Our results will help guide the design of desired microstructures and post-thermal treatment for improved materials performance, providing an accelerated path to practical applications. Mission Relevance Advanced materials characterization coupled with validated predictive modeling will provide the fundamental knowledge of microstructure evolution under extreme conditions. This capability supports the advanced materials and manufacturing core competency for the Laboratory's strategy relevant to addressing scientific and engineering challenges of accelerating the design, fundamental understanding, and deployment of new materials and manufacturing processes.
In addition, this effort contributes to stockpile stewardship science to meet NNSA's current and future national security requirements related to the design of materials with tailored properties as part of cost-efficient, timely approaches to warhead life-cycle support with new capabilities. FY16 Accomplishments and Results During FY16, our experimental effort focused on alloy systems with lower melting temperatures to facilitate data acquisition and provide critical input data to modeling efforts see figure.
A time-resolved image sequence acquired with the dynamic transmission electron microscope DTEM during rapid solidification of a copper—nickel alloy thin film is shown at top. From measurements of the evolution of melt pool area and size with time, the solidification velocity can be calculated and related to the observed microstructure evolution middle. The high-energy density sources have been used by the electronics industry because of the high degree of control and the resulting narrow tolerances. By exercising a careful control on the energy input, it is possible to heat- treat the surface without melting it.
The phenomenon of nucleation during solidification The occurrence of nucleation during solidification is a thermally activated process that involves a fluctuation growth in the sizes of clusters of the solid [80, 81]. Changes in size of the cluster are considered to occur by a single atom addition or by a removal exchange between the cluster and the surrounding undercooled liquid. For the smallest sizes, the clusters are referred to as embryos. These are more likely to dissolve than grow into macroscopic crystals. Homogeneous nucleation For the case of homogeneous nucleation, the solid forma- tion occurs without an involvement of any extraneous impurity atoms or other surface sites in contact with the melt.
As a simplification, only the case of isotropic inter- facial energy is considered. However, it is recognized that anisotropic behavior yields faceted shapes for the clusters. In this equation, AG V is volume free energy change, AT the degree of undercooling, T f the equilibrium temperature in the furnace, A H f the latent heat of solidification, and V M the molar volume.
However, a stable growth of the nucleus occurs when the cluster size exceeds r cr. To over- come this barrier, classical theory predicts that large und- ercooling values are required. However, in practice, an undercooling of only a few degrees or less is the common observation with most castings. This behavior is accounted for by the operation of heterogeneous nucleation in which foreign bodies such as impurity inclusions, oxide films, and walls of the containing crucible promote crystallization by lowering the critical free energy required for nucleation.
Since only a single nucleation event is required for the freezing of a liquid volume, the likelihood of finding a heterogeneous nucleation site in contact with the bulk liquid is appreciable. In conventional metal casting crystal, growth and solidification are functions of atomic mobility. Both ther- mal and kinetic factors must be considered when deter- mining whether crystal growth will be inhibited or accelerated. Whether spherical or needle-like in configu- ration, the metal particles tend to behave differently depending on their location within the composition, i.
While metals such as aluminum and copper have one crystal structure at all temperatures face centered cubic , few other metals such as iron, cobalt, and titanium have different crystal structure at different temperatures for example, iron is fee at elevated temperatures and bcc at room temperature. The higher the melting point of the metal, the larger is the latent heat of fusion. Therefore, solidification processing is governed by the nature of heat extraction from the cooling liquid.
During cooling, most metals tend to shrink as they gradually solidify. The shrinkage results in the formation of micro- porosity, voids, and shrinkage cracking during solidifica- tion. A conjoint influence of all these intrinsic aspects does exert its influence on heterogeneous nucleation of crystals. Springer J Mater Sci The thermal contraction of the solid that occurs during subsequent cooling tends to increase the risk of shrinkage if adequate care is not taken during casting of the metal , The kinetics governing nucleation and crystal growth An important yet difficult problem in the study of phase transitions and particularly those that take place during rapid solidification is an in-depth analysis of the first stages of the transformation.
The new phase forms or initiates by a clustering or aggregation process that produces a definite distribution of nuclei having a variety of sizes. Some of the nuclei are large enough called critical that they can resist the destabilizing effects of the surface energy and can subsequently grow by incorporating additional molecules.
Initially, the smaller nuclei decrease their free energy by re-dissolving in the parent matrix phase . The pres- ence of impurities and foreign substrates plays an impor- tant role in initiating nucleation. The presence of foreign substrates, by reducing the destabilizing influence of sur- face energy, allows the smaller clusters to become critical and initiate the growth process.
In impure metals, little chemical driving force is essential to initiate the nucleation stage of the phase transformation. A necessary requirement for the occurrence of clustering is the existence of adequate supercooling in the melt to act as a driving force for the nucleation process. It is, therefore, required to determine the maximum degree of undercooling that a given melt can be subjected to. Physically, the melt must be supercooled to the point where homogeneous nucleation i. The formation of metallic glass The problems related to nucleation and subsequent growth can be related to the process of formation of glass from a cooling melt.
