Displacement reactions between binary and ternary ceramics in the Ti–W–C system and reactive gaseous atmospheres are investigated in this work. Specifically, WC and 50:50 wt% TiC:WC solid solution powders were exposed to flowing hydrogen gas, or equilibrated against an excess of titanium in the presence of iodine, to form metallic tungsten and TiC solid products. In the case of pure WC reacting with hydrogen, transformation to metallic tungsten occurred as a result of removal of chemically bound carbon as gaseous hydrocarbons. In the case of pure WC reacting with titanium iodide vapors, transformation was accompanied by the appearance of TiC as a solid product formed at the gas‐solid interface. In the case of 50:50 wt% TiC:WC solid solution powders, hydrogen was generally found to be an ineffective displacing reagent, whereas reaction with titanium iodide vapors was observed to proceed virtually to completion, resulting in a two phase product mixture comprising metallic tungsten and TiC. For the latter case, a variety of microstructures could be observed within a given batch, including tungsten platelets and/or lamellae in a TiC matrix, or coarse tungsten grains interspersed with TiC grains. These morphological variations are speculated to arise from compositional variation in the starting material and the occurrence of local rapid coarsening along fast diffusion pathways within reacting agglomerates and polycrystalline primary particles. The observed reaction products and relative efficacy of gaseous reagents to promote displacement reactions in the Ti–W–C system are rationalized on the basis of thermodynamic predictions. The reaction between 50:50 wt% TiC:WC solid solution powders and titanium iodide vapors constitutes the first known report of an internal displacement reaction proceeding via gaseous intermediates in a nonoxide ceramic system.
Shape memory alloys (SMAs), most notably those based on nearequiatomic NiTi, feature distinctive diffusionless phase transformations that give rise to useful functionalities (e.g., to generate force/motion or to store/dissipate deformation energy) that have resulted in wide-scale commercial applications across aerospace, automotive, biomedical, robotics, and other technological fields. [1] Many SMA components feature complex architectures ranging from honeycombs to cellular structures designed to lightweight system components, to match stiffness with adjoined materials, or to intensify deformation responses. [2] Unfortunately, these same phase transformations and resultant thermal-mechanical behaviors that make SMAs desirable also make their manufacture notoriously challenging. NiTi-based SMAs particularly exhibit acute sensitivity to chemistry and microstructure that can be strongly affected by thermal-mechanical histories developed during processing and service. [3] Numerous strategies have been explored to manufacture complex-shaped NiTi SMA components, including various methods for metal joining and metal additive manufacturing. [4,5] While these techniques greatly expand the catalog of accessible SMA architectures for specialized technological applications, such high-temperature processing techniques can have difficulty maintaining precise control over material chemistry. For example, selective laser melting and fusion welding techniques can cause NiTi in localized regions to greatly exceed its melting point (%1310 C), resulting in incongruent volatilization with preferential loss of nickel. The ensuing compositional shift leads to Ti-rich precipitate formation and products whose ductility and shape memory behavior may be significantly impaired. Optimization of processing parameters can reduce the magnitude of these effects, [5] but must be performed on a part-by-part basis. The development of post-processes that can precisely restore the elemental composition of heat-affected material could therefore help enable the wide application of these and other metal manufacturing techniques for NiTi-based SMAs.This work describes a method, SMAs via halide-activated pack equilibration (SHAPE), to manufacture NiTi-based SMAs with precisely controlled chemistries. The SHAPE method improves upon existing reaction-based processing routes employed to synthesize NiTi via solid-state diffusion of titanium into a target substrate. In prior work, drive-in diffusion from a gaseous phase has been accomplished by the use of titanium sponge as a vapor source in the presence of a halide transport agent [6] ; however, titanium cannot be in equilibrium with the desired product NiTi under typical process conditions (i.e., in accordance with the Ni-Ti phase diagram, [7] below 942 C titanium can only be in equilibrium with Ti 2 Ni). Consequently, target substrates in previous studies have been subject to over-titanization resulting in undesired Ti 2 Ni formation and ultimately degrading SMA performance. In the SHAPE method, targe...
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