B2 Phase Research Articles

Phase transformation mechanism and microstructure of a Y-doped TiAl gas-atomized powders

In this study, the phase transformation mechanism of a yttrium-containing β-solidified TiAl alloy (Ti-43Al-9 V-0.3Y at.%), prepared by gas atomization, was systematically investigated. X-ray diffraction, electron backscatter diffraction, scanning electron microscopy, and transmission electron microscopy were utilized to comprehensively analyze the morphology and microstructure of powders with varying sizes as well as the form and distribution of yttrium and its influence on the phase transformation of the powders. The results show that the solidification phase structure of the powders exhibits significant variations: the ultra-fine powder consists of α’ martensite and remaining β phase, while the medium-sized powder is solely composed of β0 phase. The large-sized dendritic powder comprises β0, α’ martensite and α2 phase. With an increase in powder size, there is a corresponding increase in the content of α2 phase, whereas the content of martensite initially rises and subsequently declines. Additionally, yttrium is present in the form of multiscale Y-rich precipitates (YAl2 and Y2O3) within the matrix, and the segregation degree gradually increases with increasing powder size. The primary factors contributing to the disparity in solidification structure include cooling rate and segregation defects. A faster cooling rate and a higher supercooling degree will inhibit the β → α transition, while the Y-rich precipitated phase forms a pre-existing strain zone around it, providing an effective site for martensitic nucleation. In summary, these findings offer novel insights into the mechanism of phase transformation in yttrium-containing β-solidified TiAl alloy, thereby contributing to further advancements in the theory of rapid solidification for TiAl alloys.

Weldability and microstructural characterisation of gas tungsten arc welded Al0.3CoCrFeNi high entropy alloy

ABSTRACT High entropy alloys (HEAs) have received significant interest from researchers due to their exciting set of properties such as an optimum combination of strength and ductility. The Al0.3CoCrFeNi alloy has shown promising properties for applications such as structural components in marine environments, cryogenic applications, and antiballistic armour. This study investigates the weldability of Al0.3CoCrFeNi using gas tungsten arc welding (GTAW). GTAW of Al0.3CoCrFeNi results in a higher tensile strength than that of BM. Microstructural investigations were carried out with X- ray diffraction (XRD), scanning electron microscopy (SEM), electron backscattered diffraction (EBSD), accompanied by tensile and hardness testing. The BM and welded samples were found to have yield strengths of 255 MPa and 405 MPa, respectively. However, the welded sample has a lower ductility (20% elongation) than the BM (53% elongation). The high strength and lower ductility of the weld are mainly attributed to the evolution of the BCC/B2 phase and carbide phases in the microstructure of the welded sample. The precipitation of the BCC/B2 phase in the welded sample is a consequence of the lower free energy associated with the B2 phase at intermediate temperatures.

Investigation of phase transformation and mechanical properties of silicon addition on AlCrFeMnNi high entropy alloys

Abstract This paper examines the impact of silicon in the AlCrFeMnNi high-entropy alloy system, focusing on both its microstructural and mechanical properties. Alloys with varying silicon content (x = 0, 0.3, 0.6, 0.9 atomic ratio) were synthesized using vacuum arc melting. The phase formation of these high-entropy alloys was analyzed using X-ray diffraction to comprehend the alloying process behaviour. The findings revealed that the solidification of the AlCrFeMnNi alloy occurred in dendritically, with dendrite cores containing Cr, Fe, and Ni, while interdendritic regions were enriched in Al and Ni after adding Silicon. Increasing the silicon content from 0 to 0.9 led to significant improvements in microhardness and wear resistance. This improvement is attributed to the reinforcement of grain boundaries provided by silicon. The formation of an Al and Ni rich B2 phase is crucial in resisting dislocation motion and preventing further deformation. Additionally, the addition of silicon led to improved corrosion resistance, as demonstrated by potentiodynamic polarization measurements. However, a trade-off was observed between compressive strength and ductility: compressive strength increased with higher silicon concentrations, but at the expense of ductility.

