Transient Heat Flow Conditions Research Articles

Hypereutectic Zn–Al Alloys: Microstructural Development under Unsteady-State Solidification Conditions, Eutectic Coupled Zone and Hardness

The present study investigates the effects of Al content and solidification thermal parameters on the microstructural development under transient heat flow conditions for two hypereutectic Zn–Al alloys: Zn-6wt.%Al and Zn-11wt.%Al. The alloys were directionally solidified and had experimental cooling profiles monitored permitting cooling rates and growth rates to be determined along the length of the directionally solidified (DS) castings. The microstructure of the Zn-6wt.%Al alloy is shown to be formed by eutectic colonies, constituted by a eutectic mixture of (Zn) and (Al′) phases in the form of lamellae and the Zn-11wt.% Al alloy by the pro-eutectic (Al′) dendrites and the eutectic mixture in the interdendritic regions. Growth laws are experimentally determined relating eutectic and dendritic spacings to the growth rate and cooling rate. A diagram exhibiting the coupled zone of Zn–Al alloys as a function of cooling rate is proposed, which shows different microstructural morphologies influenced by composition and thermal parameters, that is, growth rate and the temperature gradient, synthesized by the cooling rate (Ṫ = G.V). The microhardness of both Zn-6wt.%Al and Zn-11wt.%Al alloys were shown not to depend on the length scale of the resulting microstructure.

Microstructural Patterns, Microsegregation, Porosity, and Mechanical Properties of Hypoeutectic Al-Fe Alloy, and its Dependency with Solidification Thermal Parameters

Al-Fe alloys are usually used as packaging and structural materials, but in the recent years, there have been considered for possible applications in aerospace field. The solidification sequence in pure aluminum containing 1 wt.% Fe is described in term of the formation of macrostructure, microstructure, microsegregation, porosity and mechanical properties. This material was studied in the upward unidirectional solidification system under transient heat flow conditions. Differences in microstructure, microsegregation, porosity and mechanical properties such as ultimate tensile strength, elongation and microhardness, due to the thermal parameter effects were observed and discussed. Experimental growth laws relating cellular spacing to the cooling rate and solidification speed have been determined, indicating that the increase in thermal parameter have induced a refinement effect on cell morphology. Microsegregation profiles of Fe solute were experimentally determined from the central region of the cell to the intercellular region under different solidification speeds. The Fe microsegregation determined from central region of the cell (FS = 0) to the intercellular region (FS = 1) show a growing profile, in any case considered. However, the profiles move upward with the increase in solidification speed, which indicates that Fe solubility in solid, increases with the increase in solidification speed. The effect of the solidification thermal parameters and cellular spacing on the porosity content were experimentally investigated. The value of porosity content increased along the casting. These results have pointed out that porosity content is affected by solidification parameters and cellular patterns. Further, measurable effects of the thermal parameters, cellular spacing and porosity content on the mechanical properties were experimentally determined. It stands out among experimental results the influence of porosity on the mechanical properties of as-cast material. In any case analyzed, mechanical properties increase with decreasing porosity content.

Evaluating Microstructure, Wear Resistance and Tensile Properties of Al-Bi(-Cu, -Zn) Alloys for Lightweight Sliding Bearings

One of the most important routes for obtaining Al-Bi-x monotectic alloys is directional solidification. The control of the thermal solidification parameters under transient heat flow conditions can provide an optimized distribution of the Bismuth (Bi) soft minority phase embedded into an Al-rich matrix. In the present contribution, Al-Bi, Al-Bi-Zn, and Al-Bi-Cu alloys were manufactured through this route with their microstructures characterized and dimensioned based on the solidification cooling rates. The main purpose is to evaluate the influence of typical hardening elements in Al alloys (zinc and copper) in the microstructure, tensile properties, and wear of the monotectic Al-Bi alloy. These additions are welcome in the development of light and more resistant alloys due to the growing demands in new sliding bearing designs. It is demonstrated that the addition of 3.0 wt.% Cu promotes microstructural refining, doubles the wear resistance, and triples the tensile strength with some minor decrease in ductility in relation to the binary Al-3.2 wt.% Bi alloy. With the addition of 3.0 wt.% Zn, although there is some microstructural refining, little contribution can be seen in the application properties.

