149 research outputs found
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.
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Fatigue threshold R-curves predict small crack fatigue behavior of bridging toughened materials
Small crack fatigue is a widely recognized problem in the fatigue of materials; however, there has been limited progress in developing methods for predicting small crack fatigue behavior. In this paper, small crack effects due to crack bridging are addressed. A fatigue threshold R-curve was measured for a 99.5% pure polycrystalline alumina using standard compact tension specimens and it was used to 1) determine the bridging stress profile for the material and 2) make fatigue endurance strength predictions for realistic semi-elliptical surface cracks. Furthermore, is has been shown that the fatigue threshold R-curve can equivalently be determined by measuring the bridging stress distribution, in this case using fluorescence spectroscopy, using only a long crack compact tension specimen without the need for difficult small crack experiments. It is expected that this method will be applicable to a wide range of bridging toughened materials, including composites, toughened ceramics, intermetallics, and multi-phase materials.Keywords: fracture, toughness, fatigue, crack bridgingKeywords: fracture, toughness, fatigue, crack bridgin
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Designed heterogeneities improve the fracture reliability of a Zr-based bulk metallic glass
Bulk metallic glasses (BMGs) have been reported to have large variability in mode I fracture toughness. In this study, alternating soft and hard regions were created in a Zrā
ā.ā
Tiā
CuāāNiāā.ā
Alāā (at%) BMG via mechanical imprinting at room temperature. While only 50% of as-cast samples demonstrated plastic deformation during mode I fracture tests, 100% of imprinted samples demonstrated measurable plasticity and the scatter in measured mode I fracture toughness was significantly reduced. Mechanical treatments show promise for improving the fracture reliability of BMGs.Keywords: Fracture toughness, Fracture morphology, Shear bands, Ductility, Bulk metallic glasse
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Size-dependent fracture toughness of bulk-metallic glasses
The fracture toughness is a critical material property that determines engineering performance.
However, as is well known for crystalline materials, if certain sample geometry and size
requirements are not met, test results become sample-size dependent and difficult to compare
among different studies. Here, the room-temperature fracture toughness of the Zr-based bulkmetallic
glass (BMG) Zrā
ā.ā
Cuāā.āNiāā.āAlāāTiā
(Vitreloy 105) was evaluated using compacttension,
as well as single-edge notched bend, specimens of different sizes to measure KIc values
according to ASTM standard E399 and JIc values according to ASTM standard E1820. It is
concluded that the ASTM standard E399 sample-size requirements should be cautiously
accepted as providing size-independent (valid) KIc results for BMGs; however, it is also
concluded that small sized samples may result in a wider scatter in conditional toughness KQ
values, a smaller yield of valid tests, and possibly somewhat elevated toughness values. Such
behavior is distinct from crystalline metals where the size requirements of ASTM standard E399
are quite conservative. For BMGs, KQ values increase and show a larger scatter with decreasing
uncracked ligament width b, which is also distinct from crystalline metals. Samples smaller than
required by ASTM standards for KIc testing are allowed by the J-integral based standard E1820; however, in this study on BMGs, such tests were found to give significantly higher toughness
values as compared to valid KIc results. Overall, the toughness behavior of BMGs is more
sensitive to size-requirements than for crystalline metals, an observation that is likely related to
the distinct size-dependent bending ductility and strain softening behavior found for metallic
glasses. It is concluded that toughness values measured on BMG samples smaller than that
required by the KIc standard, which are common in the literature, are likely sample size- and
geometry-dependent, even when they meet the less restrictive valid JIc requirements
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Mechanical performance of novel bioactive glass containing dental restorative composites
OBJECTIVES. Bioactive glass (BAG) is known to possess antimicrobial properties and release ions needed for remineralization of tooth tissue, and therefore may be a strategic additive for dental restorative materials. The objective of this study was to develop BAG containing dental restorative composites with adequate mechanical properties comparable to successful commercially available composites, and to confirm the stability of these materials when exposed to a biologically challenging environment. METHODS. Composites with 72 wt.% total filler content were prepared while substituting 0 ā 15% of the filler with ground BAG. Flexural strength, fracture toughness, and fatigue crack growth tests were performed after several different soaking treatments: 24 hours in DI water (all experiments), two months in brain-heart infusion (BHI) media+S. mutans bacteria (all experiments) and two months in BHI media (only for flexural strength). Mechanical properties of new BAG composites were compared along with the commercial composite Heliomolar by two-way ANOVA and Tukeyās multiple comparison test (pā¤0.05). RESULTS. Flexural strength, fracture toughness, and fatigue crack growth resistance for the BAG containing composites were unaffected by increasing BAG content up to 15% and were superior to Heliomolar after all post cure treatments. The flexural strength of the BAG composites was unaffected by two months exposure to aqueous media and a bacterial challenge, while some decreases in fracture toughness and fatigue resistance were observed. The favorable mechanical properties compared to Heliomolar were attributed to higher filler content and a microstructure morphology that better promoted the toughening mechanisms of crack deflection and bridging. SIGNIFICANCE. Overall, the BAG containing composites developed in this study demonstrated adequate and stable mechanical properties relative to successful commercial composites.Keywords: Fatigue, Hydration, Bacteria, Bioactive Glass, Fracture Toughness, Resin Composite, StrengthThis is an author's peer-reviewed final manuscript, as accepted by the publisher. The article is copyrighted by the Academy of Dental Materials and published by Elsevier Ltd. It can be found at: http://www.demajournal.com/
