Solidification Sequence Research Articles

Adjustment of Hardness of (CrFeMn)x(NiyAl)1−x, (y = 1,3 and x = 0.6,0.72,0.80) High Entropy Alloys by Deliberate Control of Intermetallic Phase Formation: Microstructural Evolution, Hardness and Dry-Sliding Wear Response

In this work, the softening of the hard, NiAl containing, Cr–Fe–Mn–Al–Ni high entropy alloy system by the introduction of Ni3Al, an intrinsically ductile intermetallic compound, is reported. The alloys were prepared by vacuum arc melting and their phase formation prediction models were verified in terms of the actual presented microstructural features. X-ray analysis revealed the presence of NiAl and Ni3Al intermetallic compounds in each HEA system group. The microstructure of all six examined alloys in their as-cast condition consisted of two phases (NiAl-rich B2 and FeCr-rich A2) featuring various eutectic morphologies, while spinodal decomposition was evident in every case. The NiAl–Cr pseudo-binary phase diagram was used for the explanation of the solidification sequence in each alloy. In terms of mechanical properties, HRC hardness measurements showed that higher amounts of Ni3Al can lower the alloys’ hardness up to half its initial value. Finally, dry-sliding wear testing revealed an oxidation-delamination mode of material removal in both HEAs group, while the Ni3Al containing alloys also exhibited parts of abrasive wear mode. Hardness vs composition diagram. It can be seen that by adjusting the type and the extent of the intermetallic phase being formed (a—NiAl, b—Ni3Al), the hardness can be controllably modified within a range of 21 to 58 HRC. The related wear rates are also included

Relationship between solidification sequence and toughness of carbon steel weld metal

This work aimed to investigate the segregation sequence of impurity elements in the weld metal of carbon steel with various solidification sequences, such as δ single-phase, hypo-peritectic, hyper-peritectic, and γ-single-phase solidification. Moreover, the relationship between the solidification sequence and toughness of the weld metal of carbon steel was studied. The solidification sequence was controlled by the nickel content of the base metals. Various amounts of sulfur and phosphorous were added to the base metals as impurity elements, and the segregation amount of the impurity elements in the weld metals increased in the order of δ-single-phase, hypo-peritectic, hyper-peritectic, and γ-single-phase. The amount of sulfur was higher than that of phosphorous in each solidification sequence. In δ-single-phase solidification, sulfur segregated during solidification and relaxed during cooling after solidification. In contrast, in γ-single-phase solidification, sulfur continued to segregate during and after solidification. The segregated sulfur was in the grain boundaries of the prior austenite at low temperatures. Charpy impact test results revealed that the ductile-brittle transition temperature (DBTT) increased, and the upper shelf energy decreased with an increase in the nickel content, specifically, only for the specimen of γ-single-phase solidification with S addition. In particular, the tendency in the specimens containing sulfur was more significant compared to the specimens containing phosphorous. Moreover, the intergranular fracture surface was only observed in the γ-single-phase solidification specimen, and sulfur was concentrated on the surface. Therefore, the superposition of segregation during and after solidification induced intergranular embrittlement and deterioration of toughness.

Influence of B and Nb Additions and Heat Treatments on the Mechanical Properties of Cu-Ni-Co-Cr-Si Alloy for High Pressure Die Casting Applications

During the high pressure die casting process (HPDC), it is necessary to develop new designs and alloys for the copper plungers. In this research, two alloys Cu-9Ni-1Co-1.6Cr-2Si-1.3Fe-0.25B wt.% (A1) and Cu-9Ni-1Co-1.6Cr-2Si-0.1Fe-0.2Nb wt.% (A2) under different heat treatments (HT) were studied. Optical microscopy technique was applied to reveal the regions of dendritic morphology, also lower Secondary Dendrite Arm Spacing (SDAS); and different grain orientations. The results reveal that the solidification sequence is primary Cu dendrites and secondary intermetallics; heat treatments increase the redistribution of alloying elements in the interdendritic regions. During the heat treatments, some precipitates were found in the grain boundary after aging heat treatments for both alloys, which were determined by X-ray diffraction. Hardness test HRB presented a decrease with the solution heat treatment and an increase with the aging heat treatments proposed for both alloys. Finally, the wear resistances for both alloys were compared with a commercial alloy C17530, with decreased A1 with B additions having the best result in the as-cast condition 4.07 × 10−4 mm3/Nm, while for A2 with Nb additions wear resistance increased, with the best result in the one with aging heat treatment 1.69 × 10−4 mm3/Nm while for the C17530 alloy this was 2.74 × 10−4 mm3/Nm.

