0.2, 0.5, 1, 1.25, 1.5, 2, wt%) alloys were investigated. With increasing the Al content from 0.2 wt% to 2 wt%, the texture changed from the TD-split texture to the quadrupolar texture at the Al content of 1.25 wt%, and finally became the RD-split texture with an increased texture intensity. The change of texture can be attributed to the formation of Al2Ca phase, which weakens the segregation of Ca at grain boundaries due to the decreased amount of Ca solute atoms in the α-Mg matrix. In addition, Al-Mn precipitates changed from a spherical shape to a rod shape accompanied by a largely increased Al/Mn ratio and a decreased amount of precipitates, when the Al content was increased to be larger than 1 wt%. The 0.2–1.5 wt% Al added alloys exhibited excellent stretch formability with the Erichsen values of 8.4–9.1 mm, which were much larger than that (6.8 mm) of the 2 wt% Al added alloy due to texture effect. The 1 wt% Al added alloy had the largest Erichsen value of 9.1 mm, while the 0.2 wt% and 2 wt% Al added alloys exhibited much higher yield strength (165 and 171 vs. 144 MPa). The thermal conductivity continuously decreased with increasing the Al content. Compared to the 2 wt% Al added alloy, the 0.2 wt% Al added alloy exhibited a much higher thermal conductivity (140 vs. 105 W/(m·K)) benefiting from the low concentration of Al solute atoms in the α-Mg matrix. Consequently, a combination of strong mechanical strength, excellent stretch formability and high thermal conductivity was achieved in the low Al-containing alloys with 0.2–0.5 wt% Al.

Comparative Study of Biocompatible Titanium Alloys Containing Non-Toxic Elements for Orthopedic Implants

Titanium alloys, particularly Ti6Al4V, are commonly used in biomedical applications. However, the inclusion of aluminum (Al) and vanadium (V) in this alloy can cause cytotoxic effects in the human body, resulting in Alzheimer’s disease and cancer. This study compares the performance of biocompatible alloys containing non-toxic elements, such as tin (Sn) and niobium (Nb), which are considered safe for implantation. Two sets of alloys were selected, Ti5Sn and Ti5Sn5Nb, and their properties were compared to Ti6Al4V. First, the alloys were prepared using a power metallurgical technique. Then, their phase analysis, hardness, wear resistance, strength, and corrosion performance in simulated body fluid (SBF) solution were characterized. Optical microscopy was used to study the microstructure, XRD was used to identify phases, and electrochemical testing was conducted to assess the alloys’ anodic and cathodic characteristics. Nanoindentation techniques were used to analyze surface characteristics, such as elastic modulus, nano hardness, and wear resistance. The results showed the alloys containing Nb and Sn had lower corrosion rates in SBF solution compared to Al-containing alloys. Moreover, Nb-containing alloys exhibited the highest hardness, 72% higher than Al-containing alloys. The corrosion-resistant properties of the alloys containing Nb and Sn were higher than those without Nb or Sn, suggesting they may be ideal for orthopedic implants in humans.