The effects of Cr addition on microstructure, hardness and tensile properties of as-cast Al–3.8wt.%Cu–(Cr) alloys

An investigation is made on the effects of Cr addition (0.25 and 0.50wt%) on the solidification evolution, microstructure formation, hardness and tensile properties of Al–3.8Cu–(Cr) alloys. Solidification experiments were carried out in a vertical furnace using a metallic mold cooled from the bottom, thus permitting solidified ingots under transient heat flow conditions to be obtained. The solidification system was instrumented with thermocouples at positions along the length of the ingot to permit the solidification cooling rate to be determined from bottom to top of the ingots. The ingots were transversally and longitudinally sectioned to extract specimens for metallographic analyze, hardness and tensile tests. The macrostructures are shown to be entirely columnar along the length of the Al–3.8Cu and Al–3.8Cu–0.25Cr alloys ingots, whereas the Al–3.8Cu–0.50Cr alloy is shown to have a columnar-to-equiaxed transition close to the top. For any alloy ingot examined, the Al-rich matrix is characterized by a cellular morphology for high cooling rates followed by cellular/dendritic transitions with the decrease in cooling rate. The addition of Cr to the Al–3.8Cu alloy promoted the formation of Al23CuFe and Al7CuCrFe phases precipitated in the matrix and eutectic mixture, preventing formation of isolated Al–Fe intermetallic compounds. Hardness and tensile strength increased with increasing alloy Cr content, with highest values of both properties associated with the Al–3.8Cu–0.50Cr alloy ingot. Experimental equations correlating hardness/tensile strength and cellular spacing are proposed. These equations were coupled to the theoretical Hunt–Lu model to estimate hardness and ultimate tensile strength as a function of the cellular spacing.

Transient directional solidification of a eutectic Al–Si–Ni alloy: Macrostructure, microstructure, dendritic growth and hardness

The Al-11 wt%Si-5 wt%Ni eutectic alloy was directionally solidified (DS) under transient heat flow conditions at cooling rates in a range of about 1–25 °C/s along the length of the casting. For comparison purposes binary Al-11 wt%Si and Al-5 wt%Ni alloys were also solidified under similar experimental conditions. Initially characterized by columnar grains, the macrostructure of the ternary DS alloy casting evolved during solidification to a columnar to equiaxed transition (CET) at critical cooling rates of about 1.3–1.6 °C/s, i.e. more than 7 times higher than the CET critical value determined in the literature for Al–Ni and Al–Si alloys. The microstructure of the ternary alloy casting is shown to be constituted by primary Si crystals and α-Al dendritic branches surrounded by the eutectic phase (α-Al+Si+Al3Ni), with the Al3Ni phase having initially a plate-like morphology, which with the decrease in the solidification kinetics, evolved to a fishbone morphology. The pertinent scaling laws representing the dendritic growth of the eutectic ternary alloy, properly compared to those of the examined Al–Si and Al–Ni binary alloys are outlined. The scale of the dendritic α-Al phase is shown to affect the Vickers hardness (HV) along the length of the ternary eutectic alloy casting, and a Hall–Petch type equation is proposed relating HV to the secondary dendritic arm spacing.