In situ X-ray observations of transient states in arc weld pools

Metallic alloys coalesce via extremely rapid melting and subsequent solidification to form fusion welded joints. The melt pool evolution in melting and solidification sequences during the welding process determines the formation of the final weld joint shape, microstructure and defects. The scientific insight on weld pool evolution and related phenomena can be a key contribution to enhance structural integrity and resilience of the welded structures or components. However, inherent complexity with multi-physics phenomena, associated high temperatures and the rapidness of the processes make direct experimental investigation of welding is extremely demanding. Thus, internal flow behaviour during welding or other melt-pool-based metal processing such as additive manufacturing remains unclear and hinders progression to process optimisation. In this contribution we report the observation of melt pool dynamics that take place during electric arc welding, obtained through in situ synchrotron imaging at millisecond scale. The analysis flow patterns along with the quantified weld pool surface dynamics revealed us to how different contributing forces dictate the flow conditions over the distinct durations of the relatively short existence of the liquid phase. Our preliminary results suggest the existence of arc, surface tension and gravity dominant regimes during the evaluation of the weld pool. Further, we present our observations on how different welding parameters influence these regimes and develop into different transient conditions.

Numerical investigation of an in-situ observed flow regimes during solidification of an NH4Cl – 70 wt%H2O solution

A volume-average mixed columnar-equiaxed solidification model, where the dendritic morphology of crystals is described with the dendrite envelope, was developed by the current authors. As an evaluation effort, this model is used to simulate a solidification experiment of ammonium chloride water (NH4Cl – 70 wt%H2O) solution. The solution was cooled in a water cooled copper mould (10×8×1 cm3), and the solidification process and the flow dynamics during solidification were in-situ observed/recorded with a Particle Imagine Velocimetry technique. The striking features of solidification in this experiment are that the crystal morphology is dominant in columnar structure, and typical 4 flow regimes in the bulk solution during solidification appear: pure thermal convection (TH), unstable (turbulent) flow caused by double diffusive convection (TU), turbulent-stratified flow (TU-ST), meandering flow (MF). These flow regimes correspond to different cooling/solidification stages, and reflect their interactions with the developing mushy zone. The calculated solidification sequence and the flow dynamics (4 flow regimes) agree with the experimental ones. The experimentally observed phenomena were successfully explained by the simulation.

Three-dimensional cellular automaton modeling of silicon crystallization with grains in twin relationships

A three-dimensional model is proposed to simulate the grain structure in directionally solidified silicon. This includes the nucleation and growth of grains in twin relationship whose formation is very frequent during silicon solidification. Based on analyses of in-situ and real-time observations of the crystallization front, a three-dimensional cellular automaton method is developed to model the dynamic of {111} facets, groove formation at grain boundaries, nucleation and growth of grains in twin relationship. The model is applied to well-characterized experiments assuming the frozen temperature approximation. A comparison of solidification sequences, crystallographic orientation maps, and coincidence site lattice maps in both experiment and simulation results is achieved. Results demonstrate that the model could be applied to optimize crystallization processes for both polycrystalline and cast-mono silicon fabrication processes.

