Precipitation of metastable phases and its effect on electrical resistivity of Al-0.96Mg_2Si alloy during aging

The precipitation behavior and its influence on the electrical resistivity of the Al-0.96Mg2Si alloy during aging were investigated with in-situ resistivity measurement and transmission electron microscopy （TEM）. The precipitates of the peak aged alloy include both β＂ and if, but the amount ratio of β＂ to β＂ varies with the aging temperature and time increasing. The precipitates during aging at 175 ℃ are dominated by needle-like β＂ phases （including pre-β＂ phase）, the size of which increases with the time prolonging, but does not increase substantially after further aging. The evolution of electrical conductivity is directly related to such microstructural evolution. However, the hardness of the alloy stays at the peak value for a long term. When the alloy is aged at 195 ℃, the ratio of β＂ to β＇ becomes the main factor to influence relative resistivity （Ap） value. The higher the temperature is, the smaller the ratio is, and the faster the Ap value decreases. Moreover, the hardness peak drops with the decrease of the ratio. With the size and distribution parameters measured from TEM images, a semi-quantitative relationship between precipitates and the electrical resistivity was established.

The Stable or Metastable Phases in Compressed Zn-O Systems

Variable-composition evolutionary structure searches are used to explore stable stoichiometries for the Zn-O system below 300 GPa. Our results confirm the previous structural phase transition sequence of pressurised ZnO. ZnO is thermodynamically stable up to 300GPa and zinc peroxide （Zn02, space group Pa3） is metastable under lower pressure. Insulating I4/mcm-Zn02 is thermodynamically stable between 128.3-300 GPa. Insulated metastable P3121-Zn02, controlling the pressure range of 51.5-128.3 GPa, has a wide band gap compared to the Pa3-Zn02 and I4/mem-Zn02. Phonon and elastic constant calculations conclude the dynamical and mechanical stability for the explored thermodynamically stable or metastable structures.

PREPARATION OF ULTRAFINE POWDER OF Fe AND ZrO_2 WITH METASTABLE PHASES BY HIGH POWER LASER BEAMS

The radiation of material surface by high intensity laser beams is used to produce the uhrafine powder of pure Fe and ZrO_2.The morphology,size and phases of the powder were examined by X-ray diffraetometer,scanning electron microscopy as well as transmission electron microscopy.In pure Fe a considerable quantity of γ-phase was found in the powder.In ZrO_2 powder,instead of stable phase,two metastable phases appeared.

METASTABLE PHASES IN RAPIDLY-SOLIDIFIED ALUMINIUMRICH Al-Sm ALLOYS

The splat foils of Al-Sm alloys with 0.04-0.06mm in thickness were made by hammeranvil technique. The estimated cooling rute was 106K/ order of magnitude. The extended solid solubility of Sm in α-Al reached 0.5at. % Sm. The intermediate phase in the rapidlysolidified Al-Sm alloys is Al11Sm3 phase, which is stable only at temperatures above 1339K in equilibrium state and retained to ambient temperature as a metastable phase, whereas the equlibrium intermediate phase, Al3Sm, was restricted to occur.

Metastable face-centered cubic ruthenium-based binary alloy for efficient alkaline hydrogen oxidation electrocatalysis

Metastable nanostructured electrocatalyst with a completely different surface environment compared to conventional phase-based electrocatalyst often shows distinctive catalytic property.Although Ru-based electrocatalysts have been widely investigated toward hydrogen oxidation reaction(HOR)under alkaline electrolytes,these studies are mostly limited to conventional hexagonal-close-packed(hcp)phase,mainly arising from the lack of sufficient synthesis strategies.In this study,we report the precise synthesis of metastable binary RuW alloy with face-centered-cubic(fcc)phase.We find that the introduction of W can serve as fcc phase seeds and reduce the formation energy of metastable fcc-RuW alloy.Impressively,fcc-RuW exhibits remarkable alkaline HOR performance and stability with the activity of 0.67 mA cm_(Ru)^(-2)which is almost five and three times higher than that of hcp-Ru and commercial Pt/C,respectively,which is attributed to the optimized binding strength of adsorbed hydroxide intermediate derived from tailored electronic structure through W doping and phase engineering.Moreover,this strategy can also be applied to synthesize other metastable fcc-RuCr and fcc-RuMo alloys with enhanced HOR performances.

On the Formability of Some Metastable Alloy Phases

Pattern recognition and neural network methods have been used to investigate the formability of metastable alloy phases, It has been found that some chemi cal bond parameters Such as valence electron number, electronegativity and metallic radii of cor-nponent elements are the dominating fac tors affecting metastable alloy phase formation. Some semi-empirical rules found in this way may be useful for the construction of expert system for materials design.

