Intentionally seeding pores in additively manufactured alloy 718: Process parameters, microstructure, defects, and fatigue
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36 MATERIALS SCIENCE↗
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In-situ re-crystallization of heavily-irradiated Gd 2 Ti 2 O 7
Gadolinium titanate (Gd 2 Ti 2 O 7 , or GTO) and other lanthanide pyrochlores are interesting candidates for actinide waste disposal and fast ion conduction because the relevant material properties are intimately dependent on local cation structure. Therefore, a deep understanding of the kinetics associated with cation ordering and disordering is required if such material properties are to be tuned for specific device designs. To this end, single crystals of GTO were irradiated with 190 keV helium ions to a total fluence of 1 × 10 17 ions/cm 2 , amorphizing the sample surface to a depth of ~1 μm and resulting in significant He bubble accumulation. FIB lamellae lifted out from the irradiated sections were examined during heat treatment in the (scanning) transmission electron microscope. Two distinct stages of the re-crystallization of the amorphized material were observed. The material near the end of the ions’ range transformed first and with the same orientation as the pristine material. This was due to the close proximity of the pristine material and the presence of small defect fluorite seeds, but the propagation of this growth front was frustrated by the large pores (formerly bubbles) in the He accumulation layer. This was followed by heterogeneous nucleation of new crystallites at random orientations at the top of the He accumulation layer, which is attributed to the high surface area associated with the many small He bubbles in that region. Finally, it is inferred that the kinetics of grain growth in this material are significantly faster than the kinetics of grain nucleation.
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
Micromechanical and fatigue in situ synchrotron characterization of an additively manufactured superalloy with porosity
Additive manufacturing (AM) has the potential to transform component production, but its widespread adoption is constrained by defects, such as pores, that compromise structural integrity. Here, this study investigates the influence of porosity on the micromechanical and fatigue response of AM Inconel 718 (IN718), a widely used aerospace superalloy. One baseline specimen with minimal stochastic porosity and another intentionally seeded with lack of fusion (LOF) pores were examined using high-energy X-ray diffraction microscopy (HEDM) and micro-computed tomography during cyclic loading. Fatigue cracks in the LOF specimen initiated more frequently and at lower cycle counts than in the baseline specimen. The number fraction of fatigue cracks that grew was comparatively higher in the LOF specimen. Grain-level metrics, including stress and diffraction spot widths, were quantified using far-field HEDM. Across the thousands of grains detected in both specimens, the pores in the LOF specimen increased the variability of stress and spot width evolution in the azimuthal direction, the latter serving as a surrogate measure of plastic deformation. However, no correlations emerged between these metrics and grain proximity to fatigue crack initiation sites or pores, underscoring the limitations of grain-averaged metrics for predicting fatigue. Nonetheless, the rich dataset reported here provides a foundation for future modeling efforts.
High-energy X-ray diffraction microscopy↗
Effect of osteoblastic culture conditions on the structure of poly(DL-lactic-co-glycolic acid) foam scaffolds
Poly(DL-lactic-co-glycolic acid) (PLGA) foams are an osteoconductive support that holds promise for the development of bone tissue in vitro and implantation into orthopedic defects. Because it is desirable that foams maintain their shape and size, we examined a variety of foams cultured in vitro with osteoblastic cells. Foams were prepared with different porosities and pore sizes by the method of solvent casting/porogen leaching using 80, 85, and 90 wt% NaCl sieved with particle sizes of 150-300 and 300-500 microm and characterized by mercury intrusion porosimetry. Foams seeded with cells were found to have volumes after 7 days in static culture that decreased with increasing porosity: the least porous exhibited no change in volume while the most porous foams decreased by 39 +/- 10%. In addition, a correlation was observed between decreasing foam volume after 7 days in culture and decreasing internal surface area of the foams prior to seeding. Furthermore, foams prepared with the 300-500 microm porogen had lower porosities, greater mean wall thicknesses between adjacent pores, and larger volumes after 7 days in culture than those prepared with the smaller porogen. Two culture conditions for maintaining cells, static and agitated (in a rotary vessel), were found to have similar influences on foam size, cell density, and osteoblastic function for 7 and 14 days in culture. Finally, we examined unseeded foams in aqueous solutions of pH 3.0, 5.0, and 7.4 and found no significant decrease in foam size with degradation. This study demonstrates that adherent osteoblastic cells may collapse very porous PLGA foams prepared by solvent casting/particulate leaching: a potentially undesirable property for repair of orthopedic defects.
NASA Program Fundamental Space Biology↗
Nondestructive evaluation of structural ceramics by photoacoustic microscopy
A photoacoustic microscopy (PAM) digital imaging system was developed and utilized to characterize silicon nitride material at the various stages of the ceramic fabrication process. Correlation studies revealed that photoacoustic microscopy detected failure initiating defects in substantially more specimens than microradiography and ultrasonic techniques. Photoacoustic microscopy detected 10 to 100 micron size surface and subsurface pores and inclusions, respectively, up to 80 microns below the interrogating surface in machined sintered silicon nitride. Microradiography detected 50 micron diameter fracture controlling pores and inclusions. Subsurface holes were detected up to a depth of 570 microns and 1.00 mm in sintered silicon nitride and silicon carbide, respectively. Seeded voids of 20 to 30 micron diameters at the surface and 50 microns below the interrogating surface were detected by photoacoustic microscopy and microradiography with 1 percent X-ray thickness sensitivity. Tight surface cracks of 96 micron length x 48 micron depth were detected by photoacoustic microscopy. PAM volatilized and removed material in the green state which resulted in linear shallow microcracks after sintering. This significantly limits the use of PAM as an in-process NDE technique.
Khandelwal, Pramod K.↗