Peritectic Phase Research Articles

Liquid state property and rapid peritectic solidification of refractory Mo-33.3 at.% Zr alloy under electrostatic levitation condition

The thermophysical properties and rapid solidification mechanism of liquid Mo-33.3 at.% Zr peritectic alloy were investigated by electrostatic levitation technique, which attained a maximum undercooling of 387 K (0.16 TL). At its liquidus temperature, the density, surface tension, viscosity and solute diffusion coefficient of this refractory alloy were determined as 8.14 g/cm3, 1.60 N/m, 11.32 mPa·s and 1.41 × 10−9 m2/s, respectively. The primary (Mo) dendrite growth started from multi-point nucleation, while its growth velocity agreed well with the prediction of LKT/BCT dendrite growth theory and reached an upper value of 43 mm/s. The subsequent peritectic transition was characterized by a power-law kinetics relation between the nominal growth velocity of peritectic Mo2Zr phase and peritectic undercooling, displaying a maximum velocity of 46 mm/s. The increase of liquid undercooling facilitated the completion of peritectic transition and refined the microstructure of residual primary (Mo) phase, thus enhancing the Vickers hardness of this alloy gradually up to 1190.9 HV at the maximum undercooling.

Metastable solidification mechanisms and micromechanical property modulations of rapidly crystallizing Si66.7Ni33.3 peritectic alloy

Metastable solidification mechanisms and micromechanical property modulations of rapidly crystallizing peritectic Si66.7Ni33.3 alloy were systematically investigated by glass fluxing and arc melting techniques. Glass-fluxed samples were undercooled up to 230 K, and the solidification microstructures consisted of Si, NiSi2 and NiSi phases. As the undercooling rose, primary Si phase grew faster along with the enhancement of volume fraction and the morphology transitioned from lamellar structure to dendritic structure. Meanwhile, the thickness of lamellar Si phase basically remained the same, and its length and width decreased gradually. The subsequent peritectic reaction became more intense and peritectic NiSi2 phase became narrower. The 123 K was the critical undercooling for the abrupt rise of eutectic growth velocity, the morphology transition of regular eutectic in the interdendritic gap of NiSi2 to the anomalous eutectic, and the sudden drop of the sample microhardness. The solute Ni content in NiSi2 and NiSi phases increased linearly with a rise in the undercooling, indicating the substantial expansion of solid solubility and the occurrence of an evident solute trapping phenomenon. For arc-melted samples with 8 mm diameter, the solidification microstructure mainly appeared as primary lamellar Si phase surrounded by peritectic NiSi2 phase plus eutectic (NiSi + NiSi2) phase, and the farther away from the sample bottom the smaller the undercooling and the higher the sample microhardness. As the sample diameter decreased to 2 mm, peritectic NiSi2 phase directly nucleated in the sample bottom and the microstructure composition in other regions resembled the large samples. The Si phase possessed lamellar and dendritic structures in the sample top and middle regions, respectively. Moreover, a maximum microhardness was determined in the lower middle region. The research has both scientific and industrial relevance, offering insights that could be applicable to the development and processing of complex alloy systems.

From source to surface: clues from garnet-bearing Carboniferous silicic volcanic rocks, Iberian Pyrite Belt, Portugal

This work investigates the relationships between partial melting, melt extraction, pluton growth and silicic volcanism in garnet-bearing felsic volcanic rocks that were extruded in the Iberian Pyrite Belt, at ca. 345 Ma. The garnets are of peritectic origin, displaying textural and chemical features of disequilibrium crystallization during partial melting reactions involving biotite at high temperatures (up to 870 °C) in the middle-lower crust. Major element composition suggests compositional equilibrium with the entrained and pinitized peritectic cordierite, but reveals some subsequent homogenization by diffusion. Trace element maps and spot analyses of garnet show, nonetheless, significant trace element variations, reflecting biotite and Y-REE-P-rich accessory phase breakdown during partial melting reactions. Peritectic garnet and cordierite growth resulted in the preservation of Th- and Y-rich prograde suprasolidus monazite, which constrains the timing of partial melting of the metapelitic protolith at ca. 356.8 ± 2.4 Ma. The zircon cargo further shows that a significant amount of zircon crystals from previously crystallized felsic melts were also remobilized and erupted. These were likely stored in an upper crustal pluton that grew episodically since ca. 390 Ma during voluminous melt generation periods within the middle to lower crust, which also resulted in voluminous volcanism. The geochemical trends of the felsic volcanic rocks reflect the entrainment of xenoliths of peritectic garnet, cordierite and feldspar, and as such, the garnet-bearing felsic volcanic rocks represent an erupted mixture of a lower-temperature (ca. 770 °C) silicic melt and autocrysts, and peritectic phases and zircon crystals from previously crystallized and stored felsic melts.

