Engineering Heterogeneous Nucleation During Solidification Of Multiphase Cast Alloys: An Overview

The theory of heterogeneous nucleation was initially developed as a part of condensed matter physics, and later it was used as an important engineering tool to design metallurgical processes. This success has led to wide applications of the theory in metallurgical practice. For example, engineering heterogeneous nucleation in ductile iron has been used to reduce shrinkage defects, suppress cementite formation, and modify the size and shape of microstructural constituencies. This demonstrates how theoretical knowledge could benefit industry practice. This overview aims to summarize the authors\u27 published studies in co-authorship with colleagues and students, which covers different aspects of engineering heterogeneous nucleation in multiphase cast alloys. Several approaches for engineering heterogeneous nucleation using thermodynamic simulation as well as practical methods for improving efficiency of nucleation using the co-precipitation technique and a local transient melt supersaturation are suggested. Automated scanning electron microscopy/energy-dispersive X-ray (SEM/EDX) analysis and high-resolution transmission electron microscopy (TEM) were used to verify the simulation predictions. Practical examples of controlling microporosity shrinkage in cast irons with spheroidal graphite are presented to illustrate the power of engineering heterogenous nucleation

Microgravity noncontact temperature requirements at NASA Lewis Research Center

NASA Lewis Research Center is currently supporting 66 microgravity science and applications projects. The 66 projects are separated into 23 flight projects and 43 ground-based projects. The part of the NASA Lewis program dealing with flight experiments is divided into six areas: Combustion Science, Materials Science, Fluid Physics, Instrumentation/Equipment, Advanced Technology Development, and Space Station Multi-User Facility studies. The part of the NASA Lewis program dealing with ground-based experiments is coincidentally also divided into six areas: Electronic Materials, Combustion Science, Fluid Dynamics and Transport Phenomena, Metals and Alloys, Glasses and Ceramics, and Physics and Chemistry Experiments. Several purposes exist for ground-based experimenting. Preliminary information is necessary before a decision can be made for flight status, the short low gravity durations available in ground facilities are adequate for a particular study, or extensive ground-based research must be conducted to define and support the microgravity science endeavors contemplated for space. Not all of the 66 microgravity science and application projects at NASA Lewis have temperature requirements, but most do. Since space allocation does not permit a review of all the pertinent projects, a decision was made to restrict the coverage to the science flight projects, flight projects minus the advanced technology development, and multiuser facility efforts. Very little is lost by this decision as the types of temperature requirements for science flight projects can be considered representative of those for the ground-based projects. The noncontact temperature needs at NASA Lewis, as represented by the science flight projects are discussed by describing briefly the experiments themselves, by displaying an illustration of each experimental setup, and by specifying their temperature requisites

From atom scale to casting : A contemporary monograph on cast irons microstructure

The origin of this monograph lies in a simple fact: there is a paradigm with cast irons, namely that these alloys have been produced and cast for thousands of years, yet they are among the most complicated alloys if we consider the formation of their microstructure by solidification and transformations in the solid state. The positive side of this complexity is that it offers a wide range of possibilities for manipulating their microstructure. With the evolution of melting furnaces in the 19th century, the silicon content of cast irons increased, leading to the development of the silicon cast irons that are the subject of this monograph. The essential step, however, was the discovery in the middle of the 20th century that it is possible to change the shape of graphite by transforming the interconnected lamellae into discrete spheroids. In this way, cast iron became a material for safety parts and was no longer limited to construction. This historical development and the research effort during the first part of the 20th century have been described in reviews published in the 1960s. Since then, developments in metallographic analysis have led to a wealth of research aimed at describing and understanding graphite formation and the microstructure of cast irons during solidification and heat treatment. This monograph is not intended to be an exhaustive review of the literature of the last 50 years, but aims to propose a coherent vision of the formation of the microstructure of cast irons based on the work carried out jointly or in parallel by its authors. Current controversies are sometimes indicated but are not discussed in order to emphasize open questions. Finally, we should mention that our work has greatly benefited from the dynamism of the European Cast Iron group (ECI), an informal forum open to all (European academics and industrialists) for more than 10 years. It is in this same spirit of discussion that this monograph is freely available. \\// Cette monographie trouve son origine dans un fait simple : il existe un paradigme avec les fontes, à savoir que ces alliages sont produits et coulés depuis des milliers d'années, mais qu'ils comptent pourtant parmi les alliages métalliques les plus compliqués si l'on considère la formation de leur microstructure par solidification et transformations à l'état solide. Le côté positif de cette complexité est qu'elle offre un large éventail de possibilités pour la manipulation de leur microstructure. Avec l'évolution des fours de fusion au 19ème siècle, la teneur en silicium des fontes a augmenté conduisant au développement des fontes au silicium qui font l'objet de cette monographie. L'étape essentielle, cependant, a été la découverte au milieu du 20ème siècle qu'il est possible de changer la forme du graphite en transformant les lamelles interconnectées en sphéroïdes disjoints. Les fontes sont ainsi devenues un matériau pour pièces de sécurité et n'ont plus été limitées à la construction. Cette évolution historique et l'effort de recherche durant la première partie du 20ème siècle ont été décrits dans les revues publiées dans les années 1960. Depuis cette époque, l'évolution des analyses métallographiques a généré nombre de recherches destinées à décrire et comprendre la formation du graphite et de la microstructure des fontes lors de leur solidification et des traitements thermiques. Cette monographie ne se veut pas une revue exhaustive de la littérature de ces 50 dernières années, mais vise à proposer une vision cohérente de la formation de la microstructure des fontes graphitiques basée sur les travaux conduits en commun ou en parallèle par ses auteurs. Les controverses actuelles sont quelquefois indiquées mais ne sont pas discutées afin de mettre l'accent sur les questions ouvertes. Enfin, nous avons plaisir à mentionner que notre travail a largement profité du dynamisme du groupe européen de la fonte (ECI), forum informel ouvert à tous (universitaires et industriels européens) depuis plus de 10 ans. C'est dans ce même esprit de discussion que cette monographie est en accès libre