When the cooling rate imposed on the ori- ginal melt is sufficiently high such that there is not enough time for the nucleation and growth processes to take place, the melt will not crystallize but the molecules will become more rigidly bound and eventually solidify without the formation of crystalline phases . Researchers working on oxide glasses first established this phenomenon.
How- ever, it is the formation of metallic glasses by rapid solidification that attracted much attention to this technique . The unexpected properties shown by metallic glasses led to an increased interest in the use of RSP . Interface stability and solute redistribution Upon the formation of a stable crystalline nucleus in a supercooled melt and concurrent growth of the crystal, another important problem surfaces.
This relates to whether the moving solid-liquid interface will remain flat and sta- ble or if it will develop morphological instabilities . An understanding of this is important since it can be related to the microstructure developed in the solidified product. Even pure materials have the tendency to develop side- branching instabilities during solidification when the melt is rapidly undercooled [18, 19].
Rapid Solidification Processing: Melt Spinning of Al-High Si Alloys
During RSP of alloys, the metastable supersaturated solid phases are frequently obtained . The supersatu- rated phases result when the solidification rate is so large that there is not enough time for the occurrence of required solute partitioning that is commonly observed under conventional solidification conditions.
An analysis of solute segrega- tion during directional solidification allows for the cal- culation of an instantaneous concentration field in the solid and liquid phases both during and after solidification . A key assumption underlying these studies related to the solute redistribution problem is that of local equilib- rium at the solid-liquid interface, that is, the equilibrium phase diagram gives the concentration of solute on both the liquid and solid sides of the interface. Although the existence of local equilibrium at the interface is appro- priate for a wide range of cases in conventional solidifi- cation processing, the concept may not hold under rapid solidification conditions.
Early experimental observations of the solute-trapping phenomenon provided the desired incentive for a fresh look at the thermodynamics of solidification . A careful consideration of the free energy-composition diagram for a binary system reveals that solid phases having a wide range of solute content can be produced from the melt of a given composition. Further, a solid with a maximum amount of the solute can be produced by rapid solidification of a liquid with composition given by the intersection of the free energy curves of the solid and liquid phases in the phase diagram.
Early estimates of the solute redistribution during rapid solidification were obtained from a simple model of understanding the solidification kinetics of an undercooled binary alloy melt . The early model provided one of the first quantitative explanations for the occurrence of solute trapping in metal-based alloys. In this model, the solidifi- cation process is considered to take place in three distinct stages . As a direct result of Springer J Mater Sci the rapid freezing, the sample temperature rises from the nucleation value to the solidus. Under these conditions, the concentration of solute at the interface is estimated from the phase diagram of the metal or alloy.
Rapid solidification processing of constituents of a composite materials The RSP methods, currently utilized for the manufacture of advanced metallic, intermetallic, and the emerging gener- ation of discontinuously reinforced metal-based compos- ites, are grouped according to the temperature of the metal during processing.
The solid phase processes The fabrication of reinforced metal matrices, particularly particulate reinforced, from bonded elemental powder involves a number of stages prior to final consolidation. The salient attributes of three key processes, i. Powder metallurgy processing Solid phase processes essentially involve a thorough blending of powders of the rapidly solidified alloy with the reinforcing phase, i.
The sequence of steps includes the following: a sieving of the rapidly solidified particles; b precision blending of the reinforcement, i. Secondary fabrication of the composite billet normally involved all other applied metal working practices [ 94]. With time, this technology has been extended and developed to various degrees of commercial success by few other manufacturers.
An important step in the manufacturing sequence of discontinuously reinforced metal matrices involves a proper selection of the discontinuous reinforcement and the matrix alloy. The key criteria for the selection of a reinforcement phase includes the following : elastic modulus, tensile strength, density, melting temperature, thermal stability, compatibility with the metal-matrix, thermal coefficient of expansion, size and shape, and cost. The most widely chosen and used metal matrices and ceramic reinforcements for the synthesis of metal- matrix composites MMCs are summarized in Table 2.
The properties of the reinforcement are summarized in Table 3. The blend is then consolidated into billets by cold compaction, Fig.
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The cold compaction density is controlled in order to maintain an open, interconnecting porosity. This is essential during the following stage of degassing, or out- gassing, during the pressing operation. The process of outgassing involves the removal of: a the adsorbed gases, b chemically combined water, and c other volatile species, through the synergistic action of heat, vacuum, and inert gas flushing .
An advantage of the PM technique is an ability to use the improved properties of rapidly solidified powder technology for the manufacture of MMCs . In the process developed by the Advanced Composite Materials Corporation ACMC to make, discontinuously reinforced metal-matrix composites DRMMCs , the atomized metal powders, and the reinforcing particulates are mixed using proprietary techniques.
