Tungsten (W) refractory alloys are of high importance in the development of high temperature application products seen in furnace elements, the aerospace industry, and many other areas. Traditional manufacturing processes produce tungsten alloys with undesirable mechanical properties due to a large grain microstructure. Commercial sintering techniques yield low density products due to the poor sinterability of tungsten alloys. Field Assisted Sintering Technology (FAST) is used in this work to produce tungsten alloys with high density, and acceptable microstructures. Limited research has been done with hafnium carbide (HfC) as grain growth inhibitors. 100% W, W-1 vol%HfC, W-2vol%HfC, and W-5vol%HfC were sintered at 2100 degrees C, 35 MPa, for 25 min. Microstructure of each composition was characterized and reported. For volume additions of 2% or more of hafnium carbide shows a decrease in grain size of over 67% while increasing the hardness by over 19% when compared with a pure tungsten composition. These results include imaging between the W-HfC interfaces gathered from high resolution transmission electron microscopy (HRTEM).

ZrB2 and HfB2 with incorporation of SiC are being considered as structural materials for elevated temperature applications. We used high energy ball milling of micron-size powders to increase lattice distortion enhanced inter-diffusion to get uniform distribution of SiC and reduce grain growth during Spark Plasma Sintering (SPS). High-energy planetary ball milling was performed on ZrB2 or HfB2 with 20vol% SiC powders for 24 and 48 hrs. The particle size distribution and crystal micro-strain were examined using Dynamic Light Scattering Technique and x-ray diffraction (XRD), respectively. XRD spectra were analyzed using Williamson-Hall plots to estimate the crystal micro-strain. The particle size decreased, and the crystal micro-strain increased with the increasing ball milling time. The SPS consolidation was performed at 32 MPa and 2,000°C. The SEM observation showed a tremendous decrease in SiC segregation and a reduction in grain size due to high energy ball milling of the precursor powders. Flexural strength of the SPS consolidated composites were studied using Four-Point Bend Beam test, and the micro-hardness was measured using Vickers micro-indenter with 1,000 gf load. Good correlation is observed in SPS consolidated ZrB2+SiC with increased micro-strain as the ball milling time increased: grain size decreased (from 9.7 to 3.2 μm), flexural strength (from 54 to 426 MPa) and micro-hardness (from 1528 to 1952 VHN) increased. The correlation is less evident in HfB2+SiC composites, especially in micro-hardness which showed a decrease with increasing ball milling time.

Tantalum carbide ceramics with high volume fractions of the ζ-Ta4C3 phase have been shown to exhibit high fracture strength and toughness as compared to those in absence of this phase. In this work, we investigated how microcracks propagated in this these high toughness ceramics using Knoop and Vickers microindentation. The Knoop indentations demonstrated that cracking preferentially occurred parallel to the lath structure in ζ-Ta4C3; however shorter cracks did form between the laths when a sufficient driving force was present. The resulting crack path was tortuous providing direct evidence for toughening through crack deflection; however, the microscale nature of the work cannot rule out crack bridging as a toughening mechanism as well. Plasticity is also observed under the indents, but is likely a result of the high confining pressures that occurred during indentation allowing for plastic flow.

Refractory metal alloys such as tungsten and tantalum have long been primary materials of use in high temperature applications. The difficulty of processing tungsten and tantalum alloys makes the job of economically fabricating components from these alloys challenging. Furthermore, despite their intended use at elevated temperatures, limited research work has been to done to evaluate mechanical properties of these alloys at elevated temperatures. In this work, the mechanical properties of three tantalum alloys, pure Ta, Ta-10 vol%W, and Ta-10 vol%W-1.5 vol%TiC are studied using a range of mechanical testing procedures. Pure tantalum is found to display higher ductility at both room and elevated temperatures in comparison to alloyed samples. Addition of both 10 vol%W and 1.5 vol%TiC addition is found to result in comparable high temperature strength and ductility in comparison to 10 vol%W addition, and improved hardness and flexural strength at room temperature. XRD and EDS analysis paired with microscopy of sintered materials and there fracture surfaces suggest this improvement is associated with solid solution strengthening occurring at Ta-W grain boundary regions. Analysis of sample fracture surfaces reveals indications of increased sample ductility at elevated temperatures for all alloys shown by tearing of tensile bar surfaces near the fracture regime and other ductile features.

