Understanding fracture in laser additive manufactured bulk metallic glass through small-scale mechanical measurement

Bulk metallic glasses (BMGs) are amorphous metal alloys formed by fast cooling that display high strength and toughness with good resistance to corrosion and wear. One traditional limitation has been that BMG castings are often limited to \u3c1 cm dimensions due to the high cooling rates needed. The recent development of selective laser melting (SLM) of metallic glasses opens up the possibility of creating large BMG components with complex geometries. However, we have recently shown that additive manufactured BMGs exhibit poor ductility and toughness when compared to their traditionally as-cast (AC) counterparts (Fig. 1 A-C). Our work investigates how the processing route influences the structure of a Zr-based BMG, and how this is linked to mechanical performance. Evaluation at the micro-scale is critical, as thermal influences on the microstructure from laser-processing and melt-pool solidification exist at these length-scales. Experimental calorimetry results have shown enthalpic relaxation variation between cast Zr-based glasses and those manufactured with SLM-processing, suggesting differences in free volume for different processing routes. The effect on the fracture properties was studied using single edge notched beam bending tests: SLM-processed alloy showed significantly lower fracture toughness when compared with the as-cast alloy, and this was explained by energetic barriers for activating shear transformations in the glass, elucidated in detail using micro-pillar compression testing (Fig. 1 D/E). These results are further related to the glassy laser-processed structure through advanced structural analyses using synchrotron X-ray diffraction and nanoindentation. Please click Additional Files below to see the full abstract

Improving Participation of Engineering Students Studying Abroad: An International Dual Degree Program in Materials Science and Mechanical Engineering

Without AbstractThis is an author's peer-reviewed final manuscript, as accepted by the publisher. The published article is copyrighted by Springer and can be found at: http://link.springer.com/journal/11837

Sea Urchins Predation Facilitates Coral Invasion in a Marine Reserve

Macroalgae is the dominant trophic group on Mediterranean infralittoral rocky bottoms, whereas zooxanthellate corals are extremely rare. However, in recent years, the invasive coral Oculina patagonica appears to be increasing its abundance through unknown means. Here we examine the pattern of variation of this species at a marine reserve between 2002 and 2010 and contribute to the understanding of the mechanisms that allow its current increase. Because indirect interactions between species can play a relevant role in the establishment of species, a parallel assessment of the sea urchin Paracentrotus lividus, the main herbivorous invertebrate in this habitat and thus a key species, was conducted. O. patagonica has shown a 3-fold increase in abundance over the last 8 years and has become the most abundant invertebrate in the shallow waters of the marine reserve, matching some dominant erect macroalgae in abundance. High recruitment played an important role in this increasing coral abundance. The results from this study provide compelling evidence that the increase in sea urchin abundance may be one of the main drivers of the observed increase in coral abundance. Sea urchins overgraze macroalgae and create barren patches in the space-limited macroalgal community that subsequently facilitate coral recruitment. This study indicates that trophic interactions contributed to the success of an invasive coral in the Mediterranean because sea urchins grazing activity indirectly facilitated expansion of the coral. Current coral abundance at the marine reserve has ended the monopolization of algae in rocky infralittoral assemblages, an event that could greatly modify both the underwater seascape and the sources of primary production in the ecosystem

A highly efficient degradation mechanism of methyl orange using Fe-based metallic glass powders

A new Fe-based metallic glass with composition Fe76 B12 Si9 Y3 (at. %) is found to have extraordinary degradation efficiency towards methyl orange (MO, C14 H14 N3 SO3) in strong acidic and near neutral environments compared to crystalline zero-valent iron (ZVI) powders and other Fe-based metallic glasses. The influence of temperature (294-328 K) on the degradation reaction rate was measured using ball-milled metallic glass powders revealing a low thermal activation energy barrier of 22.6 kJ/mol. The excellent properties are mainly attributed to the heterogeneous structure consisting of local Fe-rich and Fe-poor atomic clusters, rather than the large specific surface and strong residual stress in the powders. The metallic glass powders can sustain almost unchanged degradation efficiency after 13 cycles at room temperature, while a drop in degradation efficiency with further cycles is attributed to visible surface oxidation. Triple quadrupole mass spectrometry analysis conducted during the reaction was used to elucidate the underlying degradation mechanism. The present findings may provide a new, highly efficient and low cost commercial method for azo dye wastewater treatment

On the fatigue behavior of g-based titanium aluminides: role of small cracks

AbstractÐGamma-TiAl based alloys have recently received attention for potential elevated temperature applications in gas-turbine engines. However, although expected critical crack sizes for some targeted applications (e.g. gas-turbine engine blades) may be less than 0500 mm, most fatigue-crack growth studies to date have focused on the behavior of large (on the order of a few millimeters) through-thickness cracks. Since successful implementation of damage-tolerant life-prediction methodologies requires that the fatigue properties be understood for crack sizes representative of those seen in service conditions, the present work is focused on characterizing the initiation and growth behavior of small (a025±300 mm) fatigue cracks in a g-TiAl based alloy, of composition Ti±47Al±2Nb±2Cr±0.2B (at.%), with both duplex (average grain size of 017 mm) and re®ned lamellar (average colony size of 0145 mm) microstructures. Results are compared to the behavior of large (a>5 mm), through-thickness cracks from a previous study. Superior crack initiation resistance is observed in the duplex microstructure, with no cracks nucleating after up to 500 000 cycles at maximum stress levels (R = 0.1) in excess of the monotonic yield stress, s y. Comparatively, in the lamellar microstructure cracks nucleated readily at applied maximum stresses below the yield stress (85% s y) after as few as 500 cycles. In terms of crack growth, measurements for small fatigue cracks in the duplex and lamellar microstructures showed that both microstructures have comparable intrinsic fatiguecrac

Elevated temperature mechanical properties of TiCN reinforced AlSi10Mg fabricated by laser powder bed fusion additive manufacturing

This short communication presents 2–4 μm sized TiCN reinforced AlSi10Mg composites (TiCN/AlSi10Mg) fabricated by laser powder bed fusion (LPBF) with refined Al-Si eutectic microstructure consisting of equiaxed bi-modal α-Al grains and enhanced elevated temperature tensile strength. The formation mechanism of reported bi-modal structure is related to the modified temperature gradients and induced heterogeneous nucleation in LPBF. The enhancement in elevated temperature tensile strength is mainly attributed to refined bi-modal Al-Si microstructure and thermal stability of TiCN particles

Helical and Bouligand Porous Scaffolds Fabricated by Dynamic Low Strength Magnetic Field Freeze Casting

Porous Fe3O4 scaffolds were fabricated while subject to a low (7.8 mT) magnetic field applied in helical and Bouligand motions using a custom-built tri-axial nested Helmholtz coils-based freeze-casting setup. This setup allowed for the control of a dynamic, uniform low-strength magnetic field to align particles during the freezing process, resulting in the majority of lamellar walls aligning within ± 30° of the magnetic field direction and a decrease in porosity by up to 42%. Similar to how helical and Bouligand structures in nature promote impact resistance, these magnetic field motions produced structures with improved high strain rate mechanical properties. Strain at failure was increased by up to 2 times as cracks deflected to match the applied angles of rotation of the magnetic field