Matrix design of a novel ductile cast iron modified by W and Al: A comparison between thermodynamic modeling and experimental data

In high-temperature applications of ferrous materials, as in the case of exhaust manifolds, high thermal and mechanical stability are required. Stainless steels and Ni-resist alloys having austenitic matrices are good candidates to meet these requirements at elevated temperatures; however, they are expensive materials and present difficulties in casting. Ferritic ductile cast irons, like the commercial SiMo alloy, are comparatively cheaper materials with better castability but they cannot be used above approximately 800 °C. Thus, to meet the requirements with low-cost materials having improved high-temperature properties, new alloys must be developed by ferrite forming elements having the potential to increase equilibrium temperature. In this study, initially, a novel ductile cast iron matrix was designed using 1 W and 0-4 Al wt.-% and their phases stable at room temperature, transformation temperatures, solidification sequences and thermal expansivity values were determined using thermodynamic calculations with Thermo-Calc software. Computational studies revealed that (i) designed alloy matrices had graphite and M6C type carbides embedded in a ferritic matrix at room temperature as expected, (ii) A1 temperature increased as aluminum content increased. The obtained values were all above that of commercial SiMo alloy, (iii) the detrimental effect of increased aluminum addition on graphite content, and thermal expansivity was observed. Secondly, microstructural and thermal characterizations of cast alloys were performed for validation – the obtained data were in good agreement with the thermodynamic calculations.

Microstructure Evolution Mechanism and Corrosion Behavior of Transient Liquid Phase Bonded 304L Stainless Steel

Transient liquid phase (TLP) bonding of 304L austenitic stainless steel was carried out using MBF-20 interlayer at 1070 °C with different holding times. The effect of bonding time on the microstructure and corrosion resistance of the TLP bonded samples was investigated aiming to obtain the optimal bonding time. The results showed that isothermal solidification was completed within 45 min at 1070 °C and shorter holding times gave rise to the formation of intermetallic compounds at the joint centerline. It was found that the solidification sequence in the joint region is as follows: (1) isothermal solidification of γ solid solution, (2) formation of ternary eutectic of γ + Ni boride + Cr boride, and (3) formation of ternary eutectic of γ + Ni boride + Ni–Si–B compound in the athermally solidified zone. Final stage of the phase formation at the joint centerline was the precipitation of Ni3Si particles in the vicinity of eutectic microconstituents via a solid state transformation. Extensive Cr-rich borides were formed in the diffusion affected zone as a result of B diffusion into the base metal during bonding process. Corrosion behavior of the TLP bonded samples was studied in 3.5% NaCl solution. The joint corrosion resistance was improved with the increase of the bonding time to 30 min and then decreased when the holding time was prolonged.

Thermodynamic evaluation and investigation of solidification microstructure in the Fe–Cr–Ni–C system

Based on a critical review on various experimentally measured phase relations and thermodynamic properties, a thermodynamic update of the Fe–Ni–C ternary system was firstly carried out by the CALPHAD (CALculation of PHAse Diagrams) method. The thermodynamic database of the Fe–Cr–Ni–C system was then established by extrapolating the description of the accepted ternary systems. No quaternary parameters were introduced in the present work. Good correspondence between calculated and experimental data has been achieved both in Fe–Ni–C and Fe–Cr–Ni–C systems. Eight as-cast alloys were studied to complement the solidification behaviors of the Fe–Cr–Ni–C system by X-ray diffraction analysis and scanning electron microscope. The solidification sequences and volume fractions of precipitated phases predicated from the modified Scheil-Gulliver solidification simulations agree reasonably with the presently measured data, which further validates the reliability of the present thermodynamic description.

Effects of Bi addition on solidification behavior of Mg–Al–Zn casting alloys using experiments and CALPHAD calculations

A comprehensive understanding of solidification behavior of Mg–Al–Zn–Bi alloys is necessary for designing novel casting Bi-modified AZ alloys. In this paper, 21 Mg–Al–Zn–Bi alloys were prepared to investigate the phase constituents, morphologies, grain sizes and area fractions of precipitates in as-cast microstructures, as well as their phase transition temperatures and solidification paths by using X-ray diffraction, scanning electron microscopy, optical microscopy and differential scanning calorimetric techniques. It was found that the Bi addition leads to the formation of α-Mg3Bi2 phase, which became coarse flake as Bi content increased. Besides, the Bi addition can modify the morphology of eutectic γ-Al12Mg17 phase and refine the grain size of primary phase. Moreover, the measured area fractions of precipitates increased steadily as Bi content increased, which were comparable with the model-predicated values. After that, the calculated liquidus projections in the Mg-rich corner of Mg–Al–Zn–Bi quaternary system were employed for demonstrating the effects of Bi addition on the solidification sequences. Additionally, the phase transition temperatures predicted via Scheil solidification simulations agreed well with measured values. The present work indicated that the combination of experimental investigations and thermodynamic calculations can serve as an efficient method to obtain accurate and comprehensive solidification behavior of Bi-modified Mg–Al–Zn alloys.

