Solidification structure formation in undercooled Fe–Ni alloy

The Fe alloy melts containing 7.5, 15, 22.5 and 30 at% Ni were bulk undercooled to investigate the structure evolution. When the undercooling of the four melts is lower than the critical value 110, 125, 175 and 325 K, respectively, only the stable face-centered cubic phase crystallizes. In this case a grain refinement caused by solid superheating is observed in all the alloys, but another grain refinement induced by recrystallization can merely occur in the Fe–30 at%Ni alloy undercooled by 190–325 K. Alternate crystallization of the metastable body-centered cubic phase occurs above the critical undercooling. It is indicated that the subsequent heterogeneous nucleation of the stable phase in the metastable solid and remaining liquid coexisting system is influenced not only by the morphology and surface area of the metastable solid, but also by the effective undercooling of the remaining liquid. On the basis of the experimental results and the theoretical analyses, a structure evolution map for bulk Fe–Ni system is constructed.     

Solidification kinetics and phase formation of undercooled eutectic Ni–Nb melts

The non-equilibrium solidification behaviour of undercooled eutectic Ni₈₄Nb₁₆ and Ni_59.5Nb_40.5 melts has been analysed by in situ observation of recalescence events during electromagnetic levitation of undercooled melts. Levitated drops of controlled undercooling were quenched onto chill substrates and subjected to phase and microstructure analysis. For Ni₈₄Nb₁₆ a maximum melt undercooling of 276 K has been achieved. A transition from coupled eutectic to primary supersaturated α-Ni dendrite growth was revealed beyond a critical undercooling of 30 K. Beyond 110 K the primary Ni₃Nb phase occurred for substrate quenching. The undercooling of Ni_59.5Nb_40.5 melt was limited to 135 K. Bulk amorphous samples up to 2 mm thick have been prepared by quenching of undercooled Ni_59.5Nb_40.5 melts. On slow cooling the metastable phases decompose into an anomalous eutectic microstructure.     

Solidification structure and mechanical properties of Al–Li–Cu–Zr cast alloys

Abstract

The microstructure and properties of three Al–3Li–1Cu ternary alloys have been studied, in particular the effect of Zr additions on the microstructure, precipitation and mechanical properties. The results showed that, for these Al–Li casting alloys, Zr content up to 0.2 wt-% was acceptable, and the Zr additions appeared to refine the grain structure. During aging, the Zr rich phase provided nucleation sites for δ' phase and promoted δ' phase refinement and homogenisation. Under optimised conditions, the tensile strength and elongation to failure of the Al–Li–Cu–Zr casting alloys were 400 MPa and 2.5%, respectively.     

Solidification structures of Ti–Al–Cr alloys

Phase transformations in the ternary Ti–Al–Cr alloy system have been studied by combining preliminary phase equilibria calculations and microstructural studies of a number of model alloys. The results have contributed to a better understanding of phase equilibria in the Ti–Al–Cr alloy system above 1273 K. A liquid surface projection has been tentatively proposed. Micro-twins have been observed in the monolithic γ phase within a B2 matrix. This supports a previously proposed mechanism for the formation of such a structure in a B2 matrix. The results also suggest that there is no representative orientation relationship between γ and the Ti(Cr,Al)₂ Laves phase. The L1₂ τ phase can be in direct equilibrium with the liquid phase. The ω phase stability has also been studied. The stability of the ω phase is attributed to the electron density of the prior B2 phase. This leads to changes in the effective heat of formation of the ω structure, as concluded from total energy LMTO calculations.     

Rapid solidification of undercooled Cu–Ge peritectic alloy

Bulk samples of Cu–14.4 wt% Ge peritectic alloy has been undercooled by up to 200 K (0.166 T_L) with a glass fluxing technique. The solidified microstructures are mainly characterized by α-Cu dendrites plus ζ phase which forms in the interdendritic areas within the whole undercooling regime. With the increase of undercooling, both the secondary arm spacing of primary α-Cu dendrite and the layer thickness of the peritectic ζ phase decrease. The primary trunk and secondary arm of α-Cu dendrites show round shape under small undercooling condition, whereas they keep a good dendritic shape within a large undercooling regime, indicating that the peritectic reaction proceeds for a relatively longer period of time in the former case. The volume fraction of peritectic ζ phase increases with undercooling, but that of α-Cu dendrite shows a decreasing tendency. Furthermore, drop tube experiments were also performed to reveal the competitive nucleation and growth mechanisms of primary α-Cu dendrite and peritectic phase ζ. Calculations based on the current dendritic growth model are made to analyze the crystal growth kinetics during the rapid solidification of undercooled Cu–Ge peritectic alloy.     

