Wearable gas sensors have received lots of attention for diagnostic and monitoring applications, and two-dimensional (2D) materials can provide a promising platform for fabricating gas sensors that ...can operate at room temperature. In the present study, the room temperature gas-sensing performance of Ti3C2T x nanosheets was investigated. 2D Ti3C2T x (MXene) sheets were synthesized by removal of Al atoms from Ti3AlC2 (MAX phases) and were integrated on flexible polyimide platforms with a simple solution casting method. The Ti3C2T x sensors successfully measured ethanol, methanol, acetone, and ammonia gas at room temperature and showed a p-type sensing behavior. The fabricated sensors showed their highest and lowest response toward ammonia and acetone gas, respectively. The limit of detection of acetone gas was theoretically calculated to be about 9.27 ppm, presenting better performance compared to other 2D material-based sensors. The sensing mechanism was proposed in terms of the interactions between the majority charge carriers of Ti3C2T x and gas species.
The thermal evolution of nanoscale oxide inclusions in 316L stainless steel (SS) manufactured by laser powder bed fusion additive manufacturing (AM) was explored. The size, chemical composition, ...morphology, and distribution of the oxides were characterized as the function of heat treatment conditions. The study revealed the mechanistic driving force of the rapid oxide coarsening during recrystallization. Ostwald ripening governs oxide coarsening. The active grain boundary-oxide interaction at the early stage of recrystallization accelerated oxide coarsening via enhanced solute diffusion along grain boundaries. Pipe diffusion along dislocation cellular boundaries has a negligible contribution to oxide coarsening. At high temperatures (T > 1065 °C), although lattice diffusion primarily controlled the oxide growth, the contribution from the grain-boundary diffusion was necessary. The transformation from MnSiO3 to CrMn2O4 took place in the un-recrystallized grains but was not observed when recrystallization started. The interaction of grain boundary and oxides during recrystallization resulted in a high fraction of oxides accumulated at grain boundaries. While oxide coarsening does not significantly alter the toughness value, grain-boundary oxides promote microvoid formation and intergranular fracture under Charpy impact in the recrystallized AM 316L SS.
The thermal stability of dislocation cellular structures in three additively manufactured (AM) austenitic stainless steels (SSs), 316L SS, 304L SS, and Al modified 316L SS (316L(Al)), were studied. ...Minor alloying elements, Mo and Al, were found affecting the stability of the cellular structures in AM austenitic SS, resulting in a stability ranking of AM 316L SS > AM 304L SS > AM 316L(Al) SS. As a result, their abilities towards recrystallization also differed. Owing to the high stacking fault energy (SFE) due to Al addition, AM 316L(Al) SS had the least stable subgrain cellular structure and exhibited the lowest recovery temperature. Although 316L SS possessed slightly higher SFE than 304L SS, the pinning effect due to Mo segregation at the cellular walls in AM 316L SS significantly enhanced its thermal stability. While the low-SFE AM 316L SS and AM 304L SS recovered their cellular structures via the equiaxed cell growth, the dislocation cellular walls in high-SFE AM 316L(Al) SS continuously vanished along a preferred direction. The fast recovery of cellular structures led to recrystallization retardation. The Hall–Petch model was found incapable of correlating cell size to strength because of the continuous weakening of cellular walls during heat treatment.
This work reports on employing X-ray computed tomography (XCT) to develop a predictive model aimed at optimizing laser process parameters for laser powder bed fusion. A commercially available ...statistical analysis software was successfully combined with XCT obtained porosity data obtained from 316L stainless steel to develop an accurate model that predicted the parameter sets and ranges with the lowest porosity. The predictions indicated that laser velocity and hatch spacing had a numerically linear relationship with laser power and can be combined to minimize porosity at any selected laser power in a specific range. In fact, the predictions indicated that the minimum porosity at any laser power is associated with a specific line energy input of approximately 0.13 J/mm for this alloy. The lowest predicted porosity at each laser power was fabricated and tested with the 85- and 92-W powers confirming ultra-low porosity. Lower laser powers, however, exhibited significantly higher porosity in contrast with the prediction. This resulted from the lower hatch spacing and velocity causing higher energy density and metallurgical defects from macro-balling. Thermodynamic calculations in the optimum laser power range yielded a line energy of 0.131 J/mm, which agrees rather well with the XCT predicted line energy and indicates that porosity generation is governed by the thermo-physical behavior of the alloy. A parameter space in the optimum range was fabricated and confirmed that the lowest porosities exist along a line energy of 0.13 J/mm, where melt pool temperature was predicted to be between 2526 and 2785 °C.
