Adiabatic Shear Banding in Nickel and Nickel-Based Superalloys: A Review

This review paper discusses the formation and propagation of adiabatic shear bands in nickel-based superalloys. The formation of adiabatic shear bands (ASBs) is a unique dynamic phenomenon that typically precedes catastrophic, unpredicted failure in many metals under impact or ballistic loading. ASBs are thin regions that undergo substantial plastic shear strain and material softening due to the thermo-mechanical instability induced by the competitive work hardening and thermal softening processes. Dynamic recrystallization of the material’s microstructure in the shear region can occur and encourages shear localization and the formation of ASBs. Phase transformations are also often seen in ASBs of ferrous metals due to the elevated temperatures reached in the narrow shear region. ASBs ultimately lead to the local degradation of material properties within a narrow band wherein micro-voids can more easily nucleate and grow compared to the surrounding material. As the micro-voids grow, they will eventually coalesce leading to crack formation and eventual fracture. For elevated temperature applications, such as in the aerospace industry, nickel-based superalloys are used due to their high strength. Understanding the formation conditions of ASBs in nickel-based superalloys is also beneficial in extending the life of machining tools. The main goal of the review is to identify the formation mechanisms of ASBs, the microstructural evolutions associated with ASBs in nickel-based alloys, and their consequent effect on material properties. Under a shear strain rate of 80,000 s−1, the critical shear strain at which an ASB forms is between 2.2 and 3.2 for aged Inconel 718 and 4.5 for solution-treated Inconel 718. Shear band widths are reported to range between 2 and 65 microns for nickel-based superalloys. The shear bands widths are narrower in samples that are aged compared to samples in the annealed or solution treated condition

Expression of a Novel Chimeric Truncated t-PA in CHO Cells Based on in Silico Experiments

Tissue plasminogen activator (t-PA) is one of the fibrin-specific serine proteases that play a crucial role in the fibrinolytic system. The rapid clearance of the drug from the circulation, caused by its active uptake in the liver, has lead to complicated clinical applications. Different forms of plasminogen activators have been developed to treat thrombotic disease. Deletion of the first three domains of t-PA by gene manipulation techniques has shown a significant increase in its plasma half life. In order to compensate the disadvantage of higher bleeding risk, a novel chimeric truncated form of t-PA with 394 amino acids and more fibrin affinity compared to the truncated form was designed to be expressed in Chinese Hamster Ovarian (CHO) cells. The recombinant chimeric plasminogen activator consists of kringle 2 and serine protease (K2S) domains of t-PA, namely GHRP-SYQ-K2S. The level of expression was found to be 752 IU/ml with 566,917 IU/mg specific activity, based on amidolytic activity. The fibrin binding of this novel chimeric truncated t-PA was 86% of the full length t-PA at a fibrinogen concentration of 0.2 mg/ml. This could be a promising approach with more desirable pharmacodynamic properties compared to existing commercial forms

Human Tissue Plasminogen Activator Expression in Escherichia coli using Cytoplasmic and Periplasmic Cumulative Power

Abstract Tissue plasminogen activator (tPA) is a serine protease, which is composed of five distinct structural domains with 17 disulfide bonds, representing a model of high-disulfide proteins in human body. One of the most important limitations for high yield heterologous protein production in Escherichia coli (E. coli) is the expression of complex proteins with multiple disulfide bridges. In this study the combination of two distinct strategies, manipulated cytoplasm and native periplasm, was applied to produce the functional full length tPA enzyme in E. coli. Using a PelB signal peptide sequence at 5' site of tPA gene, the expression cassette was prepared and subsequently was transformed into a strain with manipulated oxidizing cytoplasm. Then the induction was made to express the protein of interest. The SDS-PAGE analysis and gelatin hydrolysis confirmed the successful expression of functional tPA. The results of this study showed that complex proteins can be produced in E. coli using the cumulative power of both cytoplasm and periplasm

Thermoelectric and Mechanical Characterization of Synthesized One-Dimensional Nanostructures