Ultra-high temperature ceramics such as ZrB 2 and HfB 2 with small percentage of SiC are useful as structural materials for applications in leading edge of hypersonic vehicles [1].Spark plasma sintering (SPS) technique is used for densifying the UHTCs under the influence of uniaxial pressure and pulsed direct current [2].Fine grain, low porosity, and high densification yield higher micro-hardness ceramics those can be used for high temperature oxidation resistant materials.Here we made a comparative study between Vickers and Knoop micro-hardness of SPS consolidated UHTC composites starting with micron-and nano-powders.

Spark Plasma Sintering (SPS) consolidated ZrB2+SiC composites using nano-powders (around 40 nm) showed smaller grains compared to those using micron size powders and segregation of SiC into islands is minimal but with higher oxidation of ZrB2 to form ZrO2 in nano-composites. Argon-gas purging prior to SPS consolidation at around 2000 °C and 40 MPa of ZrB2+20vol.%SiC nano-powders was used to minimize the oxidation and obtain fine granules with high densification. The densification of the Argon-gas purged nano-composites is higher compared to those consolidated without Argon gas purging. The EDX analysis showed a strong reduction in the oxygen peak for the Argon gas purged composites. The XRD spectra also support this observation with less ZrO2 phase composition in Argon gas purged composites. The Vickers micro-hardness showed slightly lower values for Argon gas purged composites though they have higher densification.

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Ultra-High-Temperature Ceramics (UHTCs) such as ZrB2 and HfB2 with incorporation of SiC nanofiller are useful as structural materials for applications in propulsion and thermal protection systems such as turbine-engine hot section components, leading edge of hypersonic vehicles, where extremely high heat fluxes generate very high temperatures and steep temperature gradients [1]. Spark plasma sintering (SPS) technique is used for densifying the UHTCs under the influence of uniaxial pressure and pulsed direct current [2]. Here, we study the densification, grain growth, and microhardness of ZrB2 nanocomposites with 15% and 20% SiC consolidated using SPS.

A polycrystalline α-Ta2C bar, fabricated by hot isostatic pressing, was tested in four-point bending at approximately 1930°C. The sample showed significant plastic deformation. Transmission electron microscopy, using two-beam defect analysis, confirmed pyramidal dislocation slip of the type a/3〈112¯3〉{101¯1}. Basal and prismatic slip of a/3〈112¯0〉 type could not be determined because of significant contrast interference from stacking faults that formed in the material. The increase in stacking fault density observed after thermomechanical loading is not believed to be caused by deformation but rather is due to growth faults that formed from the slight carburization of Ta2C from the graphite heating filaments in the testing apparatus. The significant plasticity accommodation in Ta2C is a result of basal and non-basal slip and the wide spacing of the Ta–Ta metallic bonds that form inherent to its crystallography.

A series of XTa:(1 − X)C (0.5 < X < 1) compositions have been fabricated by hot isostatic pressing (HIP) of Ta and TaC powder blends. Depending upon the targeted stoichiometry, single- or multiple-phase microstructures formed. The single-phase microstructures of both TaC and Ta2C had equiaxed grain morphologies. The multiphase microstructures had either equiaxed TaC grains with a crisscross pattern of Ta4C3 laths or acicular grain morphologies with rafts of TaC, Ta4C3 and Ta2C laths running parallel to the major axis of the grains. The effect of phase transformations on the microstructure of these specimens is discussed and compared to those microstructures seen in a reaction diffusion couple formed between Ta and TaC powders processed under the same HIP conditions. This couple revealed the depletion of carbon from the TaC phase and its reaction with the tantalum metal to form the various Ta-rich carbide phases. The precipitation sequence was found to be paramount in controlling the grain morphology. A close-packed plane and direction orientation relationship was seen between all the phases. The crisscross pattern of Ta4C3 precipitation in TaC was a consequence of TaC's multiple variant {1 1 1} orientations and had little or no effect on the grain morphology. In contrast, the single variant close-packed plane {0 0 0 1} in Ta2C resulted in the parallel alignment of the precipitated phases within its grain and an anisotropic growth direction that facilitated the acicular grain morphology.