Using transient temperature–depth profiles to assess vertical groundwater flow across semi-confining layers in the Chianan coastal plain aquifer systeme, southern Taiwan

The quantification of vertical groundwater fluxes across semi-confining layers is fundamental to evaluate groundwater recharge and discharge rates to and from aquifer systems. Methods to estimate vertical groundwater fluxes from temperature–depth profiles have been available since the 1960s. While some methodologies assume steady-state conditions, changes in land-surface temperatures as well as hydrogeological conditions can lead to transient heat flow conditions. Indeed, many studies have indicated that transient temperatures in deeper confined aquifers are widespread. A study is presented that uses transient-temperature–depth curves obtained from groundwater observation wells in the Chianan coastal plain in southern Taiwan. In this area, sedimentary aquifer systems consist of a stack of alternating sand and mud layers, over several hundred meters in thickness. Groundwater has been abstracted from these aquifers for decades, resulting in large hydraulic gradients between the shallow and deeper aquifers. Hence, vertical groundwater flow is likely enhanced across finer-grained, semi-confining units. A set of temperature–depth profiles is available from this area. Constrained by these profiles, numerical models of one-dimensional transient heat transfer were used to infer vertical fluxes of 3.3 × 10−8 to 3.9 × 10−8 m/s using thermal data from 2013 to 2016. An analytical solution was also employed that assumes steady-state conditions. Calculated fluxes using the latter approach were lower, at approximately 1.1 × 10−8 to 1.6 × 10−8 m/s. The study suggests that vertical fluxes derived from using Bredehoeft and Papadopulos’s analytical solutions result in underestimates of actual vertical seepage rates across aquitards.

Effects of Thermal Variables of Solidification on the Microstructure, Hardness, and Microhardness of Cu-Al-Ni-Fe Alloys.

Aluminum bronze is a complex group of copper-based alloys that may include up to 14% aluminum, but lower amounts of nickel and iron are also added, as they differently affect alloy characteristics such as strength, ductility, and corrosion resistance. The phase transformations of nickel aluminum–bronze alloys have been the subject of many studies due to the formations of intermetallics promoted by slow cooling. In the present investigation, quaternary systems of aluminum bronze alloys, specifically Cu–10wt%Al–5wt%Ni–5wt%Fe (hypoeutectoid bronze) and Cu–14wt%Al–5wt%Ni–5wi%Fe (hypereutectoid bronze), were directionally solidified upward under transient heat flow conditions. The experimental parameters measured included solidification thermal parameters such as the tip growth rate (VL) and cooling rate (TR), optical microscopy, scanning electron microscopy (SEM) analysis, hardness, and microhardness. We observed that the hardness and microhardness values vary according to the thermal parameters and solidification. We also observed that the Cu–14wt%Al–5wt%Ni–5wi%Fe alloy presented higher hardness values and a more refined structure than the Cu–10wt%Al–5wt%Ni–5wt%Fe alloy. SEM analysis proved the presence of specific intermetallics for each alloy.

Investigation of Thermal Parameters Effects on the Microstructure, Microhardness and Microsegregation of Cu-Sn alloy Directionally Solidified under Transient Heat Flow Conditions

The microstructure and mechanical properties of cast materials are strongly dependent on the thermal history during solidification process. The global casting industry has faced major challenges, i.e., customers increasingly demand completely finished cast parts as well as complexity of casting alloys are rising. Therefore, the previous knowledge the solidification conditions effects on the as-cast ingot microstructure and mechanical properties resulting is very useful in the casting industry in order to improve the casting quality. In this present paper, the thermal parameters effects on the microstructure, microhardness and microsegregation of a Cu- 20 wt.% Sn alloy under transient heat flow conditions were experimentally investigated. The experimental observations indicate that the tertiary dendritic arm spacing (λ3), microsegregation and microhardness are affected by the thermal parameters (solidification speed and cooling rate). Solidification speed (SS) associated with increasing cooling rate (RC) are found to contribute to the decreasing tertiary dendritic arm spacing (λ3). However, cooling rates from 11.36 °C/s to 0.65 °C/s were not found to affect significantly the microhardness along the ingot length. The solute concentration (Sn) in solid region were calculated by Scheil and Clyne-Kurz equation, and used in the predictions of microsegregation profiles. The results calculated by equations, have shown deviations from the experimental data. It is well known that it is very difficult to calculate these concentration profiles using the equilibrium partition coefficient, since frequently castings solidify under non-equilibrium conditions and the solidification process is known as non-equilibrium solidification. For this reason, effect of solidification speed (SS) was considered into equations through effective partition coefficient (kef) that has been determined for the range of solidification speed experimentally examined between 0.19 and 0.89 mm/s. However, results calculated by Scheil and Clyne-Kurz equation using the effective partition coefficient (kef), yielded discrepancies from the experimental results. Because of its deviations between calculated and experimental data, an experimental equation with effective partition coefficient (kef), is considered in present work, showing an excellent agreement with the experimental data.

