B2 Phase Research Articles

Regulating microstructure, mechanical properties and corrosion resistance of laser direct energy deposited Al0.5Mn0.5CoCrFeNi high-entropy alloy by annealing heat treatment

ABSTRACT A novel Al0.5Mn0.5CoCrFeNi high-entropy alloy (HEA) was additively manufactured by laser direct energy deposition (LDED) process and annealed at 900 ℃. The microstructure, mechanical properties and corrosion resistance of the prepared HEA were investigated and regulated by annealing time. The as-deposited microstructure is made up of well-developed columnar dendrites featured by face-centered cubic phase (FCC) and mature interdendritic regions co-structured by body-centered cubic (BCC) and Cr-Fe-rich σ duplex phases. The annealing heat treatment noteworthily enhances the hardness and improves the ultimate tensile strength with a deterioration of toughness by facilitating the precipitation of nanoscale B2 phases within the FCC matrix and the coarsening of σ phases at interdendritic regions, successfully regulating the mechanical performance between high-ductility (670.0 MPa, 25.3%) and high-strength (1015.6 MPa, 5.9%). The corrosion resistance is significantly improved by 2-hour annealing, and then deteriorates due to the severe passive film destruction of the FCC matrix by gradually coarsened B2 nanoparticles and the stability and compactness reduction of passive films by the coarsening of σ phases and formation of high-valence metal cations. These results contribute to exploring and regulating novel high-performance HEAs by the LDED process and annealing heat treatment.

Impact of NiTi Shape Memory Alloy Substrate Phase Transitions Induced by Extreme Temperature Variations on the Tribological Properties of TiN Thin Films

NiTi alloys and thin film/NiTi composites are extensively utilized in frictional environments, particularly those experiencing extreme temperature fluctuations. Current studies mainly focus on preparing wear-resistant films on NiTi alloy surfaces but neglect the potential impact of temperature-induced phase transitions in the NiTi substrate on thin films’ performance. This study examines the effect of NiTi alloy phase transitions, induced by extreme temperature variations, on the tribological properties of TiN thin films on NiTi substrates. TiN films (1 μm thick) were deposited on NiTi alloy surfaces using magnetron sputtering technology. The transition of the main phase in the NiTi substrate between the R phase and the B19′ phase was achieved via liquid nitrogen cooling (−196 °C) and water bath heating (90 °C). XRD, EDS, SEM, and tribological tests analyzed the phase structure, elemental composition, micromorphology, and tribological behavior. Fatigue wear was identified as the predominant wear mechanism for the TiN films, with minor contributions from oxidative and abrasive wear. Phase transition from the R phase to the B19′ phase in the NiTi substrate induced by temperature change couls reduce the wear rate of the TiN film by up to 41.97% and decrease the friction coefficient from about 0.45 to about 0.25. Furthermore, the shape memory effect of the NiTi alloy substrate, caused by B19′ → B2 phase transition, resulted in the recovery of the TiN thin film wear track depth from 920 nm to 550 nm, manifesting a “self-healing” phenomenon. The results in this study are important and necessary for the provision of thin film/NiTi composites in frictional environments.

DFT investigation on the effect of asymmetry on electro-optical properties of bent-core liquid crystals

ABSTRACT In this paper, we have studied the effect of asymmetry and bent-core LC phases (B1 & B2) on electro-optical and thermal parameters utilizing the DFT/B3LYP/6-31G** technique on 4-((3-((4-(((4-(decyloxy)phenyl)imino)methyl)benzoyl)oxy)phenoxy)carbonyl)phenyl-4-(alkyloxy)benzoate [DIBCPnB] homologous molecules. Raman spectra of homologous molecules (10a-10g) have been analyzed using the DFT method, while UV-vis spectra are analyzed by both the TD-DFT and ZINDO methods. The effects of asymmetry and phase on electro-optical properties e.g. dipole moment, total energy, anisotropic polarizability, mean polarizability and molar refractivity, as well as global parameters like electronegativity, electron affinity, ionization potential, electrophilicity index, chemical hardness, electron-accepting capability and electron-donating capability have been investigated. The dependence of thermal properties on asymmetry and phases has been examined. This study reveals that asymmetry does not affect total energy, polarizability and molar refractivity. We report that homologues exhibiting the B1 phase possess lower anisotropic polarizabilities than those in the B2 phase.

