Phase and microstructure pattern selection of Zn-rich Zn–Cu peritectic alloys during laser surface remelting

Peritectic solidification has attracted increasing attention as a lot of important binary alloys, such as Fe–Ni, Zn–Cu, Fe–C and Ti–Al, exhibit peritectic reaction during solidification. In order to investigate the solidification behavior of Zn-rich Zn–Cu peritectic alloy containing nominally up to 7.8 wt.% Cu, a series of laser surface remelting experiments were performed. With the increase in growth velocity, Zn–Cu alloys with Cu content below 3.0wt.% showed an evolutional sequence from low-velocity η planar interface?→?lamellar structures?→?η shallow cells and finally to high-velocity η planar interface. The Zn-4.0 wt.%Cu alloy showed a similar transitional sequence except that irregular η cells appeared when low-velocity planar interface became unstable. In contrast, ε cell/dendrite was the typical microstructure of the Zn-7.8 wt.% Cu alloy within the whole scanning velocity range. Based on the maximum interface temperature criterion, a eutectic growth model under rapid solidification conditions (TMK model) and a self-consistent numerical model for the cellular and dendrite growth were applied to establish a phase and microstructure pattern selection map, which drew a clear whole picture of the relationship between phase/microstructure and solidification conditions of this series of alloys. Regarding the microstructure feature, our investigation revealed the range of the solidification velocity and chemical composition of lamellar structures as dominant microstructure and their lamellar spacing displayed a considerable range of the average value as a function of growth velocity. The relationship between the lamellar spacing and the growth velocity was further analyzed by using the TMK eutectic model, and the results showed the same overall trend as the experimental results.

Graphical abstract

A phase and microstructure pattern selection map of Zn-rich Zn–Cu peritectic alloys. Regression analysis of the average spacing of lamellar structures.

     

Parameters controlling the microstructure of Al–11Si–2.5Cu–Mg alloys

This study investigated the effects of cooling rate during solidification, heat treatment, and the addition of Mn and Sr on the formation of intermetallic phases in Al–11Si–2.5Cu–Mg alloys. Microstructures were monitored using optical microscopy and EPMA techniques. The results reveal that the volume fractions of intermetallic phases are generally much lower in the furnace-cooled samples than in the air-cooled ones due to the dissolution of the β-AlFeSi and Al₂Cu phases during slow cooling at critical dissolution temperatures. Strontium additions increased the volume fraction of the Al₂Cu phase in the as-cast conditions at low and high cooling rates, as well as at varying ranges of Mn levels. Platelets of the β-AlFeSi phase were to be observed in the microstructure of the as-cast air-cooled samples with a DAS of 40 μm at both Mn levels, while none of these particles were to be found in the furnace-cooled samples with a DAS of 120 μm. Sludge particles were observed in almost all of the air-cooled alloys with sludge factors of between 1.4 and 1.9. These particles, however, were not observed in the furnace-cooled alloys with similar sludge factors. Solution heat treatment coarsens the Si particles in the non-modified alloys under both sets of cooling conditions studied. In the Sr-modified alloys, solution treatment has varied effects depending on the cooling rate and the level of Mn present.     

Growth rate of Nb<Subscript>3</Subscript>Sn for reactive diffusion between Nb and Cu–9.3Sn–0.3Ti alloy

In order to examine experimentally the growth behavior of Nb₃Sn during reactive diffusion between Nb and a bronze with the α + β two-phase microstructure, a sandwich (Cu–Sn–Ti)/Nb/(Cu–Sn–Ti) diffusion couple was prepared from pure Nb and a ternary Cu–Sn–Ti alloy with concentrations of 9.3 at.% Sn and 0.3 at.% Ti by a diffusion bonding technique. Here, α is the primary solid-solution phase of Cu with the face-centered cubic structure, and β is the intermediate phase with the body-centered cubic structure. The diffusion couple was isothermally annealed at temperatures between T = 923 and 1,053 K for various times up to 843 h. Owing to annealing, the Nb₃Sn layer is formed along each (Cu–Sn–Ti)/Nb interface in the diffusion couple, and grows mainly into Nb. Hence, the migration of the Nb₃Sn/Nb interface governs the growth of the Nb₃Sn layer. The mean thickness of the Nb₃Sn layer is proportional to a power function of the annealing time. The exponent of the power function is close to unity at T = 923 K, but takes values of 0.8–0.7 at T = 973–1,053 K. Consequently, the interface reaction at the migrating Nb₃Sn/Nb interface is the rate-controlling process for the growth of the Nb₃Sn layer at T = 923 K, and the interdiffusion across the Nb₃Sn layer as well as the interface reaction contributes to the rate-controlling process at T = 973–1,053 K. Except the effect of Ti, the growth rate of the Nb₃Sn layer is predominantly determined by the activity of Sn in the bronze and thus the concentration of Sn in the α phase. As a result, the growth rate is hardly affected by the volume fraction of the β phase, though the final amount of the Nb₃Sn layer may depend on the volume fraction.     

