Studying the Fatigue Behavior of Zr-Based Bulk-Metallic Glasses and Composites

As excellent candidates for structural materials, the mechanical behavior of bulk metallic glasses (BMGs) is being widely studied. Although their fatigue behavior is very important for engineering applications, there are few studies on their fatigue behavior. Moreover, the understanding of the fatigue behavior is pretty limited. High-cycle fatigue experiments were conducted on notched Zr-based BMGs under tension-tension loading in air and vacuum at room temperature. A sparking phenomenon was found by infrared camera at the final fracture moment of Zr50Al10Cu30Ni10 in air. The fatigue-endurance limit (983 MPa) of Zr50Cu37Al10Pd3 was greatest among these Zr-base BMGs in air. The fatigue lives in vacuum and air are generally similar. LM001 and LM002 are commercial Zr-based BMGs. The X-ray diffraction results show that LM001 is a monolithic BMG and LM002 is a BMG composite containing crystalline phases. The fatigue-endurance limit (239 MPa) of LM002 was found to be significantly lower than that (567 MPa) of LM001, which indicates that the crystalline phase could degrade the resistances to fatigue. The compression strengths of Zr-based BMGs with partial crystallization are comparable to those of their fully amorphous alloys. However, the fatigue-endurance limits of these BMGs with partial crystallization were much lower than those of their fully amorphous alloys, which suggested that the fatigue behavior of a BMG is very sensitive to the microstructure. Fatigue cracks initiate from the outer surface of the sample, inclusions, and/or porosity. The propagation region exhibits a typical striation-type fracture. The final fast fracture region was very rough and occupied most of the fracture surface. The vein pattern and droplets with a melted indication were observed in the apparent melting region. A mechanistic understanding of the fatigue behavior of the Zr-based BMGs is suggested. The ratio of the fatigue-endurance limit to the tensile strength seems to increase with increasing Poisson’s ratio. Moreover, The fatigue endurance limits of Zr-based, Cu-based, and Fe-based BMGs and composites were comparable to those of ductile crystalline alloys. The fatigue-endurance ratios of Zr-based BMGs were found to be comparable with those of high strength crystalline alloys

Compression-compression fatigue of Pd_(43)Ni_(10)Cu_(27)P_(20) metallic glass foam

Compression-compression fatigue testing of metallic-glass foam is performed. A stress-life curve is constructed, which reveals an endurance limit at a fatigue ratio of about 0.1. The origin of fatigue resistance of this foam is identified to be the tendency of intracellular struts to undergo elastic and reversible buckling, while the fatigue process is understood to advance by anelastic strut buckling leading to localized plasticity (shear banding) and ultimate strut fracture. Curves of peak and valley strain versus number of cycles coupled with plots of hysteresis loops and estimates of energy dissipation at various loading cycles confirm the four stages of foam-fatigue

Development of a Non-invasive Deep Brain Stimulator With Precise Positioning and Real-Time Monitoring of Bioimpedance

Methods by which to achieve non-invasive deep brain stimulation via temporally interfering with electric fields have been proposed, but the precision of the positioning of the stimulation and the reliability and stability of the outputs require improvement. In this study, a temporally interfering electrical stimulator was developed based on a neuromodulation technique using the interference modulation waveform produced by several high-frequency electrical stimuli to treat neurodegenerative diseases. The device and auxiliary software constitute a non-invasive neuromodulation system. The technical problems related to the multichannel high-precision output of the device were solved by an analog phase accumulator and a special driving circuit to reduce crosstalk. The function of measuring bioimpedance in real time was integrated into the stimulator to improve effectiveness. Finite element simulation and phantom measurements were performed to find the functional relations among the target coordinates, current ratio, and electrode position in the simplified model. Then, an appropriate approach was proposed to find electrode configurations for desired target locations in a detailed and realistic mouse model. A mouse validation experiment was carried out under the guidance of a simulation, and the reliability and positioning accuracy of temporally interfering electric stimulators were verified. Stimulator improvement and precision positioning solutions promise opportunities for further studies of temporally interfering electrical stimulation