Reasons for good high-temperature tensile rupture property of high-W content (Nb,W) co-alloying TiAl-based alloy under low stress

In order to explore why the high-W content TiAl-based alloy under low stress has better tensile rupture property, we still choose the vacuum-consumable-melted Ti-44Al-4Nb-1W-0.1B alloy. We carried out tensile rupture property testing at different time periods and different temperatures. The W content in (α2+γ) lamella matrix and microstructure before and after tensile rupture property testing are studied. At 800 °C, the B2 phase content gradually decreases and the W content in (α2+γ) lamella matrix gradually increases with the extension of time periods. For example, at 800 °C for 1370h, the B2 phase content has decreased from initial 12.024% to 1.188%. And, W content in matrix has increased from initial 0.88at.% to 1.07at.%. The tensile stress accelerates the radial diffusion of W atom. Importantly, EBSD analysis shows that, under low stress, there is no obvious stress concentration in the B2 phase region (≤800 °C), which avoids premature fracture, and makes B2 phase playing a good role in coordinating stress and strain, thus improving the tensile rupture property. However, at 900 °C, there is a slight stress concentration in the microstructure, leading to the formation of many holes, which is harmful to the tensile rupture property.

Mechanical and corrosion behavior of CoCrFeNiAl0.3 high entropy alloy seamless tubes

An urgent need arises for alloys that possesses superior ductility and good corrosion resistance, essential for applications in highly harsh environmental conditions. The corrosion resistance and mechanical properties of CoCrFeNiAl0.3 high-entropy alloy seamless tubes (HEASTs) are investigated in this study. Electron Channeling Contrast Imaging (ECCI) and Electron Backscatter Diffraction (EBSD) techniques reveal the presence of abundant dislocations within the inner crystals, highlighting the alloy's excellent deformation ability. The formation of the B2 phase significantly influences corrosion behavior, with higher B2 content promoting pit propagation and enhancing pitting corrosion. Corrosion test results demonstrate the superior pitting corrosion resistance of TR-22 tubes, attributed to their lower B2 phase content and effective coverage by a protective passive film formed by Al, Cr, and Fe. X-ray photoelectron spectroscopy (XPS) analysis further elucidates the correlation between B2 phase content and corrosion resistance, emphasizing the importance of controlling B2 content to optimize microstructure. Overall, this study provides valuable insights into enhancing the corrosion resistance and mechanical properties of high-entropy alloy seamless tubes, contributing to the development of advanced materials in engineering applications.

Optimizing the synergy of strength and ductility in a Fe–18Mn–8Al–1C–5Ni lightweight steel by adjusting the distribution, volume fraction and size of B2 phases and κ-carbides

Optimizing the synergy of strength and ductility in a Fe–18Mn–8Al–1C–5Ni lightweight steel by adjusting the distribution, volume fraction and size of B2 phases and κ-carbides

Microstructure, mechanical properties and corrosion resistance of laser direct energy deposited AlMo0.25FeCoCrNi2.1 high-entropy alloy: adjusted by annealing heat treatment

ABSTRACT A novel AlMo0.25FeCoCrNi2.1 high-entropy alloy (HEA) was additively manufactured using laser direct energy deposition (LDED) process. The microstructure, mechanical properties and corrosion resistance of the prepared alloy were investigated and adjusted by annealing heat treatment. The results showed that inadequate melting and diffusion of Mo powders cause the nonuniform Mo concentration and finally result in the microstructure diversity in the banded layer. Fine lamellar eutectic structures composed of Mo-rich σ phase and body-centered cubic (BCC) phase form within Mo supersaturated FCC phases. The annealing heat treatment promotes the precipitation and coarsening of needle-like B2 phases within the incipient FCC phases and granular σ phases at phase boundaries between FCC and BCC/B2, synergistically improving the toughness of this alloy with slight deterioration of ultimate tensile strength. The corrosion resistance of the alloy is notably improved by a 2-hour annealing heat treatment. These results contribute to rapidly designing/manufacturing/adjusting novel HEAs by LDED process.