Critical-point wetting at the metastable chemical binodal in undercooled Fe–Cu alloys

Complementary results of differential thermal analysis and microstructural examination on Fe–Cu alloys provide the first evidence for critical-point wetting occurring at a completely metastable miscibility gap. The perfect wetting conditions hold for a composition range of 50–65 at.% Fe in the vicinity of the critical concentration. For samples encased with a glass slag, the Cu-rich liquid completely wets the glass upon undercooling to the metastable miscibility gap. In the perfect-wetting range, the metastable homogeneous liquid phase exhibited phase separation without undercooling below the chemical binodal. At deep undercooling, solidification of alloys with phase separated liquids results in a coarse scaled two-phase microstructure. In contrast, the homogeneous liquid phase of samples with compositions outside the perfect wetting range did undercool below the equilibrium onset of the metastable phase separation reaction. The phase separation in these samples occurred on a much finer scale. For samples without a glass encasement and thus in the presence of the Al₂O₃ crucible and an iron oxide layer, perfect wetting occurred near the consolute point on both sides of the metastable miscibility gap. This demonstrates that critical-point wetting is independent of the surface environment, but the wetting phase selected is surface sensitive.     

Microstructure evolution and solidification kinetics of undercooled Co–Ge eutectic alloys

The Co-29.7% Ge eutectic and Co-33% Ge hypereutectic alloys processed in drop tube attain large undercoolings and show “lamellar eutectic to anomalous eutectic” and “dendritic to equiaxed” morphology transitions respectively. The eutectic coupled zone is calculated on the basis of current eutectic and dendritic growth theories, which takes the shape of peanut and leans toward Co-rich side.     

Reexamination of the solidification behavior of undercooled Ni–Sn eutectic melts

Employing a high-speed video, in situ recalescence behaviors of Ni–Sn eutectic melts under different undercooling conditions have been investigated. Based on the surface morphologies and cross-sectional microstructures that consist of independent eutectic colonies, copious nucleation is proposed to take place in Ni–Sn eutectic melt regardless of melt undercooling in the unconstrained solidification. The boundary with sharp contrast between the crystallized bright solid and undercooled dark liquid during recalescence is not the solid/liquid interface but the simultaneous thermal release of concentrated crystallizing eutectic colonies. It may not be feasible to measure growth velocities in free solidification of the system due to copious nucleation. The fluctuations in undercooled melts are proposed to activate a chain-like successive nucleation reaction once an effective nucleus is formed. The current observation urges that we should deliberate further the conventional concept that once an effective nucleus is formed in a deeply undercooled melt, the growth will be initiated promptly and the solid/liquid interface will sweep across the entire sample rapidly. The origin of anomalous eutectic formation and the growth have been discussed when the current nucleation phenomenon is taken into account. Based on the copious nucleation theory and the improved driving force for growth, the present recalescence frames and some other experimental phenomena concerning the free growth of the undercooled eutectic system can be well clarified.     

Experimental observation of solidification of undercooled single phase alloys

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Structure and properties of cast and splat-quenched high-entropy Al–Cu–Fe–Ni–Si alloys

The effect of the composition and cooling rate of the melt on the microhardness, phase composition, and fine-structure parameters of as-cast and splat-quenched (SQ) high-entropy (HE) Al–Cu–Fe–Ni–Si alloys was studied. The quenching was performed by conventional splat-cooling technique. The cooling rate was estimated to be ~10⁶ K/s. Components of the studied HE alloys were selected taking into account both criteria for designing and estimating their phase composition, which are available in the literature and based on the calculations of the entropy and enthalpy of mixing, and the difference between atomic radii of components as well. According to X-ray diffraction data, the majority of studied Al–Cu–Fe–Ni–Si compositions are two-phase HE alloys, the structure of which consists of disordered solid solutions with bcc and fcc structures. At the same time, the Al_0.5CuFeNi alloy is single-phase in terms of X-ray diffraction and has an fcc structure. The studied alloys in the as-cast state have a dendritic structure, whereas, after splat quenching, the uniform small-grained structure is formed. It was found that, as the volume fraction of bcc solid solution in the studied HE alloys increases, the microhardness increases; the as-cast HE Al–Cu–Fe–Ni–Si alloys are characterized by higher microhardness compared to that of splat-quenched alloys. This is likely due to the more equilibrium multiphase state of as-cast alloys.     

