Functionally graded materials (FGMs) are of increasing interest due to their ability to join dissimilar alloys with spatially tailored properties. However, joining incompatible materials such as steels and refractory alloys poses difficulties in processing, particularly regarding the formation of brittle intermetallic phases (e.g. σ) and cracks. The precipitation and growth of these phases are dependent on composition, which can be mitigated by Calphad-based path planning approaches, and kinetics, which are in turn dependent on the thermal history throughout the part. In this work, we propose guidelines for path planning using an effective temperature range that can capture relevant formation of deleterious phases, demonstrated on a gradient system between SS316 and titanium-zirconium-molybdenum (TZM). Computational path planning between SS316-Cr-TZM was used to predict σ phase formation from SS316 to Cr using “effective lower temperatures” (defined as the temperature above which phase transformation kinetics are assumed to become relevant) between 500 and 1000 °C, which were then validated with FGM builds produced by directed energy deposition (DED). Compositions and phase fractions were measured experimentally for direct comparison to Scheil and equilibrium predictions. Several phase transformations mechanisms are discussed to give insights into next-generation path planning accounting for microstructural evolution. Paths both avoiding and containing σ phase are fabricated and compared to investigate sources of cracking; in all cases, solidification and ductile-to-brittle transition (DBTT) cracking were identified as the main crack formation mechanisms and a new path from Cr to TZM was proposed, incorporating Nb and V to reduce crack susceptibility.

Calcium- and sulfur-rich deposits have been linked to failure of turbine components as a consequence of high temperature exposures (> 1000 °C). There are only limited studies on the effects of these deposits on the degradation behavior of turbine alloys. To gain further understanding of this phenomenon, a systematic study was undertaken with model binary nickel–chromium alloys. Three alloys with different chromium contents—low, medium and high—represented by Ni-5Cr, Ni-10Cr and Ni-18Cr, were exposed to CaSO 4 -deposit-induced corrosion in the 900–1100 °C temperature range. At 1000 and 1100 °C, the decomposition of CaSO 4 (either by decomposition to CaO and SO 3 or by reacting with Cr 2 O 3 ) led to the formation of calcium chromates and chromium sulfides. At the lower temperature, 900 °C, the limited decomposition of CaSO 4 allowed the formation of a continuous Cr 2 O 3 scale.

Functionally graded materials have the potential to improve upon monolithic parts by locally tailoring compositions to surrounding environmental conditions. Difficulties arise when designing composition gradients as incompatible materials can result in detrimental phase formation and failure of the gradient joint. As many alloys are multi-component, designing a composition gradient free of detrimental phases is difficult due to the large composition space available to explore. A framework was developed that improves the path planning algorithm and surrogate models with adaptive sampling schemes specific to their problem definition. A cost function was created to minimize a property (such as cracking susceptibility) along a path. This framework was applied to the Mo-Nb-Ta-Ti system as a case study to showcase the efficiency in building the surrogate models and in iterating different optimal compositionally graded paths.

Additive manufacturing has ushered in a new paradigm of bottom-up materials-by-design of spatially non-uniform materials. Functionally graded materials have locally tailored compositions to provide optimized global properties and performance. In this letter, we propose an opportunity for the application of graded magnetic materials as lens elements for charged particle optics. A Hiperco50/Hymu80 (FeCo-2 V/Fe-80Ni-5Mo) graded magnetic alloy was successfully additively manufactured via Laser Directed Energy Deposition with spatially varying magnetic properties. The compositional gradient is then applied using computational simulations to demonstrate how a tailored material can enhance the magnetic performance of a critical, image-forming component of a transmission electron microscope.

Functionally graded materials (FGMs) combining two dissimilar steels, stainless steel 316L and high-strength low-alloy steel, were additively manufactured using directed energy deposition. High-throughput characterization of the dissimilar steel FGM led to the discovery of a low Manganese (Mn<1wt.%) TRIP (transformation-induced plasticity) and TWIP (twinning-induced plasticity) steel with a metastable microstructure enabled by additive manufacturing. Microsegregation from non-equilibrium solidification caused heterogeneity in the phase stability and stacking fault energy, leading to TRIP and TWIP effects in the as-built condition without heat treatment. Tensile testing of the new as-built TRIP and TWIP steel resulted in ultimate tensile strength of 960MPa, yield strength of 415MPa, total elongation of 26%, and a unique strain hardening rate that increases after yielding. We compare experimental measurements of microsegregation with thermodynamic modeling to discuss the impact of microsegregation on phase stability, stacking fault energy, and solidification cracking susceptibility in additively manufactured FGMs. The highlights of this work include the discovery of a novel pathway for achieving TRIP and TWIP effects in additively manufactured steels without heat treatment. This work also shows that the TWIP effect can be introduced without high Manganese content playing a critical role in adjusting stacking fault energy.