The fabrication of lightweight mirror assemblages via a replication technique offers great potential for eliminating the high cost and schedule associated with the grinding and polishing steps needed for conventional glass or SiC mirrors. A replication mandrel is polished to an inverse figure shape and to the desired finish quality. It is then, coated with a release layer, the appropriate reflective layer, and followed by a laminate for coefficient of thermal expansion (CTE) tailorability and strength. This optical membrane is adhered to a mirror structural substrate with a low shrinkage, CTE tailored adhesive. Afterwards, the whole assembly is separated from the mandrel. The mandrel is then cleaned and reused for the next replication run. The ultimate goal of replication is to preserve the surface finish and figure of the optical membrane upon its release from the mandrel. Successful replication requires a minimization of the residual stresses within the optical coating stack, the curing stresses from the adhesive and the thermal stress resulting from CTE mismatch between the structural substrate, the adhesive, and the optical membrane. In this paper, the results on replicated trials using both metal/metal and ceramic/ceramic laminates adhered to light weighted structural substrates made from syntactic foams (both inorganic and organic) will be discussed.

The first Optical Reflector Materials Experiment (ORMatE-I) is on-board MISSE-6. The follow-on experiment, ORMatE-II, is part of MISSE-7. Both these projects are a collaborative effort among The Aerospace Corporation, the US Naval Research Laboratory (NRL), and the Air Force Research Laboratory Materials Directorate (AFRL/ML). ORMatE-I is a study of optically reflective materials focused on SiC for use as a lightweight mirror substrate. Several types of SIC material grown by different methods and vendors are included as well as diverse coating materials and deposition techniques. Advanced glass substrate technologies, like ULE and corrugated borosilicate, are also on-board. Additional SiC and composite materials will be evaluated on ORMatE-II along with silver mirrors deposited by various means. A description of both experiment suites and a summary of the pre-flight optical characterization will be presented.

: The use of monolithic glass and beryllium to produce lightweighted aerospace mirror systems has reached its limits due to the long lead times, high processing costs, environmental effects and launch load/weight requirements. New material solutions and manufacturing processes are required to meet DoD's directed energy weapons, reconnaissance/surveillance, and secured communications needs. Over the past several years the Air Force, MDA, and NASA has focused their efforts on the fabrication, lightweighting, and scale-up of numerous silicon carbide (SiC) based materials. It is anticipated that SiC can be utilized for most applications from cryogenic to high temperatures. This talk will focus on describing the SOA for these (near term) SiC technology solutions for making mirror structural substrates, figuring and finishing technologies being investigated to reduce cost time and cost, and non-destructive evaluation methods being investigated to help eliminate risk. Mirror structural substrates made out of advanced engineered materials (far term solutions) such as composites, foams, and microsphere arrays for ultra lightweighting will also be briefly discussed.

The use of monolithic glass to produce large, rigid segmented members for lightweight space-based mirror systems appears to have reached its limits due to the long production lead times, high processing costs, and launch load/weight requirements. New material solutions and processes are required to meet the US Air Force's optical needs for directed energy, reconnaissance/surveillance, and communications. Mirror structural substrates made out of advanced materials (metal, ceramic, and polymer), composites, foams, and microsphere arrays should allow for CTE and modulus tailorability, low-density, and high values in strength, stiffness, thermal conductivity and toughness. Conventional mechanical polishing to visual specifications for figure and surface finish roughness requirements will be difficult, due to the multi-phase complexities of these new systems. Advances in surface removal technologies as well as replication processes will be required to produce the required optical finishes with reduced schedule and cost. In this paper selected material and process solutions being considered will be discussed.

POINTS, an optical astrometric interferometer to be operated in space, would be a means of performing a wide variety of astrophysical studies, including a vastly improved deflection test of general relativity, a precise and direct calibration of the Cepheid distance scale, and the determination of stellar masses. The nominal 5μas uncertainty in the measurement of the angular separation of two stars about 90^deg^ apart in the sky and the estimated measurement rate of 60 star pairs per day would support a rich mixture of scientific projects during the nominal mission life of ten years. Useful results would be available after less than a year. The key to the instrument's success is the control of systematic error, which we address by instrumentation and postanalysis of the astrometric data.

This paper describes methods used to simulate the performance of Z-package infrared focal plane arrays. Signal generation and transfer, noise generation, and bandpass filtering are discussed for nonspecific (generic) Z-packaged focal plane arrays.