Investigation on effect of L12 and B2 phases on tensile properties and mechanisms of Al0.3CoCrFeNi high entropy alloy through aging treatment

Al0.3CoCrFeNi high entropy alloys (HEAs) have excellent mechanical properties due to the large tunability of microstructures, which makes them one of the most promising candidates for engineering application. In the aspect of enhancing tensile properties, precipitation strengthening was verified effective in this HEA and more studies are needed to deepen relative understanding. In this work, Al0.3CoCrFeNi HEAs with coarse-grained single FCC (A) and fine-grained triplex FCC + B2 + σ (B) structures were aged at different temperatures and times to precipitate second phases, and the tensile properties and mechanisms of aged specimens were studied. The results showed that aging of A resulted in the main precipitation of L12 phases throughout FCC matrix and increasing aging temperature obviously increased L12 size, whereas increasing annealing time had minor effect on phase coarsening. At small L12 size of ~2.0nm, stacking fault shearing mechanism was activated during tension and planar dislocation configurations were formed, leading to simultaneous increase of strength and ductility. With increasing L12 size to ~10.0nm, the deformation mechanism was transformed into anti-phase boundary shearing and Orowan bypassing and wavy dislocation configurations predominated, which resulted in the significant increase of yield strength from ~263MPa to ~433MPa without sacrificing notable ductility. On the other hand, aging of B induced the formation of fine B2 phases at residual dislocations and enhancement of tensile yield strength from ~705MPa to ~759MPa. The ductility decreased slightly from ~32% to ~28%, which was related to the plastic deformation of B2 phases. However, it was found that the tensile strengths could not be continuously increased by further increasing aging temperature or time. These findings provide insights into the development of precipitation strengthened HEAs with good mechanical properties.

Atomic mechanisms for the fracture of AlMo0.5NbTa0.5TiZr refractory high entropy superalloy

Refractory high entropy superalloys (RHESs), known for their excellent high temperature performance, exhibit promising characteristics but are challenged by significant brittleness. Efforts to enhance plasticity through microstructure regulation have achieved only limited success, largely due to the unclear underlying fracture mechanisms of the superstructure. In this study, we systematically investigate the fracture mechanisms of the AlMo0.5NbTa0.5TiZr RHES from microscopic to electronic scales. Interestingly, both experimental and simulation results reveal that the ordered B2 phase demonstrates non-negligible plastic deformation capabilities during fracture, including deformation twinning and amorphization. Despite this, the fracture resistance of the B2 phase is lower compared to the A2/B2 interface and disordered A2 phase, even though the A2 phase shows less twinning and amorphization. Ab initio molecular dynamics simulations, combined with electronic behavior analysis, indicate that bonds involving Al and Zr in the B2 phase often exist in an anti-bonding state, making them more prone to breaking under load. This study provides deeper insights into the fracture mechanisms of the A2/B2 superstructure and its constituent phases at both atomic and electronic levels, offering a systematic approach to improving the fracture properties of such RHESs.

Investigation of B2 phase formation by thermal annealing of Fe(Cu)/Rh multilayers

Investigation of B2 phase formation by thermal annealing of Fe(Cu)/Rh multilayers

