Verification of metastable phase diagrams byin situ measurements on undercooled melts

The electromagnetic levitation technique combined with pyrometric methods have been applied to verify the metastable phase diagram of Fe Cr Ni alloys byin situ observation of phase selection processes in undercooled melts. The temperatures of levitated drops prior to solidification were determined by a two-color pyrometer and the recalescence behavior of the undercooled melt was recorded from a high-speed photosensing device with a sampling rate of 1 MHz. Fe₆₉Cr₃₁ ,Ni, alloy melts with different Cr/Ni ratios were investigated for undercooling levels up to 320 K. The transition from single- to double-recalescence behavior was found beyond a critical undercooling level for primary austenitic alloys with Cr Ni < 1.5. For the first time, this gives direct evidence of metastable phase formation in this alloy system fromin situ observations. The dendrite growth velocity displayed a sudden drop at the critical undercooling level, confirming the metastable phase formation. The intermediate arrest temperature of the double-recalescence event fits well with the calculated metastable liquidas line derived from a subregular solution model for the Gibbs free energy.Paper presented at the Fourth International Workshop on Subsecond Thermophysics, June 27–29, 1995, Köln, Germany.     

Effects of initial undercooling on microstructure formation and recrystallisation of undercooled melts

The critical undercoolings for the two grain refinement events and the onset of recrystallisation event are determined by detailed analysis of the microstructure evolution of bulk undercooled Ni–20?at.-%Cu alloy melts. The first grain refinement event occurred in the low undercooling range was explained by dendrite remelting. The second grain refinement event occurred in the high undercooling range was due to the combined effects of dendrite remelting stress-induced dendrite breakup during recalescence and recrystallisation during the near-equilibrium solidification stage after recalescence. The micro-stress induced by the solidification contraction during recalescence in the so called ‘first mushy zone’ would lead to distortion and breakup of primary dendrites. The stress-induced broken-up dendrites have sufficient driving force for recrystallisation.     

快速凝固Fe-Co合金中的亚稳相

采用熔融玻璃净化和循环过热相结合的方法,研究了Fe-Co合金在深过冷快速凝固中的亚稳相。结果表明,当熔体的过冷度达到某一临界数值(△T_(crit))时,亚稳相(b.c.c)在竞争形核中优先形核生长,并在随后的冷却过程中保留在凝固组织中。在γ单相区内等温退火3 h后,亚稳相完全转变为稳定相;在过冷Fe-Co合金的凝固组织中亚稳相是竞争形核-重熔-γ相外延生长-不完全固态相变的产物。     

Metastable phase diagrams of Cu-based alloy systems with a miscibility gap in undercooled state

Some Cu-based alloy systems with a large positive enthalpy of mixing display a eutectic or peritectic phase diagram under equilibrium conditions, but show a metastable liquid miscibility gap in the undercooled state. When the melt is undercooled below certain temperature beyond the critical liquid-phase separation temperature, it separates into two liquids with different compositions. The compositions of the two liquids change successively upon the metastable phase diagram before solidification occurs. The shape and position of the metastable miscibility gap are dependent of the alloy components and their interaction features. This study reviews the metastable phase diagrams of Cu-based alloy systems, which are derived from experiments and thermodynamic calculations.     

Metastable phase formation in a laser-irradiated silver-germanium alloy

The structure of laser-irradiated regions in a silver-21 at. % germanium alloy was studied by X-ray and metallographic techniques. A hexagonal close-packed metastable phase was detected in the irradiated zone. Its formation is explained in terms of the ultrarapid cooling of a thin molten zone in contact with a thermally highly conductive solid substrate.On leave from the Faculty of Engineering, University of Tehran, Tehran, Iran.On leave from the Department of Metallurgy, Institute of Technology, Banaras Hindu University, Varanasi, India.     

Effect of phase on the formation of excited particles under ion bombardment of Fe-Co alloys

Investigations of the main parameters of ion-photon emission during Ar⁺ ion bombardment of Fe and Co metals and Fe-Co alloys have been made. It was shown that the dependence of the quantum yields of the emissions of ejected excited Fe and Co atoms on the concentration of corresponding elements in the alloys generally decreased but showed local maxima. These maxima are related to concentrations of Fe and Co at which intermetallic compounds (FeCo, Fe₃Co, FeCo₃) are present. It is suggested that this local increase in excited metal atom signals is related to the breaking of intermetallic bonds.     

Metastable phase formation in laser-processed Al-Ge sputtered thin films

The formation of a metastable phase in Al₅₉Ge₄₁ thin films under microsecond laser irradiation is reported. Thin films prepared by co-sputtering are amorphous as-grown. Upon laser processing, amorphous and crystalline phases are detected and analysed using TEM and EDX. A metastable structure formed by laminar-textured aluminium alternating with an hexagonal metastable phase is observed.     