In situ observation of the phase transformation kinetics of bismuth during shock release

A time-resolved x-ray diffraction technique is employed to monitor the structural transformation of laser-shocked bismuth.Results reveal a retarded transformation from the shock-induced Bi-Ⅴphase to a metastable Bi-Ⅳphase during the shock release,instead of the thermodynamically stable Bi-Ⅲphase.The emergence of the metastable Bi-Ⅳphase is understood by the competitive interplay between two transformation pathways towards the Bi-Ⅳand Bi-Ⅲ,respectively.The former is more rapid than the latter because the Bi-Ⅴto B-Ⅳtransformation is driven by interaction between the closest atoms while the Bi-Ⅴto B-Ⅲtransformation requires interaction between the second-closest atoms.The nucleation time for the Bi-Ⅴto Bi-Ⅳtransformation is determined to be 5.1±0.9 ns according to a classical nucleation model.This observation demonstrates the importance of the formation of the transient metastable phases,which can change the phase transformation pathway in a dynamic process.

Stable and metastable phase equilibria in binary Mg-Gd system: A comprehensive understanding aided by CALPHAD modeling

In this paper,a comprehensive understanding of stable and metastable phase equilibria in binary Mg-Gd system was conducted with an aid of the CALculation of PHAse Diagram(CALPHAD)modeling.Firstly,thermodynamic descriptions of all the stable phases in the Mg-Gd system were re-assessed by considering all the experimental data in the literature.The discrepancy between the phase equilibria and thermochemical properties existing in the previous assessments was eliminated,and the better agreement with the experimental data was achieved in the present assessment.Secondly,the Gibbs energies for metastable β"-Mg3Gd and β"-Mg7Gd were constructed based on the first-principles and CALPHAD computed results as well as their correlation,and then incorporated into the CALPHAD descriptions.The model-predicted solvuses of(Mg)in equilibrium with the metastable β"-Mg7Gd and β'-Mg7Gd compounds showed very good agreement with the limited experimental data.Finally,the presently obtained thermodynamic descriptions of both stable and metastable phases in the binary Mg-Gd system were further validated by realizing the quantitative Scheil-Gulliver solidification simulations of 5 as-cast Mg-Gd alloys,and the successful prediction of the precipitation sequences in Mg-15Gd and Mg-12Gd alloys during the aging process.

Non-equilibrium microstructure and the metastable phase of Al-9.6wt%Mg Alloy solidification under high pressure

The non-equilibrium microstructure and a new metastable phase of Al-9.6wt%Mg alloy solidified at 6 GPa were studied by optical microscope,differential scanning calorimetry,X-ray diffraction and transmission electron microscope.The results showed that dendrite microstructure was refined,and the solid solubility of Mg in α-Al phase increased greatly.Correspondingly,the lattice parameter of α-Al phase increased.Al3Mg2 phases disappeared under high pressure solidification.In particular,a metastable phase with small size(20 nm or so) was produced in the alloy,its melting temperature range was 464～518.2 ℃,which was higher than that of Al3Mg2 phase(453～465 ℃) under normal pressure.These metastable phases located in the interdendritic position.It was the first time that the metastable phase was found in Al-Mg alloy at a high pressure of 6 GPa.The formation mechanism of the metastable phases was discussed.

Metastable-phaseβ-Fe_(2)O_(3) photoanodes for solar water splitting with durability exceeding 100 h

Planar films of pure and Ti^(4+)-dopedβ-Fe_(2)O_(3)were prepared by a spray pyrolysis method.X-ray diffraction patterns and Raman spectra of the metastableβ-Fe_(2)O_(3)film showed that its thermal stability was significantly improved because of covalent bonds in the interfaces between the film and substrate,while only weak Van der Waals bonds existed at the interfaces within the particle-assembledβ-Fe_(2)O_(3)film prepared by electrophoretic deposition.The as-prepared planar films were thus able to withstand higher annealing temperature and stronger laser irradiation power in comparison with theβ-Fe_(2)O_(3)particle-assembly.Ti^(4+)doping was used to increase the concentration of carriers in the metastableβ-Fe_(2)O_(3)film.Compared with pureβ-Fe_(2)O_(3)photoanodes,the highest saturated photocurrent for water splitting over the Ti^(4+)-dopedβ-Fe_(2)O_(3)photoanode was increased by a factor of approximately three.Theβ-Fe_(2)O_(3)photoanode exhibited photochemical stability for water splitting for a duration exceeding 100 h,which indicates its important potential application in solar energy conversion.