DoctorThe relatively high cost and limited supply of fossil fuels has caused increased attention to cleaner and cheaper alternative energy sources such as solar, wind, and, as discussed here, thermoelectric energy. A new way of energy production via the conversion of thermal energy to electricity is now achievable through the application of thermoelectric effects. One-dimensional nanostructured materials have outstanding potential to enhance existing conversion devices. As tellurides of Zn, Cd and Pb have become popular for thermoelectric applications, a thorough characterization of their thermoelectric and mechanical properties is needed. In this research, the thermoelectric and mechanical properties of ZnTe nanowires (NWs) are investigated. We produced ZnTe NWs with diameters less than 30 nm via the bottom-up vapor-liquid-solid method. Bandgap engineering of single-crystalline alloy CdxZn1-xTe (0 ≤ x ≤ 1) NWs was achieved successfully through control of growth temperature and a two zone source system in a vapor-liquid-solid process. The effective process parameters on the morphologies of NWs were investigated, and the nanostructures were characterized by SEM and TEM. For the mechanical characterization, an experimental and a computational approach (molecular dynamic simulation) were used to investigate the size effects on Young’s modulus of ZnTe NWs. The mechanical properties of individual ZnTe NWs in a wide diameter range (50-230 nm) were experimentally measured inside a high resolution transmission electron microscope using an atomic force microscope probe with the ability to record in situ continuous force-displacement curves. For the smaller NWs, the computational approach was used. Mechanical characterization showed that neither molecular dynamic simulation nor experimental measurement illustrated evidence of strong size dependency of Young’s modulus of ZnTe NWs. For thermoelectric characterization, a MEMS device with two suspended thermometers was made to measure the thermal conductivity, Seebeck coefficient and electrical conductivity of the individual NWs. After positioning the NWs on the device, the aforementioned parameters were measured for NWs with different diameters. Thermal measurements concluded that the thermal conductivity of ZnTe NWs was significantly lower than that of bulk ZnTe. However, the electrical properties need to be modified in order to obtain a figure a merit comparable with or higher than bulk. The limiting factor for the use of ZnTe NWs in a wider range of applications is their high resistance. This can be removed by optimal doping or any other methods for the modification of their electrical conductivity. Also, more studies need to be done in order to achieve Ohmic electrical contacts with ZnTe NWs for a better investigation of their thermoelectric potential. Overall, it was concluded that ZnTe nanowires have the potential to be used in thermoelectric applications because of their relatively high strength and low and size dependent thermal conductance

A Review of Laser Peening Methods for Single Crystal Ni-Based Superalloys

Single crystal Ni-based superalloys are often used to create gas turbine engine blades for their high strength under intense thermo-mechanical loading. Though they are remarkably capable under these conditions, a particular class of premature failure mechanisms known as surface-initiated damage mechanisms can lead to the early fracture of an otherwise healthy blade. This review paper discusses the current progress of post-processing techniques that can greatly mitigate the potency of surface-initiated damage mechanisms. In particular, laser peening (LP) is of significant interest due to the relatively low amount of cold work it induces, greater depth of compressive residual stresses than other cold working methods, ability to accommodate complex part geometries, and the minuscule effect it has on surface roughness. The residual stresses imparted by LP can greatly hinder crack growth and consequently allow for enhanced fatigue life. Given that turbine blades (constructed with single crystal Ni-based superalloys) are prone to fail by these mechanisms, LP could be a worthy choice for increasing their service lives. For this reason, initiative has been taken to better understand the mechanical and microstructural modifications imparted by LP on single crystal Ni-based superalloys and a summary of these investigations are presented in this review. Results from several works show that this class of alloy responds well to LP treatment with improvements such as ~30–50% increase in microhardness, 72% increase in low cycle fatigue life, and elevated resistance to hot corrosion. The primary objective of this review is to provide insight into current state-of-the-art LP techniques and summarize the findings of numerous works which have utilized LP for increasing the service lives of single crystal Ni-based superalloy components

