Solidification Sequence Research Articles

Mechanism of the Mg3Bi2 Phase Formation in Pb-Free Aluminum 6xxx Alloy with Bismuth Addition

In the present work, free-cutting aluminum alloy AA6026 with 1.1 wt.% bismuth addition was fabricated by different melt preparation methods in order to investigate whether the melt preparation route affects the solidification sequence and thus has an influence on the machinability of the alloy. All experiments were designed to simulate variable industrial conditions: addition of bismuth in an induction melting furnace, addition of bismuth in an electric resistance holding furnace, addition of bismuth together with grain refiner, effect of holding time and melt temperature before casting. Detailed thermodynamic analyses (DSC, Thermo-Calc) and microstructural characterization (SEM-EDS, XRD) have been performed to explain the solidification sequence, microstructure development and especially formation of the intermetallic Mg3Bi2 phase.

Determination of Al-2.18Mg-1.92Li Alloy’s Microstructure Degradation in Corrosive Environment

The utilization of aluminum-lithium-magnesium (Al-Li-Mg) alloys in the transportation industry is enabled by excellent engineering properties. The mechanical properties and corrosion resistance are influenced by the microstructure development comprehending the solidification of coherent strengthening precipitates, precipitation of course and angular equilibrium phases as well as the formation and widening of the Precipitate-free zone. The research was performed to determine the microstructure degradation of Al-2.18Mg-1.92Li alloy in a corrosive environment using electrochemical measurements. The solidification sequence of the Al-2.18Mg-1.92Li alloy, obtained using Thermo–Calc software support, indicated the transformation of the αAl dendritic network and precipitation of AlLi (δ), Al2LiMg (T), and Al8Mg5 (β) phase. All of the phases are anodic with respect to the αAl enabling microstructure degradation. To achieve higher microstructure stability, the sample was solution hardened at 520 °C. However, the sample in as-cast condition showed a lower corrosion potential (−749.84 mV) and corrosion rate (17.01 mm/year) with respect to the solution-hardened sample (−752.52 mV, 51.24 mm/year). Higher microstructure degradation of the solution-hardened sample is a consequence of δ phase precipitation at the grain boundaries and inside the grain of αAl, leading to intergranular corrosion and cavity formation. The δ phase precipitates from the Li and Mg enriched the αAl solid solution at the solution-hardening temperature.

Developing a M6C-Reinforced High-Cr White Iron for Abrasive Wear Application

Fast removal of soft phases (e.g., pearlite and ferrite) in the iron matrix limits the wear life of high-Cr white irons. To address this shortcoming, the authors successfully produced fine networks of M6C carbide in a high-Cr white iron through extensive thermodynamic calculations. Fishbone-like networks of M6C carbides were observed with an optical microscope. It was experimentally determined that such carbide networks protected the soft matrix and increased the overall hardness. Additionally, electron backscattered diffraction was conducted, which showed that the alloy contained phases of M7C3, M6C, ferrite, and retained austenite. Solidification sequence was determined by correlating the thermodynamic equilibrium calculation results with the size and distribution of each phase. A dry sand/rubber wheel apparatus following ASTM standard G65 Procedure A was utilized to assess the abrasive wear performance of the developed alloy. Results showed that the volume loss of the developed material was 25 pct less than that of conventional high-Cr white irons. Wear scars were investigated using a scanning electron microscope, and the improved wear resistance was attributed to the “buffer” effect and plastic deformation of the introduced M6C carbide networks.

Investigation of solidification modes of high Manganese steels

ABSTRACT The solidification modes of high manganese steels were measured by experiments and comparatively calculated by different predicted methods. The DSC experiments indicate the same phase transformation sequence as the isothermal solidification experiments. It is demonstrated that the solidification modes of the Fe–0.028C–21.1Mn–5.72Al, Fe–0.28C–21.0Mn–5.80Al and Fe–0.64C–21.0Mn–5.80Al alloys are F mode, FA1 mode and FA2 mode. The change of heating/cooling rate influences the position and shape of the peaks in the DSC curves but has no effect on the solidification mode. By comparison with different calculation methods, the equilibrium model is shown to be a valuable tool to predict the solidification sequence of the high-Mn steel, from which the critical carbon range for the Fe–C–20Mn–5Al alloy is calculated to be 0.062∼0.427 wt-%.