Thermophysical properties of AlxCoCrCuFeNi high entropy alloys

The AlxCoCrCuFeNi (x = 0.25, 0.5, 1, 2) high entropy alloys (HEAs) were prepared by arc melting and spray casting techniques. Their microstructures, hardness, and thermophysical properties such as fusion enthalpy, entropy, thermal diffusion coefficients, thermal expansion coefficients, were investigated. XRD results indicated that AlxCoCrCuFeNi (x = 0.25 and 0.5) alloys were composed of a high-entropy FCC phase and Cu-rich nanophase. As the Al content increased to 1 and 2, the phase structures included the AlNi-rich B2 phase, FeCr-rich A2 phase and Cu-rich nanophase. With the increased Al content, the microstructures of AlxCoCrCuFeNi HEAs transitioned from coarse dendrites to petal-like dendrites, and the grains were continuously refined. Moreover, the Al additions reduced the density whereas increased the Vickers hardness of the alloys. The maximum hardness observed in Al2CoCrCuFeNi HEA was approximately 2.4 times greater than that of the Al0.25CoCrCuFeNi HEA. The thermal diffusion coefficients of alloys initially increased and subsequently decreased as the temperature elevated. The phase transformation induced by Al content was an effective method for rapidly homogenizing the internal temperature of alloys. Furthermore, the crystal structure, elemental segregation, lattice distortion, enthalpy and entropy of fusion, and lattice vibration frequency all mutually affected the thermal diffusion and expansion coefficients of AlxCoCrCuFeNi HEAs.

Evolution of tool wear and machining quality during dry milling of AlCoCrFeNi2.1 eutectic high entropy alloy

AlCoCrFeNi2.1, a new class of eutectic high entropy alloy (EHEA) has drawn significant interest owing to the lamellar structure of alternating face-centered cubic (FCC) and B2 phases. Properties such as high strength and high plasticity render the machining of this alloy challenging. Addressing this issue, the milling experiments were conducted under dry conditions to investigate the machining quality of AlCoCrFeNi2.1 EHEA as well as the tool wear. The cutting temperature and cutting force were measured to explain the changes in tool wear and machining quality. The tool wear mechanisms and evolution modes were clarified. Finally, the change of surface roughness and surface morphology under different parameters were analyzed and the characterization method of plastic flow marked by B2 phase was proposed. Results showed that both the cutting temperature and cutting force increase as the milling parameters become larger. The width of flank wear increases with the increase of the cutting speed and the feed rate when the volume of material removal volume is constant. The tool wear modes evolved differently with different cutting parameters, for instance, abrasive wear dominated while the cutting speed was less than 80 m/min, but for higher speeds up to 110 m/min, adhesive wear and coating peeling occurred. As the cutting speed increases further, crater wear on the rake face starts to arise in addition to the flank wear. Abrasive wear and adhesive wear with increasing feed rate were dominant until a certain threshold (0.10 mm/tooth) beyond which the coating peels off the tool. Furthermore, chipping occurred for a higher feed rate of 0.14 mm/tooth. In particular, the formation of tool adhesive wear is mainly caused by the FCC phase adheres. In terms of machining quality, larger cutting parameters will worsen the surface roughness and morphology as well as lead to deeper subsurface plastic flow. The research will be conducive to promoting the applications of AlCoCrFeNi2.1 HEA.

Tuning of the mechanical properties of a laser powder bed fused eutectic high entropy alloy Ni30Co30Cr10Fe10Al18W2 through heat treatment

Eutectic high entropy alloys (EHEA) with an exceptional combination of strength and ductility are promising candidates as advanced structural materials. However, achieving an optimal balance between the properties in additively manufactured EHEAs is an outstanding challenge. In this work, Ni30Co30Cr10Fe10Al18W2 EHEA was additively manufactured using the laser powder bed fusion (LPBF) technique. In the as-fabricated state, it exhibits a dual-phase nano-lamellar structure consisting of FCC/L12 and B2 phases. Post-fabrication heat treatments were explored for modulating microstructure and, in turn, enhancing the mechanical properties, so as to achieve an optimum balance between strength and ductility. Upon heat treatment at 750 °C for 1 h, part of the B2 phase transformed into FCC, with the appearance of the tungsten-rich precipitates inside the B2 phase. The average interlayer spacing of the FCC/L12 lamellae increased to 124 nm, while that of B2 lamellae decreased to 86 nm, resulting in an alloy with an ultimate tensile strength (UTS) of 1811 MPa, although the strain to failure (εf) decreased to 2 %. Upon increasing the heat treatment temperature to 1000 °C, the average interlayer spacings of the FCC and B2 phases increased to 210 and 187 nm, respectively, which resulted in a more balanced mechanical behavior with UTS and εf of 1332 MPa and 9.3 %, respectively. This study provides an effective approach for microstructural modulation and enhancement of mechanical properties of LPBF fabricated EHEA via post-fabrication heat treatment, offering insights for developing future high-performance alloys for advanced structural applications.

