Two-stage Crystallization Process Research Articles

Enhanced thermal stability of amorphous Al-Fe alloys by addition of Ce and Mn

The thermal stability of mechanically alloyed amorphous Al-Fe-based alloy powders, with nominal compositions Al82Fe16Ce2 and Al82Fe14Mn2Ce2, was investigated using differential scanning calorimetry (DSC), x-ray diffraction (XRD), and scanning electron microscopy (SEM) complemented by energy-dispersive x-ray spectroscopy (EDX). Analysis through DSC indicated that both Al82Fe16Ce2 and Al82Fe14Mn2Ce2 alloys undergo a two-stage crystallization process. Notably, the initial crystallization temperatures for the Al82Fe16Ce2 and Al82Fe14Mn2Ce2 alloys were determined to be approximately 525 °C and 550 °C, respectively. This high thermal stability is attributed to the delayed nucleation process induced by the presence of Ce and Mn within the Al-Fe matrix. During polymorphic crystallization, distinct phases such as β-AlFe, Al13Fe4 for Al82Fe16Ce2, and β-Al(Fe, Mn), Al13Fe4, Al10CeMn2 for Al82Fe14Mn2Ce2 were identified. Furthermore, post-annealing of these amorphous alloy powders at elevated temperatures of 600, 700, and 800 °C led to distinct morphological characteristics based on the alloy composition. For Al82Fe16Ce2, the particles preserved a nearly spherical morphology, with size distributions ranging from 1 to 5 μm. In contrast, for Al82Fe14Mn2Ce2, the particles exhibited an irregular shape with a broader size range of 1 to 15 μm.

Enhanced aromatic selectivity using the core–shell synergistic effects of HZSM-5/SAPO-5 zeolites in isobutane conversion

HZSM-5 molecular sieves are efficient catalysts in the aromatization of light hydrocarbons and the fabrication of HZSM-5-based core–shell composites has become an important strategy to improve their catalytic performance due to the simultaneous modification of the acidity and pore structures in the catalyst. In this paper, HZSM-5/SAPO-5 (CSZP and CSZC) were synthesized via a two-stage crystallization process using PDDA and CTAB as modifiers of HZSM-5, respectively. The structure, acidity, and the weight of carbon deposition of the resulting molecular sieves catalysts were characterized using various methods and their catalytic behavior in isobutane aromatization investigated. The results show that CSZP exhibits a perfect core–shell structure with a uniform and thin shell composed of nano-SAPO-5, while CSZC has an irregular coat configuration with HZSM-5 covered by a rough and thick shell consisting of flaky micro-SAPO-5 crystals. The introduction of SAPO-5 via an in-situ synthesis decreases the Brønsted acidity of the catalysts to different extents and provides a large number of intergranular mesopores due to the stacking of SAPO-5 on the surface of HZSM-5, which is favorable toward improving the BTEX selectivity (benzene, toluene, ethylbenzene, and xylene) and catalytic stability over CSZP and CSZC. Meanwhile, CSZP has more competitive advantages than CSZC in promoting the mass transfer efficiency due to its significantly thinner shell than CSZC, which combined with its moderate acidity and appropriate mesopore structure result in its excellent selectivity and stability during the aromatization of isobutane.

Ammonia removal from low-strength municipal wastewater by powdered resin combined with simultaneous recovery as struvite

Low-strength municipal wastewater is considered to be a recoverable nutrient resource with economic and environmental benefits. Thus, various technologies for nutrient removal and recovery have been developed. In this paper, powdered ion exchange resin was employed for ammonia removal and recovery from imitated low-strength municipal wastewater. The effects of various working conditions (powdered resin dosage, initial concentration, and pH value) were studied in batch experiments to investigate the feasibility of the approach and to achieve performance optimization. The maximum adsorption capacity determined by the Langmuir model was 44.39 mg/g, which is comparable to traditional ion exchange resin. Further, the effects of co-existing cations (Ca2+,Mg2+,K+) were studied. Based on the above experiments, recovery of ammonia as struvite was successfully achieved by a proposed two-stage crystallization process coupled with a powdered resin ion exchange process. Scanning electron microscopy (SEM) and X-ray diffractometry (XRD) results revealed that struvite crystals were successfully gained in alkaline conditions (pH = 10). This research demonstrates that a powdered resin and two-stage crystallization process provide an innovative and promising means for highly efficient and easy recovery from low-strength municipal wastewater.