A window on the lithospheric mantle beneath the eastern Trans-Mexican volcanic belt: Insights from pargasite-bearing mantle xenocrysts from the Holocene Ocotenco maar volcano

In this short communication, we report and discuss for the first time the occurrence of pargasite-bearing pyroxenite mantle macro-xenocrysts in a basaltic-andesite juvenile material erupted from the Holocene Ocotenco maar at the Serdán-Oriental Basin Volcanic field, situated in the eastern sector of the Trans-Mexican Volcanic Belt. Through an integrated approach based on microtextural observations, mineral chemistry, geothermobarometric modelling, and comparisons with the existing literature, we determined the xenocrysts equilibrated at ca. 0.8 GPa and 1100 °C, compatible with a pargasite-bearing, upper lithospheric mantle setting. The different textures (resorbed vs. unzoned) of xenocrysts, not only provide evidence of a mantle melting event, but also enabled the discrimination between disrupted mantle fragments and peritectic solid phase generated during the pargasite breakdown. Moreover, peculiar characteristics of the xenocrysts such as the dark pargasite rims and the melt embayment in clinopyroxene, together with the lack of phenocrysts from magma fractionation, the swallow-tail texture of groundmass plagioclase, and a vesicle-rich textures, cumulatively suggest a short-term (days to months?) eruptive scenario with a fast melt extraction from the source region to a rapid ascent through the crust and a sudden explosive eruption by the intense interaction of the ascending melt with a source of water close to the surface.

Microstructure and hydrogen storage properties of the Mg2−xYxNi0.9Co0.1 (x = 0, 0.2, 0.3, and 0.4) alloys

Rare earth elements have excellent catalytic effects on improving hydrogen storage properties of the Mg2Ni-based alloys. This study used a small amount of Y to substitute Mg partially in Mg2Ni0.9Co0.1 and characterized and discussed the effects of Y on the solidification and de-/hydrogenation behaviors. The Mg2−xYxNi0.9Co0.1 (x = 0, 0.2, 0.3, and 0.4) hydrogen storage alloys were prepared using a metallurgy method. The phase composition of the alloys was studied using X-ray diffraction (XRD). Additionally, their microstructure and chemical composition were studied using scanning electron microscopy and energy-dispersive X-ray spectroscopy, respectively. The hydrogen absorption and desorption properties of the alloys were studied using pressure-composition isotherms and differential scanning calorimetric (DSC) measurements. The structure of the as-cast Mg2Ni0.9Co0.1 alloy was composed of the peritectic Mg2Ni, eutectic Mg–Mg2Ni, and a small amount of pre-precipitated Mg–Ni–Co ternary phases, and was converted into the Mg2NiH4, Mg2Ni0.9Co0.1H4, and MgH2 phases after hydrogen absorption. Furthermore, the XRD patterns of the alloys showed the MgYNi4 phase and a trace amount of the Y2O3 phase along with the Mg and Mg2Ni phases after the addition of Y. After hydrogen absorption, the phase of the alloys was composed of the Mg2NiH4, MgH2, MgYNi4, YH3, Y2O3, and Mg2NiH0.3 phases. With the increase of Y addition, the area ratios of the peritectic Mg2Ni matrix phase in the Mg2−xYxNi0.9Co0.1 (x = 0, 0.2, 0.3, and 0.4) alloys gradually decreased until they disappeared. However, the eutectic structure gradually increased, and the microstructures of the alloys were obviously refined. The addition of Y improves the activation performance of the alloys. The alloy only needed one cycle of de-/hydrogenation to complete the activation for x = 0.4. The DSC curves showed that the initial dehydrogenation temperatures of Mg2Ni0.9Co0.1 and Mg1.8Y0.2Ni0.9Co0.1 were 200 and 156 °C, respectively. The desorption activation energies of the hydrides of the Mg2Ni0.9Co0.1 and Mg1.8Y0.2Ni0.9Co0.1 alloys calculated using the Kissinger method were 94.7 and 56.5 kJ/mol, respectively. Moreover, the addition of Y reduced the initial desorption temperature of the alloys and improved their kinetic properties.

