Single-phase Alloys Research Articles

Nonequilibrium solidification velocity, recalescence degree and grain refinement of highly undercooled Ni-based single-phase alloys

The solidification velocity-undercooling relationship of infinite miscible solid solution Ni-Cu alloys was established. The research shows that the growth velocity follows four stages of “exponential growth—platform period—linear growth—platform period” within the undercooling range. The solute trapping occurs in the linear growth stage with a discontinuity velocity jump which makes the migration interface smooth and solidification uniform. According to the recalescence degree-undercooling function, the relationship among solute composition, non-equilibrium solidification degree and grain refinement was established. It is found that higher recalescence degree enhances the non-equilibrium solidification effect and the primary solid fraction formed is higher, which makes it easier for grain refinement to occur at high undercooling. By using EBSD characterization, the obvious misorientation and high-density twin boundaries were observed in the crystal structure at high undercooling. The microhardness decreases sharply after critical undercooling, and HRTEM analysis shows that the structural orientation of adjoining grains is totally different, which can provide comprehensive and reliable evidence for recrystallization mechanism.

Обчислення лінійного коефіцієнта термічного розширення багатоелементних, однофазних металевих сплавів з перших принципів

One of the possible ways to calculate the coefficient of thermal expansion is a method based on determining the dependence of the total energy of the electron-ion system on the parameters of the crystal lattice at different temperatures. There is a relationship between the calculated values of the linear coefficients of thermal expansion and the melting point of the material. For metals and multi-element single-phase alloys, the dependence of the function V = α·Tmax on the parameter T/Tmax (α — the linear coefficients of thermal expansion, Tmax — melting point of the material) is obtained from the first principles, which has the same form for all single-phase multi-element metal alloys and is presented analytically. Using the method of pseudopotential and quasiharmonic approximation, the linear coefficients of thermal expansion of multi-element metal alloys are calculated. The temperature dependence of the coefficient of thermal expansion, after approximating the results of the computational experiment, is presented in analytical form. The results were compared with known tabular data. To confirm the reliability of the model, the calculation was performed for a number of pure metals. The consistency of the calculated and experimental data on the coefficient of thermal expansion of single-phase alloys calculated from the first principles is observed. There is a relationship between the calculated values of the linear coefficients of thermal expansion and the melting point of the material. For metals and multi-element single-phase alloys, the dependence of the function V = α·Tmax on the parameter T/ Tmax (α — the linear coefficients of thermal expansion, Tmax — melting point of the material) is obtained from the first principles, which has the same form for all single-phase multi-element metal alloys and is presented analytically. Keywords: Electron-ion system energy, interatomic interaction potential, force constants, quasiharmonic approximation, coefficient of thermal expansion.

Microstructure evolution and mechanical property of quaternary Cu-7%Al-4%Ni-2.5%Mn alloy solidified within ultrasonic field

An intensive field of 20 kHz power ultrasound with various amplitudes was applied during the solidification process of quaternary Cu-7%Al-4%Ni-2.5%Mn single-phase alloy to investigate its dynamic solidification mechanism and mechanical performance improvement. It is found that α(Cu) phase nucleates at small undercoolings and mainly grows along (111) crystalline plane direction into coarse dendrites with developed secondary arms. With the rise of amplitude, ultrasonic wave increases the nucleation rate by improving the wetting effect between alloy melt and impurities and also breaks some growing dendrites into fragments. Once the ultrasonic amplitude reaches the maximum of 15.2 µm, the prominent cavitation effect further increases the nucleation rate by three orders of magnitude, and also leads to the symmetrical distribution of temperature field, solute field and flow field in the solidification front, eventually resulting in the formation of tiny equiaxed α(Cu) grains without any obvious preferred growth orientation, inside which the solute distribution also tends to become uniform. Meanwhile, the compressive yield strength of Cu-7%Al-4%Ni-2.5%Mn alloy is significantly increased by 3 times after ultrasonic solidification. Theoretical analysis indicates that grain strengthening and dislocation strengthening are the two main reinforcement factors induced by power ultrasound.

An ab-initio based semi-empirical thermal conductivity model for multiphase uranium-zirconium alloys

In this work, a three-stage thermal conductivity model for complex metal alloys is developed, based on density functional theory (DFT) calculations, physics rules, and experimental data. The model is broken down into three stages for incorporating incremental complexity—Stage 1: modeling single-phase alloys with fixed compositions over a range of temperatures; Stage 2: modeling single-phase alloys across a range of compositions; and Stage 3: modeling multi-phase mixtures. The model is demonstrated using U-Zr and U-Mo alloys and displays good agreement with experimental data, with a root-mean-square error (RMSE) of about 1.3 W/m-K (~10% error against experimental data) over the typical operating temperature range for all phases of U-Zr. This model delivers the same level of accuracy as existing models for U-Zr and U-Mo, along with the same (or fewer) number of fitting parameters. More importantly, all the fitting parameters have clear physical meaning, providing significant insights into the physics of various electron- and phonon-scattering mechanisms and enabling predictions regarding the thermal conductivities in alloys, with a reduced reliance on experimental data.