Unsteady-State Horizontal Solidification of an Al–Si–Cu–Fe Alloy: Relationship Between Thermal Parameters and Microstructure with Mechanical Properties/Fracture Feature

Aluminum casting alloys have properties which are of great industrial interest, such as low density, good corrosion resistance, high thermal and electrical conductivities, good combination of mechanical properties, good workability in machining processes and mechanical forming. Currently, these alloys are produced in various systems and dozens of compositions. In this investigation, a mutual interaction of thermal parameters, scale of the dendritic microstructure, intermetallic compounds (IMCs), microhardness and tensile properties/fracture characteristics of a casting Al–7wt%Si–3wt%Cu–0.3wt%Fe alloy was analyzed. Solidification experiments were developed using a furnace that promoted horizontal growth under transient heat flow conditions. Then, growth rate (VL), cooling rate (CR), and local solidification time (tSL) were determined from measured temperature profiles. Secondary dendritic spacings (λ2), Si particles, Fe-rich and Al2Cu intermetallic phases were characterized by optical and SEM microscopy, as well as the area mapping and point-wise EDS microanalysis. Hence, the interrelations involving Vickers microhardness (HV), yield strength (σYS), ultimate tensile strength (σUTS) and elongation (E%) with microstructural features were evaluated by mathematical equations. IMCs as well as morphologies of Si were also analyzed in the fracture regions. In addition, the experimental growth law of λ2 = f(tSL) proposed in this study was compared with a predictive theoretical model reported in the literature for multicomponent alloys. It was observed that areas that tend to grow faster (lowest λ2 values) were associated with the highest σUTS and E% values, while HV and σYS properties were not affected by the thermal and microstructural parameters (CR and λ2). In addition, less extensive cleavage planes accompanied by small dimples in were observed in fractured samples with lower λ2 values.

Effect of Solidification Processing Parameters and Silicon Content on the Dendritic Spacing and Hardness in Hypoeutectic Al-Si Alloys

Aluminium with silicon as alloying element, form a class of material providing the most significant part of all material casting manufactured materials. These alloys have a wide range of applications in the automotive and aerospace industries. It is widely recognized that moderate addition of silicon to aluminum, significantly improves the resulting mechanical properties. Hypoeutectic Al-Si alloys were used in the present experimental study to investigate the solidification parameters effect and Si 3-5 wt% addition on the microstructural features and resulting hardness in a vertical directional solidification system. The hypoeutectic alloys were directionally solidified under transient heat flow conditions in a range of cooling rates from 0.3 to 4 oC/s. Characterization analyses by optical microscopy indicate clearly that secondary dendritic arm spacing (λ2) increases significantly with the decrease in solidification speed (SP) and cooling rate (Ṫ). On the other hand, experimental growth laws relating the secondary dendritic arm spacing (λ2) to the solidification speed (SP) and cooling rate (Ṫ) indicate that Si addition from 3 wt% to 5 wt% has induced a thickening effect leading to increase in λ2 . While, experimental results have shown that the resulting hardness increase as solidification processing parameters (SP and Ṫ ) increase. Results of Vickers hardness test for Al-5 wt% Si alloy has hardness increase of 18 % from that of Al-3 wt% Si alloy with mean values of 26 and 22 HV, respectively.