Ba-Mg isotopic evidence from an OIB-type diabase for a big mantle wedge beneath East Asia in the Early Cretaceous

Beneath Eastern Asia, the world's spatially most extensive, and possibly extremely long-lived big mantle wedge (BMW) occurs, formed by the Izanagi-Pacific slab residing in the mantle transition zone (MTZ). It provides a fascinating model accounting for the intraplate magmatism and earthquake >1800 km distal from the trench which are incompletely considered by classic plate tectonics. Since when the BMW exists is crucial for global and regional geodynamic reconstructions and modeling of material transfer. However, temporal constraints from the geophysical, geodynamic model, tectonic, and magmatic studies yield conflicting results. To contribute to this endeavor, we have characterized BaMg isotopic features of the ∼123 Ma-old, OIB-type Daixi diabase in Eastern China, which is located ∼300 km west of the inferred Izanagi subduction zone. Results show that the diabase has δ138/134Ba (−0.08‰ to 0.04‰) slightly lower than the terrestrial mantle (∼0.03 to 0.05‰). The δ138/134Ba values are negatively correlated with (87Sr/86Sr)i, K/Ti, K/La, and K/Na of the diabase, pointing to the incorporation of subducted sediments into the mantle source. In addition, the diabase also has lower δ26Mg (−0.67‰ to −0.35‰) than the terrestrial mantle (−0.25 ± 0.04‰) and low (87Sr/86Sr)i (0.704254–0.706300), reflecting source contributions of marine Mg‑carbonates. Thus, BaMg isotopes record the source contribution of carbonate-bearing sediments which must have melted in the MTZ (20–22 GPa) according to experimental investigations. High K/U (∼30,500) and Hf/Hf* (∼1.68) ratios of the melts derived from the enriched source moreover require partial melting in the presence of peritectic liebermannite and majorite. These two peritectic phases are formed in the MTZ (410–660 km) after early incipient melting of carbonate-bearing sediments during which the loss of Na-rich, Ca‑carbonate melts has resulted in the removal of Th, U, and some LREE and retention of K, Ba, Zr, and Hf. These findings provide robust evidence that the Daixi diabase originated from an enriched mantle source metasomatized by melts/fluids derived from the stagnant slab at the MTZ. They also place important constraints on the existence of the Izanagi BMW beneath eastern Asia in the Early Cretaceous (∼123 Ma). It reconciles magmatic with previous tectonic and metallogenetic models that showed lithospheric thinning (peak at ∼125 Ma) and gold mineralization flare-up (peak at ∼120 Ma) at the Eastern China continental margin. Thus, our study is important to understand not only the materials cycling and geodynamic processes within a BMW system but also the tectonic-magmatic effects and interactions between the subducted slab and the overlying continental plate.