This work reports on employing X-ray computed tomography (XCT) and optical microscopy to investigate the causal relationships between printing parameters, pore properties, and porosity in 316L ...stainless steel samples additively manufactured by Laser Powder Bed Fusion (LPBF). The porosity is very similar for both investigation methods. XCT provides more accurate results for large lack of fusion pores, while optical results are more accurate for small keyhole pores. These results were employed to develop mathematical models to determine how printing parameters influence pore properties and overall porosity. The developed optical and XCT mathematical models reveal that power is the most significant factor affecting pore properties and overall porosity. Pore number and mean diameter decrease and sphericity increases with increasing power. Overall porosity is negatively correlated with power, indicating that the higher the power, the lower the overall porosity. Attention should also be paid to the quadratic effects of power, velocity and hatch spacing on porosity, revealing an inverse change in porosity after a certain threshold. Power interacts with velocity and hatch spacing, suggesting that changes in power affect the influence of velocity and hatch spacing on porosity, and vice versa. The interaction of velocity and hatch spacing is not significant. Both models successfully predicted optimal printing parameter sets as validated by experimental measurements.
This work investigates the impact of Bi content on the microstructure of solder joints under various thermal cycling conditions. Five lead-free solder alloys were used to assemble test boards and ...subjected to thermal cycling according to JEDEC standards. After reflowing, the boards were thermally cycled after 10 days of storage at room temperature. The thermal profile JESD22-A104E was designed for cycling from −40°C to +125°C with ramp and dwell times of 16.5 and 15 minutes, respectively, at extreme temperatures. The microstructural evolution was studied using scanning electron microscopy and energy-dispersive spectroscopy at various cycle intervals. Backscatter electron imaging was used to produce electron micrographs of the specimens for each test combination. Four high-strain localized regions were identified to examine the Ag
3
Sn particles for each test condition. The multiple threshold approach was used to analyze particle size, number, and density using the surface imaging and analysis software Mountains® 9. The results revealed that the effect of adding Bi on the microstructure was instantly apparent, as solder joints with 3% Bi had more uniformly dispersed precipitates. During the accelerated temperature cycling process, the Ag
3
Sn precipitates underwent strain-enhanced coarsening and localized re-crystallization. SAC-3Bi exhibited the slowest growth of Ag
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Sn particle size compared to other SAC-based alloys. In addition, the growth rate of the intermetallic compound layer for SAC with Bi alloys was slower compared to SAC305. This was due to the solid solution hardening processes, in which the presence of micro-alloys plays a vital role in strengthening solder joints.
This study introduces a novel technique to implement a locking hole system into AM patient-specific implants without the need of additional post-processing steps such as mechanical machining. This ...has the potential to decrease the time and cost of manufacturing these implants, providing surgeons with an additional option, that is better suited in cases where the underlying bone is already weakened or bone porosis is an inherent risk. A commercially available locking system was chosen and replicated using high-resolution X-ray CT. A biocompatible material, 316L stainless steel was used to print specimen on a L-PBF machine in different orientations. The specimen were heat treated to tune the mechanical properties to enable the locking system to work. The accuracy of the printed holes was confirmed using a nominal/actual comparison between the original and printed holes. The strength of the system was evaluated by measuring the force needed to push the screw out of the locking plate. The 316L stainless steel samples, when annealed to tailor hardness, performed similarly to the commercial system. This included different build orientations that suggest the locking system can be included in AM implants without the need for additional post-processing steps.