Vertically Resolved Magma Ocean-Protoatmosphere Evolution: H2, H2O, CO2, CH4, CO, O2, and N2 as Primary Absorbers.

The earliest atmospheres of rocky planets originate from extensive volatile release during magma ocean epochs that occur during assembly of the planet. These establish the initial distribution of the major volatile elements between different chemical reservoirs that subsequently evolve via geological cycles. Current theoretical techniques are limited in exploring the anticipated range of compositional and thermal scenarios of early planetary evolution, even though these are of prime importance to aid astronomical inferences on the environmental context and geological history of extrasolar planets. Here, we present a coupled numerical framework that links an evolutionary, vertically resolved model of the planetary silicate mantle with a radiative‐convective model of the atmosphere. Using this method, we investigate the early evolution of idealized Earth‐sized rocky planets with end‐member, clear‐sky atmospheres dominated by either H2, H2O, CO2, CH4, CO, O2, or N2. We find central metrics of early planetary evolution, such as energy gradient, sequence of mantle solidification, surface pressure, or vertical stratification of the atmosphere, to be intimately controlled by the dominant volatile and outgassing history of the planet. Thermal sequences fall into three general classes with increasing cooling timescale: CO, N2, and O2 with minimal effect, H2O, CO2, and CH4 with intermediate influence, and H2 with several orders of magnitude increase in solidification time and atmosphere vertical stratification. Our numerical experiments exemplify the capabilities of the presented modeling framework and link the interior and atmospheric evolution of rocky exoplanets with multiwavelength astronomical observations.

Effects of solution treatment on microstructure and tensile properties of as-cast alloy 625

Effects of solution treatment between 1050 and 1250 °C on microstructure and tensile properties of as-cast alloy 625 were investigated. The microstructure and solidification characteristics of the alloy were studied by SEM, EDS, EPMA and DTA. The results showed that the solidification sequence of the alloy should be written as L → L + γ → L + γ +MC→ L + γ +MC+ γ /Laves→ γ +MC+ γ /Laves. After solution treatment at 1225 and 1250 °C, incipient melting of Laves phase was observed. The ultimate tensile strength decreased monotonically with the increase of solution treatment temperature, and the yield strength had no significant variation. The elongation increased slightly at first and then reached a minimum value at 1250 °C. The fracture mechanism changed from transgranular mode to intergranular mode after solution at 1250 °C for the reason that numerous Laves phases melted at grain boundaries and microcracks nucleated in the molten pool. The suitable solution treatment temperature of this alloy was 1200 °C.

Microstructural Characterization and Failure Analysis of a Wear-Resistant, High Molybdenum and Chromium White Iron Used for Metal-to-Metal Wear Systems

An Fe-15Mo-18Cr-3.3C-1.65V white iron has been widely used in industries for many years due to its excellent wear performance, but never been reported to academic circle. This paper studied its phase constituents in as-cast condition and its wear performance for metal-to-metal contact wear applications. Microstructural analyses were performed using optical microscopy and scanning electron microscopy with phase identification performed using a transmission Kikuchi diffraction (TKD) technique. A solidification sequence was proposed by correlating thermodynamic equilibrium calculations with the distribution and size of the phase constituents. Field-returned components were analyzed after approximately 10,000 service hours. Micro-galling defects exhibited regions of material flow, white layer formation, subsurface cracking, and material loss. For the first time, white layer was found in a highly alloyed white iron, which was determined to be nanocrystalline with an average grain size less than 15 nm and a phase constitution similar to the base metal. Material loss contributed to abrasive wear scars and eventual leakage of the lubricant protecting the wear components.

Experimental Liquidus Surface Projection and Isothermal Section at 1000 °C of the V-Ni-C System

Twenty-three V-Ni-C alloys were produced and analysed in the as-cast condition, as well as after annealing at 1000 °C for 1000 h. The phases were identified using SEM–EDX and XRD. The extensions of the binary phases were determined. The liquidus surface projection was derived from the overall compositions, primary phases and the solidification sequences. Four invariant reactions on solidification were found. The isothermal section at 1000 °C was also derived, which included the ~ Ni2V4C ternary phase.