Microstructure, thermophysical properties and neutron shielding properties of Gd/316 L composites for spent nuclear fuel transportation and storage

In this paper, (0.5–5.0 wt%)Gd/316 L composites were prepared by vacuum arc melting, and the microstructure, thermophysical properties, and neutron shielding properties were analyzed deeply. The results showed that the Gd/316 L composites started solidification in primary ferrite mode and terminated solidification by a peritectic-type reaction forming (Fe, Ni, Cr)3Gd that is rich in Ni and Gd. In the peritectic-type reaction, local convection in the Ni-rich and Gd-rich peritectic liquid phase leads to the formation of multiple variants of (Fe, Ni, Cr)3Gd. The higher the Gd content, the greater the amount of (Fe, Ni, Cr)3Gd and ferrite, and the harder it is to keep the austenitic matrix of 316 L stainless steel. The solidification temperature range of Gd/316 L composites is 1074–1460 °C, which severely limits the ability to further process it by high-temperature deformation techniques such as hot rolling or hot forging. (Fe, Ni, Cr)3Gd preferentially nucleates and grows in the triangular region of austenite grain boundaries at 1074 °C. At 50–500 °C, with increasing Gd content, thermal expansion and thermal diffusion coefficients become decrease, and thermal conductivity and specific heat capacity increase. The trend of thermophysical property changes is favorable for SNF transportation and storage applications. The thermal neutron shielding properties of 1.0 wt%Gd/316 L composite is comparable to that of 4.45 wt% B content boron steel and 15 wt%B4C/Al composite. The Gd content in Gd/316 L composites material should not exceed 2.5 wt%. If the Gd content continues to increase, it is of little significance to improving shielding properties and material thinning. A shielding thickness of 3 mm is more appropriate for Gd/316 L composites. Too thin would not provide sufficient mechanical properties and increase processing and manufacturing costs.

Observations of the occurrence and disappearance of peritectic couple growth performed under microgravity conditions

Peritectic couple growth (PCG) is an unusual growth form at which the peritectic and pro-peritectic solid phases solidify like a regular eutectic. To investigate the dynamics of PCG formation and growth, experiments with the transparent, organic peritectic alloy Tris(hydroxymethyl)aminomethane–Neopentylglycol are performed at the International Space Station. Fully planar and partly planar/cellular PCGs are found and analyzed. In any case, PCG starts with the formation of lateral bands from the peritectic phase that overgrow most of the pro-peritectic phase but not completely. During coupled growth, the remaining patches transform into rods and/or lamellae. Depending on the conditions, PCG either terminates by the disappearance of pro-peritectic rods or by transforming the rods into cells. The second mechanism goes hand in hand with the formation of a single-phase 2D layer at the top of a tilted interface. At low pulling velocities, planar pro-peritectic phase growth transforms into planar peritectic phase growth via PCG.

Liquid state properties and rapid solidification mechanism of Fe-Nd-B alloy investigated under space simulation conditions

The thermophysical properties of liquid Fe84Nd10B6 alloy from superheated to undercooled state were investigated by space simulation technology combined with ab initio molecular dynamics (AIMD) simulations. The liquid density at liquidus temperature and its temperature coefficient were determined as 7.17 g·cm−3 and 5.51 × 10−4 g·cm−3·K−1, while its liquid thermal expansion coefficient was measured as 7.69 × 10−5 K−1. The ratio of specific heat to emissivity decayed as a quadratic power function. Under electrostatic levitation (ESL) condition, even though the cooling rate of liquid alloy was only 23.6–35.3 K/s, a maximum 131 K undercooling was realized. As alloy undercooling rose, the grain size of primary α(Fe) dendrite was refined, whose volume fraction decreased from 26.2% to 22.0%. Meanwhile, the volume fraction of major peritectic τ1(Nd2Fe14B) phase was increased from 68.6% to 76.8%, which was accompanied by the reduction of peri-eutectoid τ2(Nd1.1Fe4B4) phase down to 1.2%. In the case of microgravity processing by drop tube, liquid undercooling and cooling rate respectively reached up to 260 K (0.16 TL) and 3.9 × 104 K/s for the falling alloy droplets. Instead of τ1 phase, the primary α(Fe) phase occupied the largest volume fraction of 49.7% in large alloy droplets. At enhanced undercooling and cooling rate, the peritectic τ1 phase only increased from 26.2% up to 43.5%, whereas those of α(Fe) and τ2 phases declined to 44.0% and 12.5%, respectively. It was found that the increase of liquid alloy undercooling promoted the peritectic transformation to some extent, but high cooling rate greatly restricted peritectic reaction by reducing reaction time. The coupled effect of liquid undercooling and cooling rate controlled the volume fraction of each phase in rapidly solidified microstructures.