Development of process parameters for selective laser melting of a Zr-based bulk metallic glass

Parameters for selective laser melting of Zr59.3Cu28.8Al10.4Nb1.5 (trade name AMZ4), allowing crack-free bulk metallic glass with low porosity, have been developed. The phase formation was found to be strongly influenced by the heating power of the laser. X-ray amorphous samples were obtained with laser power at and below 75 W. The as-processed bulk metallic glass was found to devitrify by a two-stage crystallization process within which the presence of oxygen was concluded to play an essential role. At laser powers above 75 W, the observed crystallites were found to be a cubic phase (Cu2Zr4O). The hardness and Young’s modulus in the as-processed samples was found to increase marginally with increased fraction of the crystalline phase.

Structure and Properties of Mg-Cu-(Y,Ca) Bulk Metallic Glasses / Struktura I Własności Masywnych Szkieł Metalicznych Mg-Cu-(Y,Ca)

The work presents preparation methods, structure characterization and mechanical properties analysis of Mg-based bulk metallic glasses in as-cast state and after crystallization process. The studies were performed on Mg60Cu30Y10 and Mg37Cu36Ca27 glassy alloys in the form of plates and rods. The X-ray diffraction investigations revealed that the tested samples with different thicknesses and shapes were amorphous. The characteristics of the fractured surfaces showed mixed fractures with the “river” and “mirror” patterns, which are characteristic for the glassy materials and some “smooth” areas. The samples of Mg37Cu36Ca27 alloy presented a two-stage crystallization process, but addition of Y caused a single stage crystallization behavior. Qualitative phase analysis from the X-ray data of examined alloys annealed at 473 K enabled the identification of Mg, Mg2Cu, Cu2Mg and CaCu crystalline phases. The changes of compressive strength as a function of annealing temperature for studied rods were stated. The best mechanical properties including microhardness and compressive strength were obtained for the alloy with the addition of Y in as-cast state.

On the Structural Stability of Melt Spun Ribbons of Fe95−x Zr x B4Cu1 (x = 7 and 9) Alloys and Correlation with Their Magnetic Properties

Melt spun ribbons of Fe95−x Zr x B4Cu1 with x = 7 (Z7B4) and 9 (Z9B4) alloys have been prepared, and their structure and magnetic properties have been evaluated using XRD, DSC, TEM, VSM, and Mossbauer spectroscopy. The glass forming ability (GFA) of both alloys has been calculated theoretically using thermodynamical parameters, and Z9B4 alloy is found to possess higher GFA than that of Z7B4 alloy which is validated by XRD results. On annealing, the amorphous Z7B4 ribbon crystallizes into nanocrystalline α-Fe, whereas amorphous Z9B4 ribbon shows two-stage crystallization process, first partially to bcc solid solution which is then transformed to nanocrystalline α-Fe and Fe2Zr phases exhibiting bimodal distribution. A detailed phase analysis using Mossbauer spectroscopy through hyperfine field distribution of phases has been carried out to understand the crystallization behavior of Z7B4 and Z9B4 alloy ribbons. In order to understand the phase transformation behavior of Z7B4 and Z9B4 ribbons, molar Gibbs free energies of amorphous, α-Fe, and Fe2Zr phases have been evaluated. It is found that in case of Z7B4, α-Fe is always a stable phase, whereas Fe2Zr is stable at higher temperature for Z9B4.

