Phase selection in undercooled ti3sn melts

Short-range order and solid–liquid interfaces in undercooled metallic melts

Spontaneous grain refinement in undercooled metallic melts has been a topic of enduring interest within the solidification community since its discovery more than 50 years ago. Here we present a comparative study of the solidification microstructures and velocity-undercooling behaviour in two dilute Cu-Ni alloys (3.98 & 8.90 wt.% Ni), which have been undercooled by a melt encasement (fluxing) method. Cu-3.98 wt.% Ni shows grain refinement at both low and high undercooling, with a dendritic growth regime separating the two grain refined regions. Within the grain refined region dendritic fragments are clearly evident in the centres of the refined grains and on the surface of the undercooled droplet, suggesting a dendritic fragmentation mechanism. Cu-8.90 wt.% Ni displays also grain refinement at both high and low undercoolings. In the low undercooling grain refined region the samples display curved grain boundaries with a dendritic substructure that extends across grains, indicative of a recovery and recrystallisation mechanism. Conversely, prior to the onset of the high undercooling grain refinement transition extensive regions of dendritic seaweed are observed, suggesting that it is remelting of a dendritic seaweed that gives rise to this structure. Consequently, in two closely related Cu-based systems we have strong microstructural evidence for the operation of all three mechanisms currently considered to give rise to grain refinement. This may help to resolve the grain refinement controversy, although it remains to be determined what factors determine which mechanism operates in any given system.

The Ni–38 wt% Si alloy whose eutectic products are two stoichiometric intermetallic compounds (i.e., NiSi and NiSi2) was undercooled by the melt fluxing technique. After in situ observations of the recalescence processes using a high-speed camera and by electron back-scattering diffraction analysis of the solidification microstructures, the crystal growth velocities, phase selection, and microstructure evolutions were studied. Due to a growth-controlled mechanism, the primary phase changes from the NiSi to the NiSi2 phase at a critical undercooling ΔT ≈ 48 K. Even in the absence of the driving force of chemical superheating, the transition from regular eutectics to anomalous eutectics happens. The reason is that the single-phase dendrite of NiSi2 phase solidifies firstly and then the NiSi phase grows epitaxially to form an uncoupled eutectic-dendrite at high undercooling. The present work provides further experimental evidences for the dual origins of anomalous eutectics (e.g., uncoupled eutectic-dendrite growth during the recalescence stage and coupled lamellar eutectic growth at low undercooling during the post-recalescence stage) and is helpful for understanding of non-equilibrium phenomena in undercooled melts.

Undercooling of superalloy melts: basis of a new manufacturing technique for single-crystal turbine blades

In this work, we study the effect of fluid flow on the growth dynamics during solidification of tetragonal Ni2B from the undercooled melt. Different experimental techniques are applied to generate varying fluid flow velocities in undercooled samples, electromagnetic levitation under 1 g conditions (1 g EML) and in reduced gravity (μg EML) as well as the melt-fluxing technique and electrostatic levitation. The propagation of the solid-liquid interface apparent on the surface of solidifying samples is observed by high-speed video imaging while the non-contact measurement of the undercooling prior to solidification is accomplished by IR-pyrometry. It is demonstrated that fluid flow has a substantial influence on the growth kinetics. As revealed by an extended sharp-interface model, this effect is mainly attributed to the substantial change in growth kinetics caused by a convection induced transition from dendrites to more faceted solidification structures.

Transformation kinetics of the metastable bcc phase during rapid solidification of undercooled Fe-Co alloy melts

The maximum undercooling of about 240 K of Ni90Cu10 alloys was achieved by using molten glass purifcation and cyclic superheating methods.The solidification structure of the undercooled Ni90Cu10 alloy under different undercooling was observed by optical microscope. The microstructure evolution of the undercooled Ni90Cu10 alloy under different undercooling was systematically studied, and the grain refinement structure was obtained under low undercooling and high undercooling,respectively. High-speed camera was used to capture the solidification changes of the solid-liquid interface, and it can be seen that the morphology of the solidification front was also different in different undercooling intervals.Moreover, with the difference of initial undercooling, the starting position of solidification nucleation point in the whole undercooled melt is also different.The driving force of recrystallization mainly comes from the deformation energy accumulated during the recalescence of undercooled melt. It was confirmed by optical microscopy and electron back scattering diffraction analysis that recrystallization under high undercooling is the main mechanism of grain refinement.