Nucleation of crystals in deeply undercooled alloy melts

The CALPHAD approach coupled with modelling of solid-liquid interfacial energy has been used to calculate the driving force for nucleation in undercooled melts. Thermodynamic parameters needed in nucleation have been evaluated using simplified formulae or numerical methods from assessed phase diagrams. Various models for the interfacial energy and its temperature dependence have been used. Phase selection on solidification and devitrification of glasses as well as the range of amorphous phase formation have been predicted in the Al-Ce and Fe-B systems and compared with those experimentally determined. Furthermore, the formation of quasicrystals in the Al-Mn and the competition with other compounds has been investigated.     

Metastable order-disorder transition and sigma phase formation in Fe-V binary alloys

The influence of binary and ternary alloying compositions and heating rate on theσ formation reaction are studied. Several ways are developed to supress the stableα phase formation and to understand the metastable CsCl-type order-disorder transition in Fe-V alloys. The Fe-V ordered phase has a metastable critical transition temperature of between 850 and 880° C at equiatomic composition, and the so called “650° C anomaly” corresponds to the 550° C anomaly in Fe-Co alloys. Theσ formation reaction does not commence most easily at equiatomic composition but at the iron-rich side of the 40 at. % V alloy. The Influence of ternary alloying additions on theσ formation in Fe-V alloys depends on the relative stabilities of the binaryσ phase with iron or vanadium.     

Subjective luminous line perception under changing g-load and body-positions in parabolic flight

Five healthy subjects were subjected to parabolic flight with laterally tilted head, trunk, or body position. A vertical luminous line was viewed by the subjects in a head-fixed goggle device. During normal, hyper- and hypogravity phases subjective luminous line orientation was measured. The data imply that stimulation of the neck position receptors markedly influences the perception of the subjective vertical as well as mechanisms of convergence of otolithic signals and visual information within the brain.     

Dendrite morphologies of the metastable phase from undercooled Fe-30at % Co melts and its stabilities

Five typical dendrite morphologies of the metastable bcc phase fromundercooled Fe-30 at % Co melt have been observed by TEMtechnique. The morphologies of the metastable phase exhibitedwell-developed dendrite with the primary trunk and second arms,well-developed second arms, radiated structure, lath structure,and bifurcated structure. The crystal growth mode and theformation of different dendrite morphologies were discussed onthe basis of the morphological patterns from undercooled melts.In the mean while, the breakage mode for the primary dendritewas suggested according to the observation of microstructures ofthe alloy solidified at various undercoolings. The EDS (EnergyDispersive Spectrum) analysis has confirmed enrichment of thesolute Co in metastable dendrite cores in comparison to thatpredicted from the view of equilibrium solidification. Furtherinvestigation after annealing showed that the solute diffusioncontrolled the stability of the metastable phase; thedisappearance of dendrite morphologies was mainly attributed tothe constituent homogenization within dendrite cores and thedecrease in the number of dendrite cores was chiefly owing tothe solute diffusion between dendrite cores and the subsequentlysolidified equilibrium phase.     

The effect of pressure on metastable phase formation in the undercooled Bi-Sn system

A metastable phase was produced by the solidification of highly undercooled Bi-48.6 at% Sn alloy droplet samples. During heating the metastable phase was observed to melt at 116° C at ambient pressure. The onset of the metastable endotherm was found to increase with increasing pressure, while the liquidus and eutectic temperature for the structure stable at ambient pressure decreased with increasing pressure. Based on the pressure dependence of the melting trend, the metastable phase will be stable at the expense of the stable ambient pressure structure under high hydrostatic pressure conditions (above 1 GPa). Both microstructural observations and X-ray examinations at ambient pressure revealed that the metastable phase was present in droplet samples and that the X-ray diffraction pattern was close to that of the high-pressure stable phase previously reported as a rhombic cell. High-pressure thermal analysis has also allowed for identification of the effect of pressure in promoting favourable formation kinetics and the kinetic transition from the equilibrium phases to the metastable phase at high undercooling.     

The growth velocity-undercooling relationship of highly undercooled Ni and Ni-Cu melts

Solidification velocities as a function of initial bulk undercoolings were systematically investigated using high speed thermal imaging of highly undercooled pure Ni and Ni-Cu alloy samples. For pure Ni, the dendrite growth velocity increased continuously to a maximum value of about 52 m?s^?1 at a maximum undercooling of 320 K. For Ni-Cu alloy, the dendrite growth velocity first increased continuously and then discontinuously to a maximum value of about 55 m?s^?1 at 300 K. A clear discontinuity appeared in the growth velocity-undercooling curve is considered to be caused by the finite speed of solute diffusion in the bulk liquid ahead of the migrating solid–liquid interface.     