The effect of α1 plates on the martensitic transformation and shape memory effect in a β Cu–Zn alloy

The martensitic transformation and shape memory effect (SME) in a β Cu–Zn alloy containing various amounts of α₁ plates have been investigated. The results showed that the characteristic temperature of martensitic transformation decreased as the isothermal aging time increased. The crystal structure, twin relationships between martensite variants, and characteristics of martensitic transformation of the β Cu–Zn alloy were not affected by the existence of α₁ plates. However, the α₁ plates were distributed homogeneously in the parent phase and functioned as grain boundaries, hindering the progress of martensite variants, and reducing the effective grain size of the parent phase and the size of self-accommodating plate groups formed upon cooling. In addition, the strain recovery due to the SME decreased as the isothermal aging time increased (the quantity of α₁ plate increased) and/or imposed prestrain increased. Nevertheless, the SME mechanism in the β Cu–Zn alloy containing α₁ plates was not affected by the presence of the α₁ plates.     

Effect of magnesium content on thermal and structural parameters of Al–Mg alloys directionally solidified

In this study, the influence of magnesium content on thermal and structural parameters during the unsteady-state unidirectional solidification of Al–Mg alloys is analyzed. Using a special device, Al–Mg alloys containing 5, 10, and 15 wt% Mg were submitted to unidirectional solidification. Using a data acquisition system, the temperature variations along the casting during solidification were measured. From these results, the variations of solidification parameters as growth rate of dendrite tips, thermal gradient, cooling rate, and local solidification time were determined. The variation of global heat transfer coefficient at metal/mould interface was estimated through the adjustment of experimental temperature variation close to the interface and numerical predictions. Primary and secondary dendrite arms spacing variations during solidification were measured by optical microscopy. From these results, comparative analysis were developed to determine the influence of magnesium content.     

Microstructure and XRD analysis of brazing joint for duplex stainless steel using a Ni–Si–B filler metal

Brazing of a nitrogen-containing duplex stainless steel was preformed using a nickel-based filler metal (Ni-4.5wt.%, Si-3.2wt.%, B). The microstructure of the brazed joint was investigated using scanning electron microscopy, energy dispersive spectrometry, electron probe microanalyzer, and layer-by-layer X-ray diffraction analysis. The results indicated that before completion of isothermal solidification, BN, Ni₃B and Ni₃Si precipitates formed at the interface, in the athermally solidified zone and isothermally solidified zone, respectively. After isothermal solidification, only γ-Ni phase appeared in the brazed interlayer. The appearance of hardness peak values in the athermally solidified zone and the interface most probably corresponded to the formation of Ni₃B and BN, respectively.     

Characterization of transient-liquid-phase-bonded joints in a duplex stainless steel with a Ni–Cr–B insert alloy

Transient liquid-phase bonding of a duplex stainless steel was performed with a Ni–Cr–B insert alloy. The microstructure of the joint region was investigated by cross-sectional and layer-by-layer characterization. According to the experimental studies, prior to completion of isothermal solidification, the bond microstructure can be expressed as γ-Fe + δ-Fe/γ-Fe + δ-Fe + BN/γ-Ni(Fe) + BN/γ-Ni + Cr-rich borides/γ-Ni + Ni₃B + Cr-rich borides (CrB, CrB₂, Cr₂B₃, Cr₃B₄, Cr₅B₃ and CrB₄), from the base metal side to the bonded-interlayer side. Complete isothermal solidification occurred at 1090 °C within 3600 s. Only the γ-Ni solid solution phase was present in the bonded interlayer, and BN precipitates were not removed after isothermal solidification. The formation of secondary-phase precipitates might be responsible for the presence of peak microindentation hardness in the bond region.     