Isothermal nanocrystallization behavior of bulk metallic glass composites in supercooled liquid region

Bulk metallic glass composites (BMGCs) reinforced by micro- (spherical size of about 10 μm) and nano-sized (20–50 nm) Al3Zr crystals were synthesized by copper mold suction casting method. The isothermal crystallization of Cu40Zr44Ag8Al8 BMGCs in the supercooled liquid region was studied by differential scanning calorimeter (DSC), X-ray diffractometer (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The mechanisms for nucleation and growth of the crystalline phases precipitated in two-stage crystallization process and the effect of the isothermal treatment on the in situ crystalline phases were discussed. Results show that the BMGCs experience a primary crystallization process. Nano-sized Al3Zr and Cu10Zr7 phases precipitate from the amorphous matrix successively. Furthermore, isothermal annealing has no effect on the pre-existing particles in the as-cast sample, which creates a new method to synthesize BMGCs strengthened by duplex crystalline phases with a large size difference. And the reinforced phase could also be dominated with intention if the as-cast BMGCs preparation and isothermal treatment process can be controlled more accurately.

De-vitrification of nanoscale phase-separated amorphous thin films in the immiscible copper–niobium system

Copper and niobium are mutually immiscible in the solid state and exhibit a large positive enthalpy of mixing in the liquid state. Using vapour quenching via magnetron co-sputter deposition, far-from equilibrium amorphous Cu–Nb films have been deposited which exhibit a nanoscale phase separation. Annealing these amorphous films at low temperatures (~200 °C) initiates crystallization via the nucleation and growth of primary nanocrystals of a face-centred cubic Cu-rich phase separated by the amorphous matrix. Interestingly, subsequent annealing at a higher temperature (>300 °C) leads to the polymorphic nucleation and growth of large spherulitic grains of a body-centred cubic Nb-rich phase within the retained amorphous matrix of the partially crystallized film. This sequential two-stage crystallization process has been investigated in detail by combining transmission electron microscopy [TEM] (including high-resolution TEM) and atom probe tomography studies. These results provide new insights into the crystallization behaviour of such unusual far-from equilibrium phase-separated metallic glasses in immiscible systems.

Crystallization behavior and resistance change in eutectic Si 15Te 85 amorphous films

Crystallization process and the corresponding electrical resistance change were investigated in eutectic Si 15Te 85 amorphous thin films. The Si 15Te 85 amorphous film showed two-stage crystallization process upon heating. In the first stage, the Si 15Te 85 amorphous crystallized into Te crystals at 175 °C. In the second stage, the residual amorphous phase crystallized into Si 2Te 3 crystals at above 300 °C accompanying the resistance drop. Before the second crystallization, the electrical resistance once increased in the temperature range of about 250–295 °C. This phenomenon can be explained by considering the formation of amorphous phase with a high electrical resistivity.

Transformation of Two-Line Ferrihydrite to Goethite and Hematite as a Function of pH and Temperature

Under oxic aqueous conditions, two-line ferrihydrite gradually transforms to more thermodynamically stable and more crystalline phases, such as goethite and hematite. This temperature- and pH-dependent transformation can play an important role in the sequestration of metals and metalloids adsorbed onto ferrihydrite. A comprehensive assessment of the crystallization of two-line ferrihydrite with respect to temperature (25, 50, 75, and 100 °C) and pH (2, 7, and 10) as a function of reaction time (minutes to months) was conducted via batch experiments. Pure and transformed phases were characterized by X-ray diffraction (XRD), X-ray absorption near-edge spectroscopy (XANES), atomic force microscopy (AFM), and scanning electron microscopy (SEM). The rate of transformation of two-line ferrihydrite to hematite increased with increasing temperature at all pHs studied and followed first-order reaction kinetics. XRD and XANES showed simultaneous formation of goethite and hematite at 50 and 75 °C at pH 10, with hematite being the dominant product at all pHs and temperatures. With extended reaction time, hematite increased while goethite decreased, and goethite reaches a minimum after 7 days. Observations suggest two-line ferrihydrite transforms to hematite via a two-stage crystallization process, with goethite being intermediary. The findings of this study can be used to estimate rates of crystallization of pure two-line ferrihydrite over the broad range of temperatures and pH found in nature.

