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1. Microstructural Impact on Flank Wear of Various Ti-6Al-4V Alloys

   Nguyen, D. ( July 2018 , WeAr)

   Titanium alloys typically do not contain hard inclusion phases typically observed in other metallic alloys. However, the characteristic scoring marks and more distinctive micro- and/or macro-chippings are ubiquitously observed on the flank faces of cutting tools in machining titanium alloys, which is the direct evidence of abrasive wear (hard phase(s) in the microstructure abrading and damaging the flank surface). Thus, an important question lies with the nature of the hard phases present in the titanium microstructure. In this work, we present a comprehensive study that examines the microstructural impact on flank wear attained by turning various Ti-6Al-4V bars having distinct microstructures with uncoated carbide inserts. In particular, four samples with elongated, mill-annealed, solution treated & annealed and fully-lamellar microstructures were selected for our turning experiments. After turning each sample, the flank surface of each insert was observed with confocal laser scanning microscopy (CLSM) and analyzed to determine the flank wear behavior in relation to each sample' distinct microstructures. To characterize the microstructure, scanning electron microscopy (SEM) together with Orientation imaging microstructure (OIM) was used to identify and distinguish the phases present in each sample and the content and topography of each phase was correlated to the behavior of flank wear. The flank wear is also affected by the interface conditions such as temperature and pressure, which were estimated using finite element analysis (FEA) models. The temperature dependent abrasion models enable us to estimate the flank wear rate for each microstructure, and are compared with the experimentally measured wear data. 

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2. Morphology and kinetics of asphalt binder microstructure at gas, liquid and solid interfaces

   https://doi.org/10.1111/jmi.12842  

   RAMM, A. ; DOWNER, M. C. ; SAKIB, N. ; BHASIN, A. ( November 2019 , Journal of Microscopy)

   We combined optical and atomic force microscopy to observe morphology and kinetics of microstructures (typically referred to as bees) that formed at free surfaces of unmodified Performance Graded (PG) 64‐22 asphalt binders upon cooling from 150°C to room temperature (RT) at 5°C min^–1, and changes in these microstructures when the surface was terminated with a transparent solid (glass) or liquid (glycerol) overlayer. The main findings are: (1) at free binder surfaces, wrinkled microstructures started to form near the crystallization temperature (∼45°C) of saturates such as wax observed by differential scanning calorimetry, then grew to ∼5 µm diameter, ∼25 nm wrinkle amplitude and 10–30% surface area coverage upon cooling to RT, where they persisted indefinitely without observable change in shape or density. (2) Glycerol coverage of the binder surface during cooling reduced wrinkled area and wrinkle amplitude three‐fold compared to free binder surfaces upon initial cooling to RT; continued glycerol coverage at RT eliminated most surface microstructures within ∼4 h. (3) No surface microstructures were observed to form at binder surfaces covered with glass. (4) Submicron bulk microstructures were observed by near‐infrared microscopy beneath the surfaces of all binder samples, with size, shape and density independent of surface coverage. No tendency of such structures to float to the top or sink to the bottom of mm‐thick samples was observed. (5) We attribute the dependence of surface wrinkling on surface coverage to variation in interface tension, based on a thin‐film continuum mechanics model.

   Asphalt binder, or bitumen, is the glue that holds aggregate particles together to form a road surface. It is derived from the heavy residue that remains after distilling gasoline, diesel and other lighter products out of crude oil. Nevertheless, bitumen varies widely in composition and mechanical properties. To avoid expensive road failures, bitumen must be processed after distillation so that its mechanical properties satisfy diverse climate and load requirements. International standards now guide these mechanical properties, but yield varying long‐term performance as local source composition and preparation methods vary.In situdiagnostic methods that can predict bitumen performance independently of processing history are therefore needed. The present work focuses on one promising diagnostic candidate: microscopic observation of internal bitumen structure. Past bitumen microscopy has revealed microstructures of widely varying composition, size, shape and density. A challenge is distinguishing bulk microstructures, which directly influence a binder's mechanical properties, from surface microstructures, which often dominate optical microscopy because of bitumen's opacity and scanning‐probe microscopy because of its inherent surface specificity. In previously published work, we used infrared microscopy to enhance visibility of bulk microstructure. Here, as a foil to this work, we use visible‐wavelength microscopy together with atomic‐force microscopy (AFM) specifically to isolatesurfacemicrostructure, to understand its distinct origin and morphology, and to demonstrate its unique sensitivity to surface alterations. To this end, optical microscopy complements AFM by enabling us to observe surface microstructures form at temperatures (50°C–70°C) at which bitumen's fluidity prevents AFM, and to observe surface microstructure beneath transparent, but chemically inert, liquid (glycerol) and solid (glass) overlayers, which alter surface tension compared to free surfaces. From this study, we learned, first, that, as bitumen cools, distinctly wrinkled surface microstructures form at the same temperature at which independent calorimetric studies showed crystallization in bitumen, causing it to release latent heat of crystallization. This shows that surface microstructures are likely precipitates of the crystallizable component(s). Second, a glycerol overlayer on the cooling bitumen results in smaller, less wrinkled, sparser microstructures, whereas a glass overlayer suppresses them altogether. In contrast, underlying smaller bulk microstructures are unaffected. This shows that surface tension is the driving force behind formation and wrinkling of surface precipitates. Taken together, the work advances our ability to diagnose bitumen samples noninvasively by clearly distinguishing surface from bulk microstructure.