Evaluating the Effects of Scan Strategy on AM Annealed Fe-3Si steel through Understanding of Solidification Conditions and Thermal Stresses

Soft magnetic steels have seen recent adoption in additive manufacturing (AM) due to the prospect of reducing eddy currents and hysteresis losses through leveraging of complex geometries and microstructural control. An annealing step will be a significant step for these alloys produced in AM to increase grain size and further reduce hysteresis losses. In this study, thin wall Fe-3Si samples were produced using laser powder bed fusion (L-PBF) using two different scan strategies, with a subset of samples annealed at 1200°C for 5 minutes. The effects of the two different scan strategies on microstructure in the as-built and annealed samples were analyzed through EBSD where it was found that the scan strategy does have an effected on annealed microstructure. Thermal simulations using OpenFoam were used to rationalize the differences in microstructure formation between the two scan strategies for the as-built scan strategies by looking at the thermal gradients and solidification velocity, while explanations on why there is a difference in resulting annealed microstructure was made by looking at the grain orientation, size and misorientation. Further, thermal-mechanical simulations were conducted using Abaqus to see if differences in the resulting elastic and plastic strains due to differences in thermal stresses related to the two difference scan strategies could be a mechanism causing differences in annealed microstructure to occur

Rapid solidification of zinc based alloys

This work presents an investigation into two areas : the development of zinc based brazing alloys for the joining of copper produced by the rapid solidification technique; the study of the development of microstructures in rapidly solidified dilute zinc alloys. A new model has been proposed, based on the present experimental results and the available data from the literature, to account for the ribbon formation mechanism. Within this model ribbon formation in melt spinning is mainly determined by the behaviour of the viscosity in undercooled melts. It has been shown that, contrary to previous studies, the present model can be used to predict the ribbon thickness of both crystalline and amorphous alloys, and the agreement with the measured values is excellent. The development of microstructures in dilute zinc alloys during rapid solidification has been studied by the deliberate addition of impurity elements. Significant differences in structures are observed between the dilute zinc alloys and high purity zinc. The high purity zinc exhibits a strong preferred orientation with the basal plane parallel to the ribbon surface. The severity of this texture markedly reduces with the formation of cellular substructures in dilute zinc alloys. The morphology of the cells depends on the type of impurity elements present. The presence of impurity elements which expand on solidification leads to the development of an eutectic-like structure consisting of regular lamellae. This requires reorientation of the basal plane and instability of the solid liquid interface during solidification. The instability conditions of the planar solid liquid interface during rapid solidification of these alloys have been examined by using the morphological stability criterion. A possible mechanism which accounts for the formation of unusual structures observed in rapidly solidified dilute zinc alloys has been proposed. The observed variety of microstructures of binary Zn-Mg alloys, and the structural transitions across the ribbon thickness have been reported. Comparisons are made between as-cast and rapidly solidified materials and the resulting structures have been described using a growth rate composition map. Rapidly solidified eutectic and hypereutectic alloys show a tendency to form an amorphous phase. A total of more than 50 zinc based alloys have been investigated for the development of brazing filler metal. Alloys have been examined, and their compositions optimized, in terms of their spreading and wetting abilities and brazing performances to obtain a suitable candidate. Based on the experimental results it has been suggested that Zn-Mg alloys could be used for the joining of copper. They exhibit a lower surface tension, a lower density and comparable mechanical properties to high temperature high strength silver based brazing alloys