An In Situ High-Temperature X-Ray Diffraction Study of Early-Stage Crystallization in Lithium Alumosilicate Glass-Ceramics

In situ high-temperature X-ray diffraction data have been collected for the crystallization of high-quartz solid solution (hq-ss) in a Li2O–Al2O3–SiO2 glass–ceramic. The temporal development of the number density, size, and unit cell parameters of hq-ss crystals indicated a two-stage crystallization process. At the early stage, lattice parameters of hq-ss crystals were constant. Their number density increased at a maximum rate of 1.7 × 1012·(mm3·min)−1 up to 1.4 × 1014 mm−3, while hq-ss grew linear with time at a maximum rate of ≈1 nm/min up to a size of ≈21 nm. In contrast, a decrease of the a-unit cell parameter, characteristic for increasing Si/Al ratios of the hq-ss crystals, was evident at the later stage. At this stage, the average crystal size was >21 nm. Their number density was constant or decreased linearly with time. The growth kinetics were very shallow with the exponent ≈1/50, but reached for crystals >30 nm cube root dependence typical for diffusion-controlled ripening. Avrami analysis of the volume fraction Vf confirmed these findings and resulted in a kinetic exponent in the range from 3.5 to 4.8 for the first (Vf<45%) and <0.3 for the second stage.

The influence of nanoscale phase separation and devitrification on the electrical transportproperties of amorphous Cu–Nb alloy thin films

The formation of amorphous phases in immiscible alloys with a large positive enthalpy ofmixing is thermodynamically unfavorable. Co-sputter deposited Cu–Nb filmsexhibit a nanoscale phase separation into Cu-rich and Nb-rich amorphous regions.They show relatively high room temperature resistivity, a negative temperaturecoefficient of resistance (TCR), and an incomplete superconducting transitionwith onset at 3.7 K. Annealing the nanophase-separated amorphous films at200 °C results in the nucleation of fcc Cu-rich nanocrystals within an Nb-rich amorphous matrix.This film exhibits multiple resistance steps, eventually showing a sharp drop with(TC)onset = 3.7 K.Annealing at 350 °C leads to complete devitrification via the formation of large bcc Nb-rich grains encapsulatingthe existing fcc Cu nanocrystals. These films show low room temperature resistivity,positive TCR, and a sharp superconducting transition with onset at 5.2 K. The electricaltransport and superconducting behavior appear to be consistent with a two-stagecrystallization process.

Crystallization and gelation in colloidal systems with short-ranged attractive interactions

We systematically study the relationship between equilibrium and nonequilibrium phase diagrams of a system of short-ranged attractive colloids. Using Monte Carlo and Brownian dynamics simulations we find a window of enhanced crystallization that is limited at high interaction strength by a slowing down of the dynamics and at low interaction strength by the high nucleation barrier. We find that the crystallization is enhanced by the metastable gas-liquid binodal by means of a two-stage crystallization process. First, the formation of a dense liquid is observed and second the crystal nucleates within the dense fluid. In addition, we find at low colloid packing fractions a fluid of clusters, and at higher colloid packing fractions a percolating network due to an arrested gas-liquid phase separation that we identify with gelation. We find that this arrest is due to crystallization at low interaction energy and it is caused by a slowing down of the dynamics at high interaction strength. Likewise, we observe that the clusters which are formed at low colloid packing fractions are crystalline at low interaction energy, but glassy at high interaction energy. The clusters coalesce upon encounter.