Formation and strengthening mechanism of high entropy nugget for aluminum/steel resistance rivet welding

Resistance rivet welding (RRW) was a promising joining process for aluminum (Al) alloy and steel. The high entropy mixed nugget (HEMN) with the composition of Al0.8CoCrFe1.6Ni was obtained by introducing a high entropy alloy (HEA) rivet in RRW process. The high entropy played a significant role in facilitating the formation of solid solution. The HEMN microstructure exhibited a nanometer-sized weave-like structure caused by spinodal decomposition, comprising both BCC and B2 phases. The average hardness of the HEMN exceeded that of the Al/Steel mixed nugget, resulting in higher lap-shear strength and ductility. The analysis of strengthening mechanism demonstrated that the lattice misfit and order strengthening were the primary contributors. This study provided a novel strategy for joining dissimilar metal materials.

Theoretical aspects of structure, electronic, optical and elastic properties of Ca(1-x)CxSe mixed alloys in binary and ternary phases

The structural, optoelectronic, and elastic properties of Carbon doped Calcium Selenide binary and ternary semiconductor alloys with the general form of Ca(1-x)CxSe, 0 ≤ x ≤ 1 in B1 and B3 phases have been scrutinized adopting augmented plane wave plus local orbital methods within density functional theory applying different approximations like the Local density approximation (LDA), Wu-Cohen-Generalized Gradients Approximation (WC-GGA) and modified Becke-Johnson (mBJ). Structural properties summarize the crystal structure, relevant atomic positions, lattice constant, Bulk modulus B0, minimum energy E0, and minimum volume V0. Elastic constants confirm the structure stability of the B1 and B3 phases. The band gap was modified from a wider for appropriate optoelectronic devices application to narrow that enhanced the range of applicability of these binary and ternary semiconductor compounds for optoelectronic device applications. Whereas optical properties which include the study of zero frequency limit of static dielectric constant ε0(ω) with its real and imaginary parts, static refractive index n(0), optical reflectivity R(ω), optical absorption coefficient α(ω), optical conductivity σ(ω), and loss function L(ω) studied comprehensively to get real electrical and optical properties of these materials and give semiconductor market best alternative candidate which shows its effectiveness in the visible, ultraviolet and infrared region of the electromagnetic spectrum. Obtained results compared together and with the available experimental along with theoretical work on these binary and ternary Carbon-doped Alkaline earth Selenide.

Achieving a remarkable strength–ductility combination in a novel casting AlCoCrNi high entropy alloy

We prepared a novel cast high-entropy alloy (HEA) that comprises a dual-phase microstructure with L12 and B2 phases. Our designed HEA achieved a remarkable strength–ductility balance (tensile strength of ∼1025 MPa and uniform elongation of ∼30 %, which well outperforms many as-cast HEAs including arc-melting and directly cast HEAs.

Influence of phase transition on the mechanical and optical properties of SrSe and SrTe compounds via ab initio calculations

Elastic and optical properties of strontium chalcogenides SrX (X = Se and Te) were studied under hydrostatic pressure using the flat wave method (PP-PW) implemented in the CASTEP code. The main parameter carried out was the pressure impact at ambient conditions. Indeed, under high pressures, both compounds have undergone a structural phase transition from B1 to B2 phases at ∼15.5 GPa and ∼12 GPa, respectively. Results also showed that in the B2 phase, the two postulated compounds have changed their electronic nature to become metals at metallic pressures equal to ∼16.8 GPa (SrSe) and ∼17.7 GPa (SrTe). Also, the elastic properties were calculated at different pressure values in NaCl and CsCl structures, including their elastic constants as a function of pressure for SrSe and SrTe. Their optical properties were also computed from the spectral data obtained through their dielectric function, refractive index (2.1, 2.3) and others in the visible range.