Metastable phase separation and rapid solidification of undercooled Co_(40)Fe_(40)Cu_(20) alloy

The metastable liquid phase separation and rapid solidification behaviors of Co_（40） Fe_（40） Cu_（20） alloy were investigated by using differential thermal analysis（DTA） in combination with glass fluxing and electromagnetic levitation（EML） techniques. The critical liquid phase separation undercooling for this alloy was determined by DTA to be 174 K. Macrosegregation morphologies are formed in the bulk samples processed by both DTA and EML. It is revealed that undercooling level, cooling rate, convection, and surface tension difference between the two separated phases play a dominant role in the coalescence and segregation of the separated phases. The growth velocity of the（Fe,Co） dendrite has been measured as a function of undercooling up to 275 K. The temperature rise resulting from recalescence increases linearly with the increase of undercooling because of the enhancement of recalescence. The slope change of the recalescence temperature rise versus undercooling at the critical undercooling also implies the occurrence of liquid demixing.

Formation of a FCC Al_(54)Ti_(23)C_(23) Metastable Phase by Ball Milling

The powder mixture of Al, Ti and graphite has been mechanically alloyed in a planetary ball mill.The structural evolution of as-milled powder sample has been characterized by XRD, DTA. The results show that the amorphous phase is formed first at an early milling stage, then crystallization occurs during further milling, leading to formation of a nanocrystalline fcc metastable phase. In contrast, during annealing the amorphous phase is crystallized to the equilibrium phase instead of the fcc phase. This indicates that crystallization during ball milling is different from that induced by annealing

Metastable phase separation and rapid solidification of undercooled Co-Cu alloy under different conditions

The metastable liquid phase separation and rapid solidification behaviours of Co61.8Cu38.2 alloy were investigated by using differential thermal analysis （DTA） in combination with glass fluxing, electromagnetic levitation （EML） and drop tube techniques. It is found that the liquid phase separation process and the solidification microstructures intensively depend on the experimental processing parameters, such as undercooling level, cooling rate, gravity level, liquid surface tension and the wetting state of crucible. Large undercooling and surface tension difference of the two liquids tend to facilitate further separation and cause severe macrosegregation. On the other hand, rapid cooling and low gravity effectively suppress the coalescence of the minority phase, Severe maerosegregation patterns are formed in the bulk samples processed by both DTA and EML. In contrast, disperse structures with fine spherical Cu-rich spheres homogeneously distributed in the matrix of Co-rich phase have been obtained in drop tube.

METASTABLE CHARACTERISTICS OF β-W

The nature of the oxygen contained in β-W was investigated with DSC,X-ray analysis on quenched samples and Auger spectroscope.It was shown that the oxygen contained in nonpyrophoric β-W consists of the reversibly chemisorbed oxygen and the interstitial oxygen which could form an interstitial solid solution with β-W.It seems better to consider β-W to be a metastable phase of tungsten with the interstitial oxygen as stabilizer.

Ultra-fast phosphating synthesis of metastable crystalline phase-controllable ultra-small MPX/CNT(M = Pd, Pt, Ru) for polyalcohol electrooxidation

A general approach is reported for ultra-fast phosphating synthesis of a series of ultra-small(<5 nm)noble metal phosphides(MPX/CNT,M=Pd,Pt,Ru)which are successfully produced in just 75 s for the first time.The catalytic performance of the catalysts can be optimized by controlling the nanomaterials as the metastable crystalline phases.By altering the phosphorus source under the same conditions,the hexagonal structured Pd_(7)P_(3)(NaH_(2)PO_(2).H_(2)O as P source)and monoclinic structured Pd_(6)P(Na_(4)P_(2)O_(7) as P source)can be prepared successfully.Both of them exhibit excellent polyol oxidation performance in alkaline media.Monoclinic Pd_(6)P/CNT and hexagonal Pd_(7)P_(3)/CNT have large ECSA which are confirmed as 82.1 m2 g^(-1)and 86.2 m2 g^(-1),respectively.Hexagonal Pd_(7)P_(3)/CNT has the highest mass activity of 6.14 A mgPd^(-1)(3.21 A mgPd^(-1)for Pd_(6)P/CNT)for GOR,which far exceeded Pt/C(2.81 A mgPt^(-1)).Meanwhile,the mass activity of monoclinic Pt_(5)P_(2)/CNT for EGOR achieved 12.4 A mg_(Pt)^(-1),which far exceeded Pt/C(6.8 A mg_(Pt)^(-1)).The stability test proved that the activity decay of these catalysts was negligible after the 12-hour durability test.Meanwhile,they have excellent CO anti-poisoning abilities.