Studies on crystallization behaviour and mechanical properties of Al-Ni-La metallic glasses

Alloy ingots with nominal composition, Al92−x Ni8La x (x = 4 to 6) and Al94−x Ni6La x (x = 6, 7), were prepared by induction melting in a purified Ar atmosphere. Each ingot was inductively re-melted and rapidly solidified ribbons were obtained by ejecting the melt onto a rotating copper wheel in an argon atmosphere. The crystallization behaviour of melt-spun amorphous ribbon was investigated by means of differential scanning calorimetry (DSC), X-ray diffractometry and transmission electron microscopy. DSC showed that Al86Ni8La6 alloy undergoes a three-stage and rest of the alloys undergo a two-stage crystallization process upon heating. The phases responsible for each stage of crystallization were identified. During the first crystallization stage fcc-Al precipitates for low La-containing alloys and for higher La-containing alloys a bcc metastable phase precipitates. The second crystallization stage is due to formation of intermetallic compounds along with fcc-Al. Microhardness of all the ribbons was examined at different temperatures and correlated with structural evolutions. Precipitation strengthening of nano-size fcc-Al is responsible for maximum hardness in these annealed alloys.

Magmatic volatile contents and degassing-induced crystallization at Volcán Jorullo, Mexico: Implications for melt evolution and the plumbing systems of monogenetic volcanoes

Monogenetic basaltic cinder cones are abundant on Earth and exhibit a wide range of eruptive styles, including violent explosions. However, the mechanisms driving explosive cinder cone eruptions are still poorly understood. Here we investigate relations between volatiles, degassing, and crystallization in a long-lived, historical, cinder cone eruption to better understand the plumbing systems of monogenetic volcanoes. We present volatile (H 2O, CO 2, S, Cl), major, and trace element data for olivine-hosted melt inclusions, estimates of olivine residence times based on Fe–Mg zoning, and measurements of groundmass crystallinity for tephra from the eruption of Volcán Jorullo, Mexico. Jorullo melt inclusions trapped some of the most volatile-rich (≤ 5.3 wt.% H 2O, ≤ 1000 ppm CO 2), primitive (≤ 10.5 wt.% MgO) melts yet measured in an arc setting, as well as more degassed, evolved compositions. Furthermore, the melt inclusions record temporal changes in both melt composition and crystallization. Early erupted inclusions are Mg-rich and record variable trapping pressures (10–400 MPa), whereas late inclusions were trapped only shallowly beneath the volcano (3–19 MPa) and contain increasingly evolved melts. Disparities between the compositions of the melt inclusions and the whole-rock lava samples provide evidence for a two-stage crystallization process: 1) cooling-induced fractionation of amphibole + olivine ± clinopyroxene in the lower crust, which drove the bulk melt evolution over time; and 2) degassing-induced crystallization of melts during ascent at pressures < 400 MPa. Additionally, olivine residence times calculated from diffusion profiles suggest that as the eruption progressed, olivine crystals were being stored for longer periods of time (up to 1300 days) within more evolved melts that had risen from depth. These data, taken together with temporal decreases in crystallization depths and increases in groundmass crystallinity, suggest the formation of a shallow reservoir (or simply an enlarged region of the conduit) beneath the volcano late in the eruption. Shallow storage permitted degassing and crystallization of the Jorullo melts, and facilitated assimilation of the host rock by the resident magma.

Production of Cu-Hf-Ti Bulk Glassy Composites by Mechanical Alloying and Spark-Plasma Sintering

This work reports on the production of Cu-Hf-Ti bulk glassy composites through a powder metallurgical route, i.e. by mechanical alloying and subsequent spark-plasma sintering. Powders of Cu60Hf30Cu10 and Cu60Hf25Ti15 composition were prepared using a high-energy planetary ball-mill. Both alloys nearly showed a fully amorphous structure with only a small fraction of residual HCP Hf grains left after 50 h of milling. Differential scanning calorimetry (DSC) analyses of the milled glassy powder revealed a two-stage crystallization process for both compositions. However, the released crystallization enthalpy was substantially larger for the Cu60Hf25Ti15 alloy than for the Cu60Hf30Ti10 alloy, suggesting that the former comprises a higher fraction of the amorphous phase than the latter. Both powders showed distinct glass-transition with a large super-cooled liquid region. Consolidation of Cu60Hf25Ti15 powder was carried out by means of spark-plasma sintering at applied pressures of 200 and 500 MPa, choosing a sintering temperature within the super-cooled liquid region (T = 753 K). The sintered compacts exhibited some pores and interparticle boundaries.