Stress-induced crystallographic and microstructural evolution for hierarchically twinned martensite in single- and dual-phase Ni-Mn-Ga alloys

Polycrystalline Ni54+xMn25Ga21-x high temperature shape memory alloys were developed to examine the Ni-content dependent crystallographic features, phase transformation and deformation behavior. The alloys were characterized by a hierarchically twinned martensite structure with the co-existence of ductile γ phase in Ni57 and Ni58 alloys. Two types of twinning relationships existed, i.e., (112) compound twin in the internal nano-lamellae and 11¯2 type-I twin in the adjacent micro-variants. The martensitic start temperature, compressive strength and ductility increased with increasing Ni content. However, the corresponding shape recovery capability was significantly deteriorated in the higher Ni-containing alloys. For the single-phase alloy, the detwinning/reorientation of internal nano-lamellae and activation of deformation twins contributed to the macroscopically recoverable strain in case of low pre-strains. In contrast, large pre-strains led to piles-up of dislocation, bending and kinking of twinning interfaces, formation of deformation bands and crossing of twin structures. These irreversible processes produced massive unrecoverable strain. Almost no detwinning and twining processes were observed in the dual-phase alloys due to the severe lattice distortion. Instead, most deformation occurred via dislocation motion in γ phase.

Bulk nanostructured Al-Si alloy with remarkable improvement in strength and ductility

Refinement of microstructure for metallic materials alloyed by low-soluble elements has always been a difficult task. Here we propose a novel two-step route to achieve this goal, which has been successfully applied to a hypoeutectic Al-Si alloy. Al-Si single-phase alloy first prepared by solution treatment under ultra-high isostatic pressure was then subjected to high-pressure torsion, obtaining a bulk nanostructured hypoeutectic Al-Si alloy for the first time. The ultrafine Al grains, Si nano-particles, concentrated Si solutes and dislocations greatly contributed to the yield strength. Meanwhile, the brittle fracture of micrometer-sized Si phase in common Al-Si alloys was avoided by refining Si to nanoscale, and the ductility was significantly enhanced. Consequently, the nanostructured Al-Si alloy exhibited a high yield strength of 403 MPa and a large total elongation of 32%. This work provides a promising approach for realizing bulk nanostructured alloys with low-soluble components.

Recent development of coloring alloys

In this study, we systematically reviewed the strategy of the tailoring intrinsic color characteristics of chromatic and achromatic metallic materials. Its scientific contribution involves defining and expanding the range of coloration of metallic materials, including single-phase solid-solution alloys and multiphase alloys that includes specific phases with chromatic colors. In chromatic single-phase alloys, the modulation of optical properties depends on the transition energy of the solid solution phase. The additional alloying elements significantly influence the optical properties, i.e., the reflectivity in the visible spectrum, by varying the transition energy of the solid solution phase. Another coloring alloy design strategy varies and realizes the intrinsic color based on the multiphase system. In particular, the microstructural design based on the multiphase forming alloy system, including the chromatic coloring phase, effectively improves the chromaticity of the alloys by blending effect from the constitutive phases. A methodological framework is subsequently suggested to realize chromatic characteristics based on the juxtaposition method. The novel alloy design method using the juxtaposition method can be considered an important strategy for developing novel coloring alloys with high chromatic color durability from achromatic metallic materials.

Magnetic, electrical and mechanical properties of Fe40Mn40Co10Cr10 high entropy alloy

