Peritectic Alloys Research Articles

Peritectic reaction and its influences on the microstructures evolution during directional solidification of Fe–Ni alloys

Systematic directional solidification experiments were carried out in Fe–Ni peritectic alloys to investigate the influences of peritectic reaction on the microstructures evolution. Various microstructures were observed in the directionally solidified samples, such as bands, mixed bands, island banding, tree-like oscillatory pattern, coupled and cellular coupled growth structures. It was found that peritectic reaction occurs near the trijunctions during the two-phase growth at different interface morphologies of the primary phase observed as planar, cellular and dendritic. Peritectic reaction makes the liquid/δ/γ trijunctions region quite irregular and makes the trijunctions dynamics quite complicated, and thus plays an important role in the formation of various microstructures.

Progress in Research on Solidification in a Strong Static Magnetic Field

Strong magnetic fields available from superconducting magnets are opening a way to new phenomena that could lead to new methods in materials processing including solidification. The principal research involving solidification in strong static magnetic fields is emphasizing four aspects: control of crystal orientation, convection damping, thermoelectric magnetohydrodynamics (TEMHD) and change in thermodynamics. Under high magnetic intensity, aligned structural textures are induced in both magnetic and non-magnetic materials. Since in strong magnetic field the melt flow is suppressed by convection damping, the microstructure being formed during solidification is affected heavily; this phenomenon applies to eutectic, monotectic and peritectic alloys as well as to dendritic morphologies typical of directional solidification. If strength and orientation of a magnetic field are controlled appropriately, this strong damping effect will generate more homogeneous crystals as a result of achieving diffusion-controlled solute transport conditions. TEMHD more easily occurs in strong magnetic fields, resulting in equiaxed crystals even under directional solidification. It is evidenced experimentally and theoretically that the thermodynamics of phase transformation and nucleation are changed by strong magnetic fields.

Numerical model for nucleation of peritectic alloy during unidirectional solidification

Based on transient nucleation theory, a numerical model has been constructed to describe the nucleation process of a new phase in front of the liquid–solid interface of a prior steady-growth phase in peritectic alloy with the combination of the concentration field calculated by a self-consistent numerical model for cellular/dendritic growth. The results show that the nucleation incubation time of a new phase varies with the solidification rate during unidirectional solidification. During unidirectional solidification of the Zn–4.0 wt.% Cu alloy, the incubation time changes very slightly when the solidification rate increases from 50 to 500 μm/s, but it increases significantly when the solidification rate exceeds 500 μm/s. The calculated results show a reasonable agreement with the experimental ones. This model reveals that nucleation of a new phase is time-dependent and reasonably explains the effect of the solidification velocity on the behaviors of nucleation and growth of ɛ dendrites in the matrix of the η phase in unidirectional solidification of Zn rich Zn–Cu alloys.

Non-equilibrium solidification of concentrated Fe–Ge alloys

The solidification of concentrated alloys containing ordered compounds is less well understood. These alloys often exhibit complex phase change like peritectic reaction during liquid to solid transformation. The Fe-rich part of Fe–Ge binary alloy system consists of several critical points and ordered–disorder transitions and can be used as a model system to study the effect of departure from equilibrium on the solidification microstructure. In order to understand the phase selection and morphological transitions, undercooling and recalescence behaviour; growth rate of the solidifying phases and microstructure need to be explored. In the present paper, we summarise the results obtained in several iron rich alloy compositions (Fe–(14–25) at.% Ge) using techniques of melt quenching, levitation and laser resolidification. These results provide insight to the current theories of dendritic growth and reveals possibility of a new pathway for phase evolution in peritectic alloys at high undercooling involving a massive transformation.

Decomposition behavior and properties for tin–antimony alloy with bismuth content

The peritectic alloy Sn–10Sb, was chosen as one of the lead-free solder alloys to be subjected to many studies. Bi atoms were added to this alloy in the range of 1–3 wt%. The experimental results obtained from this study show a complete solubility of Bi atoms when added by 2 wt%. More additions of Bi tend to increase the formation of the intermetallic compound SnSb by reducing the precipitations of Sb as a single phase. This intermetallic compound with the precipitation of Bi as a third phase tends to improve the mechanical properties of this alloy.

