On the coefficients of distribution of components and some regularities of the crystallization of alloys in three-component systems

The computation-graphical technique was used to study the equilibrium and nonequilibrium crystallization of the three-component solid solution alloys. Complex regularities of the component redistribution in the solid and liquid phases at the crystallization have been revealed. It is shown that during the crystallization of a number of alloys the distribution coefficient values of some components may vary from k < 1 to k = 1 and then k > 1. A new graphical analysis method of the three-component alloy crystallization has been proposed. Compound diffusion processes going on at the crystallization of such alloys have been revealed and the peculiarities of the component redistribution at the directional solidification of samples of such alloys, being important for the crystallization refining, have been defined.     

Solidification structures in submicron spheres of iron-nickel: Experimental observations

Alloys of 0, 30, 40 and 50 at.% nickel in iron have been processed in vacuum by electrohydrodynamic atomization (EHD) to produce submicron droplets. The as-solidified spheres are studied to determine which of several solidification phases has appeared. Field-emission scanning transmission electron microscopy (STEM) is used to determine the microstructure, composition and crystal structure of the 10–150 run diameter spheres. It is believed that homogeneous nucleation must be the predominant nucleation mechanism in EHD droplets during free flight. The alternative crystallization phase, b.c.c., in the Fe-Ni alloy system, is found in 30 and 40 at.% Ni alloys but not in the 50 at.% Ni alloy. A new hexagonal crystal structure of Fe-50 at.% Ni is discovered. Furthermore, the smallest spheres of each alloy (<50 nm diameter) including pure iron are found to be amorphous. These findings are consistent with calculations (detailed in a second paper [Acta metall.36, 2537 (1988)], based on classical nucleation theory which predict the conditions under which alternative crystallization phases may appear.     

Equilibrium crystallization of alloys of ternary solid solutions

Equilibrium crystallization of continuous solid solutions in ternary systems is considered. It is shown that in each ternary and multicomponent alloy with unlimited solubility of components in the liquid and solid states, equilibrium crystallization is realized similarly to a binary alloy due to diffusion decomposition of liquid and its diffusion interaction with the previously precipitated solid phase. The distinction is the formation, at a decreased temperature, of nonequilibrium compositions of the liquid and solid phases, with conservation of the composition of the solid phase, which had been at equilibrium for the temperature before the decrease; this takes place owing to the decomposition diffusion of the liquid. The diffusion interaction of all phase components leads to the formation of equilibrium of liquid and solid phases for a new temperature. By plotting and calculation, it is shown that in ternary systems, for certain alloy compositions at definite stages of crystallization, the fraction of the solid phase can decrease due to the diffusion interaction, which is excluded for solid solutions of binary alloys.     

Glass formation and nanocrystallization in Zr based alloys

In the present paper bulk glass forming behaviour of some of Zr based Ni, Cu, Al and Ti bearing alloys has been investigated. Examination of the as solidified microstructure of these alloys in the partially crystalline state has shown that the predominance presence of the Zr₂Ni or its derivatives phases as the phases competing with glass formation whereas in the fully amorphous microstructure, quenched-in nuclei and the atomic short range order existing in the amorphous phase were observed. The aim of the microstructural examinations of the fully amorphous phase was to ascertain the nature and morphology of the quench-in nuclei. In the partially crystalline microstructures, study of the crystalline phases competing with glass formation has helped in better understanding of the solidification process during BMG formation. The kinetics of crystallization of the as solidified Zr based bulk metallic glasses were studied by differential scanning calorimetry (DSC) and crystallized microstructures were examined by conventional and high-resolution electron microscopy. The activation energies of crystallization and the Avrami exponent have been evaluated. The Avrami exponent values have been rationalized in terms of the observed nucleation and growth behaviour of the phases forming on crystallisation. Conditions of crystallization leading to the formation of nanocrystals have been identified.     