A careful control of the processes is critical for the production of homoge- neous, high performance composite materials. The metal powder-reinforcement particulate mixture is then loaded into a die, degassed, and subsequently vacuum hot pressed at a temperature above the solidus temperature of the matrix alloy. The presence of molten metal during hot pressing allows for a full densification of the billet at moderate pressure, even when small high surface area reinforcements are used . Springer J Mater Sci The high-energy rate processes An approach that has been successfully utilized to con- solidate rapidly quenched powders containing a fine dis- persion or distribution of ceramic particulates is known as the high-energy rate processing [98, 99].
In this technique, the consolidation of the metal-ceramic mixture is achieved using high energy and at high rates from a homopolar generator. The homopolar generator converts the stored rotational kinetic energy into electrical energy using the Faraday effect. In this particular process, the degassed powder is subjected to a pulsed high current discharge under low pressure. The current pulse is high, reaching a maximum of 1.
At the onset of the pulse, pressure is stepped up with the primary objective of ensuring electrical conductivity to the powder mixture and to concurrently aid in smooth and easy consolidation of the powder mixture by applying a mild forging. The high-current pulse is transmitted between two rams that are driven by a hydraulic press. A pair of floating plungers made from copper then compresses the metal-ceramic powder reinforcement mixture.
The cop- per plungers serve as an effective heat sink to accelerate rapid cooling of the consolidated powder by conduction . The ultimate bonding, which occurs during consoli- dation, results from the pulse resistive heating that is pro- duced at the inter-particle surfaces. The hot and rapidly applied thermal cycle facilitates in consolidation of the powder mixture, without the occurrence of coarsening, when compared one-to-one with the conventional consol- idation techniques. Dynamic powder compaction Refined and at times required metastable microstructures, in metal-based and intermetallic-based systems, can be obtained by the use of rapid solidification techniques.
The consolidation step in the process involves prolonged ther- mal exposure with a resultant coarsening of the fine micro structural features. A method, which safely avoids such thermal excursions and concomitant microstructural changes, is dynamic powder compaction DPC [, ]. The dynamic compaction and static compaction processes differ in the following : i the speed at which the compaction process occurs; ii the physical mechanisms taking place at the very microscopic level.
In the static compaction process, a relatively uniform pressure is applied and either concurrently or subsequently a high temperature is imposed to initiate and bring about long-range diffusion. This is essential for the following reasons: a causing and promoting particle bonding while con- currently either eliminating or minimizing porosity; b to enable the metastable material to decompose. The dynamic compaction occurs by the passing of an intense shock wave from one side of the material to the other side while being compacted.
Very high pressures are produced and the resultant densities facilitate little or no requirement for the occurrence of long-range diffusion. Further, the occurrence of localized shearing between the particles results in excessive heating of the surface. This facilitates in promoting and enhancing an effective bonding between the powder particles.
The dynamic powder com- paction technique is useful for the purpose of compaction of thermally unstable materials. This technique is rapidly growing and commercially viable since it offers the potential for exploiting all of the benefits of rapid solidi- fication technology . Consolidation processes Clearly, the rapidly solidified particulates must normally be consolidated into fully dense bodies before they can be of any commercial interest.
The consolidation sequence for rapidly solidified PM particulates essentially consists of the following i cold pressing, ii degassing, and iii hot consolidation, followed by iv hot working step see Table 4. This allows for an effective evacuation of the volatile species in the powder. In general, the processes for consolidation and densification are grouped together as follows . Forging and rolling: in order to be able to handle the end-product, these methods require the preparation of a preform either by hot mechanical pressing, cold pressing, and sintering, or some other practical means.
Hot isostatic pressing HIPing : involves placing the powder in a container that plastically deforms at the consolidation temperature. The powder is then brought up to temperature with subsequent compression under a high-pressure fluid usually argon. This results in a fully dense piece, but the presence of oxide particles at the prior particle boundaries does severely limit the utility of this process to aluminum alloys. Pseudo-hip processes: because of the high costs and long cycle times associated with conventional HIPing, a number of pseudo-HIP processes have emerged .
However, the crucible ceramic mold process makes use of conventional HIP equipment. In order to compact the Al-Mg-Si alloy , the powder is cold pressed into pellets prior to consolidation. A large pressure is applied in a time span of few seconds using a computer controlled ram . Hot extrusions and other hot working processes: the powders are packed into containers, sealed, evacuated, and then hot extruded. The transfer of PTM aluminum is carried out in ambient air. Dynamic consolidation methods, such as explosive and other high-energy rate processes: these employ high pressures produced by the impact of fast moving punches or shock waves to i cause densification of the particulates, and ii promote interparticle bonding .
During dynamic compaction, the presence of large amount of energy at the powder particle contact points is conducive for the initiation of surface melting and inter-particle bonding. Each of these approaches has some noticeable disad- vantages, but all have been utilized to varying degree to produce materials having an attractive combination of properties.