Investigation of stress exponent in the room-temperature creep of Sn–40Pb–2.5Sb solder alloy

Creep behavior of Sn–40%Pb–2.5%Sb peritectic solder alloy having two different grain sizes was studied by indentation, impression, stress relaxation and conventional creep tests at room temperature ( T > 0.65 T m) in order to evaluate the correspondence of the creep results. Stress exponent values have been determined through these methods and in all cases the calculated exponents are in good agreement. The stress exponent values in the range 2.55–3.20, depending on the testing method and grain size of the materials, indicate that grain boundary sliding is the possible mechanism during room-temperature creep deformation of this alloy.

Metallurgy in Space

Over the past five years, an application-oriented research strategy has been initiated by ESA to permit valuable microgravity research in a broad range of physical sciences. The main objective is to integrate ESA, national activities and industry into an overall European strategy, which will allow research to be performed aboard the International Space Station (ISS), as well as other microgravity platforms, like unmanned space capsules, sounding rockets and parabolic flights. A key area of microgravity research is centred on metallurgy in space. The principal aims of this research field are (i) to investigate various physical phenomena during solidification processes and (ii) to determine the thermophysical properties of important liquid alloys. A number of metallurgical sub-topics have been identified in the ESA research programme, including the columnar-to-equiaxed transition during solidification; metastable and non-equilibrium solidification; multiphase multicomponent alloy solidification; eutectic, peritectic, monotectic and intermetallic alloy growth; fluid flow effects on mushy zone formation; and the measurement of thermophysical properties of liquid alloys. This review paper will therefore highlight the theoretical, experimental and modelling efforts currently being undertaken in the ESA programme.

Impression creep of Sn–40Pb–2·5Sb peritectic solder alloy

The power law creep behaviour of the Sn–40Pb–2·5Sb peritectic solder alloy was investigated using an impression test apparatus. The tests were carried out under constant stress in the range 17 to 39 MPa and at temperatures in the range 296 to 363 K. Assuming a power law relationship between the impression velocity and stress, power law stress exponents in the range 1–3 were determined. Analysis of the data showed that for all loads and temperatures, the activation energy was stress independent with values in the range 51–56 kJ mol-1. Based on the stress exponents obtained and activation energy data, it is proposed that grain boundary diffusion is the major mechanism for creep of the Sn–Pb–Sb peritectic alloy under these test conditions.

Solidification of undercooled peritectic Fe–Ge alloy

Bulk samples of Fe–25 at.% Ge peritectic alloy are undercooled up to 260 K using electromagnetic levitation technique. The growth rate of the primary phase is measured using a capacitance proximity sensor technique. Solidification microstructure is studied as a function of undercooling. The microstructure of samples at low undercoolings consists of a residual primary phase α 2, peritectic phase ε and inter-dendritic ε–β eutectic. Microstructure at higher undercoolings is nearly phase-pure ε. Time resolved diffraction analysis of the levitated droplets using synchrotron radiation indicates the nucleation of primary α 2 in all cases. The growth rate is analysed using current theories to explain the experimental observations. Interfacial undercooling is found to play an important role in the growth kinetics. Our results also suggest suppression of peritectic reaction.

Primary cellular/dendritic spacing selection in rapidly solidified peritectic alloy

Primary cellular/dendritic spacing plays an important role in the microstructure control under rapid solidification conditions, here laser resolidification experiments have been performed with Ti–Al alloys to clarify the growth mechanism of peritectic reaction. By taking both transverse and longitudinal sections of the directional growth cells/dendrites in laser traces, the primary phase separation, primary cellular/dendritic spacing, as well as the local solidification velocity are quantitatively determined by means of SEM and TEM. With the solidification velocity increasing, the primary phase evolves from phase α, two-phase ( α + γ), to single-phase γ, and the corresponding primary cellular/dendritic spacing decreases gradually. The experimental observation of primary spacing evolution exhibits a good agreement to the prediction of rapid dendritic growth model in all single-phase regions.