A Review of Laser Peening Methods for Single Crystal Ni-Based Superalloys

Single crystal Ni-based superalloys are often used to create gas turbine engine blades for their high strength under intense thermo-mechanical loading. Though they are remarkably capable under these conditions, a particular class of premature failure mechanisms known as surface-initiated damage mechanisms can lead to the early fracture of an otherwise healthy blade. This review paper discusses the current progress of post-processing techniques that can greatly mitigate the potency of surface-initiated damage mechanisms. In particular, laser peening (LP) is of significant interest due to the relatively low amount of cold work it induces, greater depth of compressive residual stresses than other cold working methods, ability to accommodate complex part geometries, and the minuscule effect it has on surface roughness. The residual stresses imparted by LP can greatly hinder crack growth and consequently allow for enhanced fatigue life. Given that turbine blades (constructed with single crystal Ni-based superalloys) are prone to fail by these mechanisms, LP could be a worthy choice for increasing their service lives. For this reason, initiative has been taken to better understand the mechanical and microstructural modifications imparted by LP on single crystal Ni-based superalloys and a summary of these investigations are presented in this review. Results from several works show that this class of alloy responds well to LP treatment with improvements such as ~30–50% increase in microhardness, 72% increase in low cycle fatigue life, and elevated resistance to hot corrosion. The primary objective of this review is to provide insight into current state-of-the-art LP techniques and summarize the findings of numerous works which have utilized LP for increasing the service lives of single crystal Ni-based superalloy components

A Study on the Stoichiometry of One-Dimensional Nanostructures

While attributes such as small dimensions, low power consumption, fast sensor response, and a wide range of detection give one-dimensional nanostructures excellent potential to revolutionize sensor and detector industries, challenges to achieving uniform stoichiometry pose significant obstacles to their commercial use. Diverse characteristics arise from nanostructures with variable compositions and morphologies. Thus, investigation of physical properties of nanostructures would be pointless if one cannot assure the exact stoichiometry of the material. We studied the stoichiometry of ZnTe nanowires grown via the vapor-liquid-solid method. Different microscopy and composition analysis methods were exploited to study the stoichiometry of the nanowires. It was observed that nonstoichiometric wires had relatively higher defect concentrations. The temperature profile along the substrate during nanowire growth was found to be the reason for the formation of nanowires with different stoichiometries

Adiabatic Shear Banding in Nickel and Nickel-Based Superalloys: A Review

This review paper discusses the formation and propagation of adiabatic shear bands in nickel-based superalloys. The formation of adiabatic shear bands (ASBs) is a unique dynamic phenomenon that typically precedes catastrophic, unpredicted failure in many metals under impact or ballistic loading. ASBs are thin regions that undergo substantial plastic shear strain and material softening due to the thermo-mechanical instability induced by the competitive work hardening and thermal softening processes. Dynamic recrystallization of the material’s microstructure in the shear region can occur and encourages shear localization and the formation of ASBs. Phase transformations are also often seen in ASBs of ferrous metals due to the elevated temperatures reached in the narrow shear region. ASBs ultimately lead to the local degradation of material properties within a narrow band wherein micro-voids can more easily nucleate and grow compared to the surrounding material. As the micro-voids grow, they will eventually coalesce leading to crack formation and eventual fracture. For elevated temperature applications, such as in the aerospace industry, nickel-based superalloys are used due to their high strength. Understanding the formation conditions of ASBs in nickel-based superalloys is also beneficial in extending the life of machining tools. The main goal of the review is to identify the formation mechanisms of ASBs, the microstructural evolutions associated with ASBs in nickel-based alloys, and their consequent effect on material properties. Under a shear strain rate of 80,000 s−1, the critical shear strain at which an ASB forms is between 2.2 and 3.2 for aged Inconel 718 and 4.5 for solution-treated Inconel 718. Shear band widths are reported to range between 2 and 65 microns for nickel-based superalloys. The shear bands widths are narrower in samples that are aged compared to samples in the annealed or solution treated condition