An Experimental Study of the Solidification Thermal Parameters Influence upon Microstructure and Mechanical Properties of Al-Si-Cu Alloys

Metals solidification involves the transformation of the molten metal back into the solid state. Solidification structures impact heavily on the final product's characteristics. The microstructure effects on metallic alloys properties have been highlighted in various studies and particularly the dendrite arm spacing influence upon the mechanical properties such as tensile strength has been reported. In the present investigation, Al-10wt%Si-2wt%Cu and Al-10wt%Si-5wt%Cu alloys were directionally solidified upward under transient heat flow conditions. The experimental results include solidification thermal parameters such as tip growth rate and cooling rate, optical microscopy, volume fraction of the eutectic mixture, primary dendritic arm spacing and ultimate tensile strength. Experimental growth laws of primary dendrite arm spacing as a function of the solidification thermal parameters are proposed. The Hall-Petch mathematical expressions were used to correlate the ultimate tensile strength as a function of the primary dendritic arm spacing. It was found that the alloy with higher copper content had a more refined structure. More refined structures had higher ultimate tensile strength values.

Microindentation Hardness-Secondary Dendritic Spacings Correlation with Casting Thermal Parameters in an Al-9wt.%Si Alloy

Experiments were carried out to analyze the effect of growth rates (VL) and cooling rates (TR) on both secondary dendritic arm spacings (λ2) and Vickers microhardness (HV) of an Al-9wt.%Si alloy during the horizontal directional solidification under transient heat flow conditions. A water-cooled solidification experimental apparatus was developed allowing a wide range of TR (from 0.2 to 3.5 oC/s) to be experienced. Five computer guided thermocouples were connected with the metal, and the time-temperature data were recorded automatically. The solidification path was also calculated by Scheil model in Thermo-Calc software. Casting samples were characterized by the combined analyses of optical microscopy (OM) and scanning electron microscopy coupled with energy dispersive spectrometry (SEM-EDS) revealing a complex arrangement of phases including binary (α-Al + Si) and ternary (α-Al + Si + β-AlFeSi) mixtures within interdendritic regions. It was observed that power law functions characterize the variation of λ2 as a function of VL and TR with exponents of -2/3 and -1/3, respectively. Finally, experimental laws of power and Hall-Petch types are proposed relating the resulting HV to the λ2. According to these results, it was found that, for increasing values of λ2, the results of HV decrease.DOI: http://dx.doi.org/10.5755/j01.ms.24.1.17319

Dendritic Growth, Solidification Thermal Parameters, and Mg Content Affecting the Tensile Properties of Al-Mg-1.5 Wt Pct Fe Alloys

Al-Mg-Fe alloys are appointed as favorable ones with respect to the costs and all the required properties for successful vessel service. However, the experimental inter-relations of solidification thermal parameters, microstructure, and mechanical strength are still undetermined. In the present research work, the dependences of tensile properties on the length scale of the dendritic morphology of ternary Al-1.2 wt pct Mg-1.5 wt pct Fe and Al-7 wt pct Mg-1.5 wt pct Fe alloys are examined. Transient heat flow conditions during solidification have been achieved by the use of a directional solidification system, thus permitting a comprehensive characterization of the dendritic microstructures to be performed. Thermo-Calc computations, X-ray diffraction, and scanning electron microscopy analyses are carried out to give support to the extensive microstructural evaluation performed with both ternary Al-Mg-Fe alloys. Experimental growth relations of primary, λ 1, and secondary, λ 2, dendrite arm spacings with cooling rate ( $$ {\dot T}_{\rm{L}} $$ ) and of tensile properties with λ 2 are proposed. For both alloys examined, Hall–Petch type formulas show that the tensile strength increases with the decrease in λ 2. The soundest strength–ductility balance is exhibited by the Al-7 wt pct Mg-1.5 wt pct Fe alloy specimen with refined microstructure. This is shown to be due to a more homogeneous distribution of intermetallic particles in connection with solid solution strengthening propitiated by Mg. Functional experimental inter-relations of tensile properties with growth (V L) and cooling rates ( $$ {\dot T}_{\rm{L}} $$ ) for both ternary Al-Mg-Fe alloys have also been derived.