The particular process, which has been most widely used and for which the largest database is available is hot extrusion. The salient advantages of the hot extrusion process lie in its ability to provide large amounts of mechanical working while concurrently yielding a fully dense product. During the extrusion process, large hydro- static compressive forces coupled with a normal force component make the preform flow through a die. The shear component that results form frictional forces in the deforming metal takes up one-half of the total energy required for extrusion.
The extrusion process provides far more plastic deformation than any other single metal working step [, ]. The three basic methods most commonly used for the extrusion of metal powders are : a filling the extrusion container with loose metal powder and then extruding; b cold or hot pressing coupled with sintering the powder immediately followed by extrusion; c cold pressing in a can followed by extrusion of both the can and powder.
When using the first two methods, alloys sensitive to oxidation i. Of the several techniques that have been developed and implemented during the recent decade, the technique of spray processing offers an opportunity to synergistically utilize the benefits associated with fine particulate technology. A few of the notable benefits include: a refinements in intrinsic micro structural fea- tures, b modifications in alloy chemistry, c improved microstructural homogeneity, d in situ processes and, in a few cases, e near-net shape manufacturing.
In the time spanning the last two decades, the technique based on principles of spray technology has rapidly evolved, and there presently exists a variety of spray-based methods. These include the following: a spray atomization and deposition processing [ ]; b low-pressure plasma deposition ; c modified gas welding technique ; d high-velocity oxyfuel HVOF thermal spraying [, ]. For the case of composite materials, the technique of spray processing involves a thorough mixing of the dis- continuous reinforcements with the matrix material under conditions of non-equilibrium.
In this section, we provide a detailed overview of the technique of spray atomization and deposition processing and a succinct overview of the techniques of low-pressure plasma deposition, modified gas welding, and HVOF thermal spraying. Spray atomization and deposition processing Commercialization of the technique of RSP has been lim- ited due to difficulties associated with the production of bulk shapes. These problems resulted from the presence of oxides on the reinforcing particulates.
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In an effort to con- trol the volume fraction of oxide particles present in the consolidated rapidly solidified alloy obtained by the PM technique, and to concurrently simplify the overall pro- cessing, the technique of spray atomization and deposition processing emerged. During the last three decades, i. Singer at the University College of Swansea, United Kingdom pioneered the basic principles of spray deposi- tion during the s. Singer proposed the production of rolled strip directly from molten metal as an attractive and economic alternative to the conventional practice of casting and rolling of large ingots [, ].
Singer examined and studied the spray rolling of metals as an alternative to the process initially developed at the Rey- nolds Metal Company in . In this process, an aluminum alloy was centrifugally atomized, reheated, fed into roll gaps, and subsequently hot rolled to produce strips in a continuous operation . The general prin- ciple of spray atomization and deposition is to atomize a stream of molten metal using high-velocity inert gases, such as argon or nitrogen, and to direct the resulting spray of molten metal onto a shaped collector, which can be either a mold or a substrate.
On impact with the substrate, the very fine particles in the molten metal stream flatten and weld together to form a hot, high-density preform, which is conducive for being readily forged to form a fully dense product. The surface of the substrate must be prepared in such a way that the first layer of atomized droplets will adhere onto it to form a smooth quenched layer. Singer performed his studies on aluminum alloys [, ].
The atomized droplets were approximately pm in diameter, and were produced by using conventional atomization equipment. Subsequent to this development, Osprey Metals Ltd Neath, South Wales implemented the early ideas of Singer for the production of forging preforms. This effort was undertaken as a direct result of the increasing pressure imposed on the metal-forming industry to a reduce manufacturing costs, b improve material utilization, c improve material structure and properties, and d enhance efficiency .
The melting is undertaken by induction heating in a furnace, which is directly linked with a tundish. The molten metal then flows from the tundish into the gas atomizer. The melting and dispensing opera- tion is carried out in a vacuum chamber Production of metal -based MMCs and intermetallic- based IMCs matrix composites is accomplished by injecting the reinforcing ceramic particulates into the atomized stream of molten metal leading to co-deposition with the atomized metal or intermetallic onto the substrate.
A careful control of the atomizing conditions and particu- late feeding conditions is both required and essential in order to facilitate a near homogeneous distribution of the reinforcing ceramic particulates in the micro structure [, ]. The primary attraction of this technique is the rapid production of semi-finished composite products in a com- bined atomizing, particulate injection and deposition operation. Further, this method offers notable cost savings compared to the other RSP techniques for the manufacture of both metal-matrix based and intermetallic-based com- posite materials.
The versatility of the technique has been demonstrated by an ability to spray diverse metal matrices such as commercially pure alumi- num and the emerging generation of aluminum-lithium based alloys. The final quality of the aluminum alloy-based MMCs are critically dependent on composition of the matrix alloy. The salient advantages of the spray atomization and deposition process is that it inherently avoids the extreme thermal excursions, and the resultant degradation of mechanical and interfacial properties and extensive macro- segregation, normally associated with conventional casting processes.
