Tertiary Dendrite Arm Spacings Research Articles

Thermal analysis via horizontal solidification of Al3Cu2Si (mass%) alloy: thermal and microstructural parameters, intermetallic compounds and microhardness

A transient horizontal solidification experiment was performed to investigate the role of the thermal parameters, such as growth and cooling rates (GR and CR, respectively), on the microstructure and microhardness (HV) of the Al2Si3Cu (mass%) alloy. Thermal data were generated using a water-cooled solidification device. This allowed to determine a large range of GR and CR. As-cast samples from the cooled surface were characterized by optical microscopy and scanning electron microscopy (MO and SEM). The resulting microstructure was characterized by an Al-rich dendritic primary phase (Alα) and by an interdendritic eutectic mixture composed of Alαeutectic + Si particle + (Al2Cu and Fe) intermetallic compounds. The Alα phase was characterized by primary, secondary and tertiary dendrite arm spacings (λ1, λ2 and λ3, respectively). The λ1, λ2 e λ3 dependence on GR and CR was characterized by power-type mathematical equations. It was evidenced the tertiary branches occurrence for GR > 0.95 mm s−1 and CR > 6.5 °C s−1. Microhardness (HV) was measured at the center of the dendritic primary phase and within the interdendritic regions. Higher HV values were observed in the eutectic mixture. Experimental power and Hall–Petch mathematical equations were proposed to characterize HV variation within the interdendritic region as a function of λ1, λ2 and λ3. A parametric factor given by the expression $$\varepsilon = \frac{\text{CR}}{{\left( {\mathop \sum \nolimits_{1}^{\text{n}} C_{\text{on}} } \right)/n}}$$ that allows to predict the appearance of tertiary dendritic arms for the assumed solidification conditions was proposed by the present work, for future predictions. A comparison with the literature was conducted.

Effect of pouring temperature on microstructure and microsegregation of as-cast aluminum alloy

An experimental investigation of pouring temperature effects on the thermal parameters, microstructure, and microsegregation during directional solidification of the hypoeutectic Al–Cu alloy is presented and discussed. The hypoeutectic Al–4.0 wt.% Cu alloy with superheat temperature is poured at temperatures 682.5, 747.5, and 812.5 °C. The thermal parameters (cooling rate, solidification speed, and local solidification time) are affected by pouring temperature. These, in turn, affects the microstructure arrangement and microsegregation profiles. Experimental growth laws of tertiary dendrite arm spacing relating to the cooling rate have been determined, indicating that increase in thermal parameter was responsible for the refinement effect on dendritic morphology. Results showed that pouring temperatures nearer to the liquidus temperature produced a dendritic array refined, i.e., pouring temperatures few degrees Celsius higher than the liquidus temperature favored the reduction of the tertiary dendrite arm spacings. The results of chemical composition obtained by fluorescence X-ray spectrometry technique confirmed that microsegregation profiles were influenced by pouring temperature, i.e., the microsegregation profiles were shown to move upward with the decrease in the pouring temperature.

Microstructure and microhardness in horizontally solidified Al–7Si–0.15Fe–(3Cu; 0.3Mg) alloys

ABSTRACTTransient horizontal directional solidification (THDS) experiments have been carried out with Al–7wt.%Si–0.15Fe, Al–7wt.%Si–3wt.%Cu–0.15wt.%Fe and Al–7wt.%Si–0.3wt.%Mg–0.15wt.%Fe alloys, to identify experimental relationships between growth rates (GR), cooling rates (CR), tertiary dendrite arm spacings (λ3) and microhardness (HV). Optical microscopy and scanning electron microscopy/energy-dispersive spectrometry (SEM/EDS) were used to perform a comprehensive microstructural characterisation of the β-Al5FeSi, ω-Al7Cu2Fe, θ-Al2Cu, π-Al8Mg3FeSi6 and α-Mg2Si intermetallic phases. The addition of Cu and Mg to the Al–7wt.%Si–0.15wt.%Fe alloy led to the precipitation of ω and π phases from the β phase. It has been found for all analysed alloys that power experimental functions given by λ3 = constant.(GR)-1.1 and λ3 = constant.(CR)-0.55 best describe the variation of λ3 with corresponding thermal and microstructural parameters.