Compatibility of container materials with peritectic phase change material for high-temperature thermal energy storage

Compatibility of storage and container materials is a well-known problem for high-temperature thermal energy storage (TES) technology, which often limits the use of the most economic and best performing materials. Peritectic phase change materials (PCMs) were recently demonstrated as promising TES materials that provide an alternative for molten salts and metallic alloys PCMs. However, unlike molten salts and alloys, peritectic PCMs have not yet been explored in terms of corrosion. This work presents the first compatibility study between Li4(OH)3Br peritectic and common construction materials such as carbon steel and stainless steel 316. Mass gain, XRD, and EDX-SEM surface as well as cross-section analyses reveal that the studied peritectic is recommended for long-term use with carbon steel under air atmosphere and is fully compatible with SS316 under both air and argon atmosphere. Both experimental observations as well as chemical reactions analysis suggest that carbon steel should be used under air rather than inert atmosphere to maximize its compatibility with Li4(OH)3Br peritectic.

Fluid-fluxed partial melting of the Buncheon granitic gneiss in the Yeongnam Massif, Korea: Protracted (ca. 1.86–1.84 Ga) reworking of the Paleoproterozoic Korean arc

The fluid-fluxed melting of igneous protoliths provides an opportunity to assess the role of fluid for the formation and modification of continental crust during high-temperature metamorphism. In order to investigate such an anatexis in the Buncheon granitic gneiss, Yeongnam Massif, we combined field, microstructural, and geochemical studies with the U-Pb and oxygen isotopic analyses of zircon and monazite. The granitic gneisses comprise metatexitic migmatites containing leucosomes, although neither melanosomes nor peritectic phases are associated with neosomes. These migmatites underwent syn-kinematic wet melting in the presence of ca. 0.9–1.4 wt% H2O (estimated from the pseudosection modeling) via the reaction, plagioclase + K-feldspar + quartz + H2O = melt, that took place at ∼650–750 °C and 4–6 kbar. The leucosomes differ in the degree of Eu anomalies and have variable amounts of rare earth elements, Zr, Y, and Th. Such variations are attributed to the segregation and migration of melts as well as the entrainment of residual phases. Sensitive high-resolution ion microprobe (SHRIMP) U-Pb ages of zircon and monazite were determined from two granitic gneisses and three leucosomes together with an adjacent, undeformed Hongjesa granite. Zircon grains are oscillatory-zoned, except for homogeneous overgrowth rims rarely present in two leucosomes. Both zircon domains were dated at ca. 1.98–1.97 Ga and ca. 1.86 Ga, respectively, suggesting that igneous protoliths were partly recrystallized during the late Orosirian. Monazites from two gneiss samples contain submicroscopic polycrystalline inclusions of K-feldspar + quartz + apatite, reflecting the presence of former melts, and yielded a pooled 207Pb/206Pb age of 1859 ± 6 Ma (n = 30). In contrast, monazites from a leucosome are inclusion-free, and yielded the youngest age of 1840 ± 16 Ma (n = 6). This result is interpreted to represent the melt crystallization at ca. 1.84 Ga following the ca. 1.86 Ga anatexis. On the other hand, monazite from the Hongjesa granite, yielded a weighted mean 207Pb/206Pb age of 1878 ± 5 Ma (n = 11), most likely reflecting a prograde metamorphic growth. The δ18O values of ca. 1.98–1.97 Ga igneous zircon in granitic gneisses are 7.1–7.2 ‰, but those from the leucosome are reduced at 5.9–6.7 ‰. The latter is largely identical to 5.7–6.7 ‰ estimated from ca. 1.86 Ga domains of zircon in the leucosome and monazite in the granitic gneiss; in contrast, a slightly lower δ18O value of 5.46 ± 0.61 ‰ is recorded in the ca. 1.84 Ga monazite of leucosome. The relatively low δ18O values are most likely accounted for by an influx of fluid that has apparently triggered partial melting in the granitic gneiss, and the fluid itself is possibly linked to the crystallization of mafic magmas in an arc complex. Taken together, in contrast to the majority of zircons preserving the crystallization age of igneous protoliths, monazites are more susceptible to recrystallization during the fluid-present melting and permit us to decode a protracted (∼20 m.yr.) episode from partial melting to melt crystallization in the Yeongnam Massif. Such a longevity of melt-present system typifies the prolonged crustal reworking of Paleoproterozoic Korean arc situated at the eastern margin of the North China Craton.