Thermal and microstructural characterization of a novel ductile cast iron modified by aluminum addition

In high-temperature applications, like exhaust manifolds, cast irons with a ferritic matrix are mostly used. However, the increasing demand for higher-temperature applications has led manufacturers to use additional expensive materials such as stainless steels and Ni-resist austenitic ductile cast irons. Thus, in order to meet the demand while using low-cost materials, new alloys with improved high-temperature strength and oxidation resistance must be developed. In this study, thermodynamic calculations with Thermo-Calc software were applied to study a novel ductile cast iron with a composition of 3.5wt% C, 4wt% Si, 1wt% Nb, 0–4wt% Al. The designed compositions were cast, and thermal analysis and microstructural characterization were performed to validate the calculations. The lowest critical temperature of austenite to pearlite eutectoid transformation, i.e., A1, was calculated, and the solidification sequence was determined. Both calculations and experimental data revealed the importance of aluminum addition, as the A1 increased by increasing the aluminum content in the alloys, indicating the possibility of utilizing the alloys at higher temperature. The experimental data validated the transformation temperature during solidification and at the solid state and confirmed the equilibrium phases at room temperature as ferrite, graphite, and MC-type carbides.

Effect of Fe on microstructure and properties of AlCoCrFexNi (x=1.5, 2.5) high entropy alloy coatings prepared by laser cladding

In this study, low-cost AlCoCrFexNi (x = 1.5, 2.5) high entropy alloy (HEA) coatings were prepared by laser cladding and their microstructural evolutions, mechanical properties and wear behaviors were investigated. Both Fe–Cr rich disordered BCC(A2) phase and Al–Ni rich ordered BCC(B2) phase were identified in these coatings. At x = 1.5, the coating consisted of B2 matrix and nano-scale A2 precipitates. While, at x = 2.5, A2 matrix with B2 precipitates formed. CALPHAD revealed that addition of Fe changed the solidification sequence of coatings, and was beneficial to the formation of A2 phase, resulting in different A2/B2 morphologies. AlCoCrFe2.5Ni coating exhibited higher strength and better wear resistance, due to the strengthening effect of B2 precipitates was stronger. It was found that the properties of coatings could be improved by regulating the A2/B2 morphology. Moreover, increasing Fe content while maintaining a morphology of A2 matrix with B2 precipitates was a strategy to design high strength HEA coatings.

Microstructure of Compacted Graphite Iron Near Critical Shrinkage Areas in Cylinder Blocks

To study the microstructure of CGI near areas prone to shrinkage, a special sample was developed that resembles critical areas in cylinder blocks. Foundry trials were conducted with different magnesium contents and inoculation amounts. Using color etching techniques to follow the solidification sequence, four areas were observed with different macrostructure: (i) direct chill and columnar region near the surface; (ii) small eutectic cells and small intercellular space; (iii) large eutectic cells and large intercellular space; and (iv) eutectic cells with carbides in the last to freeze area. By increasing the amount of inoculation, the size of the eutectic cells in the zone with large eutectic cells (iii) is reduced, and the eutectic cells are now smaller and more evenly distributed through the section. Increasing the magnesium content brings a similar effect. In zone (iv), the samples with less inverse chill formation show smaller shrinkage porosities, which are located in the same regions where the inverse chill are, between the eutectic cells, in the last to freeze areas; they also have higher amounts of spheroidal graphite in the last-to-freeze areas. The amount of inverse chill carbides observed in zone (iv) did not present any clear relation with inoculation levels or magnesium content variations used in the experiments.