Microstructural Development of an AlNiBi Alloy and Influence of the Transient Horizontal Solidification Parameters on Microhardness

The combination of the structural and tribological properties presented by AlNiBi alloys has motivated us to establish, as the main objective of this study, the investigation of the microstructural evolution and its influence on the microhardness (HV) of Al-3wt pct Ni-1wt pct Bi alloy horizontally solidified via a water-cooled directional solidification device. Temperature mapping by thermocouples inserted in the metal has been performed for experimental determination of the solidification thermal parameters, such as the growth rate and cooling rate (VL and TR, respectively). The microstructure has been characterized by optical and scanning electron microscopy and by microanalysis of the composition via dispersive energy spectroscopy (EDS composition). The macrostructure of the as-solidified ingot is characterized by columnar grains, and the final microstructure consists of an Al-rich primary phase (α-Al) and a eutectic mixture composed of two phases: α-Al + Al3Ni intermetallic (β) with Bi particles anchored on the β phase. The Bi droplet scale is affected by the thermal parameters. The primary phase (α-Al) is characterized by a reverse cellular-to-dendritic microstructural transition. Cellular and dendritic microstructures have been quantified by the cell, primary dendrite arm, secondary dendrite arm, and tertiary dendrite arm spacings (λC, λ1, λ2, and λ3, respectively). The relationships of λC, λ1, λ2, and λ3 with VL and TR have been established via power-type mathematical expressions. The HV dependence on λC, λ1, λ2, and λ3 has been analyzed in both cellular and dendritic microstructural zones. It has been observed that the HV values do not vary in the dendritic zone; however, Hall–Petch’s mathematical equations characterize the HV variation with these thermal and microstructural parameters in the cellular zone.

Experimental Investigation of Ternary Al-Si-Cu Alloy Solidified with Unsteady-State Heat Flow Conditions

Ternary Al-9.0wt%Si-4.0wt%Cu alloy was solidified in a vertical directional solidification system under unsteady-state heat flow conditions. The resulting dendritic morphology and microsegregation were investigated. A more detailed analysis was dedicated to the microsegregation phenomena where a multielement interaction was observed. The solidification parameters such as: solidification speed (VL) and cooling rate (T˙) were determined from the cooling curves obtained during the solidification process. The thermal variables effect on the dendritic morphology is presented. The measurements of tertiary dendrite arm spacing (λ3) and microsegregation were performed for different positions along the casting. The experimental curves for microsegregation were obtained for Si and Cu from the center of dendritic tertiary arm to the next nearest tertiary arm. The solidification speed (VL) influence is built the effective partition coefficient (Kef_Cu and Kef_Si) that has been determined for the range of VL and microsegregation curves are calculated by Scheil's equation for comparison with experimental data. Good agreements of the Scheil's equation with experimental data on microsegregation curves of the Si and Cuwere obtained when effective partition coefficient (Kef_Cu and Kef_Si) is taken into consideration. The multielement interaction effect on the Si microsegregation is investigated. Experimental results show that, Cu-rich dendrites were accompanied by minute amounts of Si. The concentration profiles obtained experimentally point to a strong negative correlation between Si and VL on ternary Al-9.0wt%Si-4.0wt%Cu alloy.

Analytics on large microstructure datasets using two-point spatial correlations: Coarsening of dendritic structures