Isotope disequilibrium caused by the influx of fluids during crustal anatexis

Isotope disequilibrium is increasingly recognized during crustal anatexis, which is usually attributed to stoichiometry or peritectic phases of melting reactions. Instead, whether fluid influx can contribute to isotope disequilibrium during fluid-fluxed melting remains poorly constrained. Here we report combined Mg-O-Sr isotopic data for migmatites in the Dabie orogen. Leucosomes show considerably lower δ26Mg (−0.71‰ ∼ −0.23‰) and δ18O (3.7‰ ∼ 7.9‰) but higher 87Sr/86Sr(i) (0.7089–0.7131) than the corresponding melanosomes (−0.29‰ ∼ −0.11‰, 6.9‰ ∼ 10.6‰ and 0.7080–0.7119, respectively). Such isotopic difference between leucosomes and melanosomes cannot result from either fractional crystallization, equilibrium, incongruent to non-modal melting or accumulation. Instead, co-variations of δ18O, δ26Mg and 87Sr/86Sr(i) of leucosomes indicate influx of external fluids and their preferential partition into melts. The external fluids could have low δ18O and δ26Mg but high 87Sr/86Sr(i) and were most likely derived from dehydration of the surrounding eclogites and gneisses with Neoproterozoic protoliths experienced meteoric-hydrothermal exchange. One leucosome is plotted off the major trend by its rather low δ26Mg (∼ −0.71‰), and may have been overprinted by inter-diffusion. Simple diffusion modeling shows that the preservation of Mg-O-Sr isotope disequilibrium in the Dabie migmatites requires a short duration from fluid-fluxed anatexis to re-solidification (i.e., < 103–104 years). This study highlights that a combination of multiple isotopes may be conducive to understand the mechanism of disequilibrium melting.

In Situ Study of Peritectic Couple Growth Under Purely Diffusive Conditions

Herein, in a microgravity campaign onboard the International Space Station, peritectic coupled growth (PCG) is studied by solidifying alloys of the peritectic transparent organic system TRIS–NPG under purely diffusive conditions. The experiment reveals that the formation of PCG begins with coagulated lateral bands comprising the peritectic phase at temperatures below the peritectic one. After reaching the growth front of the pro-peritectic phase, growth competition between both solid phases occurs such that the patches of the pro-peritectic phase, lamellae, and fibers/rods grow in the stated order, coupled with the peritectic phase. As the entire solidification process occurs during the initial transient stage, the diffusion coupling between the two solid phases is weak. With the gradual decrease in the interface temperature during the initial transient, the necessary growth conditions for the pro-peritectic phase diminish. Additionally, different concentrations along the inclined solid/liquid interface favor the peritectic phase at the rear window and the pro-peritectic phase at the front window. The corresponding PCG lasts 19 hours while the recoil of the inclined interface continues to a temperature level that makes alterations of the ‘effective’ TRIS–NPG phase diagram necessary.

Study of the peritectic phase transformation kinetics with elastic effect in the Fe–C system by quantitative phase-field modeling

A quantitative phase-field model developed for two-phase growth process (Folch and Plapp, 2005) is applied to the simulation of peritectic phase transformation in a Fe–C binary alloy system with elastic effects due to the misfit on the solid/solid interface. The model includes the anti-trapping current, elastic effect, and the quantitative model parameters estimated based on the thin-interface limit. The diffusion of carbon in the solid phases (i.e. δ and γ phases) has not been neglected and an appropriate free-energy functional has been developed such that there is no presence of any extra phase in the interface. The model’s behavior over a wide range of interface width has been studied and the effect of anti-trapping current and elastic effect on the phase transformation has been closely analyzed.