Effect of the primary phase on grain coarsening in undercooled Fe-Co alloys

Fe-Co alloy melts with Co contents of 10, 30, and 60 at. pct were undercooled to investigate the dependence of the primary phase on grain coarsening. A pronounced characteristic is that the metastable fcc phase in the Fe-10 at. pct Co alloy and the metastable bcc phase in the Fe-30 at. pct Co alloy will primarily nucleate when undercoolings of the melts are larger than the critical undercoolings for the formation of metastable phases in both alloys. No metastable bcc phase can be observed in the Fe-60 at. pct Co alloy, even when solidified at the maximum undercooling of ΔT = 312 K. Microstructural investigation shows that the grain size in Fe-10 and Fe-30 at. pct Co alloys increases with undercoolings when the undercoolings of the melts exceed the critical undercoolings. The grain size of the Fe-60 at. pct Co alloy solidified in the undercooling range of 30 to 312 K, in which no metastable phase can be produced, is much finer than those of the Fe-10 and Fe-30 at. pct Co alloys after the formation of metastable phases. The model for breakage of the primary metastable dendrite at the solid-liquid interface during recalescence and remelting of dendrite cores is suggested on the basis of microstructures observed in the Fe-10 and Fe-30 at. pct Co alloys. The grain coarsening after the formation of metastable phases is analyzed, indicating that the different crystal structures present after the crystallization of the primary phase may play a significant role in determining the final grain size in the undercooled Fe-Co melts.     

Microstructure and mechanical properties of RS NiCrAl melt-spun alloys

The microstructure and mechanical properties of three melt-spun NiCrAl alloy ribbons have been studied in the as-cast condition as well as after thermal treatments. The microstructure of the alloys is dendritic-microcellular in as-cast condition and phases present for 10 at.% Al and 30 at.% Al alloys are as is predicted by the equilibrium phase diagram. In the 20 at.% Al alloy, γ' has frozen in metastable form and partial ordering takes place during cooling in the solid state. After thermal treatments the ribbons generally maintain a refined microstructure; α phase precipitates are always found in β and γ' phases in 20 and 30 at.% Al alloys. The hardness of the alloys increases with aluminum content. The tensile strength at room temperature is related to the phases present in the material for each state of treatment. The alloys are brittle, a higher ductility always being obtained in the as-cast condition.     

Quantitative analysis of the primary crystallization of iron-containing phases as applied to aluminum alloys of various doping systems

Aluminum-based multicomponent systems are analyzed using the Thermo-Calc program in order to determine the concentration boundaries in which the first primary crystals of Fe-containing phases appear. Projections of the liquidus surfaces are calculated as applied relative to the industrial cast alloys of three main groups, namely, Al-Si (silumins), Al-Cu (the AM5 type), and Al-Mg (magnaliums). It is shown that the primary crystallization of the following Fe-containing phases is most probable: in silumins these are the Al₅FeSi and Al₁₅(Fe, Mn)₃Si₂ phases and in magnaliums and the AM5-type alloys these are Al₃Fe and Al₆(Fe, Mn) phases. Based on the calculation of parameters of the primary crystallization of the Fe-containing phases, the possibility of evaluating the efficiency of iron removal from aluminum alloys is shown.     

Phase diagram of the Cu-Ni-Mn system

Part of the phase diagram of the Cu-Ni-Mn system from 0 to 20% Ni and from 30 to 50% Mn is refined with the help of a theoretical analysis and based on the experimental data. It is shown that the assumption that a line exists in the Cu-Ni-Mn ternary system along which the alloys with a zero crystallization range are arranged is erroneous. Only the alloys in the Cu-Mn and Ni-Mn binary systems have a zero crystallization range. The primary assumption that it is impossible to represent the nonequilibrium crystallization by Petrov-Scheil in the absence of a minimum line in the Cu-Ni-Mn system was not confirmed because previous studies were performed for a model system in which a convex liquidus line was present in the polythermal section connecting the minima in binary systems. In this study it is shown that this line is concaved towards lowering the temperature.     