Grain refinement of high alloy stainless steels in sand and directionally solidified castings

The goal of this research project is to develop an industrially viable melting process that will control the crystallization macrostructure of austenitic grades of cast steels. Titanium nitride (TiN) has proven to be an effective grain refiner of austenite. Theoretical simulation and experimental application has led to the development of a repeatable grain refining melt process for austenitic stainless steel alloys. Grain refinement of the as-cast structure of Cr-Ni stainless steel alloys solidified with primary FCC, BCC and dual FCC/BCC phases was studied experimentally. Refinement was achieved in both cast ferritic and austenitic grades. Dual solidification of FCC/BCC phases resulted in an unrefined macrostructure. It is proposed that solidification sequence can limit the grain refining capability of heterogeneous nuclei. Two inoculation-based melt practices were developed to study grain refinement in cast austenitic stainless steels. The first includes in-situ formation of TiN on to Mg-Al spinel oxides, and the second involves master alloy additions containing preformed TiN. The master alloy method extended the equiaxed zone and improved the distribution of TiN in the casting. The in-situ method showed more effective grain size refinement. The effect of the developed grain refining melt practice on the properties of cast superaustenitic stainless steel (similar to CK3MCuN) was examined. Heat treatment had no effect on the as-cast grain size. The grain refined alloy exhibited a reduction in segregation after heat treatment; an increase in ultimate tensile strength (+11%), yield strength (+13%), ductility (+8%), hardness (+2%), pitting corrosion; a decrease in impact strength and intergranular corrosion rate in comparison to the unmodified, base alloy --Abstract, page iv

Novel Processing Methods and Mechanisms to Control the Cast Microstructure in Al Based Alloys - 390 and Wrought Alloys

The enactement of the Energy Policy and conservation Act of 1975, led to a paradigm shift in material selection and design in the automotive industry. The net effect was an increased focus by the automotive industry toward the use of light metals leading for the reduction of weight and hence, the dependence of imported oil. Increasing use of aluminum was a transition in that direction. However, raw aluminum on an average is 1.5 - 2 times as expensive as steel. Near net shape manufacturing processes (Die casting, Thixo-forging, etc) provided the much needed competitive advantage vis-à-vis steel / iron parts by permitting the manufacturing of Al components. Semi solid processing involves the net shape manufacturing of alloys in a two phase region (liquid + solid). The reduced turbulence (during casting), less entrapped gases and lower operating temperatures (processes below the liquidus) make semi solid processing ideal for the manufacturing of high integrity Al parts. Traditionally, semi solid processing involved the heating of billets to a two phase region (called Thixcasting). Rheocasting is a new semi solid processing technique wherein the alloy is cooled from a liquid state (a combination of controlled heat / nucleation and growth phenomena) to yield structures similar to the Thixocasting process. Rheocasting or Slurry-On-Demand is in its early stages of development (the first industrial prototype of rheocasting was invented in the late 1990\u27s) and forms the central point of interest in this work. Much research is underway around the globe to understand the controlling mechanism as well as the structure - property relationships in rheocast parts, primarily limited to the hypoeutectic Al-Si alloys (less than 12.6% Si). This work is dedicated in the development of novel methods for the rheocasting of hypereutectic Al-Si alloys (greater than 12.6% Si) and Al based wrought alloys (alloys with Cu, Zn, Mg and Si as alloying elements). The thesis presents the problems associated with microstructure control of hypereutectic Al-Si (primary Si coarsening and accelerated growth) and Al based wrought alloys (dendritic structures and hot tearing) with currently available technologies. Novel processing techniques are presented for the casting of hypereutectic Al-Si alloys and Al based wrought alloys with a combination of industrial trials and a through analysis of the underlying mechanisms

Metallurgical and statistical approaches to the study of cast iron street furniture