A prototypical, single-phase, and non-equiatomic high entropy alloy Fe40Mn40Co10Cr10 has been mechanically deformed at room and cryogenic temperatures. Plastic deformation was accommodated via crystallographic slip at room temperature while transformation induced plasticity (TRIP) has been observed in samples deformed at 77 K. The stress-induced martensitic transformation occurred from face-centered cubic (FCC) to hexagonal close-packed (HCP) structures. A detailed electron backscatter diffraction analysis was utilized to detect phase change and evaluate the evolution of the HCP phase volume fraction as a function of plastic strain. Physical properties of undeformed and deformed samples were measured to elucidate the effect of deformation-induced phase transitions on the magnetic and electrical properties of Fe40Mn40Co10Cr10 alloy. Relatively small magnetic moments along with non-saturating magnetic field dependencies suggest that the ground state in the considered material is ferrimagnetic ordering with coexisting antiferromagnetic phase. The temperature evolution of the coercive fields has been revealed for all samples. The magnitudes of the coercive fields place the considered system into the semi-hard magnetic alloys category. The temperature dependence of the zero-field cooled (ZFC) and field cooled (FC) magnetization was measured for all samples in the low field regime and the origin of irreversibility in ZFC/FC curves was discussed. Besides, the temperature dependence of the resistivity in all samples was measured and the possible conduction mechanisms were discussed.

Grain Refinement and Solidification Velocity of Supercooled Melts

Using a high speed thermal imaging camera, solidification velocity of undercooling is accurately determined for a highly undercooled Ni70Cu30 binary concentrated single phase alloy. It is observed that the velocity of solidification is first continuously and smoothly increasing at the low and mediate undercoolings then increasing discontinuously to a maximum value at the high undercooling range. Thus, the solidifcation velocity-undercooling relation has an abrupt discontinuity, which is caused by nonequilibrium diffusion of solutes in the liquid side near the advancing solid liquid interface. Grain refinement of high undercooling is directly induced by recrystallization, which was in clear contrast with the remelting induced dendrite fragmentation in conventional alloys.

Surprising increase in yield stress of Mg single crystal using long-period stacking ordered nanoplates

Mg–Zn–Y ternary alloys containing the long-period stacking ordered (LPSO) phase exhibit superior mechanical properties. This is believed to be originating from the LPSO phase acting as the strengthening phase. However, we first clarify that the mechanical properties of the matrix Mg solid solution in the Mg/LPSO two-phase alloy are significantly different from those of pure Mg. The yield stress of a Mg99.2Zn0.2Y0.6 single crystal (matrix Mg solid solution) is almost the same as that of an LPSO single-phase alloy. This is ascribed to the formation of thin stacking-fault-like defects, named “LPSO nanoplate”. In Mg99.2Zn0.2Y0.6, kink-band formation is induced in the same manner as that in the LPSO phase in deformation, resulting in high strength accompanied with increased ductility. Our results suggest that the strengthening mechanism of the Mg/LPSO two-phase alloy must be reconsidered depending on the microstructure. Furthermore, the results suggest that new ultrahigh-strength Mg alloys, which have much lower Zn and Y contents but the mechanical properties are comparable or superior than the present Mg/LPSO two-phase alloys, are expected to be developed via the appropriate control of LPSO nanoplate microstructures.

Damping Performance of Material Candidates for Service at 350K

Vibration systems require the damping materials operating at high service temperature. In this paper, damping performance of HT100, M2052 and S316L at 350K were evaluated by applying different frequencies, strain amplitudes and heating rates. It is found that the internal friction dependence of frequency of HT100, M2052 and S316L all show a characteristic of Check function, and the resonance frequency has a negative linear correlation with the material physical parameters. The strain amplitude as well as heating rate has no obvious effect on the resonance frequencies of the materials, but significantly enhance the internal friction of the interface damping alloys such as M2052 and HT100, but small on single-phase alloys such as S316L. The internal friction mechanism for HT100 and M2052 are of static hysteresis at 350K, and HT100 and M2052 are applicable candidates for working at temperatures around 350K from the viewpoint of vibration reduction.

Phase transformation and hydrogen storage properties of LaY2Ni10.5 superlattice alloy with single Gd2Co7-type or Ce2Ni7-type structure

Abstract The structural evolution of as-cast LaY2Ni10.5 multi-phase alloy with the annealing temperature has been investigated, and the single-phase alloy with rhombohedral Gd2Co7-type or hexagonal Ce2Ni7-type structure was firstly prepared. It is found that the rhombohedral structure is more stable than the hexagonal polymorph at the low temperature, which is reversed at the high annealing temperature. Structural refinement indicates that Y atoms preferably substitute La in the [A2B4] subunit of both hexagonal and rhombohedral superlattice structures. Compared with the rhombohedral polymorph, the hexagonal phase possesses a relatively smaller volume of [A2B4] subunit and thus demonstrates higher structural stability during the hydrogen charging-discharging cycling. This work offers the guideline for the preparation process and composition optimization of single-phase rare earth-based A2B7-type alloys for nickel-metal hydride battery applications.