Crack Initiation Mechanism in Casting AC4B Aluminum Alloy Parts with Complex Structure

Hot tears were frequently formed in Aluminum (Al) parts with complex structure. In this study, OM (optical microscope), SEM (scanning electron microscope), EDS (energy dispersive spectrometer), and FDM (finite difference method) were used to reveal the hot tears mechanisms in the casted AC4B Al engine. Shrinkage porosities with the size of 300–500 μm were found in casted specimens near the parts’ surface. Many cracked brittle Fe-bearing phase were also found in crack section. The secondary dendrite arm spacing observation in five typical positions showed that the cooling rate in position 5 was faster than that in position 1. The impropriety solidification sequence in position 5 and position 1 impeded the solidification feeding of position 1, and it might lead to casting defects or even casting cracks. Numerical simulation also showed that solidification sequence promoted casting defects, which was accordant with microstructure observation. Impurity may concentrate at position 5 during filling. It is concluded that large β-Fe intermetallics and improper solidification sequence together contributed to crack initiation in the engine.

Influence of Copper Addition in AlSi7MgCu Alloy on Microstructure Development and Tensile Strength Improvement

Commercial AlSi7Mg alloy represents the usual choice for complex geometry casting production. The market imperative to improve mechanical properties imposed the design of new chemical composition of AlSi7MgCu alloy with high content of Cu (up to 1.435 wt.%). This represents a challenge in order to achieve advanced properties. The interaction of a number of alloying (Si, Mg, Cu) and trace elements (Fe, Mn) influenced a wide range of complex reactions occurring and therefore leading to intermetallic phase precipitation. The characterization of novel chemical composition interaction and its solidification sequence was achieved by modelling an equilibrium phase diagram, simultaneously performing both thermal analysis and metallographic investigations. Copper influence was indicated in the whole solidification process starting with infiltration in modified Chinese script phase Al15(Fe,Mn,Cu)3Si2, beside common intermetallic Al5FeSi. Copper addition encourages formation of compact complex intermetallic phases Al5Cu2Mg8Si6 and Al8(Fe,Mn,Cu)Mg3Si6. Solidification ended with secondary eutectic αAl + Al2Cu + βSi. Microstructure investigation allows volume reconstruction of the microstructure and distribution of particular phases. Chemical compositions enriched in copper content and developed microstructural constituent through solidification sequence of AlSi7MgCu alloy contribute to a significant increase in mechanical properties already in an as-cast state.

The partial liquidus projection and solidification sequences in Ti-(40–60) Al-(5–30) Nb alloys

The properties of Ti–Al–Nb alloys strongly depend on alloy compositions and microstructures. Proper design of these important aspects requires detailed knowledge of the solidification sequences and liquidus projection. However, there are still some conflicts between previous experimental and calculated results about the liquidus projections in the composition range of Ti-(40–60)Al-(5–30)Nb and this has hindered Ti–Al–Nb alloy development. In this present work, we explicitly investigated the as-cast microstructures and phase constitution of 50 alloys with the composition range of Ti-(40–60)Al-(5–30)Nb and clarified the solidification sequences. We re-constructed the partial liquidus projection and found that the primary β phase region extend to just below 55 at.% Al content and the primary α phase region extend to just below 24 at.% Nb content. Moreover, it was observed that the invariant reaction between Liquid, β, α and γ phases is a peritectic reaction (Liquid+β+γ→α) and all the Liquid+β+γ, Liquid+β+α and Liquid+α+γ three-phase regions are of peritectic type. The present work provides essential information necessary for optimizing the Ti–Al–Nb thermodynamic database to accelerate alloy development.