Nonequilibrium behavior in the Al-Ge alloy system: Insights into the metastable phase diagram

The competitive formation of metastable and stable phases during nonequilibrium processing of Al-Ge alloys and the corresponding metastable phase equilibria have been investigated. For germanium concentrations in the range 30 to 50 at. pct, it is shown that the four metastable phases can be ranked in order of decreasing stability as follows: monoclinic (P2₁/c), rhombohedral (R-C), orthorhombic (Pbca), and hexagonal (P6/mmm). Their formation depends not only on the transformation temperature(e.g., the liquid undercooling), but also on the presence of appropriate heterogeneous nucleation sites. For example, the orthorhombic phase has only been observed in amorphous films after rapid annealing/crystallization treatments. It is also shown that all of these phases form metastable equilibria with α-aluminum only,i.e., no metastable phase equilibria appear to exist between any metastable phase and β-germanium or between any two metastable phases. Consequently, it is not possible to draw a single metastable phase diagram that incorporates all of these phases with phase boundaries that represent their metastable equilibria; rather, separate diagrams should be drawn for each metastable phase. It is noted that these diagrams should extend only to the metastable phase field rather than all the way to pure germanium: for compositions richer in germanium, the results indicate that the metastable phase forms and then remelts upon the formation of germanium or a more stable, germanium-enriched metastable phase. Furthermore, it is proposed that this behavior is rather general in nature. Finally, it is concluded that the production of metastable phases in bulk form, in systems such as this where so many reactions occur simultaneously and competitively, might be impossible using solidification processing approaches. Formerly with the Department of Materials Science and Engineering, University of Washington, Seattle, WA 98195     

Formation and stability of metastable structures and amorphous phases in PU-V,PU-TA,and PU-YB systems with positive heats of mixing

The triode sputtering technique with a “split-target” arrangement was used to obtain metastable crystalline and amorphous phases in the Pu-V, Pu-Ta, and Pu-Yb systems. The proposed phase diagrams for these systems all exhibit liquid immiscibility. The heats of mixing are estimated to be highly positive, and the atomic radii of the component atoms differ by at least 10 pct. Extended amorphous and body-centered cubic (bcc) solid-solution regions were observed in the Pu-V and Pu-Ta systems. The corresponding lattice parameters appear to follow in each case an assumed Vegard’s Law extension. In the Pu-Yb system, no amorphous phase was obtained, but an extended face-centered cubic (fcc) solid-solution region (24 to 78 at. pct Yb) was observed with a large positive deviation of the lattice parameter (∼9 pct at 40 at. pct Yb) from a linear Vegard’s Law between the pure fcc components. The observed ranges of amorphous and metastable solid-solution phases have been interpreted in terms of predicated heats of formation for these phases using Miedema’s thermodynamic approximations that include chemical, elastic, and structural contributions. The effect of the high deposition rates on the formation of amorphous and metastable phases has also been considered. Thermal annealing of Pu-Ta amorphous alloys brings about a rapid diffusion of Pu to the free surface of the amorphous phase without crystallization of the remaining Ta-rich amorphous phase. Microhardness measurements indicate that amorphous Pu-V and Pu-Ta alloys are softer than the crystalline bcc solid-solution alloys in the same composition range. Several similarities in the formation of mixed phase regions (amorphous and solid solutions), microhardness, and resistance to decomposition on heating were noted between the Pu-Ta and Pu-V systems and the Cu-W system studied previously.     