Furthermore, this approach eliminates the need to handle fine reactive particulates as is necessary with conventional PM processing. A series of studies based on spray atomization and collection processes but using high- pressure gas atomization for the spraying of fine, rapidly quenched droplets, in a controlled atmosphere, resulted in the development and emergence of the technique of Vari- able co-deposition of multiphase materials referred to henceforth as VCM [, , ].
The variable co-deposition of multiphase materials VCM In the variable co-deposition of materials VCM process, the matrix material is disintegrated into a fine dispersion of droplets using high-velocity inert gas jets Fig. Simultaneously, one or more jets of the strengthening phase s are injected into the atomized spray at a prescribed spatial location Fig.
Good interfacial control is achieved by injecting the reinforcing ceramic particulates at a spatial location where the atomized spray of the metal contains only a limited amount of volume fraction of liquid. By this, the contact time and thermal exposure of the reinforcing ceramic particulates with the partially solidified matrix are minimized and interfacial reactions are closely controlled. Further, a tight control of the envi- ronment during processing minimizes oxidation and other environmental effects [, , , ].
For situations where reactivity between the matrix material and the reinforcement is negligible, the reinforcing phases are safely introduced into the liquid alloy prior to the initiation of spray deposition . Springer J Mater Sci Fig. The experimental set-up, shown in this figure, incorporates a series of pressure transducers and thermocouples coupled with pneumatic control of the deposition surface.
The pressure transducers are used to monitor and control the gas atomization pres- sure, the chamber pressure, and the instantaneous position on the deposition surface. The thermocouples aid in mon- itoring i the temperature of the melt, ii the temperature of the nozzle, and iii temperature of the deposition sur- face, during the experiments.
During spray atomization and deposition, the matrix material is disintegrated into a dispersion of droplets; the average size is denoted by J 50 , using high-velocity inert gas jets. The injection distance is determined based on a numerical analysis of the fraction of solid contained in the atomized matrix as a function of the flight distance. The synthesis attempts to achieve careful interfacial control by injecting the reinforcing particulates at a spatial location Zj where the atomized spray contains a limited amount of volume fraction of the liquid. Thus, i the contact time, ii thermal exposure of the reinforcing particulates with the partially solidified matrix, and iii interfacial reactions are all minimized.
The resultant micro structural characteristics of the spray-deposited material depends largely on the conditions of the droplets prior to their one-to-one impact with the substrate, that is, on the following: i the relative propor- tions of liquid and solid phases present, ii temperature, iii velocity, and iv size of the intrinsic microstructural feature in both the partially solidified and fully solidified droplets [, ].
It is important to know the size and distribution of the droplets, since the amount of heat dis- sipated by the droplets during flight is strongly dependent on their size. Overall, micro structure evolution during spray atomization and deposition can be separated into two distinct yet closely related stages. At the moment of impact of the droplets with the deposition surface, the thermal and solidification condi- tions governing droplet distribution depend on the following: i the thermodynamic properties of the material used; ii the thermodynamic properties of the gas; iii the processing parameters.
In order to develop a better understanding of the evo- lution of microstructure subsequent to deposition and impact, it is essential to establish the thermal conditions of the growing deposit. During the deposition stage, an extensive fragmentation of the dendrite arms formed during solidification occurs because of the repeated impact of the partially solidified droplets, initially with the deposition surface, and subse- quently with each other. The presence of dendrite frag- ments coupled with the development of strong convective currents in the liquid during impact and the formation and presence of a large number of solid nuclei have all been proposed as factors contributing to the development of equiaxed grain morphology during spray atomization and deposition .
It is accepted that the formation of this micro structure is intimately linked to the solidification conditions that are present during atomization and deposition. In an exhaustive study on nickel aluminide Ni 3 Al powders, it was proposed that the formation of equiaxed grains from the dendrites during annealing evolves from two distinct mechanisms : a the coarsening of secondary dendrite arms, and b the growth and coarsening of the primary dendrite arms. During the early years in an attempt to comprehensively understand, the solidification kinetics during spray atom- ization and deposition a nickel aluminide Ni 3 Al was selected as an experimental material because of interest in this class of alloys for elevated temperature applications [, ].
In a region located near the substrate, the microstructure consisted of densely packed powders that deformed upon impact with thick-prior droplet boundaries between them, giving the region a highly irregular appearance. Locally, the microstructure inside the deformed powders consisted of a mixture of deformed dendrite arms, fractured dendrite arms, and what appeared to be dendrite arms aligned perpendicularly to the deposi- tion surface. In general, microstructural observations made in this region suggest the droplets to be partially solidified upon impact with the deposition surface.
The dendrites that were examined in this region appeared to have experienced either extensive plasticity or fracture. A large fraction of the micro structure in the deposited material consisted of fine, homogeneous, and fully sphe- roidal grains with an average size of The microstructure appeared to be reasonably dense with a porosity of 1.