Kawin, a new open-source implementation of the Kampmann-Wagner Numerical model of precipitation (con-comitant nucleation, growth, and coarsening), is presented. An overview of the organization and capabilities of the program is provided, along with an outline of the constituent physics. Kawin is shown to be able to reproduce the results of state-of-the-art commercial software and experimental data for a variety of alloy systems under multiple precipitation conditions. Kawin is capable of simulating the bulk precipitation behavior of multiphase, multicomponent systems in response to complex heat treatments, and contains numerous innovative features to enhance model stability, improve flexibility and usability, and minimize computational expense. Kawin also incorporates sophisticated elastic energy calculations, traditionally ignored in this type of simulation but shown here to significantly impact the precipitation behavior of some systems. The inclusion of native strain calculations enables Kawin to predict the influence of internal or external stress fields on precipitation, as well as track the evolution of precipitate geometry throughout the course of a heat treatment. It is the hope of these authors that this software will facilitate the advancement of precipitation modeling as a tool for materials design.

The sensitivity and sophistication of spacecraft for Earth Science and Planetary Science missions are increasing with each successive mission. Advances in instrumentation, robotics, remote sensing, avionics and controls are allowing un-crewed missions to be incredibly sophisticated. Effective electromagnetic shielding is critical for high fidelity functioning of the spacecraft and onboard instrumentation. Moreover, the geometrical complexity of the shielding arrangement and the constraints of size and shape, make additive manufacturing (AM) a critical, enabling technology for current and future missions. Two AM approaches - directed energy deposition (DED) and laser powder bed fusion (LPBF) - were used to produce Fe-80Ni-5Mo alloy rings and shields. The microstructure and magnetic properties of the materials produced through these processes are reported in this paper. A mechanism relating the microstructure to the soft magnetic performance of the material is proposed. The AM material produced in this work demonstrated the highest reported magnetic permeability and lowest reported coercivity of any additively manufactured soft magnetic material. Two different combinations of bi-metallic shields, Fe-80Ni-5Mo/FeCo-2V and Fe-80Ni-5Mo/Fe-49Ni, were fabricated using the DED process. The creation of these shields as monoliths in general, and for space-related applications in particular, is a novel aspect of this work. These multi-alloy, multi-layer shields showed a nearly 10 dB increase in shielding performance over Fe-80Ni-5Mo single alloy shielding, a significant improvement. For the DED builds, the coercivity decreases and permeability increases (better soft magnetic performance) with increasing build power. With increasing build power in the DED process, the grain size in the printed part increases. The corresponding reduction in the grain boundary area results in fewer obstacles to the movement of magnetic domain walls.

Magnetic shielding in spacecraft is a mission‐critical issue that must be addressed in a timely and effective manner. The high permeability of Fe–Ni–Mo alloys, makes them excellent candidates for magnetic shielding. This article explores a new and innovative approach, enabled by additive manufacturing (AM), to design, build, and test geometrically complex magnetic shields. A Fe–79.7Ni–4.1Mo alloy is additively manufactured using blown powder laser‐directed energy deposition (DED). AM build conditions are explored in the production of magnetic test rings and magnetic shield prototypes. Magnetic hysteresis test data are obtained, allowing for the determination of magnetic permeability, saturation, and coercivity. Detailed microstructural characterization is carried out. Three different prototype shield designs are printed and magnetic shield attenuation data is obtained. The magnetic field attenuation (shield effectiveness) obtained for the AM components is comparable to wrought equivalents. The values reported here for the magnetic permeability are the highest, and that for the magnetic coercivity the lowest, for any blown powder DED‐printed material currently known. The magnetic behavior is discussed with regard to grain size and orientation, as well as grain boundary effects, with all of these attributes contributing to the ultimate performance.