Orientation Dependence of the Yield Strength of the B2 Phase in Stress-Free and Stress-Assisted Aged Ti-51.5at.%Ni Single Crystals

Orientation Dependence of the Yield Strength of the B2 Phase in Stress-Free and Stress-Assisted Aged Ti-51.5at.%Ni Single Crystals

Tuning the Microstructure and Mechanical Properties of Al-Co-Cr-Fe-Ni-Ti Compositionally Complex Alloys Manufactured by Means of L-DED

Abstract In the current study, a combinatorial high-throughput screening approach based on CALPHAD simulations and experimental validation has been utilized to explore a Co2CrFeNi2-Al-Ti CCA-system. This technique, introduced by Kaspar et al. (High Entropy Alloys Mater. https://doi.org/10.1007/s44210-023-00023-x), allows to perform an accelerated alloy development within a wide compositional range and automated fabrication of graded components with varying chemical composition and microstructure. Extended by semi-automated analytical characterization of the produced samples, this approach enables to design novel compositionally complex alloys (CCAs) with promising properties for specific requirements. In our current work, a multiphase design of L12 γ′-strengthened Co2CrFeNi2-Al-Ti CCAs partially tolerating the disordered BCC-A2 or ordered B2 phases in the alloy microstructure has been utilized. The samples with three different chemical compositions were manufactured by means of laser directed energy deposition (L-DED). By subsequent two-step heat treatment, different phase compositions and microstructures have been realized with a main objective to achieve a high volume fraction of L12 γ′ precipitations. Mechanical properties of investigated alloys were characterized by means of tensile tests. Depending on chemical and phase composition of the alloys, the ultimate tensile strength varied in the range of 1060-1150 MPa. The formation of BCC-B2 phase led to decreased yield to tensile strength and showed a detrimental effect on ductility of investigated alloys.

Effects of HIP on Microstructure and Mechanical Properties of LMD Fe36Mn21Cr1815NiAl10 High-Entropy Alloy

To reduce costs, a cobalt-free FeMnCrNi-based HEA has been proposed. Further investigation into the mechanical properties of the Fe36Mn21Cr18Ni15Al10 alloy is essential to expand its application potential. In this study, a cobalt-free Fe36Mn21Cr18Ni15Al10 HEA was fabricated using LMD, and the effects of HIP on its microstructure and mechanical properties were investigated. Results indicated that the as-printed specimen exhibited a dual-phase structure consisting of BCC and FCC phases, with the B2 phase dispersed as fine blocks. After HIP treatment, the content of the FCC phase significantly increased, displaying a lamellar distribution between the BCC phases, with secondary block-like B2 phases forming within the BCC matrix. The HIP process enhanced the density of the high-entropy alloy to 98.2%, while the tensile strength at 25 °C increased to 903.9 MPa. Additionally, the post-fracture elongation improved to 17.4%, thereby increasing the potential for industrial applications of HEAs.

Lattice and spin entropy changes in B2-type magnetocaloric Al–Mn–Ni alloy

Abstract Understanding the electronic, lattice, and magnetic contributions to the magnetocaloric effect in magnetic materials can help to elucidate and optimize their performance. In this work, the structural and magnetocaloric properties of Al–Mn–Ni alloy are experimentally determined and theoretically analyzed based on ab initio calculations. The dominating B2 phase associated with the Mn-rich sublattice is found to be responsible for the observed magnetocaloric properties. The magnetic entropy change, refrigerant capacity, and adiabatic temperature change are evaluated. Through the analysis of the data, we find that for the B2 phase, changing from ferromagnetic to paramagnetic configurations results in a pronounced elastic hardening despite the volume expansion. The decrease in lattice entropy is significant and contributes negatively to the magnetic and electronic entropy changes. Our work emphasizes the critical role of the lattice sector in the magnetocaloric effect, and provides an in-depth understanding of the individual entropy terms in magnetic solid solutions.