Revealing high-fidelity phase selection rules for high entropy alloys: A combined CALPHAD and machine learning study

We reveal high-fidelity new phase selection rules for high entropy alloys (HEAs) by combining CALPHAD calculations and the machine learning (ML) method. Employing Thermo-Calc and TCHEA3 database, we first generate more than 300,000 equilibrium phase data from 20 quinary families formed by the 8 elements of Al Co, Cr, Cu, Fe, Mn, Ni, and Ti, and choose initially 15 materials/physical descriptors. The eXtreme Gradient Boosting (XGBoost) method is then used to identify 5 most important descriptors that best delineate the single and mixed phases in the complex temperature-composition space of HEAs. The ML model trained by the 5 features is validated by 155 annealing experimental data points from 15 publications and then used to predict 213 new single-phase alloys with BCC and FCC structures of the alloy families of AlCrNiFeMn and AlCrCoNiFeTi. We also highlight the importance of equilibrium temperature and offer in-depth insights into the paradigm of composition-feature-phase of HEAs. On the basis of the 5 important features, we establish new phase selection rules for single FCC and BCC phases with a success rate above 90%, significantly outperforming all existing phase selection rules and providing a powerful tool for mapping single-phase in the complex temperature-composition space of HEAs.

The Texture and Structure of the Melt-Spun Co2MnAl-Type Heusler Alloy.

The structure of the Co2MnAl-type Heusler alloy in the form of a melt-spun ribbon was studied by electron microscopy, electron back-scattered diffraction (EBSD), and X-ray diffraction. The melt-spun ribbon consists of a homogeneous single-phase disordered Heusler alloy at the wheel side of the ribbon and an inhomogeneous single-phase alloy, formed by cellular or dendritic growth, at the free surface of the ribbon. Cellular growth causes the formation of an inhomogeneous distribution of the elemental constituents, with a higher Co and Al concentration in the centre of the cells or dendritic arms and a higher concentration of Mn at the cell boundaries. The EBSD analysis shows that the columnar crystals grow in the <111> crystal direction and are declined by about 10° against the direction of the spinning.

Grain refinement in low SFE and particle-containing nickel aluminium bronze during severe plastic deformation at elevated temperatures

The influence of particle size and morphology on grain refinement in low stacking fault energy (SFE) alloys was studied by comparing the grain structures in single- and multi-phase Al-bronze (AB) alloys following equal channel angular pressing (ECAP) between 350 and 500 °C. In particular, nickel aluminium bronze (NAB) was chosen as it contained both coarse and fine rounded particles, as well as a lamellar phase which evolved during ECAP. Grain refinement in the single-phase alloy was achieved through dynamic recrystallisation initiated at deformed twin boundaries. By contrast, different mechanisms were observed in the particle-containing NAB. Recrystallisation around the coarse κII particles (∼5 μm) was promoted through particle stimulated nucleation (PSN), whereas recrystallisation in the region of the fine κIV (∼0.4 μm) was delayed due to the activation of secondary slip. Grain refinement in areas of the lamellar κIII showed significant variation, depending on the lamellar orientation relative to the shear plane of ECAP. As the lamellae deformed, numerous high angle grain boundaries were generated between fragments and served as nucleation sites for recrystallisation, while PSN occurred around spheroidised lamellae. The spreading of the κIII particles by ECAP then enhanced the total area of recrystallised grains.

Entropy-Driven Melting Point Depression in fcc HEAs

High Entropy Alloys (HEAs) are an increasingly dominant alloy design paradigm. The premise of entropic stabilization of single-phase alloys has motivated much of the research on HEAs. Chemical complexity may indeed help stabilize single alloy phases relative to other lower-entropy competing solid phases. Paradoxically, this complexity may de-stabilize these alloys against the liquid, potentially limiting the application space of HEAs at elevated temperatures. In this work, we carry out a comprehensive investigation of the phase stability in the fcc CoCrFeMnNiVAl HEA space using a state of the art CALPHAD database. By using modern

Structural transformations of a gas-atomized Al62.5Cu25Fe12.5 alloy during detonation spraying, spark plasma sintering and hot pressing

In this work, we traced structural transformations of an Al62.5Cu25Fe12.5 alloy, in which a quasicrystalline icosahedral phase (i-phase) can be formed, upon spraying onto a substrate and consolidation from the powder into the bulk state. The Al62.5Cu25Fe12.5 powder was obtained by gas atomization and consisted of i-phase and ?-phase AlCu(Fe). The powder was detonation sprayed (DS) and consolidated by spark plasma sintering (SPS)/hot pressing (HP). During DS, the particles experienced partial or complete melting and rapid solidification, which resulted in the formation of coatings of a complex structure. The composite regions containing i-phase were inherited from the powder alloy. The fraction of the material that experienced melting solidified as ?-phase AlFe(Cu) in the coating. It was suggested that the difficulty of obtaining i-phase upon post-spray annealing is related to aluminum depletion of the alloy during DS. During SPS and HP, the elemental composition of the alloy was preserved, while the exposure to an elevated temperature led to phase homogenization. SPS and HP conducted at 700?C resulted in full densification and the formation of a single-phase quasicrystalline alloy. The sintered single-phase alloy showed a higher microhardness in comparison with the DS coatings.