We extend the existing framework [1–4] of two-point spatial correlations to allow for the quantification, analyses, interpretation and visualization of microstructure coarsening measured by time-resolved X-ray computed tomography. Specifically, extensions were made to facilitate (i) the incorporation of nonconventional local attributes such as solid-liquid interface, interface curvature, and interface velocity in the description of the local state, and (ii) the efficient computation of bulk spatial correlations when the local attributes are sparsely defined only at special locations in the three-dimensional volume (e.g., solid-liquid interfaces). We have explored multiple variants of spatial correlations, including Pearson correlation coefficients and two-point joint probabilities, and examined their relative merits in providing useful new insights into the coarsening process. Algorithmic enhancements needed to carry out these computations on the large datasets produced in the experiments are also described. The results demonstrate the remarkable ability of these new protocols in automated (unbiased) capture of the four-fold symmetry of the dendritic microstructure, and in providing quantitative and reliable estimates of the characteristic lengths associated with the dendritic microstructure (including the secondary and tertiary dendrite arm spacings, secondary dendrite arm diameter, and the solute diffusion length). These estimated quantities agree well with the direct measurements from the microstructure. The results also indicate that interfaces with high negative and near-zero mean curvature (H) have long range spatial auto-correlations, whereas all values of the interfacial normal velocity (V) are only auto-correlated in the short range in space. For mid-range (positive) values of H and non-extreme values of V, the spatial distributions are essentially random.

Effects of Solidification Thermal Parameters on Microstructure and Mechanical Properties of Sn-Bi Solder Alloys

Samples extracted along the length of directionally solidified (DS) castings of three Sn-xBi alloys (x = 34 wt.%Bi, 52 wt.%Bi and 58 wt.%Bi) were first evaluated metallographically and then subjected to scanning electron microscopy and energy-dispersive x-ray spectroscopy analyses. The characteristic length scale of both eutectic and dendritic phases forming the microstructure were determined and correlated with solidification thermal parameters (growth rate V, and cooling rate Ṫ). Tensile and Vickers hardness tests were performed to allow strength and ductility to be discussed as a function of both microstructure features and alloy solute content. The tertiary dendrite arm spacings along the length of the DS Sn-52 wt.%Bi alloy casting are shown to be lower than those obtained for the Sn-34 wt.%Bi alloy casting. The results of mechanical tests show that, with the decrease in the alloy Bi content, both tensile strength and hardness are improved. This is shown to be mainly attributed to the higher density of Bi precipitates decorating the Sn-rich dendrites, which are finer than the equivalent phase developed for the Sn-52 wt.%Bi alloy. However, the ductility is shown to be significantly improved for specimens associated with regions of more refined microstructure of the Sn-52 wt.%Bi alloy DS casting. A microstructure combining much branched dendrites, fine Bi particles within the β-Sn dendritic matrix and an important proportion of very fine eutectic formed by alternate Bi-rich and Sn-rich phase, seems to be conducive to this higher ductility. In this case, the fracture surface is shown to be more finely broken with presence of dimples for this particular condition, i.e., characteristic of a ductile fracture mode.

Assessment of Tertiary Dendritic Growth and Its Effects on Mechanical Properties of Directionally Solidified Sn-0.7Cu-xAg Solder Alloys

Although commercial SAC107/207/307 alloys are being used as alternatives to traditional Sn-Pb solder alloys, there is a lack of studies emphasizing some metallurgical aspects, for instance, the different morphologies of Cu6Sn5 and Ag3Sn intermetallic compounds (IMCs) as well as conditions for the launch of tertiary dendritic β-Sn branches and their effects on localized mechanical properties. A wide range of cooling rates during solidification is normally associated with quite different microstructural length parameters. Hence, Sn-0.7 wt.%Cu-x wt.%Ag (where x = 1.0, 2.0, and 3.0) alloys were directionally solidified under transient heat flow conditions, undergoing cooling rates varying from 0.1 K/s to 32.0 K/s. This experimental study encompasses: primary, secondary, and tertiary dendrite arm spacings (λ 1, λ 2, and λ 3) associated with the evolution of the tip cooling rate (\( \dot{T} \)) during solidification; start and growth of tertiary dendrite branches; yield (σ y) and ultimate tensile strengths (σ u); elongation to fracture (δ); and the morphology of IMCs embedded in the Sn-rich phase. A single ratio between the cooling rate (\( \dot{T} \)) and the alloy silver content (C 0-Ag) of 0.45 seems to be the parametric factor associated with the beginning of the growth of tertiary dendritic branches. SAC307 is shown to be the only alloy examined having λ 3 along the entire casting length. Despite the presence of some tertiary branches in part of the SAC207 alloy casting, for both the SAC107 and SAC207 alloys, σ u and σ y are shown to increase with decreasing λ 2, while the opposite trend is exhibited by the SAC307 alloy. It seems that the well-defined array of tertiary branches in such alloy, and consequently the more complex dendritic network, allowed the strength to increase despite the associated increase in λ 2 and the change in the morphology of the Ag3Sn IMC from spheroidal to fibrous.