The Effect of Zirconium on the Microstructure and Properties of Cast AlCoCrFeNi2.1 Eutectic High-Entropy Alloy.

To improve the performance of AlCoCrFeNi2.1 eutectic high-entropy alloys (EHEA) to meet industrial application requirements, ZrxAlCoCrFeNi2.1 high-entropy alloys (x = 0, 0.01, 0.05, 0.1) were synthesized through vacuum induction melting. Their microstructures were analyzed using X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy-dispersive spectroscopy (EDS). Additionally, the hardness, low-temperature compressive properties, nanoindentation creep behavior, and corrosion resistance of these alloys were evaluated. The results showed that AlCoCrFeNi2.1 is a eutectic high-entropy alloy composed of FCC and B2 phases, with the FCC phase being the primary phase. The addition of Zr significantly affected the phase stability, promoting the formation of intermetallic compounds such as Ni7Zr2, which acted as a bridge between the FCC and B2 phases. Zr addition enhanced the performance of the alloy through solid-solution and dispersion strengthening. However, as the Zr content increased, Ni gradually precipitated from the B2 phase, leading to a reduction in the fraction of the B2 phase. Consequently, at x = 0.1, the microhardness and compressive strength decreased at room temperature. Furthermore, a higher Zr content reduced the sensitivity of the alloy to loading rate changes during creep. At x = 0.05, the creep exponent exceeded 3, indicating that dislocation creep mechanisms dominated. In the ZrxAlCoCrFeNi2.1 (where x = 0, 0.01, 0.05, 0.1) alloys, when the Zr content is 0.1, the alloy exhibits the lowest self-corrosion current density of 0.034197 μA/cm2 and the highest pitting potential of 323.06 mV, indicating that the alloy has the best corrosion resistance.

Research on the compressive mechanical behavior and constitutive model modification of novel refractory high-entropy alloy (Ti2Zr)1.5NbVAl0.25

Abstract In recent years, high-entropy alloys have been widely applied across various fields. To explore the mechanical properties of high-entropy alloys and their characteristics under extreme environments, ingots of a quinary high-entropy alloy (Ti2Zr)1.5NbVAl0.25 were prepared. The material’s compression mechanical properties were studied under quasi-static conditions at room temperature with strain rates ranging from 0.001s−1 to 0.1s−1, dynamic compression at strain rates ranging from 1300s−1 to 3100s−1, and compression at a strain rate of 1900s−1 and temperatures from -80°C to 400°C. Impact specimens subjected to strain rates of 1300s−1 and 3100s−1 at room temperature were recovered for metallographic analysis. The results indicate that (Ti2Zr)1.5NbVAl0.25 exhibits significant strain-hardening behavior and strain rate sensitivity. Under medium to high strain rates, the alloy undergoes a phase transformation from a single-phase BCC structure to a mixture of BCC and B2 phases upon impact, with 2500s−1 being the threshold strain rate for phase transformation. The original parameters of the Johnson-Cook constitutive model were fitted based on experimental data, and modifications were made to the strain rate hardening and temperature terms in the original model. The modified Johnson-Cook constitutive model demonstrates improved prediction of the mechanical properties of (Ti2Zr)1.5NbVAl0.25 high-entropy alloy at both room temperature and under extreme environments.

Mechanical Responses and Deformation Mechanisms of Low-Cost High-Entropy Alloy AlCrFe2Ni2 with Heterogeneous Structure Subjected to High-Strain Rate Deformation