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•A locking fixation system was printed in different orientations using L-PBF•The geometry of the printed system was compared to the original using X-ray CT•The mechanical properties of the used material was characterized•The strength of the printed system was compared to the original
This work focused on ascertaining the effect of pile-up during indentation of thin films on substrates. Conventional understanding has postulated that differences in contact area resulting from ...pile-up or sink-in significantly alter the extraction of material properties. In this work, the specific case of pile-up with compliant, plastically deforming films on stiff, nonplastically deforming substrates was studied. Several literature methods to assess pile-up were leveraged, and a new technique was developed and validated to quantify projected pile-up. Indentation testing was performed on gold films of multiple thicknesses on several ceramic-based substrates. The results indicated that the degree of pile-up was solely a function of indent depth into the film. Pile-up was not influenced by film thickness or substrate elastic modulus. In other words, the pile-up development was insensitive to the presence of the substrate and how it contributes to the composite's elastic properties. In such case, if the elastic response of the film/substrate composite was independent of the degree of pile-up, then elastic data acquired from unloading did not require a contact area correction. The findings are confirmed using the Zhou–Prorok model for extracting film elastic properties for both gold and platinum films.
In this study, a new experimental method is proposed to measure the real area of contact between a ceramic sphere and an Al surface based on the adhesive transfer of the Au film and the scanning ...electron microscope (SEM) in the back-scattered mode. A thin film of Au is sputtered on the ceramic sphere before the indentation with the Al surface. The success of this method relies on the fundamental assumption that the adhesive transfer of Au only occurs everywhere inside the contact area. A thin polymer (PMMA) film is deposited between gold film and the ceramic surface to further reduce adhesive strength. After indentation, the interfaces of the ceramic sphere and Al surface are observed by SEM. Experimental evidence that the adhesive transfer of the Au film occurs inside the contact area is given. The entire contact regions on the ceramic sphere and the Al surface are captured in the second electron and back-scattered images with a magnification of 220× (resolution: 432 nm, i.e., distance between neighboring pixels). The contact area can be identified based on both the distributions of the ceramic and Au on the ceramic sphere and Al surface, respectively. The back-scattered images with the magnifications of 5000× and 10,000× (resolution: 20 and 4 nm) are captured at four different locations along the radial direction (starting from the contact center), respectively. The real area of contact decreases from the center to the contact edge.
Extensive research has been conducted on Ti-Fe-Sn ultrafine eutectic composites due to their high yield strength, compared to conventional microcrystalline alloys. The unique microstructure of ...ultrafine eutectic composites, which consists of the ultrafine-grained lamella matrix with the formation of primary dendrites, leads to high strength and desirable plasticity. A lamellar structure is known for its high strength with limited plasticity, owing to its interface-strengthening effect. Thus, extensive efforts have been conducted to induce the lamellar structure and control the volume fraction of primary dendrites to enhance plasticity by tailoring the compositions. In this study, however, it was found that not only the volume fraction of primary dendrites but also the morphology of dendrites constitute key factors in inducing excellent ductility. We selected three compositions of Ti-Fe-Sn ultrafine eutectic composites, considering the distinct volume fractions and morphologies of β-Ti dendrites based on the Ti-Fe-Sn ternary phase diagram. As these compositions approach quasi-peritectic reaction points, the α″-Ti martensitic phase forms within the primary β-Ti dendrites due to under-cooling effects. This pre-formation of the α″-Ti martensitic phase effectively governs the growth direction of β-Ti dendrites, resulting in the development of round-shaped primary dendrites during the quenching process. These microstructural evolutions of β-Ti dendrites, in turn, lead to an improvement in ductility without a significant compromise in strength. Hence, we propose that fine-tuning the composition to control the primary dendrite morphology can be a highly effective alloy design strategy, enabling the attainment of greater macroscopic plasticity without the typical ductility and strength trade-off.