Metastable crystalline and amorphous structures formed in the Cu-W system by vapor deposition

The possibility of producing nonequilibrium amorphous and crystalline phases in the Cu-W system is of interest because, under equilibrium conditions, no mutual solubility is expected between Cu and W. Triode sputtered coatings (45 to 150 μm thick, produced at deposition rates between 20 and 150 Å/s) consisted of amorphous and metastable crystalline phases. The latter remained decomposition-resistant on heating to various temperatures between 340 °C and 600 °C (the maximum temperature of exposure). The amorphous phase in such coatings crystallized on heating into a metastable body-centered cubic (bcc) phase, and the crystallization temperatureT _x was found to decrease across the phase diagram from 450 °C to 340 °C as the percentage of W increased from 26 to 60 at. pct. Samples containing amorphous phase regions, when subjected to heating between 150 °C and 250 °C, showed an unusual rapid precipitation of Cu at the sample surface, indicating an easy diffusion of the Cu component. This occurred without crystallization of the remaining slightly tungsten-enriched amorphous matrix. Microhardness measurements in sputtered two-phase amorphous and bcc regions have shown that in alloys of the same composition, the amorphous phase was always softer than the bcc solid solution phase. X-ray, microprobe, and optical evidence suggests that the amorphous films deposited at very low temperatures(i.e., at liquid N₂) may subsequently undergo a phase separation upon heating to room temperature and prior to crystallization. Earlier work and present studies of vapordeposited alloys in this system confirm that the observed phases and microstructures can be related to free energy trends estimated from thermodynamic considerations and to specific deposition parameters, such as the substrate temperature and the deposition rates, which influence the kinetics.     

The microstructure and phase relationships in rapidly solidified type 304 stainless steel powders

The microstructure and relative amounts of fcc and bcc phases have been studied for rapidly solidified Type 304 stainless steel powders produced by vacuum gas atomization (VGA) and centrifugal atomization (CA). The VGA powder solidifies with a cellular microstructure while the CA powder has a dendritic microstructure. The volume fraction of fcc phase in the CA powder is found to increase from 40 Pct to 97 Pct with increasing particle size from 30 to 125 μm. In the VGA powder, the volume fraction of fcc phase is found to decrease from about 90 Pct to 77 Pct over the same range of particle sizes. The origins of the fcc and bcc phases in each powder are considered. It is concluded that bcc is present as both a primary crystallization phase in the smaller CA particles (<75 μm) and as compositionally stabilized eutectic ferrite at the cell walls of particles of both CA and VGA powders in which fcc was the primary crystallization phase.     

Influence of the nonequilibrium crystallization on the phase composition of the Al-Mo-Ti alloy

The phase composition, microstructure, and crystal structure of the AMT TU-48-4-366 (Technical Specifications) foundry alloy (which is used as the alloying material when smelting titanium alloys) are investigated by X-ray phase analysis, electron probe microanalysis, and microscopy. Lattice parameters of ?, p, and δ phases are calculated and their elemental composition is revealed. No formation of the Mo₃Al refractory phase (t _m = 2150°C) is observed during the primary crystallization of the Al-Mo-Ti foundry alloy in nonequilibrium conditions. Its presence in the refractory phase in the foundry alloy is caused by secondary crystallization processes, during which an ultradispersed mixture of Mo₃Al + Mo₃Al₈ + TiMoAl₆ phases is formed at temperatures 1311 and 1314°C. The ultradispersed silicon-containing σ phase with the Mo_2.4Ti_2.1Si_0.8Al_4.7 average composition, which was formed in nonequilibrium crystallization conditions, is revealed. Parameters and interplanar distances of its lattice are determined. It is established that the largest nonuniformity by molybdenum in peritectics of primary crystals occurs at a high crystallization rate, i.e., in the lower part of the Al-Mo-Ti ingot.     

Effect of micro-additions of Mo,P, Si and Ti on the thermal stability of Ni24Zr76 metallic glass

The effects of micro-additions (about 1 at.%) of Mo, Ti, Si and P on the thermal stability and crystallization behavior of Ni₂₄Zr₇₆ metallic glass have been investigated. Dynamic devitrification process of these melt-spun amorphous alloys have been followed by means of differential scanning calorimetry, X-ray diffraction, and transmission electron microscopy. The last two techniques have been employed for studying the microstructural evolution and for identification of the phases formed after crystallization of these alloys. Addition of the above elements results in an increase in thermal stability as indicated by the increase in crystallization temperature, and also by the increase in difference between the crystallization temperature and the glass transition temperature. The enhancement of the thermal stability has been analyzed in terms of the atomic size difference effect, cohesive energy effect, elastic property/physical parameter and thermodynamics of alloying effect. It is found that the enhancement of thermal stability can be well correlated with the thermodynamics of alloying behavior of the third elements in Ni₂₄Zr₇₆ alloy.     