Molten Salt Reactors (MSRs) are a promising alternative to the current generation of light-water reactors. However, the corrosivity of molten salts is a significant issue that must be understood before MSRs become commercial. Ni-base superalloys are promising candidate materials that can provide corrosion resistance in molten salt environments and serve as reliable structural materials for reactor operation. In this study, immersion studies of three different Ni-base superalloys were conducted in ternary chloride salts at 750°C. Post-test characterization included mass change per unit area, X-ray diffraction, and microstructural evaluation using optical microscopy, and scanning electron microscopy coupled with energy dispersive X-ray spectroscopy. The effect of alloying additions on molten chloride corrosion is discussed.

Abstract Metallic alloys that are typically used for medical purposes include stainless steels, Ti-6Al-4V, and Co-Cr-Mo. This article discusses the relative merits of each of these alloys. The utilization of stainless steels in the biomedical industry, especially in relation to the additive manufacturing (AM) process, is the main focus of this article. The characteristics of various stainless steels are described subsequently, and the categories that are of relevance to the biomedical industry are identified. The types of stainless steels covered are austenitic, ferritic, martensitic, duplex, and precipitation-hardened stainless steels. The article discusses the potential benefits of AM for biomedical devices. It describes the types of AM processes for stainless steels, namely binder jet, directed-energy deposition, and laser powder-bed fusion. The article reviews the AM of austenitic, martensitic, and PH stainless steels for biomedical applications. In addition, the challenges and obstacles to the clinical use of AM parts are covered.

High-permeability Ni-15 Fe-5 Mo (wt.%) alloy coupons were produced using a directed energy deposition process with varying laser powers and scan speeds. The microstructure of the as-printed alloy is influenced by the printing parameters, and this influence is perceptible even after heat treatment. Electron backscatter diffraction data showed that the grain sizes in all samples were small near the build plate and increase with distance along the build direction. The average grain size of as-printed coupons decreased with increasing scan speed and lower laser power. Preferred orientation along the <110> family of directions was noted when moderate laser powers and scan speeds were utilized. Annealing resulted in a decrease in average grain size with less texturing and more high-angle grain boundaries.

The microstructure and magnetic performance of Fe49Co2V (Hiperco50) manufactured via laser‐directed energy deposition are determined. In the as‐printed form, the material displays a fine, equiaxed microstructure and magnetically “hard” behavior. With a customized post‐process annealing treatment, significant grain growth occurs, resulting in soft magnetic performance comparable to traditionally manufactured materials. An inverse correlation between laser power and post‐anneal grain size is observed, with lower laser powers resulting in samples with larger grain sizes after annealing. Samples printed with lower laser powers also display improvements in soft magnetic performance. This is consistent with the expected relationships between grain size, magnetic permeability, and magnetic coercivity, i.e., as grain size increases, magnetic permeability increases and magnetic coercivity decreases. A prototype magnetic shield for a Hall‐effect thruster is successfully printed. The magnetic performance of the printed and annealed shield is comparable to one that is traditionally manufactured.

Calculation of Phase Diagrams (Calphad) is a method of using thermodynamic models obtained from experimental data to perform thermodynamic calculations. The next step to advancing this methodology is to account for the inconsistency or lack of data when assessing thermodynamic systems. A generalized method of propagating uncertainty through Calphad calculations by local expansion is proposed such that any type of Gibbs free energy model or number of components can be used. This method is faster than Monte Carlo approaches as only a single equilibrium calculation is needed for uncertainty propagation and also improves upon previous approaches to uncertainty propagation by its generalization to any thermodynamic system. As a case study, the Mg–Si system was assessed using a Bayesian approach and various thermodynamic calculations were performed comparing uncertainty quantification by Monte Carlo and the method in this work. Sensitivities (derivatives with respect to model parameters) were calculated on the Fe–Cr–Ni system and compared with sensitivities determined by finite different method.