Introducing heterogeneous structures into metallic materials through composition design is a promising strategy for simultaneously improving the strength and ductility of the materials. In this work, a novel dual-phase low-cost AlCrFe2Ni2 high-entropy alloy (HEA) was designed and prepared. Its phase constitution, microstructure, dynamic mechanical responses at strain rates ranging from 1100[Formula: see text]s[Formula: see text] to 4800[Formula: see text]s[Formula: see text] as well as deformation mechanisms were investigated. It was found that the HEA consists of FCC, B2 and BCC phases and has a heterogeneous microstructure containing dual-phase alternate lamellar and high-density precipitates. The HEA displays the optimal mechanical properties at the strain rate of 4800[Formula: see text]s[Formula: see text], of which the yield strength, maximum flow stress and strain are 1222.9[Formula: see text]MPa, 2340.8[Formula: see text]MPa and 30.5[Formula: see text]%, respectively, better than that of most single-phase FCC or BCC HEAs, dual-phase HEAs and traditional alloys. The deformation mechanism of the HEA at the strain rate of 4800[Formula: see text]s[Formula: see text] involves both dislocation slip and deformation twinning, with the former dominating. This work could provide guidance and insights for designing metallic materials with excellent mechanical properties at high strain rates.

Formation of B2 Phase and its Phase Stability in Equiatomic Al-Cu-Fe-Ni-Ti High Entropy Alloy

In the present investigation, we have synthesized a single-phase high-entropy alloy in Al-Cu-Fe-Ni-Ti system by melting of the individual metals using a radiofrequency induction furnace under an argon environment. The as-synthesized alloy shows the formation of a B2-type ordered phase with a lattice parameter of 0.289nm. The mechanical stability of this single phase high-entropy alloy was investigated under high-energy ball milling. The milling was performed at a speed of 400rpm for 10, 20, and 40h under a hexane medium with a ball-to-powder ratio of 40:1. The formation of nano crystallites (~ 10nm sizes) body centered cubic (BCC) phase (disordered B2) has been observed after 40h of ball milling, which has been confirmed by X-ray diffraction and transmission electron microscopic investigation. The equiatomic Al-Cu-Fe-Ni-Ti high entropy alloy structure is observed to be quite stable during mechanical milling up to 40h; only grain refinements and lattice strain accumulation were observed with milling time.

Melting of B1‐Phase MgO From Simultaneous True Radiative Shock Temperature and Sound Speed Measurements to 250 GPa on Samples Preheated to 2300 K

AbstractTo refine the melting curve, equation of state, and physical properties of MgO we performed plate impact experiments spanning 170–250 GPa on MgO single crystals, preheated to 2300 K. A controlled thermal gradient in 20 mm long samples enabled radiative temperature (3%–4%) and rarefaction overtake observations (yielding sound speed 2%) close to the hot Mo driver with a free surface below 2000 K that minimized evaporation. Ta flyers were launched by two‐stage light‐gas gun up to 7.6 km/s and sample radiance was recorded with a 6‐channel (500–850 nm) pyrometer. Shock front reflectivity was measured at 198 and 243 GPa using 50/50 sapphire beam‐splitters. Most experiments show monotonic increases of shock temperature with pressure, from (168 GPa, 7100 K) to (243 GPa, 9400 K), in good agreement with predictions of our MgO B1 phase equation of state. Measured sound speeds are parallel to but 10% higher than model predictions for bulk sound speed of solid B1 MgO, confirming ductile behavior of preheated MgO. Two experiments, at 238 and 246 GPa, showed anomalously low shock temperature and sound speed, suggesting melting. Using reported MgO melting data up to 120 GPa and our data at 232–246 GPa, we constructed a maximum‐likelihood Simon‐Glatzel fit. At Earth's core‐mantle boundary pressure (135 GPa), our best‐fit interpolated MgO melting point is K. Our proposed melting line falls within the envelope of theoretical predictions but does not completely agree with any particular model curve. Our results reduce the uncertainty on MgO melting temperature at Earth's core‐mantle boundary by a factor of 17 and provide an anchor for extension to multicomponent systems.

Patterning Composites Made of Rodlike and Bent-Core Molecules for Electric Applications.

Soft materials are ideal candidates for creating tunable, self-assembled architectures. Composite materials are elaborately designed with an unusual physical performance that combines solid nanostructures and orientationally ordered soft matter. Such composites can not only inherit properties of their constituents but also exhibit excellent conductivity. Herein, we demonstrate a dewetting method to pattern binary mixtures composed of rod-like (R) 8OCB and bent-core (BC) OXD-7 molecules. By confining the R-BC composite in a geometric sandwich system with a surface-modified surface, we were able to generate R-BC microstripes that were not only nearly uniform in size but also highly ordered in alignment. It was found that these composite microstripes formed donor-acceptor systems that enhance the conductivity of the soft system while not altering the coexisting smectic phase of 8OCB and B6 phase of OXD-7.