Metastable phase formation and structural evolution of epitaxial graphene grown on SiC(100) under a temperature gradient

Multilayer epitaxial graphene was obtained from a 6H-SiC(001) substrate subjected to a temperature gradient from 1250 to 1450?°C. Scanning tunneling microscopy and x-ray diffraction were used to identify the structure and morphology of the surface, from which the formation of a metastable phase was inferred. By a comparison between microscopy and diffraction data, we report the appearance of misoriented Si-doped graphene in cold regions (1250?°C) of the substrate. This metastable phase occurs in domains where silicon sublimation is incomplete and it coexists with small domains of epitaxial graphene. At 1350?°C this phase disappears and one observes complete graphene-like layers (although misoriented), where rotational registry between the underlying epitaxial graphene and additional layers is absent. At 1450?°C the stacking among layers is established and the formation of highly oriented single crystalline graphite is complete. The stability of this Si-rich metastable phase at 1250?°C was confirmed by first-principles calculations based on the density functional theory.     

Casting process for hypermonotectic alloys under terrestrial conditions

Bearing materials are generally heterogeneous materials, containing hard as well as soft, phases. Hypermonotectic AlPb and AlBi alloys, especially, are considered as exceptionally qualified bearing materials if they also contain additional hard phases to decrease wear. Based on the considerable differences in the density of the decomposed fluid phases at high temperatures and the high velocity of separation, such alloys cannot, to date, be manufactured under terrestrial conditions. The results of microgravity experiments for the manufacturing of suitable sample material with a fine phase dispersion of the monotectic phases were rather disappointing. The cause of the rapid phase separation and local enrichment under microgravity conditions was found to be the Marangoni convection, the effects of which, to date, have been underestimated. The results of these space experiments are now utilized in a terrestrial casting process, whereby a comparatively high Marangoni convection is superposed in the opposite direction to the sedimentation action of gravity, thereby partially compensating the effects of gravity. Thus, cast strips of AlSiPb and AlSiBi alloys could be manufactured, the lead and bismuth phases being present in a characteristic fine dispersion overthe length of the cast strips. The first tribological laboratory tests give an indication of the excellent suitability of such advanced bearing material for the future.     

Parameter determination of critical nucleation frequency in solidification of undercooled metallic melts

Abstract

A simple method was proposed to calculate the essential parameters correlated with the critical nucleation frequency of undercooled metals and alloy melt. Numerical results show that the calculation accuracy from this method can be improved using the experimental data either with high undercooling or with low undercooling range (the difference of undercoolings between two solidification events). The calculations of the interfacial energy for high undercooling of silver and of the catalytic factor f(θ) for high undercooling of Al, Cu and Al–30 wt-%Cu alloy indicate that the results are consistent with the experimental measurements and with the results of Jian’s model [Metall. Trans. A, 2001, 32A, 391–395]. In addition, by analysing the differential scanning calorimetry data of pure Sn subjected to different cooling rates, similar values of catalytic factor f(θ) are obtained. This further indicates the validity of the current method.     

深过冷Cu-Pb偏晶合金的快速凝固行为和凝固组织

用熔融玻璃净化与循环过热相结合的方法,研究了亚偏晶Cu-25%Pb合金,Cu-37.4%Pb偏晶合金和过偏晶Cu-40%Pb(质量分数)合金过冷熔体凝固行为和凝固组织的演化规律,以及Cu-37.4%Pb偏晶合金的过冷度对磨损率的影响.研究表明:在过冷亚偏晶Cu 25%Pb合金熔体凝固过程中先形成α(Cu)初生相,随着过冷度的增大,凝固组织经历粗大枝晶重熔形成的细化枝晶向准球状晶粒演化的过程;在过冷Cu-37.4%Pb偏晶合金熔体凝固过程中初生相为L2相,当过冷度在20～150 K区间时,得到第二相S(Pb)弥散在α(Cu)枝晶间的凝固组织,并且在该过冷区间内随着过冷度的增加,材料的磨损率也逐渐降低;在过冷过偏晶Cu-40%Pb合金熔体凝固过程中初生相为L2相,在过冷度区间42～80 K时,得到以偏晶胞形式分布的凝固组织.     

Dispersion and re-agglomeration of graphite nanoplates in polypropylene melts under controlled flow conditions

The kinetics of GnP dispersion in polypropylene melt was studied using a prototype small scale modular extensional mixer. Its modular nature enabled the sequential application of a mixing step, melt relaxation, and a second mixing step. The latter could reproduce the flow conditions on the first mixing step, or generate milder flow conditions. The effect of these sequences of flow constraints upon GnP dispersion along the mixer length was studied for composites with 2 and 10 wt.% GnP. The samples collected along the first mixing zone showed a gradual decrease of number and size of GnP agglomerates, at a rate that was independent of the flow conditions imposed to the melt, but dependent on composition. The relaxation zone induced GnP re-agglomeration, and the application of a second mixing step caused variable dispersion results that were largely dependent on the hydrodynamic stresses generated.