    

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3. Wear Performance Evaluation of Minimum Quantity Lubrication with Exfoliated Graphite Nanoplatelets in Turning Titanium Alloy

   Nguyen, D. ( May 2019 , Journal of manufacturing science and engineering)

   This paper evaluates the performances of dry, minimum quantity lubrication (MQL), and MQL with nanofluid conditions in turning of the most common titanium (Ti) alloy, Ti-6Al-4 V, in a solution treated and aged (STA) microstructure. In particular, the nanofluid evaluated here is vegetable (rapeseed) oil mixed with small concentrations of exfoliated graphite nanoplatelets (xGnPs). This paper focuses on turning process that imposes a challenging condition to apply the oil or nanofluid droplets directly onto the tribological surfaces of a cutting tool due to the uninterrupted engagement between tool and work material during cutting. A series of turning experiments was conducted with uncoated carbide inserts, while measuring the cutting forces with a dynamometer under the dry, MQL and MQL with nanofluid conditions supplying oil droplets externally from our MQL device. The inserts are retrieved intermittently to measure the progress of flank and crater wear using a confocal microscopy. This preliminary experimental result shows that MQL and in particular MQL with the nanofluid significantly improve the machinability of Ti alloys even in turning process. However, to attain the best performance, the MQL conditions such as nozzle orientation and the concentration of xGnP must be optimized. 

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4. Quantifying Microstructure Features for High-Performance Solid Oxide Cells

   C. M. Ruse, L. A. ( April 2023 , arXivorg)

   Focused ion beam (FIB) – scanning electron microscopy (SEM) allowed the characterization of the microstructure of two solid oxide fuel cells prepared at different sintering temperatures. 3D volume reconstruction showed that a relatively low sintering temperature significantly and positively affected distribution, volume and particle size of yttria-stabilized zirconia, nickel, and pore phases inside the anode, as well as the extent of the important triple-phase boundary interface. The poor performance of the T1 sample sintered at a higher temperature is explained by the poorly connected pore network and very low-density triple-phase boundary. The pore space inside the T1 anode was unable to ensure continuous hydrogen flow from the inlet to the outlet and thus exhibited very low gas permeability. In contrast, the T2 sample sintered at a lower temperature had approximately equal amounts of YSZ and nickel and larger pores, which allowed formation of significantly more TPB electrochemical reaction sites. The higher power density of the T2 cell was also the result of its robust pore network capable of transporting hydrogen throughout the anode. The methodology used in this paper eliminates the need for employing hypothetical structures and provides accurate estimates of the investigated parameters by evaluating microstructures that were successfully reconstructed using high-resolution microscopy techniques. 

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5. Hydroxyapatite reinforced Ti6Al4V composites for load-bearing implants

   https://doi.org/doi.org/10.1016/j.actbio.2020.12.060  

   Avila, Jose ; Stenberg, Kevin ; Bose, Susmita ; Bandyopadhyay, Amit ( March 2021 , Acta biomaterialia)

   null (Ed.)

   Titanium has been used in various biomedical applications; however, titanium exhibits poor wear resistance, and its bioinert surface slows osseointegration in vivo. In this study, directed energy deposition (DED)-based additive manufacturing (AM) was used to process hydroxyapatite (HA) reinforced Ti6Al4V (Ti64) composites to improve biocompatibility and wear resistance simultaneously. Electron micrographs of the composites revealed dense microstructures where HA is observed at the β-phase grain boundaries. Hardness was observed to increase by 57% and 71% for 2 and 3 wt.% HA in Ti64 composites, respectively. XRD analysis revealed no change in the present phases. Tribological studies revealed an increase in contact resistance due to in situ HA-based tribofilm formation, reduction in wear rate when testing in DMEM with a ZrO2 counter wear ball, ˂1% wear ball volume loss, and suppression of cohesive failure of the Ti matrix. Histomorphometric analysis from a rat distal femur study revealed an increase in the osteoid surface over the bone surface (OS/BS) for 3 wt.% HA composite over the control Ti64 from 9 ± 1% to 14 ± 1%. Shear modulus was also observed to increase from 17 ± 3 MPa for control Ti64 to 32 ± 5 MPa for the 3 wt.% HA composite after 5 weeks. Our study demonstrates that the addition of HA in Ti64 can simultaneously improve bone tissue-implant response and wear resistance. 

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