High-strain-rate superplasticity of the Al–Zn–Mg–Cu alloys with Fe and Ni additions

During high-strain-rate superplastic deformation, superplasticity indices, and the microstructure of two Al–Zn–Mg–Cu–Zr alloys with additions of nickel and iron, which contain equal volume fractions of eutectic particles of Al₃Ni or Al₉FeNi, have been compared. It has been shown that the alloys exhibit superplasticity with 300–800% elongations at the strain rates of 1 × 10^–2–1 × 10^–1 s^–1. The differences in the kinetics of alloy recrystallization in the course of heating and deformation at different temperatures and rates of the superplastic deformation, which are related to the various parameters of the particles of the eutectic phases, have been found. At strain rates higher than 4 × 10^–2, in the alloy with Fe and Ni, a partially nonrecrystallized structure is retained up to material failure and, in the alloy with Ni, a completely recrystallized structure is formed at rates of up to 1 × 10^–1 s^–1.     

Peculiarities of ball-milling induced crystalline–amorphous transformation in Cu–Zr–Al–Ni–Ti alloys

An amorphization process in (Cu₄₉Zr_45−xAl_6+x)_100−y−zNi_yTi_z (x = 1, y, z = 0; 5; 10) induced by ball-milling is reported in the present work. The aim was investigation of the effect of Ni and Ti addition to Cu₄₉Zr₄₅Al₆ and Cu₄₉Zr₄₄Al₇ based alloys as well as type of initial phases on the amorphization processes. Also the milling time sufficient for obtaining fully amorphous state was determined. The entire milling process lasted 25 h. Drastic structural changes were observed in each alloy after first 5 h of milling. In most cases, after 15 h of milling the powders had fully amorphous structure according to XRD except for those ones, where TEM revealed a few nanosized crystalline particles in the amorphous matrix. In (Cu₄₉Zr₄₅Al₆)₈₀Ni₁₀Ti₁₀ alloy the amorphization process took place after 12 h of milling and the amorphous state was stable up to 25 h of milling. In the case of (Cu₄₉Zr₄₄Al₇)₈₀Ni₁₀Ti₁₀ alloy the powders have fully amorphous structure between 12 h and 15 h of milling.     

Solidification of Al alloys under electromagnetic field

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Phase selection in undercooled peritectic Fe–Mo alloys

The change of the primary solidification mode of undercooled peritectic Fe–Mo melts has been studied by in situ observation of recalescence events during electromagnetic levitation. A maximum melt undercooling up to 380 K has been achieved. Levitated drops of controlled undercooling were quenched onto chill substrates and subjected to phase and microstructure analysis. A transition from the primary bcc-Mo to the peritectic σ-phase solidification mode beyond a critical undercooling of 345 K was revealed for the Fe₄₇Mo₅₃ alloy and in a similar way for other compositions between Fe₄₅Mo₅₅ and Fe₅₄Mo₄₆. The suppression of the properitectic bcc-Mo phase was also achieved for subcritical undercooling in substrate-quenched Fe₄₅Mo₅₅ samples. In Fe₆₁Mo₃₉ a transition from the primary σ- to the peritectic R-phase solidification mode beyond a critical undercooling of 150 K was inferred from recalescence processes and X-ray investigation of as-quenched undercooled samples.     

Thermodynamics of La based La–Al–Cu–Ni–Co alloys studied by temperature modulated DSC

The onset and offset melting temperatures T_m and T_l, glass transition temperature T_g and heat of fusion ΔH_m of six La based La–Al–Cu–Ni–(Co) alloys were measured by DTA or DSC at a heating rate of 20 K/s. The absolute values of specific heat capacity for the undercooled liquid and the corresponding crystalline of these six alloys were obtained by means of temperature-modulated DSC (TMDSC). Entropy, enthalpy and Gibbs free energy differences between the undercooled liquid and crystalline of these alloys as a function of temperature have been calculated. Their glass forming ability was discussed from a thermodynamic point of view.     