Electron microscopy study of laser welded GH909 superalloy joint

The formation process of gamma/Laves eutectic constituents were systematically investigated in both heat-affected zone (HAZ) and fusion zone (FZ) of laser-welded GH909 superalloy. Solidification sequences and eutectic reactions were proposed by morphologically and chemically analyzing phase and chemical composition by utilizing various characterization methods. In FZ, the ultimate eutectic constituents experienced three stages: 1) liquid (L)→Primary gamma (γ) (1360 °C), forming a γ dendritic morphology; 2) following L→γ+(Nb, Ti)C precipitation at mushy zone; 3) terminal L→γ+Laves (1184 °C) at γ interdendritic regions. Enrichment of Nb in Laves and surrounding γ produced a high dislocation density in eutectic constituents. Continuous distribution of such γ/Laves structures promoted the formation of micro interdendritic solidification cracks under accumulated tensile stress. In HAZ, however, the fraction of γ/Laves constituents was heat input depended and increased as the reduction of distance from fusion line. An interactionally and simultaneously integrated process NbC+γ→Laves (~1323 °C) caused the formation of eutectic constituents with NbC decomposition, Laves formation, grain boundary liquation and subsequently eutectic reactions.

Flow-solidification interaction: A numerical study on solidification of NH4Cl – 70 wt.%H2O solution in a water-cooled mould with a large sample thickness

This study focuses on the flow-solidification interaction during the solidification of NH4Cl – 70.43 wt.%H2O solution in a water-cooled mould with a large sample thickness (100 × 80 × 10 mm3). First part of the paper is to use a volume-average-based dendritic solidification model to reproduce an experiment in which transient flow is monitored in-situ using particle image velocimetry, and the solidification sequence is recorded with a light camera. The striking features of the solidification experiment are that the crystal morphology is dominant in a columnar structure, and four typical flow regimes appear in the bulk solution during cooling/solidification, namely pure thermal convection, unstable turbulent flow caused by double diffusive convection, turbulent-stratified flow, and meandering flow. The results show that there is quantitative agreement between the simulation and experiment results. The focus of this paper (second part) is to study the flow-solidification interaction by correlating the above flow regimes with the solidification quantities in the mushy zone. Owing to the enhanced mass (solute) and energy transport by advection, flow suppresses the growth of the mushy zone, while speeding up the solidification inside the mushy zone, leading to the formation of dense mush and massive columnar dendrite trunks. The crystalline morphology of NH4Cl, as presented by a dendritic envelope and described by morphological parameters, is an important factor that influences the flow; as a feedback, the flow further influences the solidification inside the mushy zone. The numerical model also provides valuable information about the formation of freckle trails.

Investigation of Solidification Behavior in a New High Alloy Ni-Based Superalloy

While solidification behavior has been investigated in several nickel-based superalloys, few investigations have focused on the meticulous study of solidification behavior in such a highly alloyed and high γ′ volume fraction-containing alloy as GH4151. In this study, several material characterization techniques were employed to identify phase types, illustrate the morphology of precipitates, and detect composition characteristics. In addition, differential scanning calorimeter and isothermal solidification quenching experiments were used to explain the solidification sequence. It has been confirmed that (Nb,Ti)C carbides, η phase, eutectic (γ + γ′), Laves, Ni5Ce, and γ′ phase were found in the as-cast GH4151. The solidification sequence was determined as follows: L → L + γ → L + MC → L + η → L + (γ + γ′) → L + Laves → L+Ni5Ce, and γ → γ′. Combined with electron probe micro-analysis, the results show that Nb, Ti, Mo, and V are significantly enriched in the interdendritic region, which has elemental degrees of segregation such that Nb > Ti > Mo > V, while Al, Co, Cr, W, and Ni are enriched within the dendritic arms, whose degree of segregation is W > Al > Ni > Co > Cr.

Characterization of Microstructure and Phase Elemental Composition of 15 wt.% Cr – 2 wt.% Mo Cast Iron with Boron Addition