Single-crystal elastic moduli, anisotropy, and the B1–B2 phase transition of NaCl at high pressures

Pressure dependences of the highest and lowest possible longitudinal sound velocities in single crystals of the B1 and B2 phases of NaCl were extracted from examination of their polycrystalline samples using the technique of time-domain Brillouin scattering. Based on the data collected up to 41 GPa, we largely extended the pressure range where single-crystal elastic moduli, Cij(P), and elastic anisotropy of the two cubic phases are measured, especially the experimental data for the B2 phase formed upon the reconstructive phase transition. The B1 phase of NaCl exhibits strong and growing anisotropy with increasing pressure, while that of the B2 phase is much weaker. Comparing with the previous experimental Cij(P) of other compounds exhibiting or expected to exhibit the B1–B2 phase transition, such as CaO, MgO, and (Mg1−x, Fex)O, we concluded that the transition is initiated by the shear instability due to violation of the Born stability criterion [C44(P)–P] > 0 and predicted the presently-not-verified transition pressures for MgO and (Mg1−x, Fex)O.

Microstructural characteristics and mechanical response of transient liquid-phase diffusion bonding AlCoCrFeNi2.1 high entropy alloy utilizing BNi-5 interlayer

This study focuses on the microstructural characteristics and mechanical response of the transient liquid phase (TLP) diffusion-bonded joints of AlCoCrFeNi2.1 high entropy alloy. Bonging parameters were chosen and optimized for the AlCoCrFeNi2.1 alloy and the TLP joining mechanism. The relatively complete isothermal solidification zone (ISZ) ensured a reliable connection of the base metal. As the bonding temperature increased from 1060 °C to 1160 °C, the athermal solidification zone (ASZ) disappeared. The homogenization degree of elements in the ISZ continuously increased, the matrix γ phase became fully ordered, and the B2 phase was precipitated, achieving high entropy state in the ISZ of the joint. Additionally, semi-coherent relationships of [0, 1, 1] L12, [0, 1, 1] B2 and (1, 1̅,1) L12, (2̅, 3, 1̅) B2 were detected between the B2 precipitate phase and the L12 matrix. The phase distribution in the ISZ significantly influenced the mechanical properties of the joint. The complete ordering of the matrix phase at 1160 °C eliminated deformation incoordination between the ISZ and the diffusion-affected zone (DAZ). The B2 phase transitioned from precipitating at the grain boundary at lower temperatures to precipitating uniformly in the ISZ, eliminating the weakening effect of the grain boundary and resulting in tensile strength and elongation comparable to the base material. The fracture morphology indicated a mixed fracture mode.

Enhancing the strength-ductility synergy of dual-phase Al0.3CoCrFeNiTi0.3 high-entropy alloys through the regulation of B2 phase content

In this study, Al0.3CoCrFeNiTi0.3 high-entropy alloys (HEAs) with different phase contents and microstructures were prepared through cold rolling and recrystallization annealing. This method achieved a synergistic combination of strength and ductility, with the CR1000-1 HEA attaining an ultimate tensile strength of 1175 MPa and a uniform elongation of 21.5 %. In Al0.3CoCrFeNiTi0.3, there exist two distinct phase morphologies: dispersed structure and clustered structure. Using in-situ high-resolution digital image correlation at the microscale, it was observed that local strain in the dispersed structure tends to concentrate along the phase boundaries when a small strain is applied. As the applied strain increases, strain rapidly concentrates in the FCC phase and subsequently in the B2 phase. For the clustered structure, the strain initially concentrates rapidly within the B2 phase and then gradually diffuses into the FCC phase. The low degree of inhomogeneity of the dispersed structure reduces the likelihood of stress concentration and the risk of damage initiation. Furthermore, we observed the synergistic strain hardening effect produced by various deformation mechanisms, such as dislocation pile-ups, dislocation networks, interacting stacking faults, and deformation twins. This study provides valuable insights for the fabrication of dual-phase HEAs with enhanced strength-ductility synergy, applicable to a wide range of engineering applications.