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Mechanical properties of suture materials in general and cutaneous surgery
Comprehensive studies comparing tensile properties of sutures are over 25 years old and do not include recent advances in suture materials. Accordingly, the objective of this paper is to investigate the tensile properties of commonly-used sutures in cutaneous surgery. Thirteen 3-0 sized modern sutures (four non-absorbable and nine absorbable) were tensile tested in both straight and knotted configurations according to the procedures outlined by the United States Pharmacopeia. Glycomer 631 was found to have the highest failure load (56.1 N) of unknotted absorbable sutures, while polyglyconate (34.2 N) and glycomer 631 (34.3 N) had the highest failure loads of knotted absorbable sutures. Nylon (30.9 N) and polypropylene (18.9 N) had the greatest failure loads of straight and knotted non-absorbable sutures, respectively. Polydioxane was found to have the most elongation prior to breakage (144%) of absorbable sutures. Silk (8701 MPa) and rapid polyglactin 910 (9320 MPa) had the highest initial modulus of nonabsorbable and absorbable sutures, respectively. The new data presented in the study provides important information for guiding the selection of suture materials for specific surgeries.Keywords: modulus, sutures, strength, elongation, tensile propertie
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A highly efficient degradation mechanism of methyl orange using Fe-based metallic glass powders
A new Fe-based metallic glass with composition FeāāBāāSiāYā (at. %) is found to have extraordinary degradation efficiency towards methyl orange (MO, CāāHā4NāSOā) 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 (294ā328 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.This is the publisherās final pdf. The published article is copyrighted by the author(s) and published by Nature Publishing Group. The published article can be found at: http://www.nature.com/articles/srep2194
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Characterization of fatigue-induced free volume changes in bulk metallic glass using positron annihilation spectroscopy
Depth-profiled Doppler broadening spectroscopy of positron annihilation on the cyclic fatigue-induced fracture surfaces of three amorphous ZrāāTiāāNiāāCuāāBeāā
metallic glass specimens reveals the presence of a 30ā50 nm layer of increased free volume that is generated by the propagating fatigue crack tip. The presence and character of this fatigue transformation zone is independent of the initial amount of bulk free volume, which was varied by structural relaxation via annealing, and the voids generated in the zone by intense cyclic deformation are distinct from those typical of the bulk.Keywords: fatigue cracks,
copper alloys,
Doppler broadening,
deformation,
beryllium alloys,
voids (solid),
annealing,
zirconium alloys,
titanium alloys,
nickel alloys,
metallic glasses,
positron annihilatio
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Fatigue Crack Growth Mechanisms for Nickel-based Superalloy Haynes 282 at 550-750Ā° C
The fatigue crack growth rates for nickel-based superalloy Haynes 282 were measured at 550, 650, and 750 Ā°C using compact tension specimens with a load ratio of 0.1 and cyclic loading frequencies of 25 and 0.25 Hz. The crack path was observed to be primarily transgranular for all temperatures, and the observed effect of increasing temperature was to increase the fatigue crack growth rates. The activation energy associated with the increasing crack growth rates over these three temperatures was calculated less than 60 kJ/mol, which is significantly lower than typical creep or oxidation mechanisms; therefore, creep and oxidation cannot explain the increase in fatigue crack growth rates. Transmission electron microscopy was done on selected samples removed from the cyclic plastic zone, and a trend of decreasing dislocation density was observed with increasing temperature. Accordingly, the trend of increasing crack growth rates with increasing temperature was attributed to softening associated with thermally assisted cross slip and dislocation annihilation.Keywords: electron microscopy, mechanical characterization, fatigue, nickel-based superalloysKeywords: electron microscopy, mechanical characterization, fatigue, nickel-based superalloy
Ni-Nb-P-based bulk glass-forming alloys: Superior material properties combined in one alloy family
Ni-Nb-based bulk glass-forming alloys are among the most promising amorphous metals for industrial applications due to their incomparable combination of strength, hardness, elasticity and plasticity. However, the main
drawback is the limited glass-forming ability, narrowing the field of application to solely small components. In
this study, we show that minor additions of P to the binary Ni-Nb system increase the glass-forming ability by
150 % to a record value of 5 mm. P can be easily added by using an industrial Ni-P pre-alloy which is readily
available. The partial substitution of Nb by Ta further boosts the glass-forming ability to values 200 % higher
than that of the binary base alloy. Besides conventional X-ray diffraction measurements, the amorphous nature of
the samples is verified by high-energy synchrotron X-ray diffraction experiments. Moreover, the mechanical
properties of the new alloy compositions are characterized in uniaxial compression tests and Vickers hardness
measurements, showing a high engineering yield strength of 3 GPa, an extended plastic regime up to 10 % strain
to failure and an increase of the hardness to a maximum value of 1000 HV5. Additionally, calorimetric measurements reveal that the modified alloys feature an extended supercooled liquid region up to 69 K upon heating,
permitting thermoplastic micro molding of amorphous feedstock material
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