The directional solidification of Pb-Sn alloys

Directional solidification experiments have been carried out on different Pb-Sn alloys as a function of temperature gradient G, growth rate V and cooling rate GV. The specimens were solidified under steady state condition with a constant temperature gradient (50 °C/cm) at a wide range of growth rates ((10–400) × 10^–4 cm/s) and with a constant growth rate (17 × 10^–4 cm/s) at a wide range of temperature gradient (10–55 °C/cm). The primary dendrite arm spacing, ₁, and secondary dendrite arm spacing, ₂, were evaluated. This structure parameters were expressed as functions of G, V and GV by using the linear regression analysis. The results were in good agreement with the previous works.     

Microstructures formed by directional solidification of two ternary eutectic alloys of Bi–Cd–In system

Abstract

Microstructures of the two ternary eutectic alloys of the Bi–Cd–In system were studied using slow unidirectional solidification, followed by quenching to form a representative solid/liquid interface for subsequent observation. The eutectic reactions were found to take the form L?BiIn+BiIn₂+Cd at 77.5°C and L?BiIn₂+_?+Cd at 61.5°C. The 77.5°C eutectic was observed to be of the faceted (BiIn)–faceted (Cd)–non-faceted (BiIn₂) type, while all three phases of the 61.5°C eutectic showed faceting. The BiIn and BiIn₂ phases of the 77.5°C eutectic formed a quasiregular microstructure with the Cd phase growing relatively independently. The phases of the 61.5°C eutectic tended to form a lamellar microstructure with a BiIn₂–?–Cd–_?–BiIn₂ phase sequence. Both eutectics were observed to obey the usual phase spacing law, λ²R=constant, where λ is the phase spacing and R is the growth rate.     

On the influence of carbon on secondary dendrite arm spacing in steel

Solidification-related phenomena and the properties of the final product are strongly influenced by the developing dendritic microstructure, which is defined e.g. by the secondary dendrite arm spacing. In the past, different experimental set-ups were applied and subsequently the secondary dendrite arm spacing of certain steel grades was measured. However, it is difficult to compare the proposed relations based on either the local solidification time or the cooling rate, and they also vary over a wide range. Therefore, the present study systematically investigates the effect of carbon on the secondary dendrite arm spacing using in situ solidification experiments with accurately defined solidification conditions. The parameter K in the empirical equation was determined as a function of carbon, using an iterative procedure to calculate the local solidification time and the measured secondary dendrite arm spacings. Furthermore, these results were discussed and compared with theoretical models from the literature.     

Microstructure and solidification behavior of Cu–Ni–Si alloys

The microstructure and solidification behavior of Cu–Ni–Si alloys with four different Cu contents was studied systematically under near-equilibrium solidification conditions. The microstructures of these Cu–Ni–Si alloys were characterized by SEM and the phase composition was identified by XRD analysis. The phase transition during the solidification process was studied by DTA under an Ar atmosphere. The results show that the microstructure and solidification behavior is closely related to the composition of Cu–Ni–Si alloys. The microstructure of Cu–Ni–Si alloys with higher than 40% Cu content consists of primary phase α-Cu(Ni, Si) and eutectic phase (β₁-Ni₃Si + α-Cu(Ni,Si).When the Cu content is about 40%, only the eutectic phase (β₁-Ni₃Si + α-Cu(Ni,Si)) is present. DTA analysis shows there are three phase transitions during every cooling cycle of alloys with higher than 40% Cu content, but only one for 40% Cu content. Cu–Ni–Si alloy with 40% Cu solidifies by a eutectic reaction, but Cu–Ni–Si alloys with higher than 40% Cu content solidify as a hypoeutectic reaction.     

Numerical calculation of microsegregation in coarsened dendritic microstructures

Abstract

An earlier procedure for calculating homogenization during the solidification of binary alloys is extended to include more realistic assumptions for dendrite arm coarsening kinetics and the partition coefficient. Dendrite coarsening is shown to follow the usual exponential function which differs from that relating final dendrite arm spacing λ_f to local solidification time. This function is not sensitive to the assumption for the partition coefficient or the choice of initial spacing (between λ₀ = 0 and λ_f/2). The homogenization parameter H (normalized difference between the minimum possible concentration and the actual minimum concentration in the centre of the dendrite arm) changes only slightly with the partition coefficient. With the average coarsening parameter used by earlier authors, homogenization is overestimated and a lower amount of the non-equilibrium phase is obtained. The experimental data available for Al–Cu alloys in the range 2–20 wt-% Cu are very closely predicted by the numerical procedure used here.