Abstract The article is aimed at the study of structural features and phase chemical composition in 3.6 wt.% C-15 wt.% Cr-2 wt.% Mo cast iron prepared with adding 0.50 wt.% boron. The work was fulfilled using optical and electron scanning microscopy, EDS, XRD, hardness measurement, and thermodynamic modelling. It was found that due to boron effect the cast iron acquired hypereutectic structure consisting of the primary M7C3 carbides (5.3 vol. %) and the eutectics based on M7C3 carbide (rosette-shaped cells) and M3C carbide (coarse-net-shaped cells or Ledeburite-shaped cells). The total amount of carbide phases was 51.8 vol.% whereas the volume fractions of M7C3-eutectic cells and M3C-eutectic cells were 21.0 vol.% and 74.7 vol.% respectively. The matrix comprised austenite, martensite and pearlite whereas austenite volume fraction was measured as 32 vol.%. Chromium and molybdenum were unevenly partitioned in phases performing the opposite character of a distribution. The primary carbides contained about 37 wt.% Cr and 1.6 wt.% Mo with the stoichiometry of (Fe3.88Cr2.76Mo0.22Mn0.12Si0.02)C3. Substantial part of carbide precipitates (primary and eutectic both) exhibited the duplex nature presenting Cr-rich/Mo-depleted core (M7C3 carbide) and Cr-depleted/Mo-rich shell (M3C carbide). Duplex carbides were closely adjusted to austenite + M3C eutectic cells to be the starting interface for their solidification. A minor amount of micron-sized areas with 67.5 wt.% Mo and 4.6 wt.% Si were observed to be presumed as M6C carbide. The cast iron was subjected to holding at 950 °C for 2 hours with subsequent oil-quenching. This treatment caused the precipitation of fine secondary carbides followed by the martensite transformation which resulted in significant hardness increase from 672 HV (as-cast) to 1038 HV. The solidification sequence is discussed in terms of structure observation and Thermo-Calc modelling.

Pandat™ Simulation of the Solidification Sequence and Microstructure Development of the 2 Pct Mg-55 Pct Al-1.6 Pct Si-Zn Coating on Steel

The current study uses the Pandat™ software developed by CompuTherm to simulate the solidification sequence of the 2 pct Mg-55 pct Al-1.6 pct Si-Zn (wt pct) coating on steel. Using the Scheil model approach, the solidification process of the coating alloy is revealed to proceed in the following sequence: (i) primary (Al) formation at T = 553.3 to 475 °C, (ii) Si formation at T = 475 to 437.9 °C, (iii) simultaneous Si and Mg2Si formation at T = 437.9 to 383.6 °C, (iv) MgZn2 formation at T = 383.6 to 336.9 °C, and finally (v) simultaneous formation of (Zn) and MgZn2 at T = 336.9 °C to complete the solidification process, with (Al) being an important participating component throughout, and Si a significant component at Stages (ii) and (iii) and becoming insignificant at Stages (iv) and (v). It is also revealed that the remaining liquid composition varies significantly throughout the solidification process, and therefore, different isopleths must be produced to illustrate the starting solidification reactions at different stages. The simulation results are then used to interpret the complex microstructures observed in the coating.

Microstructural Characterization of Ni-Base Superalloy As-Cast Single Crystal (CMSX-4)

The as-cast microstructure of a single-crystal CMSX-4 Ni-base superalloy produced by directional solidification was studied. The 〈100〉 misorientation of the angled and spiral grain selectors was evaluated for 3.33 and 5.0 mm/min withdrawal rates. The dendritic microstructure presented primary dendrite arm spacing around 370 µm (for a withdrawal rate of 3.33 mm/min) and 320 µm (for 5.0 mm/min); and interdendritic γ–γ′ islands, whose volume fraction was around 5.4% (for 3.33 mm/min) and 6.1% (for 5.0 mm/min). The microsegregation profiles of one withdrawal condition (for 5.0 mm/min) were used to extrapolate the solidus and γ′ solvus temperatures using the Thermo-Calc software. These microsegregation profiles of the as-cast microstructure also affected the solid-state γ′ precipitation in terms of shape, size and volume fraction. In the center of the dendrite, the γ′ precipitates were regularly cubic-shaped with a face of approximately 0.5 µm and a volume fraction of 55%. In the periphery of the dendrite, the morphology of the γ′ precipitates was more asymmetric, and the volume fraction of the γ′ phase increased to 65%. Two morphologies of the interdendritic γ–γ′ islands, fine and coarse, were observed, and a eutectic solidification sequence was proposed. The results indicated that the chemical composition of the interdendritic liquid progressed from hypoeutectic (fine interdendritic structure) to hypereutectic (coarse interdendritic structure) during the solidification.