The spheroidal grains present in the region were not fully developed, and their size distribution was heterogeneous. Kinetics governing microstructural evolution during atomization The microstructural characteristics of spray atomized and deposited materials are critically dependent upon the solidification history of the droplets during atomization.
It is well documented that the solidification behavior of atomized metal droplets is strongly dependent on the fol- lowing: i the distribution of droplet size, and ii the flow and thermal interactions with the atomization gas. During atomization, droplets form because of momentum transfer from the gas to the liquid, promoting the formation of ligaments that eventually acquire a spherical morphology because of surface tension forces [, ]. The sphero- idization of ligaments is particularly rapid in molten metals because of their high surface tension . Therefore, it was assumed that the droplets are spherical.
The atomiza- tion gas not only provides the energy to disintegrate the liquid metal into micron- sized droplets, but also provides a cooling medium for the droplets. The disintegration and cooling of the droplets are intimately related to the exit gas properties of velocity, density, and temperature. One of the key characteristics of rapid solidification is the attainment of relatively high levels of undercooling prior to the onset of nucleation. Although the nucleation may take place heterogeneously or homogeneously, het- erogeneous nucleation governs solidification in almost all practical cases .
It is important and useful to have knowledge of the undercooling that is required for homogeneous nucle- ation, as an upper bound of the maximum level of undercooling achieved. If all of the droplets were to nucleate and solidify homogeneously, then the N droplets should exhibit the same level of homogeneous underco- oling. However, the presence of nucleation catalysts will promote or favor heterogeneous nucleation in a propor- tion of the incoming droplets, defined as N h.
Thus, the number of droplets that nucleate homogeneously is reduced to N — N h. If the undercooling required for heterogeneous nucleation is assumed negligibly small when compared to that which is required for homoge- neous nucleation, then heterogeneous nucleation is initi- ated and occurs to completion.
The droplets arriving on the deposition surface are either in the fully solid, liquid, or partially solid condition. Fur- ther, the fine dendritic micro structures that are present in the powders provide experimental support to the relatively fast rates of heat extraction that were computed based on thermal considerations. However, it was evident that there existed significant microstructural variations within a sin- gle droplet, both in scale and morphology, owing to the complex thermal history experienced by the droplet, that is, a combination of undercooling, nucleation, recalescence, and equilibrium solidification.
This highly heterogeneous mixture of microstructures arrives on the deposition sur- face, eventually collecting as a highly dense preform, whose microstructure critically depends on the following: a solidification characteristics of the impinging droplets, b the interactions of the droplets with the deposition surface, and c mutually interactive with each other. Overall, a large proportion of the micro structure present in the spray-deposited nickel aluminide Ni 3 Al exhibited spheroidal grain morphology.
Other investigators have reported the presence of spheroidal grain morphology for spray atomized and deposited aluminum and magnesium alloys . Mild steel and tool steels , nickel-based alloys , copper alloys , and even MMCs [, ]. The cooling rate is essentially controlled by the conjoint and mutually interactive inter- actions with each other of the following: a the deposit-substrate heat transfer coefficient; b thermal properties of the material.
Elevated temperature annealing of the spray-deposited material is conducive for the occur- rence of grain growth and eventual coalescence, resulting in the formation of a spheroidal grain micro structure during subsequent annealing by two distinct yet interactive mechanisms: i a homogenization of the dendrites that did not deform extensively during deposition, and ii the growth and eventual coalescence of the deformed and fractured dendrite arms.
This is shown in Fig. With this relationship, the grain size at a certain annealing temperature and time can be predicted, that is, if the annealing temperature and grain size are fixed, the annealing time can be predicted. An increase in annealing temperature T corresponds to an increase in time needed for the grains in the deposited material to grow to a particular size. In an attempt to better understand and rationalize this phenomenon Liang and coworkers [, ] subjected a nickel aluminide Ni 3 Al intermetallic to atomization using nitrogen gas at a dynamic pressure of 2.
The mass flow rates of the atomizing gas and liquid melt were adjusted to be 0. The copper deposition surface was water cooled and positioned at a distance of The overspray powders were collected in a cyclone separator for purpose of analysis. The micron-sized dendrites exhibited well-defined primary and secondary arms. The observed fine scale of the microstructure was attributed to the fast solidification front velocity resulting from the highly non-equilibrium condi- tions that are present during atomization . The extent of microstructural refinement, as measured by spacing of the secondary dendrite arms SDAS , was found to be inver- sely related to the diameter of the powder particles.
The thermal profile of three locations inside the spray-deposited mate- rial Fig. The results reveal the temperature at all three loca- tions of the spray-deposited material to remain below the solidus temperature. From this data, it was inferred that the Powder diameter pm Fig. Since the mandrel is rotating, feedstock impingement must occur slightly ahead of the laser beam for stable, steady state deposition. The process has great potential as a hardfacing treatment.
