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Refractory metals are a class of high-melting-temperature materials suitable for use in extreme environment applications. Interestingly, during additive manufacturing many pure refractory metals exhibit a switch from 001 to 111 build direction fiber preference with increasing surface energy density. We exploit this solidification physics to fabricate material with "mesoscale composite" engineered structures consisting of features with contrasting 001 and 111 build direction microtextures. Separately, elevated temperature tensile testing of EBM fabricated material with a randomized distribution of mixed 001 / 111 -fiber grains is shown to exhibit excellent properties. These results are utilized to build a crystal plasticity model for evaluating the local inelastic response of the composite mesoscale structures. Analysis of printed microstructures and microstructure-scale simulations indicate that both macro-scale and localized material behavior may be tailored. This strategy can be potentially used to synthesize materials with optimized performance for high-temperature applications. [ABSTRACT FROM AUTHOR]

Additive manufacturing enables the fabrication of complex engineering components previously inaccessible through traditional processes. Nickel-base superalloys with large γ ′ volume fraction are typically considered non-weldable and therefore exhibit a propensity for cracking during the fusion process. These crack-prone materials, however, are of great importance in gas turbine engines due to their excellent high temperature creep resistance. In this study we investigate the creep behavior of IN738LC produced by the electron beam melting process. We find that with appropriate post-build heat treatment the creep response of material oriented in the build direction exhibits deformation and rupture behavior comparable to that of conventionally cast IN738 & IN738LC. In the transverse direction properties fall below the expected cast behavior, however, we argue this is likely due to differences in grain scale and crystallographic texture. It may be possible to coarsen the grain morphology with appropriate process-parameter optimization in order to reduce the severity of intergranular fracture in the transverse direction. These results illustrate that high temperature properties exhibited by additively manufactured IN738LC are suitable for the hot section of gas turbine engines. [ABSTRACT FROM AUTHOR]

• Fabrication of highly dense crack free pure tungsten via electron beam additive manufacturing. • "Texture switch" observed along the build direction microstructure. • High temperature tensile properties show material behavior similar to recrystallized tungsten. • High dislocation densities were observed in the as-fabricated samples. • Highly anisotropic behavior due to the underlying crystallographic anisotropy. There is considerable interest in the adoption of additive manufacturing for processing refractory metals. The layer-wise fabrication approach enables opportunities for producing complex geometries which cannot be otherwise be achieved via powder metallurgy. However, the processing science is still in its nascent stages and structure–property relations are relatively unexplored. Fundamental research is needed to further develop the technology and enable the fabrication of refractory metals for high temperature applications. Here we focus on the processing of pure tungsten using electron beam melting additive manufacturing. Experimentally we develop a suitable processing window for achieving high density crack free material. Microstructural analysis reveals that the microstructure generally consists of a columnar structure with a 111 build direction fiber preference, although, fiber switching was observed. Process induced deformation is believed to drive the formation of subgrains whose boundaries exhibit a high dislocation density. High temperature tensile testing reveals that the material exhibits excellent properties closer to that of annealed tungsten. Significant mechanical anisotropy was observed to be present which is likely driven by strong crystallographic texture. [ABSTRACT FROM AUTHOR]

The Portevin-Le Chatelier (PLC) effect has been studied in the Ni-based superalloy IN100 which is currently used as a disk material in jet engines. A series of tensile tests was carried out at 588 K, 755 K, and 922 K (315 °C, 482 °C, and 649 °C) at plastic strain rates ranging from a low of 6.21 × 10 s to a high of 4.92 × 10 s. The activation energy was determined using the slope of a line on a strain rate/temperature graph which divided the area of the graph into two regions: (1) 'PLC behavior observed,' and (2) 'No PLC behavior observed.' A new statistical approach was developed to objectively differentiate between a true PLC effect and experimental uncertainty ( i.e., 'noise'). The value of the activation energy was found to be 1.14 eV/atom, which strongly suggests that the rate controlling process was bulk diffusion of C in the lattice. A qualitative model, based on the Orowan equation and slip band dislocation mechanics, was proposed, which unifies the seemingly disparate ideas of the process being controlled by a single atom/dislocation interaction while at the same time exhibiting significant strains during PLC load drops. [ABSTRACT FROM AUTHOR]

Additive manufacturing technologies have emerged as potentially disruptive processes whose possible impacts range across supply chain logistics, prototyping, and novel materials synthesis. Numerous works illustrate the ability to control microstructure in fusion based processes and a few recent authors have even produced single crystals. However, a number of questions remain open regarding the process window which enables printing of single crystals. Furthermore, it has been observed that these additively manufactured single crystals exhibit a preferred 〈 011 〉 secondary orientation parallel to the scanning direction. In this work we investigate the fabrication conditions that enable printing of single crystals via electron beam melting. A space filling design of experiments is utilized to efficiently explore the fabrication space. Single crystals are successfully obtained using both commercially available powders and custom melt alloys. Microstructures obtained via these exploratory experiments exhibited a continuum of columnar structures ranging from weakly textured polycrystals, near single crystal, and fully single crystalline material. Complex geometry experiments are performed to study the grain selection mechanism. We find that the grain selection mechanism is independent of the bulk scale geometry and must therefore be driven by local heat transfer and solidification dynamics. Furthermore, grain selection is shown to be driven by competing driving forces; one which prefers epitaxial growth and another which is driven by the imposed processing conditions. [ABSTRACT FROM AUTHOR]