MST/435     

Influence of Cu addition on the self-propagating high-temperature synthesis of Ti5Si3 in Cu–Ti–Si system

The self-propagating high-temperature synthesis (SHS) reactions can take place in Cu–Ti–Si systems with Cu additions of 10–50 wt.%, and the products only consist of Ti₅Si₃ and Cu phases, without any transient phase. In Ti–Si system, most of the Ti₅Si₃ grains synthesized exhibit the polygon-shaped coarse appearance with an obviously sintered morphology. When Cu content increases from 10 to 50 wt.%, however, the Ti₅Si₃ exhibits cobblestone-like shape with a relatively smooth surface, and its average size decreases significantly from 15 to 2 μm or less. The formation mechanism of Ti₅Si₃ in Cu–Ti–Si system is characterized by the solution, reaction and precipitation processes. Furthermore, the addition of Cu has a great influence on the volume change between green and reacted preforms. The volume change increases with Cu content increasing from 0 to 20 wt.%, and then decreases with the content further increasing from 20 to 50 wt.%. The addition of Cu to Ti–Si system significantly decreases the onset temperature of the reaction during differential scanning calorimetry process, which is even much lower than the α → β transition temperature of Ti (882 °C), suggesting that the reaction could be greatly facilitated by Cu addition. As a result, the role of Cu serves not only as a diluent but also as a reactant and participates in the self-propagating high-temperature synthesis reaction process.     

Development of solidification microstructure and tensile mechanical properties of Sn-0.7Cu and Sn-0.7Cu-2.0Ag solders

Non modified and Ag-modified eutectic Sn-0.7Cu solder alloys were directionally solidified under transient heat flow conditions. The microstructure of the Sn-0.7Cu alloy has been characterized and the present experimental results include the cell/primary dendrite arm spacing (λ₁) and its correlation with: the tip cooling rate ( $\mathop T\limits^{ \bullet }$ ) during solidification, ultimate tensile strength (σ_u) and elongation to fracture (δ). Distinct morphologies of intermetallic compounds have been associated with the solidification cooling rate for both alloys examined. For the Sn-0.7Cu alloy, cellular regions were observed to occur for cooling rates lower than 0.9 K/s, being characterized by aligned eutectic colonies. On the other hand, the alloy containing 2.0 wt %Ag enabled the launch of tertiary branches within the dendritic arrangement. The comparison of results allows stating that finer solder microstructures are shown to be associated with higher ultimate tensile strengths (σ_u) for both alloys although a more complex microstructure was found for the SAC alloy. In contrast the elongation (δ) exhibited opposite tendencies. The growth of coarse Ag₃Sn fibers and platelets within interdendritic regions seems to contribute for the reduction on ductility observed for the SAC alloy.     

Microstructural response to controlled accelerations during the directional solidification of Al-6 wt.% Si Alloys

After establishing steady-state directional growth, aluminum-6 wt.% silicon alloys were subjected to a series of programmed accelerations and decelerations. The response of the primary dendrite arm spacing (λ₁), the eutectic spacing (λ_E), and the primary dendrite trunk diameter (d) to the imposed rate changes were evaluated and compared with results from constant growth velocity experiments. It was found that both λ_E and d responded to a continually changing growth rate, whereas λ₁ did not. The implication of using these microstructural features to characterize solidification histories is discussed.     