Powder Technology Most of the effort on rapid solidification powder technology has been concerned with the further development of high performance compo- nents for gas-turbine engines, such as blades, vanes, discs, rotors, com- bustors, and bearings. After extrusion the billet is subjected to gradient annealing in order to develop a textured, columnar "rained structure. Such a textured, homogeneous structure exhibits outstanding high temperature creep properties. In order to exploit such property benefits, this same processing method has been applied to the fabrication of a demonstration "radial wafer" turbine blade.
The sequence of steps involved in the manufacture of such a blade is shown schematically in Figure 8. As illustrated in this figure, rapidly solidified superalloy powder is produced by the RSR process and consolidated by hot extrusion, with extrusion parameters adjusted to give an intrinsically superplastic prod- uct.
- In Custody.
- Science and Technology of Rapid Solidification and Processing.
- Solidification Processing!
- Rapidly Solidified Aluminum Alloys.
The billet is rolled superplastically into sheet stock by controlling temperature and deformation rate. The resulting sheet product is pho-. This is the critical step in the process, since it generates the desired network of internal cooling passages. Following directional recrystallization of the bonded structure, the actual blade profile is formed by electrochemical machin- ing. The experimental air cooled blade shown in Figure 8 has already been successfully tested in an advanced engine.
Such alloys are promising candidates for integral vane and case assem- blies in the cooler compressor section of the engine as replacements for the more expensive titanium alloys. Improvements in the rolling contact fatigue resistance of M50 bearing steel by rapid solidification processing also presents an opportunity for advancing the performance of high speed bearings.
This beneficial effect has been related to refinement of the carbide phases in M50 steel. Vacuum plasma spraying of high performance coatings has become rou- tine practice, with applications in industrial gas turbines and jet engines. Progress has also been made in the fabrication of a thin-walled com- bustor and a massive turbine disc Figure 9 , making use of the unique thick section capabilities of the process. Laboratory tests have shown that deposited materials, such as Rene 80, have superior resistance to thermal fatigue, which is a prerequisite for combustor applications. Cur- rently, efforts are being made to apply this technology to the near-net shape fabrication of general engineering components, such as extrusion dies, valve bodies, pipes, casings, and sleeves.
Allied has favored the melt spinning process, whereas Battelle has favored the melt extraction process. Many areas of application have been identified; some have already been commercialized. Castable refractories are widely used in furnaces and reactors. Con- ventional processing of steel fibers involves repeated shear-cutting of continuously drawn wires, and final embossing of the fibers to improve adhesion. The cost of processing is high, so that the 2 volume percent of fibers normally introduced into the ceramic can cost several times that of the ceramic.
Battelle was first to recognize the potential for melt extracted steel fibers in this application. High aspect ratio fibers are readily and inexpensively produced by melt extraction, using a notched wheel. The resulting fibers tend to have expanded ends dog-bone shaped , which facilitates reinforcement of the ceramic matrix.
Another advan- tage is that melt composition is no longer limited by mechanical working considerations, so that even low grade scrap can be used for melting. Resulting savings in production costs have been substantial, and many thousands of tons of melt extracted steel fibers are used today in castable ceramics. This same process is being considered for making steel fibers for reinforcing concrete.
Diffusion brazing is a method of joining materials that combines the essential features of both conventional brazing and diffusion bonding. The filler material is placed between the mating surfaces of the workplace and is permitted to alloy with it at a temperature where only the eutectic melts. Under isothermal conditions the melting point of the filler material gradually rises as the melt depressant diffuses away into the workpiece. Bonding is judged to be complete when no melt remains.
Subsequent heat treat- ment is employed to erase all traces of the original junction. Success in diffusion brazing depends not only on good design of filler material, but also on the ability to produce the material in a usable form. A particular problem has been encountered in the preparation of thin ribbon material 25 to 50 ,um thick x 2 to 5 cm wide , which is very difficult, if not impossible, to produce by conventional hot working methods because of the limited ductility of the eutectic alloy.
A solution to this problem has been to prepare the thin ribbon material by melt spinning. Considerable success has been achieved in utilizing melt spun nickel- base alloys boron added as melt depressant for diffusion brazing of gas-turbine engine components, such as blades, vanes, and even entire stator rings Figure Standard brazing alloys e. Again such eutectic alloys are essentially unworkable but are amenable to glass formation by rapid quenching from the melt. Amorphous brazing tapes have the advantages of convenience in form, chemical uniformity, and cleanliness no binders to pyrolyze, as in conventional brazing mate- rials , and they are relatively inexpensive to produce.
Commonly used soft magnetic alloys include Fe These materials are normally pro- duced by a complicated sequence of rolling operations, with critical intermediate annealing steps to develop the optimal crystallographic texture and magnetic properties. Subsequent processing may involve stress relieving and coating with polymers.
This complicated fabrication procedure contrasts with the simplicity of melt spinning, which produces ferromagnetic ribbon or tape directly from the melt at very high rates and at relatively low cost. In a finished transformer this translates into substantial energy savings over the lifetime of the installation.