Upward and downward unsteady-state directional solidification of a hypoeutectic Al-3wt.%Mg alloy

Solidification thermal parameters, such as growth rate, cooling rate and dendrite arm spacing (λ), have been measured in a hypoeutectic Al-Mg alloy directionally solidified under upward and downward transient heat flow conditions. The experimental setup used in this work consists of a water-cooled mould with heat being extracted from the bottom or the top, promoting upward and downward directional solidification, respectively. It is shown that the dendritic arm spacing are not significantly affected by interdendritic convection for both solidification configurations and single growth laws are proposed for both cases. The Bouchard- Kirkaldy model is shown to overestimate the experimental primary dendritic arm spacing, despite fitting properly the secondary dendrite arm spacing.

Tensile properties and related microstructural aspects of hypereutectic Al-Si alloys directionally solidified under different melt superheats and transient heat flow conditions

The present investigation deals with directional solidification (DS) of Al-15wt% and 18wt% Si alloys under transient heat flow conditions and further characterization of the related microstructures. Emphasis is given on the eutectic growth being affected not only by solidification kinetics but also by melt superheat, which is varied in two degrees for each alloy, i.e., 6% and 23% above the liquidus temperature. The dependences of interphase spacing (λ) on both solidification kinetics (fast, intermediate and slow cooling conditions) and on melt superheat during solidification of hypereutectic alloys are reported for the first time. Furthermore, functional experimental interrelations of tensile mechanical properties and interphase spacing between eutectic Si particles of both alloys evaluated are proposed. It is shown by these correlations that the tensile properties increase with the decrease in these spacings. More significant variations in properties are associated with a certain range of spacings (1.0µm <λ<2.3µm) in the case of the Al-15wt%Si alloy. Tensile strength and elongation-to-fracture decrease with increasing alloy silicon content. The applicability of the eutectic growth expression λ=f (V−1/2) has been verified for the present tested alloys and experimental conditions.

Thermal Parameters, Tertiary Dendritic Growth and Microhardness of Directionally Solidified Al-3wt%Cu Alloy

The main purpose of this paper is to evaluate both tertiary dendritic arm growth and microhardness of Al-3wt%Cu alloy during horizontal directional solidification under transient heat flow conditions. Experimental thermal profiles recorded during solidification process allowed to determine growth rate and cooling rate values which are associated with both tertiary dendritic arm spacings and microhardness. The results show that initial tertiary branches growth only occurs when a cooling rate value of 1.14 K/s is reached. Variation of tertiary spacings is expressed as-1.1 and-0.55 power law functions of growth rate and cooling rate, respectively. A comparative analysis with other studies published in the literature that analyze tertiary dendritic growth of Al-Cu alloys considering transient directional solidification is carried out. Dependence of microhardness on dendritic arrangement is evaluated by experimental laws of power and Hall-Petch types with a view to permitting the applicability of the resulting expressions.

Analysis of the Microstructural Evolution during the Transient Upward, Downward and Horizontal Directional Solidification of the Al-1.2wt%Pb Monotectic Alloy

Monotectic alloys are of great industrial importance because of their favorable tribological behavior. Many studies in order to better understand the morphologies obtained by monotectic reaction have been developed. To better understand the morphologies obtained by monotectic Al-1.2wt%Pb alloy reaction, especially in relation to induced convective flow, this paper aims to compare the microstructural evolution of the alloy obteind by directional solidification in transient heat-flow conditions in upward, downwand and horizontal solidification devices. It was observed a morphology transition from particles to fibers on upward and downward cases and a morphology of only particles on the horizontal case. The classical relationship used for eutectic growth, λ2v = C, which was considered applicable to monotectic reactions, didn’t seemed to be valid in the interphase spacing evolution for the downward device, however power functions (λ = C.va) were found in all cases.