Titanium-boride eutectic materials. Structure of the Ti-Nb-B alloys and phase equilibria

The structure of Ti-Nb-B alloys that are cast and annealed at subsolidus temperatures and at 1400°C is experimentally analyzed (x-ray diffraction, metallography, and electron probe microanalysis), and so are temperatures of their phase transformations (differential thermal analysis and pyrometry). No ternary phases are found in the alloys. Projections of solidus and liquidus surfaces, an isothermal section at 1400°C, and a vertical section at 7.5 at.% B are constructed. A reaction scheme is proposed for alloy crystallization. __________ Translated from Poroshkovaya Metallurgiya, Vol. 46, No. 1–2(453), pp. 72–87, 2007.     

Crystallization Kinetics of Pr_8Fe_(86-x)Zr_xB_6 Amorphous Alloys

Ｎａｎｏｃｏｍｐｏｓｉｔｅｍａｇｎｅｔｓ ,ｗｈｉｃｈｃｏｎｓｉｓｔｏｆａｔｗｏ ｐｈａｓｅｄｉｓｔｒｉｂｕｔｉｏｎｏｆｈａｒｄ ａｎｄｓｏｆｔ ｍａｇｎｅｔｉｃｇｒａｉｎｓ,ｈａｖｅａｔｔｒａｃｔｅｄｃｏｎｓｉｄｅｒａｂｌｅｉｎｔｅｒｅｓｔｓｓｉｎｃｅｔｈｅｙｃｏｕｌｄ ,ｂｙｅｘｃｈａｎｇｅｃｏｕｐｌｉｎｇ ,ｐｏｔｅｎｔｉａｌｌｙｐｒｏ ｖｉｄｅａｍａｘｉｍｕｍｅｎｅｒｇｙｐｒｏｄｕｃｔ ,(ＢＨ ) ｍａｘ,ｉｎｅｘ ｃｅｓｓｏｆ 10 0ＭＧＯｅ[1] ,ｗｈｉｃｈｉｓｍｕｃｈｌａｒｇｅｒｔｈａｎａｎｙｓｉｎｇｌ…     

Co-Al二元系中的fcc/hcp马氏体相变和形状记忆效应

It is known that pure Co undergoes martensitic transformation from γ phase (fcc) to ε phase (hcp) by the movement of a/6<112> Shockley partial dislocations at around 400 ℃, however, there have been few systematic works on the SM effect in Co and Co-based alloys. In this study, the fcc/hcp martensitic transformation and the SM effect were investigated in Co-Al binary alloys(mole fraction of Al=0～16%).The γ/ε martensitic transformation temperatures were found from the DSC measurements to decrease with increasing Al content, while the transformation temperature hystereses were observed to increase from 60 ℃ at x(Al)=0 to 150 ℃at x(Al)= 16%. The SM effect evaluated by a conventional bending test was enhanced by the addition of Al over 4%(mole fraction) and Co-Al alloys containing over 10%(mole fraction) exhibit a good SM effect associated with the hcp →fcc reverse transformation above 200 ℃. The SM effect was significantly improved by precipitation ofβ (B2) phase and the maximal shape recovery strain of 2. 2% was obtained, which can be explained by precipitation hardening. The crystallographic orientations between theβ, ε and γ phases were also determined. Finally, the magnetic properties were investigated and it was found that the Curie temperature and saturation magnetization of Co-14% Al(mole fraction) are 690 ℃and 120 emu/g, respectively. It is concluded that the Co-Al alloys hold promise as new high-temperature and ferromagnetic SM alloys.     