Phase selection during directional solidification of peritectic alloys

Directional solidification studies have been conducted using Pb-Bi peritectic alloys over a wide range of compositions, temperature gradients, and growth velocities to characterize the primary α- to primary β-phase transitions, which have been observed at both very low and very high velocities. The critical conditions for these transitions correspond to the simultaneous growth of the α and β phases at or close to a single isotherm. The low velocity transition occurs under very specific conditions of composition, temperature gradient, and growth velocity. Since the transition conditions are composition dependent, they change continuously under terrestrial conditions where rejected solute is convectively mixed into the liquid. Detailed experimental studies have been carried out to examine the phase selection in the immediate vicinity of the critical velocity for the α to β transition, and the effect of convection on this transition is examined experimentally in the Pb-Bi system. The dynamic condition, at which both phases are present at the same isotherm, was shown to depend not only on velocities, temperature gradients, and bulk (nominal) alloy compositions, but also on the volume fractions of solid. A quantitative expression for the α- to β-phase transition condition was obtained by using the boundary layer model of fluid flow, which showed good agreement with the experimental results. It is shown that the transition occurs at the volume fraction where the bulk composition reaches the critical composition value predicted by the diffusive model. The modification in the microstructure map for the trailing planar or nonplanar β phase is discussed.

Superplastic indentation creep of Sn–Pb–Sb peritectic

Abstract Creep behaviour of the Sn–40% Pb–2.5% Sb peritectic alloy having different grain sizes was studied by long time Vickers indentation testing at room temperature. Based on the steady-state power law creep relationship, the stress exponents were determined. The corresponding strain rate sensitivity (SRS) indices of 0.38 and 0.34, depending on the grain size of the materials, indicate that grain boundary sliding is the main mechanism during room temperature creep deformation of these superplastic alloys.

Microstructure refinement of Sn-Sb peritectic alloy under high-intensity ultrasound treatment

In this paper the solidification behavior of Sn-Sb peritectic alloy and the mechanism of grain refinement in solidification process under high-intensity ultrasonic field are investigated. Three different powers of high-intensity ultrasound are introduced into molten Sn-Sb peritectic alloy to study the refining effectiveness. The results show that the application of high-intensity ultrasound during solidification process of Sn-Sb peritectic alloy can refine α phase and β phase and eliminate gravity segregation of the alloy. As acoustic intensity is increased from 400 W to 800 W, not only the homogenous fine structure can be obtained, but also the cubic β phase crystals tend to be spherical. Microstructure of the sample treated by 600 W high-intensity ultrasound demonstrates the best refining effect.

Electrical property of rapidly solidified Co-Cu peritectic alloys

The microstructure formation and phase selection characteristics of Co-Cu peritectic alloys during rapid solidification are investigated experimentally. The re lationships between such physical factors as cooling rate, microstructure morpho logy, crystal orientation, and alloy resistivity are further analyzed. The exper imental results show that rapid solidification makes the Co solubility in (Cu) p hase extend up to 20%Co. If Cu concentration is more than 80%Cu, the peritectic transformation L+αCo→(Cu) is suppressed, and the (Cu) phase can directly preci pitate from the undercooled alloy melt. When the Cu concentration is in the rang e of 40%-70%Cu, the liquid phase separation of the alloys is also suppressed, and the microstructure in the direction of ribbon thickness consists of two crysta l zones. In the fine crystal zone, the αCo and (Cu) phases nucleate and grow co mpetitively, the tiny equiaxed (Cu) dendrites are distributed in the αCo matrix homogeneously, whereas in the coarse crystal zone the αCo is the leading phase , where Cu-rich phase exists in the grain boundary of αCo dendrites. With the increase of cooling rate, the microstructure is refined and the amount of grain boundary increases, resulting in the rise of alloy resistivity. If the grain bou ndary reflection coefficient r＝0996—0999, the electrical resistivity o f Co-Cu peritectic alloys can be predicted theoretically.

On secondary dendrite arm coarsening in peritectic solidification

Abstract A model for isothermal coarsening of secondary dendrite arms in peritectic reaction and transformation (liquid + primary-phase → peritectic-phase) is proposed to evaluate the secondary dendrite arm spacing ( λ 2 ) of the primary phase in directional solidification of peritectic alloys. The model defines three stages for thin-arm dissolution (or thick-arm coarsening), i.e. the initial, intermediate and final stages: the initial thin-arm dissolution through the primary phase is sustained solely by the Gibbs–Thomson effect; the intermediate thin-arm dissolution through the peritectic phase is driven by Gibbs–Thomson effect but retarded by the peritectic reaction and transformation; the final dissolution through the primary and peritectic phases is enhanced by the Gibbs–Thomson effect and the phase transformation. The kinetics of peritectic reaction and transformation were found to be crucial to determine the thin-arm dissolution, which were characterized by the reaction constant ( f ) and the diffusion coefficient of solute in solid peritectic-phase ( D S ), respectively. The present model shows that λ 2 V m is constant for a given Pb–Bi peritectic alloy, where V is growth velocity, and the factor, m , ranges from 1/3 to 1/2, rather than that normally observed (e.g. 1/3) for single-phase solidification. It is also notable that the calculated λ 2 for a Zn–7.37 wt.% Cu peritectic alloy was reasonably consistent with our earlier experiments for various growth velocities.