Effect of Travelling Magnetic Fields on the Dendritic Microstructure of AlSi and AlSiMn

The microstructure formation of AlSi alloys is known to be sensitive to specific solidification conditions. In particular, small fractions of heavier alloying atoms can lead to the precipitation of intermetallic phases. Moreover, the mainly dendritic structure is also sensitive to fluid flow in the melt. These two factors and their mutual influence is examined in this paper. The solidification of AlSi7 and AlSi7Mn1 samples was studied while inducing fluid flow by a traveling magnetic field (TMF) of approximately 5 mT strength, traveling up or down the sample axis. All samples were molten and directionally solidified at constant solidification velocities between 0.03 and 0.24 mm/s. The application of two separate heaters allowed the fixation of constant temperature gradients in the solid and liquid parts of the samples, the use of a transparent silica aerogel crucible permitted optical verification of the solidification velocity. Cross sections were cut from the processed samples and the microstructure analyzed using light microscopy and SEM-EDX. From these images, values for the primary, secondary and tertiary dendrite arm spacing were retrieved. Results are presented which show a clear effect of the TMF-induced fluid flow on the binary samples, but almost none for the ternary alloy. Finally, an explanation proposing a process of precipitate particle pushing is given.

The effects of tertiary dendrite arm spacing and segregation on the corrosion behavior of a Pb–Sb alloy for lead-acid battery components

The aim of this study is to evaluate the effects of segregation and of the presence of tertiary dendrite arms in the microstructural arrangement of a Pb–Sb alloy on its resultant corrosion behavior. In this context, a water-cooled unidirectional solidification system was used to obtain alloy samples having different dendritic patterns. Electrochemical impedance spectroscopy and potentiodynamic polarization curves were used to analyze the corrosion resistance in a 0.5M H2SO4 solution at 25°C. Three different dendritic arrays were investigated as a function of the cooling rate and antimony macrosegregation profile. It was found that the tertiary dendritic arms associated with the antimony segregation have an important role on the resulting corrosion response. It is shown that the sample with a well defined tertiary dendritic array provide a more homogeneously distributed interdendritic eutectic mixture exhibiting better corrosion protection.

Effect of Process Variables on Microstructural Features during Solidification of AM60B Magnesium Alloy

In this study, commercial AM60B magnesium alloy was studied under different solidification conditions to understand the influence of cooling rate, thermal gradient, growth velocity, Niyama criterion, solidification time and mold dimensions on microstructural features such as secondary and tertiary dendrite arm spacing, grain size, porosity, pore shape and size, local morphological and phase variations. Porosity, grain size and dendrite arm spacing were measured as functions of the process variables. It was realized that the process of mold filling and solidification are simultaneous in nature and they significantly affect the microstructure development trends and its dependency on the process parameters. A significant effect, of the above mentioned, was found on the obtained porosity values and their variation along the casting. The results clearly indicate that rate of filling, nature of flow of liquid and shape of the mold greatly affect the solidification process and thereby the microstructure.

Growth of tertiary dendritic arms during the transient directional solidification of hypoeutectic Pb–Sb alloys

Despite the importance of a complete characterization of dendritic patterns in castings, the availability of studies on the development of tertiary dendrite arms is scarce in the literature. In the present study, the tip cooling rate, local solidification time, primary and tertiary dendrite arm spacings have been determined in Pb–Sb alloys castings directionally solidified under unsteady-state heat flow conditions. The alloys compositions experimentally examined are widely used in the as-cast condition for the manufacture of positive and negative grids of lead-acid batteries. The initial growth of tertiary dendritic arms from the secondary branches was found to occur only for a Pb–3.5 wt% Sb alloy at cooling rates in the range 0.4–0.2 K/s, with no evidence of this spacing pattern for Pb–Sb alloys having lower solute content. Tertiary dendritic branches have been observed along the entire casting lengths for alloys of the Pb–Sb hypoeutectic range having compositions higher than 4.0 wt% Sb. It is shown that a power function experimental law with a characteristic −0.55 exponent is able to characterize the tertiary spacing evolution with the solidification cooling rate for alloys compositions ≥4.0 wt% Sb. The only exception was the Pb–3.5 wt% Sb alloy for which λ 3 exhibited significant lower values when compared with the experimental values obtained for the other Pb–Sb alloys for a same solidification cooling rate.