The evolution of microstructure in relation to dating and nationality of origin was investigated in twenty-four cast iron objects of street furniture produced between XIX and XX centuries in United Kingdom, France, and Italy. Chemical composition of the metalworks was evaluated by glow-discharge optical emission spectrometry. Fragments from the cast irons were analyzed by optical microscopy and scanning electron microscopy with energy-dispersive spectroscopy. Form, distribution, and size of graphite were evaluated in the microstructure according to standard EN ISO 945-1:2008. An image analysis software was employed to quantify the area fraction of graphite in the matrix, major axis, and shape factor of graphite lamellae, area fraction of manganese sulfides (ψS), area fraction of steadite, and number of eutectic cells per area unit. All data were grouped and linear discrimination analysis (LDA) was applied to assess the group assignment and the probability of correct classification for each metalwork. The results showed that the microstructural features were compatible with those of cast irons produced in the XIX and XX centuries. Values of ψS also suggested re-melting of cast irons, associated with recycling of cast iron and/or steel scraps. The high values of steadite found in the metalworks are probably due to the excellent castability required for complex shape castings in these centuries. The LDA multivariate analysis allowed to discriminate cast irons based on the year of manufacturing and the nationality of origin

Controlling tin nucleation and grain orientations in Pb-free solders

Lead-free solder joints made with Sn-Ag-Cu or Sn-Ag solders usually contain only a single βSn grain or three twinned βSn grains which are oriented differently in every joint. Due to the anisotropy of βSn every joint therefore has unique thermomechanical properties and, in an array of numerous joints, it is likely that some will be poorly oriented and could cause early failure of a component. The problems of few grains with variable orientation can be attributed to the nucleation of βSn. This thesis explores catalyzing βSn nucleation using heterogeneous nucleation and develops methods to control βSn microstructures and orientations in Sn-3Ag-0.5Cu/Cu solder joints. The developed droplet nucleation technique in this study provides a new ‘motorway’ for heterogeneous nucleation study, and the introduced orientation control method paves the way to solve chronic problems, such as electromigration, thermomechanical fatigue, and shear fatigue in the electronic packaging industry. It is found that Co additions and Co substrates are effective at catalyzing βSn nucleation. This is demonstrated to be due to heterogeneous nucleation on αCoSn3 crystals. By using a ‘droplet nucleation technique’, in which Sn droplets are solidified directly on an intermetallic compound (IMC) particle, it is proved that αCoSn3 forms a reproducible orientation relationship (OR) with βSn that has a good lattice match. Strong grain refinement occurred in 60g Co-microalloyed Sn-3.0Ag-0.5Cu samples but only weak grain refinement occurred in 550μm solder balls and joints. When soldering Sn-3.0Ag-0.5Cu on Co substrates, βSn is always observed to grow from the interfacial αCoSn3 layer with an orientation inherited from the αCoSn3 layer texture. However, it is shown that nucleation on the αCoSn3 layer does not give useful βSn orientation control. The droplet nucleation technique was then applied to gain a deeper understanding of βSn nucleation on a range of IMC phases. A family of transition metal stannides, PtSn4, PdSn4, and βIrSn4 that have similar crystal structures to αCoSn3, were identified as potent nucleants for βSn. The common solder IMCs, Cu6Sn5, Ag3Sn, Ni3Sn4, were also investigated. It was found that reproducible ORs formed on all IMCs studied and the nucleation mechanisms were explored by combining nucleation undercooling measurements with measured ORs. The nucleation potency of all studied intermetallics is: αCoSn3>βIrSn4 >PdSn4>PtSn4 >Ni3Sn4> Ag3Sn, Cu6Sn5. The droplet nucleation technique also generated new insights into solidification twinning in solder joints. It was found that cyclic twins formed in droplets when the undercooling was sufficiently high and the liquid contained Ag, Cu and/or Ni. Complex interrelated cyclic twins were found in droplets on Cu6Sn5, Ag3Sn, and Ni3Sn4 where up to five rings of cyclic twins formed each related by a common . The twinning mechanisms in these cases were explored and discussed. The thesis then applies the new understanding developed in the previous chapters to develop a technique to reliably control the orientation of βSn in solder joints. Ball grid array (BGA) joints were fabricated reproducibly by introducing an extra step into the manufacturing process: bonding a nucleant IMC ‘seed crystal’ onto each Cu pad so as to control the nucleation location, nucleation undercooling and crystallographic orientation of βSn at the moment of nucleation. Each joint made by this technique had a uniform single-grain microstructure with the c-axis of βSn parallel with the substrate plane. This orientation is reported in the literature to give the best resistance to electromigration and shear fatigue.Open Acces