Influence of cooling rate on microstructure development of AlSi9MgMn alloy

Aluminum alloys are widely applied in automotive, aircraft, food, and building industries. Multicomponent technical AlSi9MgMn alloy is primarily intended for high cooling rate technology. Controlled addition of alloying elements such as iron and manganese as well as magnesium can improve mechanical and technological properties of the final casting depending on the cooling conditions during solidification. The aim of this investigation is the characterization of AlSi9MgMn alloy microstructure and mechanical properties at lower cooling rates than those for which this alloy was primarily developed. Thermodynamic calculation and thermal analyses revealed solidification sequence in correlation to the microstructure investigation as follows: development of primary dendrite network, precipitation of high temperature Al15(Mn,Fe)3Si2 and Al5FeSi phases, main eutectic reaction, precipitation of intermetallic Al8Mg3FeSi6 phase, and Mg2Si as a final solidifying phase. Influence of microstructure features investigation and cooling rate reveals significant Al15(Mn,Fe)3Si2 morphology change from Chinese script morphology at low, irregular broken Chinese script morphology at medium, and globular morphology at high cooling rate. High manganese content in AlSi9MgMn alloy together with high cooling rate enables the increase of Fe+Mn total amount in the intermetallic Al15(Mn,Fe)3Si2 phase and encourage favourable morphology development, all resulting in enhanced mechanical properties in as-cast state.

Thermodynamic descriptions of ternary Al–Si–Sr system supported by key experiments

Abstracts All the experimental phase equilibria and thermodynamic properties in the Si–Sr and Al–Si–Sr systems were critically reviewed. Based on the reviewed experimental data, the thermodynamic descriptions of the Si–Sr binary system were updated by using the CALculation of PHAse Diagram (CALPHAD) technique, resulting in better agreement with the experimental data, especially for the standard enthalpies of formation, than those in the previous assessment. After that, the thermodynamic assessment of the Al–Si–Sr ternary system was carried out using the CALPHAD approach. Comprehensive comparisons indicate a good agreement between the calculated phase equilibria/thermodynamic properties and the experimental/first-principles data in Al–Si–Sr ternary system. Moreover, the as-cast microstructures of 4 Sr-additional Al–Si alloys were experimentally investigated by using the scanning electron microscope and electron probe microanalysis techniques. The solidification sequence and volume fractions of precipitated phases in the 4 alloys predicated from Scheil-Gulliver solidification simulations agree reasonably with the presently measured data, which further validates the reliability of the present thermodynamic descriptions.

Elucidation of the solidification sequence of a complex graphitic HSS alloy under a combined approach of DTA and EBSD analyses

In this work, a complex graphitic high-speed steel (Gr-HSS) obtained from a spin-casting process that allows high cooling rates during solidification is considered. Differential thermal analysis is carried out at a moderate and constant heating scan speed until complete remelting. This allows linking each of the peaks observed in the DTA curve to a related phase transformation that had occurred during the initial solidification in the spin-casting process. The phases that were formed during the spin-casting process, especially solidification carbides, are characterized through light microscopy (distribution) and scanning electron microscopy (morphology) either associated with EDS (chemical composition) or associated with EBSD (crystal lattice identification). A discussion is opened about the influence of solidification rates on both supersaturation and lattice distortions in the solidification carbides. The combination of reverse DTA and SEM/EDS–EBSD applied here to the complex Gr-HSS alloy constitutes a relevant approach to understand the phase transformations during complex casting processes where relatively high solidification rates are achieved.

CALPHAD assessment of bio-oriented Ti–Zr–Sn system and experimental validation in Ti/Zr-rich alloys

The development of CALPHAD-type thermodynamic database for Ti or Zr based biomedical alloys has been spurred by the increased interest in efficiently tailoring an alloy composition to obtain high stability of β_bcc, low Young's modulus, and free of detrimental phases. However, the thermodynamic prediction is not adequate to be performed without the information of key sub-ternary Ti–Zr–Sn system. In present work, the thermodynamic assessment of Ti–Zr–Sn system is performed via a critical evaluation of phase equilibria and microstructure development in this ternary system. The partial isothermal sections at 1323 K and 1473 K with Sn content below 40 at. % are obtained by analyzing chemical compositions and crystal structures of individual phases in the annealed alloys. The composition homogeneity range of most phases is validated to favor a ternary extension paralleling to the Ti–Zr axis. Particularly, β_bcc and η phases (with the chemical composition (Ti, Zr)5Sn3+x) show complete solubility of Ti and Zr from Ti–Sn edge to Zr–Sn edge. With the database, negligible ternary solubility of Zr4Sn phase, microstructure development in the as-cast samples, and the controversial conclusions in literature are discussed. Most of the experimental findings, including equilibrium phase constitution, solidification sequence, DSC signals, projections of liquidus, are reproduced in a self-consistent way. The work moves towards the completeness of multi-component Ti/Zr thermodynamic database. It can be used for composition design of novel metastable β-type biomedical alloys.