Effect of Interfacial Heat Transfer Coefficient on Dendritic Growth and Microhardness during Horizontal Directional Solidification of an Aluminum-Copper Alloy

This paper presents a theoretical-experimental study for the prediction of the interfacial heat transfer coefficient during the horizontal directional solidification of an Al-3wt.%Cu alloy on water cooled stainless steel chill under transient heat flow conditions. Eight thermocouples were connected with the casting and the time-temperature data were recorded automatically. The thermocouples were placed at 5, 10, 15, 20, 30, 50, 70 and 90 mm from the metal-mold interface. A numerical technique which compares theoretical and experimental thermal profiles was used to measure the heat transfer coefficient values. This has permitted the evaluation of the variation of this thermal parameter along the solidification which is represented by a power equation that shows the time dependence during the process given by hi = constant (t)-n, which represents the best fit between the experimental and calculated curves. The obtained results also include the variation of both primary and secondary dendritic arm spacings of alloy analyzed as a function of heat transfer coefficient. These dendrite arm spacings were found to decrease as the values of this coefficient are increased. Finally, an experimental law of the Hall-Petch type is proposed relating the resulting microhardness to the heat transfer coefficient investigated.

Correlation between microstructure and hardness of a Bi-1.5wt%Ag lead-free solder alloy

In the present study a hypoeutectic Bi-1.5wt%Ag alloy was directionally solidified under transient heat flow conditions and the microstructure was analysed. Bi-Ag alloys are considered as potential alternatives to replace Pb-based alloys as solder materials for metallic connections under high temperatures. However, a lack of understanding regarding the effects of solidification thermal parameters (growth rate - VL, the cooling rate - Ṫ) on microstructural aspects is reported in literature. Another challenge is to improve properties and reliability. The results of the present study include the determination of the tip growth rate and the cooling rate by cooling curves recorded by thermocouples positioned along the casting length, metallography, X-ray fluorescence (XRF) and Vickers hardness. The entire directionally solidified Bi-1.5Ag microstructure was arranged by faceted Bi-rich dendrites surrounded by a eutectic mixture (Bi+Ag). The primary and secondary dendrite arm spacing (λ1 and λ2), the interphase spacing (λ) and the diameter of Ag-rich particles were also measured along the casting length; and experimental growth laws. Relating these microstructural features to the experimental thermal parameters are proposed.

Cooling thermal parameters and microstructure features of directionally solidified ternary Sn–Bi–(Cu,Ag) solder alloys

Low temperature soldering technology encompasses Sn–Bi based alloys as reference materials for joints since such alloys may be molten at temperatures less than 180°C. Despite the relatively high strength of these alloys, segregation problems and low ductility are recognized as potential disadvantages. Thus, for low-temperature applications, Bi–Sn eutectic or near-eutectic compositions with or without additions of alloying elements are considered interesting possibilities. In this context, additions of third elements such as Cu and Ag may be an alternative in order to reach sounder solder joints. The length scale of the phases and their proportions are known to be the most important factors affecting the final wear, mechanical and corrosions properties of ternary Sn–Bi–(Cu,Ag) alloys. In spite of this promising outlook, studies emphasizing interrelations of microstructure features and solidification thermal parameters regarding these multicomponent alloys are rare in the literature. In the present investigation Sn–Bi–(Cu,Ag) alloys were directionally solidified (DS) under transient heat flow conditions. A complete characterization is performed including experimental cooling thermal parameters, segregation (XRF), optical and scanning electron microscopies, X-ray diffraction (XRD) and length scale of the microstructural phases. Experimental growth laws relating dendritic spacings to solidification thermal parameters have been proposed with emphasis on the effects of Ag and Cu. The theoretical predictions of the Rappaz-Boettinger model are shown to be slightly above the experimental scatter of secondary dendritic arm spacings for both ternary Sn–Bi–Cu and Sn–Bi–Ag alloys examined.