Effect of rare-earth elements on nanophase evolution, crystallization behaviour and mechanical properties in Al-Ni-R (R = La/Mischmetal) amorphous alloys

The crystallization behaviour and evolution of nanoparticles in amorphous Al-Ni-Mischmetal (Mm) and Al-Ni-La alloys during heat treatment have been studied. Rapidly solidified ribbons were obtained by induction melting and ejecting the melt onto a rotating Cu wheel in an Ar atmosphere. The crystallization behaviour of the melt-spun ribbons was investigated using differential scanning calorimetry and X-ray diffractometry (XRD). XRD studies confirmed that all the ribbons were fully amorphous. Al-Ni-Mm systems showed a three-stage crystallization process whereas Al-Ni-La system, in general, showed a two-stage crystallization process on annealing. Crystallization kinetics was analysed by Kissinger and Johnson-Mehl-Avrami approaches. In Al-Ni-La alloys, the crystallization pathways depend on the La concentration. Microhardness of all the ribbons was examined at different temperatures and correlated with the corresponding evolution of phases.

Effect of in situ prepared silica nano-particles on non-isothermal crystallization of polypropylene

The crystallization behavior of PP/silica nanocomposites prepared in-situ via solid-state modification and sol–gel reaction is investigated. The crystallization behavior studied with DSC shows that in situ formed silica nano-particles act as nucleating agents. The non-isothermal crystallization kinetics of PP/silica nanocomposites is studied using a combined Avrami–Ozawa approach and shows a two-stage crystallization process: the primary stage is characterized by nucleation and spherulitic growth and the secondary stage is characteristic of the perfectioning of crystals. Silica speeds up the primary stage, resulting in a more narrow lamellar thickness distribution. The crystallization activation energy decreases with increasing silica content in the PP/silica nanocomposites. The nucleating efficiency of the in-situ prepared silica particles based on the scale as proposed by Lotz and co-workers is found to be 20% in the low concentration range and is higher compared to silica nano-particles as well as other nano-fillers studied. The melting behavior indicates the formation of more perfect crystals with a narrow lamellar thickness distribution and the WAXD patterns show that silica nano-particles induce the formation of crystals with the β-modification in PP at high silica content (ca. 5wt%). DMTA analysis shows a marginal lowering of the Tg and an increased mobility of the amorphous phase.

Effect of La on the crystallization behaviour of amorphous Al 94 − xNi 6La x ( x = 4–7) alloys

Melt spun and annealed Al-alloys containing 6 at.% Ni and 4–7 at.% La were investigated by means of differential scanning calorimetry (DSC), X-ray diffractometry (XRD), small angle neutron scattering (SANS), transmission electron microscopy and hardness measurements. XRD studies revealed that all the ribbons were fully amorphous, whereas SANS indicated concentration fluctuations. DSC showed that the amorphous alloys undergo a two-stage crystallization process upon heating. The details of the crystallization pathway depend on the La concentration. Upon annealing, SANS measurements revealed two different distributions of particles which can be explained by a core shell structure or two different types of particles. Moreover, significant changes in hardness occur which are attributed to changes in microstructure.

Isothermal nanocrystallization behavior of ZrTiNiCuBe bulk metallic glass in the supercooled liquid region

Abstract The isothermal crystallization of Zr41.25Ti13.75Ni10Cu12.5Be22.5 (at.%) bulk metallic glass in the supercooled liquid region is studied by X-ray diffraction, differential scanning calorimetry and transmission electron microscopy. The kinetics and the underlying mechanisms for the nucleation and growth of the crystalline phases precipitated in the two-stage crystallization process are discussed.