Effect of Ni and Si alloying elements on the phase evolution, mechanical properties, tribological behaviour of Al–Cu alloys

The present study aims to understand the solidification pathways of novel Al-rich multicomponent alloys with ultrafine eutectic with micron-scale dendrite. Four eutectic alloys Al–Cu-X (X = Ni1.5, Si5, Ni1.5 and Si5) have been designed with the help of thermodynamic simulation using the thermo-calc software, followed by synthesis by solidification processing technique. The microstructure of the studied alloys reveals the presence of dendrite phases and eutectic phase mixtures, bimodal eutectics, peritectic phase and ternary eutectics mixture (L → α-Al + Al7Cu4Ni + β-Si). Based on these results a quasi-peritectic reaction (L + Al7Cu4Ni → α-Al + Al2Cu) has been proposed for AlCuNi1.5 alloy which has confirmed with the DSC results. With the addition of Ni and Si, the tensile strength and hardness of these alloys increases to 336 MPa and 188.84 HV from 231 MPa and 134.2 HV respectively. The AlCuNi1.5Si5 alloy have better wear resistance (3.40 cm−3) as compared to other alloys.

Hypo-peritectic TRIS–NPG in a stationary temperature gradient: Thermodynamics, grain boundary migration and phase identification

At a stationary temperature gradient, the grain boundaries comprising residual melt can migrate. Their migration is governed by liquid diffusion and, in multiphase materials, depends highly on the phases present. Therefore, this phenomenon can be used for phase identification if data on metastable extensions of the corresponding phase diagram are available. Thus, we re-optimised the thermodynamic description of Tris(hydroxymethyl)aminomethane-Neopentylglycol (TRIS–NPG), a transparent peritectic alloy often used as a model alloy for metallic solidification, and used the predicted metastable liquidus and solidus curves to evaluate the grain boundary migration observations. As we found temperature gradient zone melting (TGZM) at low temperatures, the presence of the peritectic phase could be excluded even though a near-peritectic alloy had been processed. The liquid diffusivity, as a function of the position/temperature, was estimated from the TGZM velocity measurements. The data suggest that the diffusion coefficient deep in the mush is one order of magnitude smaller than that close to the liquidus temperature. This may be typical for non-dilute alloys, where the concentration of the intergranular liquid changes considerably.

Grain refinement and abnormal peritectic solidification in WxNbTiZr high-entropy alloys

The microstructures of many high-entropy alloys (HEAs) can be understood or controlled based on traditional phase formation mechanisms. In this study, peritectic solidification was introduced to HEAs by designing novel WxNbTiZr peritectic high-entropy alloys (PHEAs). The phase constitutions and solidification microstructure evolutions of WxNbTiZr were clarified by combining experiments and thermodynamic simulations. Grain refinements were achieved in WxNbTiZr, which should attribute to the high growth restriction factors introduced by W solute and the segmentations of the primary phase during peritectic transformation. The significantly refined grains can simultaneously enhance the strength and ductility of the alloy. High fractions of peritectic phases and special pearl chain structures that are highly likely the products of peritectic solidification are seen in WxNbTiZr but not in the typical binary W-Zr peritectic alloy, highlighting that the compositional complexity featured by PHEAs could heavily influence peritectic reaction. The PHEAs idea is also generalized by developing WxMoTiZr alloys.