Reaction-sintering of intermetallic alloys of the Ni–Al–Mo system

A powder metallurgy route has been used for producing binary and ternary alloys of the Ni–Al–Mo system. Elemental powder mixtures were compacted and, then, sintered in a dilatometer. In this way the dimensional changes involved with thermally induced transformations could be followed during continuous heating runs up to the sintering temperatures. Sintering was assisted by the formation of a liquid phase, promoted by the heat output coming from the intermetallic phase formation reactions. The amount of liquid phase and the efficiency of sintering was highly dependent on the heating rate. A threshold value for optimal densification was identified for some compositions. The effect of other processing parameters, such as pre-sintering compaction pressure and sintering atmosphere has been considered too. The characterisation of the final products was mainly based on X-ray diffraction analyses. The microstructural parameters and the phase composition of the sintered materials were evaluated. On the basis of these results it is possible to draw some conclusions concerning the main phenomena occurring during the sintering process.     

Solidification simulation of Al-Mg-Er alloys using CALPHAD method

With CALPHAD（calculation of phase diagrams） method, calculated with Scheil--Gulliver model in the Thermo-calc software the solidification paths of several Al-rich Al-Mg-Er alloys were And the amounts of several phases formed during solidification were also calculated. Synchronously, the heat-treatment temperatures of the same alloys were predicted by thermodynamic calculation. The calculated results show that by adding 0.4%Er （mass fraction）, the strength effect of Er in Al-Mg alloy is better. This result is in agreement with the experimental investigations.     

Solidification behaviour of Al-7 %Si-Mg casting alloys

The effects of Mg content and cooling rate on the solidification behaviour of Al-7% Si-Mg(mass fraction)casting alloys have been investigated using differential scanning calorimetry, differential thermal analysis and microscopy. The Mg contents were selected as respectively 0.00%, 0.35% and 0. 70% (mass fraction). DTA curves of Al-7%Si-0.55%Mg(mass fraction) alloy at various cooling rates were accomplished and the alloy melt was cast in different cooling rates. The results indicate that increasing Mg content can lower the liquidus and binary Al-Si eutectic transformation temperatures. Large Fe-rich π-phases (AlsFeMg3Si6) are found in the 0.70% Mg alloys together with some small β-phases (Al5FeSi) ; in contrast, only β-phases are observed in the 0.35% Mg alloys. The test results of the Al-7%Si-0.55% Mg alloys identify that the liquidus and binary Al-Si eutectic transformation temperatures decrease, and the quantity of ternary Al-Si-Mg2 Si eutectic phase decreases as the cooling rate increases.     

In situ observation of solidification behavior in undercooled Pd–Cu–Ni–P alloy by using a confocal scanning laser microscope

In the undercooled melt of Pd₄₀Cu₃₀Ni₁₀P₂₀ alloy, the solidification behavior including the nucleation and growth of crystals at the micrometer level has been observed in situ by use of a confocal scanning laser microscope combined with an infrared image furnace. The Pd₄₀Cu₃₀Ni₁₀P₂₀ alloy specimens were cooled from the liquid state to various undercooled states under a helium gas flow. Images of solidification progress were obtained by the charge-coupled device image sensor of the confocal scanning laser microscope. Depending on the degree of undercooling, the morphology of the solidification front changed among various types: faceted front, columnar dendritic front, cellular grain and equiaxed grain, etc. The velocities of the solid–liquid interface were measured to be 10⁻⁵–10−7 m/s, which are at least two orders of magnitude higher than the theoretical crystal growth rates. Combining the morphologies observed in the three undercooling regimes and their solidification behaviors, we conclude that phase separation takes place in the undercooled molten Pd₄₀Cu₃₀Ni₁₀P₂₀ alloy. The continuous-cooling–transformation (CCT) diagram was derived from the experimental time–temperature-transformation diagram constructed from solidification onset times under various isothermal annealing conditions. The CCT diagram suggests that the critical cooling rate for glassy solidification is about 1.8 K/s, which is in agreement with previous calorimetric findings.     

Tribologicai property and wear mechanism of undercooled Ni-Pb monotectic alloys

The trihological properties of Ni-31.44%Pb monotectic alloys were measured by using a SRV reciprocating triho-tester. The effects of load, sliding speed and melt undercooling on wear rate of the sample were investigated. The worn surface of Ni-31.44%Pb was examined using scanning electron microscope (SEM) and X-ray photoelectron spectroscope (XPS). The results show that the wear properties of the samples undercooled by 80 K and 310 K are obviously superior, which is attributed to more efficient transfer of Pb from the bulk material to the worn surface. The lubricating film is identified as a mixture of Ni2O3 and PbO by XPS analysis. At the same load and sliding speed, the predominant wear mechanisms can be identified as oxidative wear for the lower and larger undercooling, and plastic deformation and fracture for the medium undercooling.