High strength‐ductility combination in low‐density dual phase high entropy alloys

High strength and good ductility combination make it appealing for advanced structural and functional applications. However, enhancing strength typically reduces ductility due to their inverse correlation. FCC, BCC, L12, and B2 phases are most commonly found in high-entropy alloys (HEAs), which are the subject of extensive research to tweak their mechanical properties. We present a systematic microstructure-oriented mechanical property investigation for a newly developed low-density dual-phase high entropy alloy (LDDP-HEA) with varying phase fractions. In this study, Al15Cu20Fe25Ni30Ti10 HEA was designed by phase formation rules and heat treated at 800, 900, and 1000 °C for 24 h, followed by water quenching to investigate phase stability and their effect on mechanical properties. A combination of FESEM, EBSD, EDS, XRD, microhardness, and compression tests were carried out to investigate the microstructural characteristics and mechanical properties of the HEAs. The LDDP-HEA exhibited an excellent combination of mechanical properties with a high yield strength of ∼1675 MPa, ultimate compressive strength of ∼2250 MPa, and elongation to fracture of ∼10 % in the as-cast condition. The strength and low density of the alloy lead to superior specific compressive yield strength of 240 MPa g−1 cm3 and 121.70 MPa g−1 cm3 in as-cast and heat treated at 1000 °C conditions, respectively.

Influence of the grain size on the martensitic transformation and strain nanodomains in the Ti-Hf-Ni-Cu shape memory alloy

The martensitic transformation and strain nanodomain were studied in the Ti40.7Hf9.5Ni44.8Cu5 shape memory alloy, whose grain size varied from 130 μm to 160 nm. The largest average grain size (600 μm in length and 130 μm in diameter) was found in the CAST sample. The grains with the smallest size (160 nm) formed during the crystallisation of thin ribbons obtained by the melt spinning technique from the Ti40.7Hf9.5Ni44.8Cu5 ingot (labelled as RIBBON). The average grain size (16 μm) was observed in the sample subjected to high-pressure torsion and post-deformation annealing (labelled as HPT). It was found that the CAST and HPT samples included the NiTi-based matrix and the Ti2Ni-type precipitates, whereas the RIBBON sample contained the NiTi-based matrix only. Regardless of the grain size, all samples underwent the B2 ↔ B19’ transformation on cooling and heating. A decrease in grain diameter from 130 μm to 160 nm decreased the transformation temperatures by more than 100 °C but did not affect the sequence of the transformation. The strain nanodomains formed in all samples on cooling prior to the forward martensitic transformation. The smaller the grain size, the larger the temperature range of the strain nanodomain existence. On heating of HPT and RIBBON samples, the B19 phase transformed to the B2 phase with strain nanodomains, which disappeared only on heating over 120 °C. The influence of grain size on the martensitic transformation and strain nanodomains was analysed from the thermodynamics approach.

Temperature-dependent mechanical behavior in a novel hierarchical B2-strengthened high entropy alloy: Microscopic deformation mechanism and yield strength prediction

Integration of twinning and hierarchical microstructure into a face-centered cubic (FCC) matrix is a novel research approach for advancing high-temperature alloys, enhancing strength without the need for costly heavy elements. Here, a three-level B2 phase was incorporated into a NiCoCrFe high-entropy alloy (HEA) matrix, offering a high strengthening effect at room temperature and a good resistance to moderate-temperature softening while preserving the low stacking fault energy of the FCC matrix. The resulting NiCoCrFeAl0.3Si0.3 HEA exhibited stable yield strength, strain hardening, and deformation twinning over a broad temperature ranging from 77 to 973 K. By establishing a yield strength model based on the various strengthening mechanisms, the study highlighted the important role of the three-level B2 phase in the exceptional mechanical properties of the alloy across a wide temperature range. These findings present a promising avenue for the advancement of high-temperature structural materials.