Icosahedral and decagonal phase formation in Al-Mn alloys

The solidification conditions leading to the formation of the icosahedral phase in Al-Mn alloys have been investigated, using samples prepared by melt spinning and electron beam surface melting. It is found that the icosahedral phase can grow with a range of compositions, but that it grows in competition with another metastable phase which is decagonal. Both of these phases can displace the equilibrium intermetallic phases by nucleating ahead of them in the melt when the solidification velocity is greater than a few centimeters per second. The relative abundance of the icosahedral and decagonal phases varies with composition and solidification rate. Icosahedral crystals in electron beam melt trails are often about 25 μm in diameter, and they grow dendritically along a preferred crystallographic direction. A Guest Scientist at the National Bureau of Standards. A Guest Scientist at the National Bureau of Standards.     

Structural Changes of High-Temperature Nickel Alloy Containing Rhenium and Lanthanum on Heat Treatment

The properties of high-temperature nickel alloys for manufacturing depend on the thermal stability of the structure, the particle size, the shape, the quantity of strengthening γ' phase, and the strength of the γ solid solution. Such alloys are strengthened by the addition of rhenium and lanthanum. In the present work, the structure and phase composition of high-temperature nickel alloy with added rhenium (0.4 at %) and lanthanum (0.006 at %) are qualitatively and quantitatively investigated. The methods employed are transmission diffraction electron microscopy and scanning electron microscopy. The alloy structure is considered in three states: after directed crystallization (the initial state, sample 1); after directed crystallization, annealing at 1150°C for 1 h, and annealing at 1100°C for 480 h (sample 2); and after directed crystallization, annealing at 1150°C for 1 h, and annealing at 1100°C for 1430 h (sample 3). Primary and secondary phases are observed in the superalloy. The primary phases are γ' and γ. They form the structure of the alloy and are present in the form of γ' quasi-cuboids separated by γ layers. The secondary phases due to the presence of rhenium and lanthanum are β NiAl, AlRe, NiAl₂Re, σ, χ, and Ni₃La₂. The secondary phases seriously disrupt the structure of the γ + γ' quasi-cuboids. The rhenium and lanthanum do not uniformly fill the whole alloy volume, but only appear in local sections. Therefore, in all three states of the alloy, only some volume of γ + γ' quasicuboids is disrupted. Analysis of the secondary phases’ morphology shows that the σ particles are thin needles, whereas the Ni₃La₂ particles have internal structure with characteristic contrast and are relatively thick. Interestingly, the σ phase and Ni₃La₂ are deposited at the same locations. The introduction of rhenium and lanthanum changes the phase composition of the alloy, suppressing the formation of γ phase. The particles of secondary phase are localized in individual sections of the alloy with specific periodicity. The secondary phases are refractory: the melting point is about 1600°C for β phase, 2600°C for σ phase; and 2800° for χ phase. Thanks to the formation of refractory secondary phases and their periodic distribution in the structure, the strength of the superalloy with added rhenium and lanthanum is increased.     

金属材料熔点热物性动态测算新方法

利用物质相变相界面移动与两相热物理性质的关系，提出了一种测试相变速率与基于参数估计的相变导热逆问题数值解法相结合的金属材料熔点热物性动态测算新方法，并设计了相应的测试仪。利用设计的测算方法对已知热物性的金属如铅、锌、铝等对此方法进行了检定，测试结果与参照值误差均不超过3％。该方法的优点是通过相变过程来进行测试，确保了被测物质结构及物性的真实性，测量较简便，结果准确、可靠、误差小，并可测得多个热物性数据及用于金属及合金相变条件下热传导的研究。还对相变导热系数未知的铅锑、铅锡、铋锡、铝硅、铝铜五种共晶合金进行了测试，其结果具有较高的参考价值，丰富了热物性文献。