We describe an implementation detail of a parallel finite volume solver PPFF2 (2-dimensional parallel phase-field finite volume method solver) for the simulation of solidification in two dimensions using the phase-field model. The governing equations for phase-field and temperature field are discretized with cell-centered finite volume method in a fully implicit way. In each time step, a parallel version strongly implicit procedure is used to solve the discretized equations. The problem of the growth of a solid seed into an undercooled melt is first solved to validate the numerical algorithm. The effects of the dimensionless width (0.0025, 0.0020, 0.0015) and mesh spacing () on the structures of dendrites and interface growth rate are investigated. At last, the competitive growth of the second and ternary arms with different noise term are simulated with the parallel finite volume code developed in this work. In terms of the parallel performance, the maximum speed-up for 3 grid systems are 6.98 and 8.77, its corresponding efficiency 43.6% and 54.8%.

Review: 60 refs.

Disorder trapping in Ni 3(Al, Ti) by solidification from the undercooled melt

Dendrites are the best studied structures formed during solidification of undercooled melt. Additionally other complex structures have been found in experiments and numerical studies. We perform 2D/3D-numerical simulations of solidification processes and experiments which allow in situ investigations of three-dimensional growth of xenon crystals into undercooled pure melt. Dendrites, seaweed, doublons, triplons etc. can be produced in experiments and numerical studies depending on initial conditions. In an interplay of numerical studies and experiments we use simulations to interpret and to plan experiments. Experimental results are used as a basis of model calculations and the development of models of complex shapes. Three-dimensional growth shapes of crystals are reconstructed using sophisticated image processing combined with experimentally determined shape parameters.

Phase selection and metastable phase formation during solidification of undercooled alloy melts have been studied using Bi-Sb alloys. Undercoolings larger than 0.2T(1) were obtained for bulk Bi95Sb5 alloy melts. The factors which influence the undercooling behavior are discussed. Microstructures of the samples obtained at different undercoolings reveal that undercooling can play an important role on the formation of the final phase structure. The specific heat of the undercooled Bi95Sb5 alloy melt is measured by DSC. Results of X-ray diffraction on the samples show that a metastable phase with tetragonal structure, which is similar to that existing stable under high pressure, can form if the undercooling of Bi95Sb5 is 121 K.

<sec> The ability to undercool and solidification mechanism of liquid quinary Ni<sub>40</sub>Zr<sub>28.5</sub>Ti<sub>16.5</sub>Al<sub>10</sub>Cu<sub>5</sub> alloy are investigated by electromagnetic levitation (EML) and drop tube (DT) technique. Under the EML condition, the maximum undercooling of levitated alloy can reach up to 290 K (0.21<i>T</i><sub>L</sub>). Under the DT condition, the alloy achieves higher undercooling than EML, and solidifies finally into metallic glass. At lower undercooling, the solidification structure of the alloy is composed of primary Ni<sub>3</sub>Ti phase, secondary Ni<sub>10</sub>Zr<sub>7</sub> phase and eutectic (Ni<sub>10</sub>Zr<sub>7</sub>+Ni<sub>21</sub>Zr<sub>8</sub>) phase. With the rise of undercooling, the solidification structure displays the following evolution events: phase morphology refinement, primary phase inhibition, phase number reduction, and amorphous phase formation.</sec><sec> By using the high-speed cinematography technique, three nucleation modes are distinctly observed on the levitated alloy melt surface at the beginning of solidification, that is, single-point nucleation, multi-point nucleation and annular nucleation. The levitation state corresponding to single-point mode nucleation is relatively stable, and the alloy undercooling is also relatively low. The annular nucleation only occursin the case with high rotation speed, and the undercooling is greater than 208 K. The discrepancy between nucleation modes is due to the He gas flow for forced cooling. </sec><sec> The theoretical calculations indicate that the alloy droplets achieve high undercoolingand large cooling rate under the DT condition. The experimental results show that when the droplet diameter decreases to 498 μm, the amorphous phase begins to appear in the alloy particles. It is noteworthy that the amorphous phase is preferentially formed inside the droplet, but not on the outer surface. The morphology of solidification structure reveals that different regions of the droplet have various local undercoolings, which result in the distribution characteristics of amorphous phase. The volume fraction of amorphous phase increases linearly with the decrease of particle diameter. When the droplet diameter decreases to 275 μm, the alloy droplets are completely frozen into glassy particles.</sec><sec> The average eutecticspacing values are also measured at different alloy undercoolings. Compared with the classical binary eutectic growth model, the experimental eutectic growth law exhibits a large deviation in index. This indicates that the eutectic growth in multicomponent alloys displays more complex kinetic characteristics.</sec>

Two facilities for the complete noninvasive measurement of the electrical resistivity of liquid metals above the melting temperature and in the undercooled liquid state below the melting temperature are presented: a ground-based facility that was built up in our laboratory and the microgravity facility TEMPUS. Both facilities are unique in combining the containerless positioning method of electromagnetic levitation with the contactless, inductive resistivity measurement technique, thereby enabling for the first time measurements in the undercooled liquid state of a metal. The principles and technical realizations of the levitation and measurement technique of both facilities are presented. Furthermore, experimental resistivity data for Cu60Ni40 as well as Co80Pd20 alloy are presented which indicate atomic ordering effects in the undercooled melt.