Microstructural evolution of rapidly solidified Ti–Al peritectic alloy

Primary cellular/dendritic spacing of the solidified structures plays an important role on the microstructure control under rapid solidification conditions. In order to clarify the phase evolution and growth mechanism of peritectic reaction, here laser resolidification experiments, with different growth velocity, have been performed on Ti–Al peritectic alloys. By taking both the transverse and longitudinal sections of the directional growth cells/dendrites in the laser traces, the primary phase formation and average primary cellular/dendritic spacing, as well as the corresponding solidification velocity are quantitatively determined by means of scanning electron microscopy (SEM) and transmission electron microscopy (TEM). With the solidification velocity increasing, the separating primary phase evolves from single-phase α, two-phase (α+γ), γ, and the corresponding primary cellular/dendritic spacing decreases gradually. All experimental observations of primary spacing evolution, in all single phase regions, agree well with that predicted by rapid dendritic growth model.

Phase and microstructure selection in peritectic alloys under high G-V ratio

Abstract A new model for the case of a superposition of a phase transformation and a microstructure transition from columnar plane front to equiaxed dendritic growth for peritectic alloys is presented. This model is applied to Fe–Ni alloys where a phase transition from γ to δ takes place when the solute content decreases. The results of the model are compared with predictions of the nucleation constitutional undercooling model. They show that the peritectic two-(solid) phase region is restricted and may completely disappear depending on nucleation undercooling and dendrite growth kinetics.

Lamellar structures in laser surface remelted Zn–Cu peritectic alloy under ultra-high temperature gradient

Laser surface remelting experiments have been performed on a Zn–2.7wt.%Cu hyperperitectic alloy. The average lamellar spacing and the corresponding growth rate are quantitatively analyzed, and the results are in good agreement with the prediction of the Jackson–Hunt eutectic model. It implies that lamellar structures in peritectic alloys have a similar growth mechanism to those of eutectic alloys.

Researches on lamellar structures in the unidirectional solidified Zn–2 wt.% Cu peritectic alloy

Unidirectional solidification on Zn–2 wt.% Cu peritectic alloy was carried out with the Bridgman method. Lamellar structure consisting of regular η+ε was obtained at a solidification velocity range between 1600 and 6400 μm/s. The concentrations of Cu in each phase as well as the average composition have also been analyzed by both energy spectrum analysis (ESA) and electron probe microanalysis (EPMA). Experimental results are in good agreement with the prediction of the calculated metastable phase diagram.

Phase selection in the containerless solidification of undercooled CaO · 6Al 2O 3 melts

The CaO · 6Al 2O 3 melts were solidified on an aero-acoustic levitator under a containerless processing condition at various undercoolings. A high-speed video was operated to monitor the recalescence behavior, from which the growth velocity as a function of melt undercooling was determined. The microstructures were observed and the crystalline phases were identified using the X-ray diffraction technique, indicting that the Al 2O 3 was solidified when the melt temperature was higher than the peritectic temperature, T p. When the melt was undercooled below T p, the CaO · 6Al 2O 3 (CA 6) peritectic phase was crystallized directly from the undercooled melts. With respect to the direct formation of the peritectic phase, further analysis from the viewpoints of competitive nucleation indicated that the minimum free energy principle may be applied to elucidate the nucleation of CA 6 phase. In terms of the competitive growth behavior, the interface attachment kinetics for Al 2O 3 and CA 6 phases are calculated by using the classical BCT model indicating that although the Al 2O 3 phase doped by CaO has about four times larger interface kinetic coefficient than that of the CA 6 peritectic phase, the growth kinetics of Al 2O 3 in the melt with the CaO · 6Al 2O 3 chemical composition is not sufficiently high to replace the CA 6 phase as the primary phase. Therefore, once CA 6 is nucleated, it can develop into a macro crystal as the primary phase. The competitive nucleation and growth behavior in the CA 6 system is different from those in other well-studied peritectic alloys and the present investigation on the phase formation will be an essential supplement to the phase selection theory.