Microstructural properties and peritectic reactions in a binary Co–Sn alloy by means of scanning electron microscopy and atom probe tomography

Recent studies of lithium-ion batteries suggest that the use of binary CoSn alloys as anodes should provide an improvement over currently used anode materials. However, the implementation of CoSn alloys is challenging due to uncertainties regarding the phase transformations within this system. In order to understand these, we evaluate the compositions of different intermetallic compounds produced via the peritectic reactions, nucleate and grow within the microstructure of binary Sn − 25 at.% Co by employing atom probe tomography (APT). The stoichiometric phase, which is produced upon the cooling of the melt, is not only found as part of the peritectic solidification sequence but also as clusters within the pure Sn phase. The phase was found as a nano sized layer and is attributed to the peritectic reaction between the phase and the pure Sn phase. The production of the compound was enhanced by the phase transformation of the phase. Furthermore, clusters had formed in the pure Sn phase. A limited solubility within the pure Sn phase was also determined to be (0.6 ± 0.1) at.% Co.

Solidification and microstructural formation of a ternary eutectic Al-Cu-Si cast alloy

A novel ternary eutectic Al-27%Cu-5%Si cast alloy was investigated to understand its solidification behavior and microstructural evolution. The studied alloy was cast, remelted and subjected to a rapid cooling in ice-water mixture, and also cooled slowly in air. The solidification sequence of the alloy was studied using Thermo-CalC software with a subsequent DSC measurement. Micro- and nano-hardness tests were conducted at room temperature to assess the change of hardness associated with the microstructural changes due to the different cooling paths the alloy was subjected to. Thermodynamic analysis showed that the equilibrium solidification path of the eutectic alloy consists of three stages: precipitation of Al2Cu primary phase at 527.6 °C, evolution of binary eutectic (α-Al + Al2Cu) at 525.9 °C and the formation of ternary eutectic (α-Al + Al2Cu + Si) at 521.7 °C. DSC analysis of the alloy suggests only one endothermic peak at 536.4 ± 0.2 °C due to the melting of the alloy, which also corresponds to the ternary eutectic transition. The quenched alloy has a well-refined and uniform distribution of bimodal eutectic microstructure when compared with the slowly cooled alloy. The hardness of the quenched alloy was observed to be 16% higher than the slowly cooled alloy. The increased microhardness of the quenched alloy is attributed to the solid solution strengthening of the alloying elements and homogeneous dispersion of fine Si particles, and partly due to the higher volume fractions of both θ-Al2Cu and Si phases.

Thermodynamic descriptions of quaternary Mg–Al–Zn–Bi system supported by experiments and their application in descriptions of solidification behavior in Bi-additional AZ casting alloys

Abstract 16 Mg–Al–Zn–Bi quaternary alloys were utilized to measure the phase equilibria and transformation temperatures in the Mg-rich Mg–Al–Zn–Bi quaternary system by means of the X-ray diffraction, electron probe micro-analysis and differential scanning calorimetry techniques. The isothermal section at 400 °C and three vertical sections along Mg–8 wt%Al–0.75 wt%Zn–xBi, Mg–3.4 wt%Al–0.5 wt%Zn–xBi and Mg–6.9 wt%Al–2.3 wt%Zn–xBi in the Mg–Al–Zn–Bi quaternary system were constructed. Based on the literature data, the ternary Mg–Al–Bi and Mg–Bi–Zn systems were re-assessed using the CALculaiton of PHAse Diagram (CALPHAD) approach. The calculated phase equilibria agree well with the measured data. By directly extrapolating the constituent sub-ternary systems, the thermodynamic database for the Mg–Al–Zn–Bi quaternary system was developed. The remarkable consistency between the predicted phase equilibria and the presently measured data in Mg–Al–Zn–Bi quaternary system further demonstrated the accuracy and reliability of the established thermodynamic database. After that, by using the newly developed thermodynamic database, the growth restrict factors and the solidification curves in Bi-containing AZ series magnesium alloys were calculated and analyzed. It was confirmed that the grain size of AZ alloys can be refined with the addition of Bi, and the component Al had larger grain refinement effect than Bi. Besides, the amount of Bi had also noticeable effect on the solidification sequence of the AZ alloys.