Fatigue deformation of microsized metallic glasses

Metallic glasses typically exhibit high strength and a high elastic limit but suffer from poor fatigue resistance. This work demonstrates that 1.6 micron diameter Zr-based metallic glass samples subjected to compressive fatigue cycling did not fail after 40 × 10^6 cycles. The fatigue endurance limit was shown to increase to more than 110% of bulk yield strength under compression–compression and up to 90% under bending; those of the same material with macroscopic dimensions are typically at 50% of bulk yield strength

Corrosion and corrosion fatigue of Vitreloy glasses containing low fractions of late transition metals

Corrosion resistance and fatigue performance of Vitreloy glasses with low fractions of late transition metals (LTMs) in 0.6 M NaCl are investigated and compared to a traditional Vitreloy glass and other crystalline alloys. Owing to their ability to form uniform passive films, low LTM Vitreloy glasses exhibit corrosion rates that are an order of magnitude lower than those of other alloys considered. The fatigue ratios in solution are substantially lower than crystalline alloys considered and similar to traditional Vitreloy glasses, which fail at low stresses due to limited repassivation in solution

Addition of Nitric Oxide Through Nitric Oxide-paracetamol Enhances Healing Rat Achilles Tendon

Nitric oxide is an important messenger molecule in many physiological processes. The addition of NO via NO-flurbiprofen enhances the material properties of healing tendon, however, flurbiprofen has a detrimental effect on healing. We asked if NO delivered by a cyclooxygenase 3 inhibitor (paracetamol/acetaminophen) would enhance healing in a rat Achilles tendon healing model. Rats were injected subcutaneously daily with NO-paracetamol, paracetamol or vehicle from two days before surgery to the day of tissue harvesting. Paracetamol had no effect on tendon healing compared with vehicle alone. NO-paracetamol did not change the failure load, but did decrease the water content, enhance the collagen content, reduce the cross-sectional area and improve the ultimate stress of healing tendon compared with paracetamol and vehicle. The collagen organization of the healing tendon in the NO-paracetamol group, as determined by polarized light microscopy, was enhanced. Our data suggests NO-paracetamol increases the total collagen content and enhances organization while decreasing the cross-sectional area of healing rat Achilles tendon and is consistent with human clinical trials where NO has improved the symptoms and signs of tendinopathy

<i>BrDMC1</i>, a Recombinase Gene, Is Involved in Seed Germination in <i>Brassica rapa</i> under Salt Stress

Recombinases are in part responsible for homologous recombination and genome integrity during DNA repair. DMC1 has a typical RecA domain, and belongs to the recombinase superfamily. The reactive oxygen species (ROS) as a potent DNA damage agent is produced during seed germination under stress conditions. DNA repair should be initiated immediately to allow for subsequent seedling development. In this study, we attempted to characterize the underlying mechanism of BrDMC1 responsiveness to salinity stress using the RNA interference approach in Brassica rapa (B. rapa). Bioinformatics and expression pattern analysis revealed that BrDMC1 only retained BrDMC1.A01 after the whole genome triplication (WGT) event and was primarily transcribed in flowers and seeds. BrDMC1 had high activity in the promoter region during germination, according to histochemical GUS staining. The data showed that salt treatment reduced the germination rate, weakened seed vigor and decreased antioxidant enzyme activity, but increased oxidative damage in BrDMC1-RNAi seeds. Furthermore, the expression of stress-responsive genes and damage repair genes was significantly different in transgenic lines exposed to salt stress. Therefore, BrDMC1 may respond to salt stress by controlling seed germination and the expression of stress-related and damage repair genes in B. rapa

Amorphous metals for hard-tissue prosthesis

Owing to a unique atomic structure lacking microstructural defects, glassy metals demonstrate certain universal properties that are attractive for load-bearing biomedical-implant applications. These include a superb strength, which gives rise to very high hardness and a potential for minimizing wear and associated adverse biological reactions, and a relatively low modulus, which enables high elasticity and holds a promise for mitigating stress shielding. There are, however, other non-universal properties specific to particular amorphous metal alloys that are inferior to presently used biometals and may be below acceptable limits for hard-tissue prosthesis. In this article, features of the performance of amorphous metals relevant to hard-tissue prosthesis are surveyed and contrasted to those of the current state of the art, and guidelines for development of new biocompatible amorphous metal alloys suitable for hard-tissue prosthesis are proposed