Nickel-base alloys are used in high-temperature applications due to their favorable mechanical properties and oxidation behavior. Some of these alloys are designed to form a protective aluminum oxide scale to achieve oxidation resistance. In some oxidizing environments, water vapor is also present. However, only limited data are available regarding the effects of water vapor on the oxidation behavior of alumina-forming alloys (“alumina formers”), especially for early stage oxidation. Moreover, the currently available mechanisms for dry oxidation of alumina formers propose different pathways leading up to the final morphology of the oxide scale. In this study, the early oxidation behavior of an alumina former (UNS N07214) in dry and humid air was conducted at 1000°C for different exposure times (1 min to 100 h). Detailed examination of the surface of this alloy reveals that an alumina film is initially (1–10 min) formed in both dry and humid conditions. For longer exposure times (1–10 h), this initial alumina film is disrupted by the formation of chromia and nickel oxide/nickel chromite islands in both cases. Subsequently, for 100 h exposures, a continuous alumina scale is re-established. A mechanism to explain the observed phenomena is proposed.

Compositionally graded cylinders comprising FeCo-2V, a soft magnetic alloy, and 316L stainless steel, were produced by directed energy deposition incorporating two different gradient lengths. Five distinct regions in the microstructure were observed in both the as-printed and annealed gradients. When normalized for the gradient length, the gradients showed similar grain size and microhardness profiles. In tensile tests, the as-printed samples failed in the 316L region with an effective total “composite” strain of 20-30% and overall strength approaching that of wrought 316L. The annealed samples failed in the FeCo-2V region. Neither exhibited failure in the gradient region. Molecular dynamics simulations were used to calculate tensile strength as a function of composition, showing good correlation to experimental trends but not absolute values. This work demonstrates the tunability of site-specific properties using blown powder directed energy deposition to gradually grade from FeCo-2V to 316L stainless steel in a monolithic component.

Fe-based amorphous coatings are typically known for their high hardness and wear resistance and are often applied using thermal spraying techniques. When cladding these materials, using laser directed energy deposition (DED), it becomes possible to vary the cooling rate to create unique microstructures. Herein, it is demonstrated that a common FeCrMoBC coating alloy, typically known for its high hardness, can be fabricated into a variety of microstructures using DED, including an amorphous metal matrix composite with a soft dendritic phase and intrinsic toughening mechanism. The work offers the promise of using DED to fabricate custom claddings with high toughness as well as providing a potential route for creating ductile-phase-reinforced metallic glass composites using additive manufacturing.

Nickel base alloys are widely used in high temperature oxidative atmospheres, such as heat exchangers and land-based gas turbines. To mitigate the deleterious effects of oxidation, chromium and aluminum additions, which promote the development of a protective layer of chromia or alumina, are made to these alloys. In dry air, the behavior of chromia- and alumina-forming alloys can be predictable. However, in several instances, high temperature applications contain a significant amount of water vapor, which can lead to adverse effects on the oxidation process. The effects of water vapor are not completely understood and thus, a comparative study was conducted on the oxidation behavior of a chromia- and an alumina- former in dry and wet air. UNS N06230, a chromia-former, and UNS N07214, an alumina former, were oxidized at 1000°C in dry and wet air (15 volume% H2O) for times of 1 minute, 10 minutes, 1, 5, 10 and 100 hours using thermogravimetric analysis. The oxidized coupons were characterized using X-ray diffraction and scanning electron microscopy. In these early stage oxidation experiments, the UNS N07214 (alumina former) had a lower oxidation rate relative to the UNS N06230 (chromia former). The alumina former showed a more complex behavior in these early stages of oxidation relative to the chromia former.

The present work investigates the hot corrosion behavior of Al0.1CoCrFeNi high entropy alloy (HEA) and Ni-base Alloy 718. Electrochemical tests were conducted to characterize the corrosion behavior of Al0.1CoCrFeNi HEA and Alloy 718 in a molten Na2SO4-NaCl eutectic mixture at 750 +/- 5 degrees C in the presence of a platinum catalyzed SO2/air mixture. The morphology of the surface of Al0.1CoCrFeNi HEA and Alloy 718 was investigated using scanning electron microscopy (SEM) as well as energy dispersive spectroscopy (EDS). The results indicate that the polarization resistance of the HEA was higher and the corrosion rate lower in comparison with Alloy 718. SEM and EDS analyses reveal the formation of a dual oxide layer on the HEA that provides a better corrosion barrier compared to the single chromia scale observed on Alloy 718. (C) The Author(s) 2019. Published by ECS.