Tertiary dendrite arm spacing during downward transient solidification of Al–Cu and Al–Si alloys

The microstructural scales of dendrites, such as primary, secondary and tertiary arm spacings, control the segregation profiles and the formation of secondary phases within interdendritic regions, which determine the properties of cast structures. Investigations on tertiary dendritic arms are scarce in the literature, and only a few studies have been conducted in order to analyze the effects of melt convection within the interdendritic region during transient solidification. In this article, the influence of thermosolutal convection on the tertiary dendrite arm spacing is experimentally examined in the downward vertical unsteady-state directional solidification of Al–Cu and Al–Si hypoeutectic alloys.

The effect of solidification variables on tertiary dendrite arm spacing in unsteady-state directional solidification of Sn–Pb and Al–Cu alloys

Abstract Solidification thermal variables, i.e., transient metal/mold heat transfer coefficient and tip cooling rate, and tertiary dendrite arm spacings have been measured in Sn–Pb and Al–Cu alloys directionally solidified under unsteady-state heat flow conditions. The tertiary arms seem to initiate from the secondary branches only when a certain value of a parametric factor ( δ= T /C 0 ) relating cooling rate and alloy solute content is attained. It was observed that a −0.55 power law characterizes the tertiary spacing variation with the cooling rate, for any hypoeutectic alloy experimentally examined. The influence of initial alloy composition on tertiary dendritic spacing is also analyzed. The insertion of analytical expressions for cooling rate into the resulting experimental equations permitting to establish empirical formulae relating tertiary dendrite spacing with unsteady-state solidification parameters like: melt superheat, type of mold and transient metal/mold heat transfer coefficient is proposed.

Characterisation of Solidified Microstructure of Solid Solutions / Charakterisierung des Erstarrungengefüges in Mischkristallen

First this paper gives a summary of the solidified structures of solid solutions, the solidification methods, and the investigation possibilities of solidified structures. Afterwards through an example the influences of the solid/liquid interface velocity on the dendrite morphology in the case of steady-state unidirectional solidification are shown. Al-4 wt% Cu alloy was used as testing material. The temperature gradient at the solid/liquid interface was 1500 K/m and the moving velocities of samples were 5 x 10 -5 , 2 x 10 -4 and 8 × 10 -4 m/s. The purpose of experiments was the accurate numerical determination of the typical parameters of the dendritic structure. The amount of the second phase (Al 2 Cu), as well as the primary, the secondary and the tertiary dendrite arm spacing were measured. For quantitative characterisation the image analyser equipment type Quantimet 500 Image Workstation was used. The measured values were studied as a function of the solid/liquid interface velocities.

Secondary and tertiary dendrite arm spacing relationships in directionally solidified Al-Si alloys

Secondary, λ2, and tertiary, λ3, dendrite arm spacings have been measured from Al-Si alloys which were directionally solidified as functions of growth velocity,V, temperature gradient,G, and composition,Co. Both λ2 and λ3 decreased as the imposed growth velocity and silicon concentrations were increased, and for each function a systematic variance in the rate was seen. Complications with measuring secondary arm spacings are shown and it was found that the tertiary arm data agree much better with coarsening theory, the implication being that λ3, when measurable, is a more representative and reliable measure of the solidification history than λ2.

A study of solidification of aluminum and aluminum-copper alloys by anodic oxidation technique

An anodic oxidation etching technique has been developed to reveal the cast morphology of aluminum and aluminum-copper alloys. The paper shows that the cell size (secondary or tertiary dendrite arm spacing) depends upon the degree of superheat, undercooling and the copper content.