Solid-liquid Metastable Equilibria in Quaternary System (NaCl+Na_2CO_3+Na_2SO_4+H_2O) at 273.15 K

The metastable phase equilibria of the quaternary system NaCl＋Na2CO3＋Na2SO4＋H2O were studied at 273.15 K. The salts＇ solubilities, densities and pH values of the equilibrated solution in this system were determined. According to the experimental data, the metastable equilibrium phase diagram, the diagram of density vs. composition and pH vs. composition diagram were plotted. The phase diagram consists of five univariant curves, four crystallization fields and two invariant points. The four crystallization fields correspond to sodium carbonate decahydrate （Na2CO3·10H2O）, sodium sulfate decahydrate（Na2SOn-10H2O）, sodium chloride（NaCl） and burkeite（2Na2SO4. Na2CO3）, respectively. The crystallization field of sodium sulfate decahydrate（Na2SO4.10H2O） is the largest, which indicates that sodium sulfate is easy to saturate and crystallize from solution at 273.15 K.

The Metastable Phase Equilibria of the MeX-MgCl_2-H_2O Systems

1 Introduction The Pingluoba brine,which characterized as high concentration with sodium,potassium,boron,lithium,and rubidium,possess great development value.The main composition of the brine can be summarized to the

Manipulation of metavalent bonding to stabilize metastable phase:A strategy for enhancing zT in GeSe

Exploration of metastable phases holds profound implications for functional materials.Herein,we engineer the metastable phase to enhance the thermo-electric performance of germanium selenide(GeSe)through tailoring the chemical bonding mechanism.Initially,AgInTe2 alloying fosters a transition from stable orthorhombic to metastable rhombohedral phase in GeSe by substantially promoting p-state electron bonding to form metavalent bonding(MVB).Besides,extra Pb is employed to prevent a transition into a stable hexagonal phase at elevated temperatures by moderately enhancing the degree of MVB.The stabilization of the metastable rhombohedral phase generates an optimized bandgap,sharpened valence band edge,and stimulative band convergence compared to stable phases.This leads to decent carrier concentra-tion,improved carrier mobility,and enhanced density-of-state effective mass,culminating in a superior power factor.Moreover,lattice thermal conductivity is suppressed by pronounced lattice anharmonicity,low sound velocity,and strong phonon scattering induced by multiple defects.Consequently,a maximum zT of 1.0 at 773 K is achieved in(Ge_(0.98)Pb_(0.02)Se)_(0.875)(AgInTe_(2))_(0.125),resulting in a maximum energy conversion efficiency of 4.90%under the temperature difference of 500 K.This work underscores the significance of regulating MVB to stabilize metastable phases in chalcogenides.

Precipitation process and its effects on properties of aging Cu–Ni–Be alloy

The precipitation process of aged Cu-Ni-Be alloy was investigated by X-ray diffraction （XRD）, trans- mission electron microscopy （TEM）, and high-resolution transmission electron microscopy （HRTEM）. The tensile strength, yield strength, and electronic conductivity of this alloy after aging were also studied. The precipitation sequence of the C17510 alloy aged at 525 ℃ is supersat-urated solid solution→G.P zones→ γ″-γ′→ γ. This transformation can be achieved by the accumulation of Be-atom layers. The G.P zones are composed of disk-shaped monolayers of Be atoms, which are formed on （001） matrix planes. The intermediate γ″ precipitate is nucleated in the G.P zones. The γ″ and γ′ precipitates have the same orientation relationship with matrix, e.g., （110）p｜｜（100）M,[001]p｜｜[001]M. The tensile strength of specimen shows a maximum during the aging process and then continuously decreases if the specimen is over aged. The strengthening effect of γ′ phase precipitated in aging at 525 ℃ for 4 h is calculated to be 436 MPa according to the Orowan strengthening, which is quite consistent with the experimental data.

In situ scanning electron microscopy observation of deformation and fracture behavior of Ti-3Zr-2Sn-3Mo-25Nb alloy

The plastic deformation and fracture processes of Ti-3Zr-2Sn-3Mo-25Nb alloy for surgical implants were directly observed by in situ scanning electron microscopy(SEM).The effect of phase transformations on deformation-induced martensitic transformation accompanying the cyclic tensile fracture processes was investigated.The results reveal that the metastable alpha martensites(α″) promotes deformation-induced martensitic transformation to ductile fracture,whereas the omega(ω) and alpha(α) phases drastically prevent slip dislocation and deformation-induced martensitic transformation to brittle fracture.