—Kuvaevite, ((Ir,Rh)5(Ni,Fe,Cu)10S16), forms small grains (up to 20 µm across) in globular inclusions hosted by grains of Os–Ir–(Ru) alloys (up to 0.5 mm) in ore occurrences along the Ko River in the Sisim placer zone, Eastern Sayans. Rh-bearing pentlandite or oberthürite (or both), the minerals of the laurite-erlichmanite series and Pt–(Pd)–Fe alloys are the main associated minerals. Kuvaevite is gray to brownish gray in color in reflected light. Its bireflectance is weak to absent. It is slightly pleochroic in gray to light brown shades, and slightly anisotropic, from gray to light yellow shades. Its calculated density is 6.37 g/cm3. According to results of microprobe analyses (n = 3) carried out using wavelength-dispersive spectrometry, WDS, the composition of kuvaevite is: Cu 5.94 (4.39–6.89), Ni 13.95 (13.80–14.24), Fe 10.95 (10.18–11.97), Co 0.07 (0.06–0.10), Ir 32.38 (32.19–32.73), Rh 7.27 (7.22–7.31), Pt 1.91 (1.67–2.06), Os 0.05 (0–0.09), Ru 0.05 (0.04–0.05), S 27.06 (26.77–27.41), total 99.63 wt.%. The empirical formulae calculated using the mean results of analyses are: (Ir3.22Rh1.35Pt0.19Ru0.01Os0.01)Σ4.78(Ni4.54Fe3.75Cu1.79Co0.02)Σ10.10S16.13 (WDS) and (Ir3.23Rh1.43Pt0.25)Σ4.91(Ni4.49Fe3.57Cu1.86Co0.06)Σ9.98S16.11 (SEM/EDS; n = 56). These are based on a total of 31 atoms according to structural data obtained for torryweiserite, the rhodium-dominant analogue. Kuvaevite forms solid-solution series with torryweiserite, tamuraite and ferrotorryweiserite, all these being isostructural. The symmetry of kuvaevite was determined using the synchrotron Laue microdiffraction; the results are in good agreement with the trigonal crystal system and give the following unit-cell parameters: a = 7.079(5) Å, c = 34.344(12) Å, V = 1490(2) Å3; Z = 3. The ratio c/a is 4.852. The probable space-group, R3m (#166), is based on structural results for torryweiserite. The strongest eight reflections in the X-ray diffraction pattern derived from the microdiffraction study [d in Å(hkl) (I)], are the following: 3.0530(201)(43), 3.0103(216)(100), 2.9962(1010)(53), 2.7991(205)(50), 2.4946(208)(31), 1.9208(3110)(41), 1.7697(410)(73), 1.7582(2016)(66). The results of the electron backscatter diffraction study (EBSD) of two kuvaevite crystals are well–indexed based on the R3m space group. Kuvaevite and related sulfides significantly vary in composition in the Ko River placer, in the entire Sisim zone, and in some other ore occurrences worldwide. Associations of platinum-group minerals observed in ore occurrences at Ko River and in the Sisim zone seem to be genetically related to bedrock zones of chromite-bearing ultramafic rocks (serpentinites) of the Lysanskiy complex. Kuvaevite and other minerals present in the polymineralic inclusions, hosted by Os–Ir–(Ru) alloys, formed from droplets of residual melt. This melt accumulated the “incompatible” elements, which could not be incorporated into the structure of the host alloy, including lithophile elements, chalcogens (S, Te), semimetals (As, Sb, Bi), base metals (Fe, Ni, Cu), as well as relatively low-temperature PGE species (Pt, Pd) and Rh. There are local data on metastable crystallization and undercooling of the silicate melt, as well as effective differentiation and fractionation of S and ore components during the crystallization of these inclusions. Kuvaevite is named after O.M. Kuvaev (1934–1975), a prominent geologist, geophysicist and writer.