Accelerated elimination of residual B2 phase in TiAl-based alloys by low-temperature annealing plus stretching

Low-temperature annealing plus stretching is an efficient method to accelerate the elimination of B2 phase in TiAl-based alloys. The stretching stress can produce radial compressive stress on atom and promote the formation of atomic vacancies, both of which promote the atom diffusion, thus accelerating the elimination of B2 phase.

Directionally solidified NiCoCrFeAlW eutectic high-entropy alloy: Microstructure and mechanical properties

The NiCoCrFeAlW eutectic high-entropy alloy exhibits a balanced combination of ductility and strength. This study examines the microstructure evolution and mechanical properties of the Ni30Co30Cr10Fe10Al18W2 alloy at varying growth velocities (10, 20, 50, 100, and 200 μm/s), which in its as-cast state, features a microstructure that includes both coarse dendrites and lamellar eutectic. The structure of the coarse dendrites is characterized by an ordered body-centered cubic (B2) phase, while the lamellar eutectic, features a combination of face-centered cubic (FCC) and ordered B2 phases. The microstructure, characterized by lamellar and dendritic structures that are parallel to the growth direction after directional solidification, undergoes a morphological transition at the solid-liquid interface from a planar to a dendritic form as growth velocity increases. The tensile fracture strength initially increases with the growth velocity, reaching a peak of 1286 MPa at a velocity of 50 μm/s, and then subsequently decreases as the growth velocity continues to increase. Directional solidification results in an increase in sample elongation with increasing growth velocity, achieving an elongation of 22.5% at 200 μm/s. An examination of the deformation mechanism reveals that the lamellar structure, composed of the more ductile FCC phase and the harder B2 phase, effectively harmonizes the plasticity and strength of the alloy. Therefore, directional solidification significantly adjusts the eutectic microstructure's orientation, thereby enhancing the mechanical properties along the direction of solidification.

Influence of hot rolling on microstructure, mechanical properties, and martensitic transformation of TiNiCuNb shape memory alloy

TiNiCuNb shape memory alloys are a promising new class of materials with the potential to be applied as elastocaloric components. However, the mechanical processing of these alloys remains a challenge and new insights on this topic must enlighten the knowledge about this system. In this work, the effects of hot rolling on Ti46Ni38Cu10Nb6 alloy were investigated. The results showed that hot rolling leads to the increase of martensitic transformation temperatures and the formation of a matrix containing both B2 and B19 phases. The coarsening of the lamellar eutectic constituent was observed, and part of the β-Nb phase precipitated into the matrix after being dissolved due to hot work. Microstructural aspects and ultra-microhardness measurements suggest that dynamic recrystallization occurred during hot rolling.

Effect of Er2O3 and Y2O3 on microstructure and mechanical properties of Ti2AlNb alloy

Rare earth oxides can significantly refine the microstructure of the cast alloys and improve their mechanical properties. In this study, Er2O3 and Y2O3 particles were chosen to add into the Ti2AlNb-based alloy with vacuum non-consumable arc melting method. For the Er2O3 particles, it has obvious dissolution and re-precipitation characteristics during melting. While for the Y2O3 particles, they are more stable, and aggregated and grew up during melting. These two kinds of Er2O3 or Y2O3 both have obvious refinement effects on the B2 grains due to their heterogeneous nucleation. Specially, adding Er2O3 particles can promote the decomposition of the B2 and α2 phases and the increases of the O-phase content. But adding the Y2O3 particles can inhibits the B2 decomposition and just only promotes the α2 decomposition into the O-phase. Additionally, the TAC-Er2O3 ingot shows the worst mechanical properties at room-temperature due to its large size and grain boundary segregation of the Er2O3 reinforcements. Conversely, the TAC-Y2O3 alloy exhibits excellent strength and ductility whatever at room- and high- temperatures owing to its fine grain strengthening and effective strengthening of the second precipitated Y2O3 phases.