Hydrogen storage and electrochemical properties of annealed low-Co AB5 type intermetallic compounds

The structure, hydrogen storage and electrochemical properties of annealed low-Co AB5-type intermetallic compounds have been investigated. La-alloy, Nd-alloy and Cr-alloy are used to represent La0.8Ce0.2Ni4Co0.4Mn0.3Al0.3, La0.6Ce0.2Nd0.2Ni4Co0.4Mn0.3Al0.3 and La0.6Ce0.2Nd0.2Ni3.8Co0.4Mn0.3Al0.3Cr0.2, respectively. The XRD results indicated that annealed samples are all single-phase alloys with CaCu5 type structure. The maximum of both hydrogen content and discharge capacity is obtained for La-alloy 1.23 wt%H2 and 321.1 mA h/g, respectively. All the investigated alloys are quiet stable with ΔH of hydrogen desorption about 36–38 kJ/mol H2. Cycle life of alloy electrode has been improved by partial substitution of La for Nd and Ni for Cr. The highest capacity retention of 92.2% after 100 charge/discharge cycles at 1C has been observed for Nd-alloy. The hydrogen diffusion coefficient measured by PITT is higher at the start of charging process and dramatically reduces by 2–3 order of magnitude with saturation of β-hydride. The highest value 6.9 × 10−13 cm2/s is observed for La alloy at 100% SOC. Partial substitution La for Nd and Cr for Ni in low-Co AB5 metal hydride alloys slightly reduces maximum discharge capacity, HRD performance and hydrogen diffusion kinetics. Low-Co alloys show good overall electrochemical properties compared to high-Co alloys and might be perspective materials for various electrochemical applications.

Superior electrochemical performance of La-Mg-Ni-based alloys with novel A2B7-A7B23 biphase superlattice structure

Nickel metal hydride (Ni-MH) rechargeable batteries hold an important position in the new-energy vehicle market owing to their key technology advantages. Their negative electrode materials—hydrogen storage alloys (HSAs) are always on the spotlight and are the key to compete with the burgeoning Li-ion batteries. Here, for the first time we report a series of biphase supperlattice HSAs with a (La,Mg)2Ni7 matrix phase and a novel (La,Mg)7Ni23 secondary phase. The biphase alloys show discharge capacities of 402–413 mAh g−1 compared with 376–397 mAh g−1 of the other multi- or single-phase alloys. These values are among the highest for superlattice HSAs. In addition, the alloy with 15.4 wt.% (La,Mg)7Ni23 phase exhibits good high rate dischargeability due to the proper compromise between the amount of crystal boundaries and equilibrium plateau voltage. The cycling stability of the biphase alloys is lower than that of the single-phase alloy but is till higher than the multiphase alloy. The novel superlattice biphase alloys with superior overall electrochemical properties are expected to inspire further design and development of HSAs as advanced electrode materials for power batteries.

Slip-twinning interdependent activation across phase boundaries: An in-situ investigation of a Ti-Al-V-Fe (α+β) alloy

Microstructural plastic strain distribution evolution is highly heterogeneous even in single-phase alloys. One of the important factors that govern this heterogeneity is slip/twinning transfer across grain/phase boundaries. In this regard, the fundamentals of transfer across grain boundaries have drawn significant attention in the literature, while the understanding of phase boundaries remains comparatively limited. (α+β) titanium alloys provide a profound platform to explore these phenomena, since: (i) both of the present phases can exhibit plastic deformation at similar microscopic strain levels; and (ii) both dislocation slip and mechanical twinning can be triggered to accommodate plastic strain. In the present work, we evidenced a deformation transfer unit involving dislocation slip in the β-phase and {101¯2}-mechanical twin in the α-phase. We revealed by crystallographic calculations that the combination of Schmid factor and the Luster-Morris compatibility factor enables a rational quantification for the inception propensity of the slip-twinning transfer event. Our in-situ strain mapping approach verified that this sort of transfer activity can plausibly alleviate strain incompatibility/localization, demonstrating the potential to facilitate deformation homogeneity.