Interfacial microstructure of Si3N4/Si3N4 brazing joint with Cu–Zn–Ti filler alloy

In this study, Si₃N₄ ceramic was jointed by a brazing technique with a Cu–Zn–Ti filler alloy. The interfacial microstructure between Si₃N₄ ceramic and filler alloy in the Si₃N₄/Si₃N₄ joint was observed and analyzed by using electron-probe microanalysis, X-ray diffraction and transmission electron microscopy. The results indicate that there are two reaction layers at the ceramic/filler interface in the joint, which was obtained by brazing at a temperature and holding time of 1223 K and 15 min, respectively. The layer nearby the Si₃N₄ ceramic is a TiN layer with an average grain size of 100 nm, and the layer nearby the filler alloy is a Ti₅Si₃N_x layer with an average grain size of 1–2 μm. Thickness of the TiN and Ti₅Si₃N_x layers is about 1 μm and 10 μm, respectively. The formation mechanism of the reaction layers was discussed. A model showing the microstructure from Si₃N₄ ceramic to filler alloy in the Si₃N₄/Si₃N₄ joint was provided as: Si₃N₄ ceramic/TiN reaction layer/Ti₅Si₃N_x reaction layer/Cu–Zn solution.     

Microstructural characterization of precipitates in Cu–10 wt.%Al–0.8 wt.%Be shape-memory alloy

Microstructural characterization of α₁-plate and γ₂ phase precipitated in hypoeutectoid Cu–10 wt.%Al–0.8 wt.%Be shape-memory alloy (SMA) aged at 200 °C for different periods of time (20–160 h) is researched in this study. High-resolution transmission electron microscope (HRTEM) was employed to investigate the α₁-plate with 18R long period stacking order structure (LPSO) in the SMA aged for 20 h. According to the atomic shuffling revealed in HRTEM-micrograph, the atomic model of the 18R LPSO is proposed. The quantitative mapping of electron energy loss spectrometry shows that the α₁-plates in the SMA aged for 160 h contain lower aluminum concentration than the parent phase matrix. The lattice image of the nanometer-sized γ₂ phase precipitated homogeneously in the SMA aged for 160 h is also revealed by using HRTEM. Precipitation of the nanometer-sized γ₂ phase cannot be impeded by means of the addition of beryllium and quenching, and such precipitate does not grow up in the SMA aged for periods of time less than 160 h.     

Theoretical strength and structural response of Cu crystal

The theoretical strength and structural response of FCC crystal Cu under uniaxial loading along one edge of a unit cell have been investigated with MAEAM. The stability criteria of the tetragonal system are extended to the generally stressed orthogonal system. The results show that, even if an orthorhombic path is applied, the deformation is spontaneously along the tetragonal Bain path till tensile elastic limit at stretch λ₁ = 1.085 where the Born criterion is violated. The branched orthogonal path is preferred over the conventional tetragonal Bain path from minimization of the stress σ₁ or the energy E. Although a stress-free BCC phase with the local maximum energy of −3.460 eV appearing either in compressive region or in tensile region is unstable and would slip spontaneously into a near neighbor stress-free mBCT phase with the local minimum energy of −3.461 eV, the initial FCC phase with the lowest energy of −3.489 eV is the most stable in correspondence with the actual behavior of pure Cu. Furthermore, the calculated elastic moduli of FCC phase are in good agreement with the experimental values. The stable region ranges from −2.89 GPa to 7.252 GPa in the theoretical strength or from 0.912 to 1.085 in the stretch λ₁ correspondingly.     

Properties and microstructure of Sn–0.7Cu–0.05Ni solder bearing rare earth element Pr

Effects of trace amount of rare earth element Pr on properties and microstructure of Sn–0.7Cu–0.05Ni solder were investigated in this paper. The solderability of Sn–Cu–Ni–xPr alloy and shear strengh of Sn–Cu–Ni–xPr soldered micro-joints were determined by means of the wetting balance method and shear test, respectively. Moreover, microstructure of solder alloys bearing Pr, as well as intermetallic compound (IMC) layer formed at solder/Cu interface after soldering were observed. It was concluded that the major benefits of rare earth element Pr on Sn–Cu–Ni lead-free solder are: improving solderability, refining microstructure, and depressing IMC (IMC) growth, which exhibited improved mechanical properties. It also revealed that (Cu,Ni)₆Sn₅ is the majority IMC phase at the interface of Sn–Cu–Ni–xPr/Cu solder joints. Ni added into the solder effectively suppressed the growth of Cu₃Sn and consequently also the total IMC layer thickness. Above all, the thickness and morphology of the interfacial (Cu,Ni)₆Sn₅ IMC were optimized due to alloying Pr. It can be inferred that Pr and Ni would play an important role in improving the reliability of Sn–Cu–Ni lead-free solder joints.