Study on high temperature solidification behavior and crack sensitivity of Fe-Mn-C-Al TWIP steel

Fe-Mn-C-Al alloy is a new steel grade of TWIP steel developed in recent years. It has an excellent combination of elongation and tensile strength, as well as good anti-delayed fracture property. However, the crack sensitivity of this new TWIP steel has not been reported yet. In this study, differential thermal analysis (DTA) method was used, combined with professional thermodynamic software ThermoCalc to analyze the solidification behavior for Fe-Mn-C-Al alloys with different chemical compositions. Based on this, the crack sensitivity of TWIP steel is further determined. Through this study, it was found that Fe-Mn-C-Al TWIP steel may have a solidification sequence with high crack sensitivity, belonging to hypo-peritectic steel. Moreover, it was found that the carbon content has a large influence on the solidification behavior, and the manganese content also affects the solidification sequence. It can make the phase transition sequence of the solidification process change significantly, which may avoid the solidification behavior of hypo-peritectic reaction. The analysis results by thermodynamic software ThermoCalc are in good agreement with the experimental results. It displays thermoCalc can be a cost-effective way to develop Fe-Mn-C-Al TWIP steel. It is of great significance for shortening the development period of new Fe-Mn-C-Al steel grades.

Solidification sequence and as-cast microstructure of a model Pb-Bi-Sn alloy with quasi-peritectic reaction

The Pb39Bi25Sn36 alloy was directionally solidified at varied withdrawal rates followed by quenching. From metallographic observation, composition analysis and differential scanning calorimetry (DSC) analysis, the solidification sequence of Pb39Bi25Sn36 alloy was characterized. The Sn-dendrite is observed as the primary phase during solidification, followed by the β(Pb7 Bi3)/Sn divorced eutectic. At lower temperatures, the β(Pb7Bi3)/Sn binary eutectic is formed in the interdendritic region. However, the quasi-peritectic reaction [L+α-Pb→β(Pb7 Bi3)+Sn] does not occur as the prediction based on ternary phase diagram analysis, which can be attributed to the unequilibrium solidfication condition. Moreover, the as-cast microstructure is significantly refined with the increasing withdrawal rates. The quenched sample only exhibits the lamellar structure of Sn, β(Pb7Bi3) and Bi phases. The results can provide instructions to optimize the as-cast microstructure of ternary alloys with quasi-peritectic reaction.

A Combination of Keyhole GTAW with a Trapezoidal Interlayer: A New Insight into Armour Steel Welding

The ballistic performance of armour steel welds using austenitic filler materials is poor on account of the disparity in the mechanical properties of the weld and base metals. Consequently, a novel Keyhole Gas Tungsten Arc Welding process with a trapezoidal AISI309 austenitic stainless steel interlayer was developed to tailor chemical composition and microstructure by controlling the solidification sequence. Results show that the dilution rate in the weld metal region can reach up to 43.5% by placing a specially designed interlayer in between the base metal, providing a major scope for microstructure modification. Detailed weld analysis was undertaken by X-ray diffraction, optical and secondary and transmission electron microscopy, energy dispersive spectroscopy and electron back-scattering diffraction. The results from Vickers hardness indents and Charpy impact toughness testing at −40 °C show that the properties of the weld metal region are comparable to that of the base metal. This is ascribed to the weld metal comprising a two phase microstructure of martensite and retained austenite, which contribute to improvements in strength and toughness, respectively. Furthermore, the tailored chemical composition, microstructure and low temperature phase transformation in the weld metal may reduce the tendency toward both solidification cracking and hydrogen assisted cold cracking.