Thus, there is a real incentive for pushing forward with the development of amor- phous cored transformers despite certain technical drawbacks related to the thin gauge of the sheet. Prototype systems have already been fab- ricated Figure 11 and are now being evaluated in actual field tests. Various electronic device applications have been considered for me- tallic glasses. Large sheets for shielding were made by simple weaving and coating with polymers. Cylindrical shields made from these woven fab- rics compared favorably in performance with conventional 80NiFe permalloy foil, except at very low fields where metallic glass loses its high permeability.
The main advantage claimed for the metallic glass fabric was its ability to be formed into the required shape without altering shielding performance. Another application that takes advantage of high permeability, coupled with high electrical resistance, mechanical hard- ness, and resistance to corrosion and wear is for audio and video recorder heads.
The preference in this application is for zero magnetostriction high-cobalt compositions with B and Si as glass farmers, and twin roller quenching to produce smooth surfaces on both sides of the tape. Overall performance is claimed to be superior to conventional ferrites and similar materials.
Rapidly Solidified Aluminum Alloys :: Total Materia Article
Other applications being considered include "stress trans- ducers," which exploit the high stress sensitivity of the magnetic prop- erties in amorphous alloys, and "acoustic delay lines," which make use of the very large values of magnetomechanical coupling and change in Young's modulus with applied field that are found in metallic glasses. Surface Modification Technology Laser or electron beam surface melting glazing has been employed to modify the surface structure and properties of very thin edges of samples using a single pass of a sharply focused beam.
On the other hand, to obtain continuous surface coverage of glazed material it has been necessary to generate a multiplicity of overlapping passes by scan- ning the focused beam over the workpiece surface or by indexing the workpiece with respect to a fixed beam.
A laser beam may be scanned by making use of special coupled arrangements of mirrors, whereas an electron beam may be scanned by electromagnetic means. Both laser and electron beam glazing treatments have been used to achieve beneficial modifications in the surface properties of materials. In sensitized stainless steel,38 laser glazing has the effect of reso- lutionizing harmful carbide phases at the grain boundaries and restores the resistance to stress corrosion cracking.
In aluminum bronze,39 laser glazing homogenizes the surface, which increases its resistance to. In a pseudobinary Fe-TiC alloy,4i electron beam glazing and tempering produce a threefold increase in the wear life in tests performed on a fully hardened M42 steel counter- face material. Laser glazing has also been applied to eutectic-type alloys that are ready glass farmers.
Thus, amorphous surface layers have been developed on crystalline substrates in Pd The high hardness and corrosion resistance of metallic glasses containing P and chromium [CrJ , together with their ability to accept and maintain a sharp cutting edge, suggests such uses as surgeon's scalpels and even long-life razor blades. Laser glazing in conjunction with surface compositional modification is also an area of obvious high potential. Methods of processing typically involve preplacement of alloying material powder, electrodeposit, etc. Much thicker deposits.
Surface alloying by this means is being developed for a wide range of applications, including hardfacing of valve seats, turbide blade tips, bearing surfaces, and gas- path seals. Experimental work has also been conducted on the fabri- cation of bulk rapidly solidified structures by incremental solidification processing. Typically the deposited material exhibits a pronounced columnar "rained dendritic structure, with grains extending through many successive layers of material.
Applications for this process are currently limited by the requirement that the deposited material possess good weld-cracking resistance and by the need to improve the shape- defining capabilities of the process. As indicated in Figure 15, the fab- rication of more complex shapes requires the use of a numerically con- trolled work station, which is capable of simultaneous motion about two or three axes.
Today's technology includes methods for the production and consolidation of rapidly solidified fine powders, fabrication and utilization of rapidly solidified thin filaments or ribbons, and rapid so- lidification surface modification of materials. Powder technology has been applied to the fabrication and coating of high-performance com- ponents for gas-turbine engines. This same technology is also being applied to airframe structural materials, such as high specific strength aluminum alloys.
The anticipated use of amorphous soft mag- netic alloy ribbons in the cores of power transformers and motors is. Surface modification technology is still in it infancy, although the benefits of rapid solidification laser or electron beam glazing treatments have been amply demonstrated. How- ever, areas of application have been targeted for development, including hardfacing of tools, dies, and valve seats.
The possible extension of this technology to bulk rapid solidification processing has also been consid- ered. NOTES 1. Duwez, R. Willens, and W. Klement, J. Klement, R. Willens, and P. Duwez, Nature, , Mehrabian, B. Kear, and M. Rapidly Solidified Amorphous and Crystalline Alloys, eds. Kear and B. Meeting at Boston, Mass. Rapidly Quenched Metals IV, eds.
Masumoto and K. Suzuki, Japan Institute of Metals, Proc. War and W. Cox, J. Moore, and E. Van Reuth, in Superalloys: Metallurgy and Man- ufacture, eds. Kear, D. Muzyka, J. Tien, and S. Cline and R.