The evolution of microstructure, micromechanical and magnetic properties of FeCoNiAlSi alloys with peritectic structure processed by high-pressure solidification

In this work, we examined the microstructure evolution, as well as the micromechanical and magnetic properties of FeCoNiAlSi high-entropy alloys (HEAs) under high pressure solidification (HPS). According to the results, FeCoNiAlSi alloy solidified at 0.1 MPa (atmospheric pressure) contains three kinds of phases: grain boundary (GB)phases, peritectic phases, and primary phases. As the solidification pressure increases to 7 GPa, the volume fraction of GB phases and peritectic phases decreases, while the volume fraction of primary phase increases by 82.8%. In addition, the Fe of the primary phase increases by 63.6 %, which enhances the ferromagnetism and exchange coupling properties of the alloy. Furthermore, HPS also reduces the magneto-crystal anisotropy of the GB phases and the pinning effect of boundaries on domain movement. Additionally, with the increasing solidification pressure, the FeCoNiAlSi HEAs shows stronger ferromagnetic. When solidified at 7Gpa, a reduction of 51.5 % in intrinsic coercivity(H MC ) was observed, as was an increase of 106 % in magnetic polarization strength (Js). As well, HPS significantly increased the high temperature magnetic properties of FeCoNiAlSi alloy as well as the microhardness(i.e. from 8.25 GPa to 10.15 GPa). According to these results, HPS is an effective method for developing new functional applications for FeCoNiAlSi HEAs. • The Js (HMC)of the FeCoNiAlSi HEAs increased (decreased) by 106% (51.5%) after HPS at 7Gpa, The Tc are also improved. • The pinning effect of boundary on magnetic domain movement were reduced after HPS and exchange coupling was enhanced. • The volume fraction of primary phase and the content of Fe in primary phase increased when solidified under 7GPa. • The promoted solution strengthening after HPS significantly improved the microhardness (i.e. from 8.25GPa to 10.15GPa).

Microstructure and mechanical behavior of Ti—Cu alloys produced by semisolid processing

The mechanical properties of Ti—Cu alloys processed via thixoforming were evaluated. Ti—Cu (25, 27, and 29 wt.% Cu) ingots were produced via arc melting, homogenization at 950 °C for 24 h, and hot-forging at 900 °C, followed by thixoforming at a speed of 8 mm/s after isothermal heat treatment at 1035 °C for 300 s. The thixoformed alloys exhibited good mechanical strength, limited plasticity under tensile loading, and reasonable plasticity under compressive loading. The mechanical strength and plasticity decreased as the Cu content increased as a result of the increasing volume fraction of the peritectic Ti2Cu phase (transformed liquid), which exhibited a lower strength and plasticity than the α + Ti2Cu regions (transformed solid). These findings indicated that the trade-off between the mechanical properties and semisolid processability is largely governed by the Cu content.

Formation of late Miocene silicic volcanic rocks in the central Tibetan Plateau by crustal anatexis of granulites

Partial melting of granulites generates melts, which causes the modification of continental crust. However, the detailed mechanisms of this partial melting and related magmatic processes are debated. In this study, we investigated how crustal melting of granulites generated late Miocene (ca. 6.0 Ma) silicic volcanic rocks in the Chibuzhangcuo area of the Qiangtang Block, Tibet. Various peritectic textures (e.g., quartz inclusions in clinopyroxene and fluorhydroxyl phlogopite) suggest that the pyroxenes were derived from a residual granulite phase and peritectic reactions, and were then entrained into the partial melts. The F-rich mica, granulite mineral assemblages, and high titanomagnetite–ilmenite (811–975 °C) and zircon saturation (836–867 °C) temperatures indicate the silicic magmas were high-temperature and water-poor. The geochemical and isotopic compositions of the silicic volcanic rocks are similar to those of middle–lower crustal intermediate–mafic granulite xenoliths in Cenozoic volcanic rocks in the Qiangtang Block. We suggest that these volcanic rocks were generated by partial melting of hydrous mineral-bearing granulites, triggered by an underlying thermal anomaly associated with the high crustal heat-flow of the central Tibetan Plateau. This work indicates that the occurrence of restitic hydrous mineral and perturbation of thermal anomaly are the primary conditions to cause the partial melting of granulite. As the melting went on, the residual granulite phase and peritectic minerals were likely to be preferentially entrained into the melt due to the erosion effect caused by the continued melt flow at the sites of melting.