Innovative materials for high temperature structural applications: 3rd Generation γ-TiAl fabricated by Electron Beam Melting

In the aeronautics industry, the propulsion systems stand among the most advanced and critical components. Over the last 50 years, gas turbine aeroengines were subjected to intensive research to increase efficiency and reduce weight, noise and harmful emissions. Together with design optimization, breakthrough in materials science for structural applications triggered the development of the most advanced gas turbine engines. For low temperatures, basically ahead of the combustion section, lightweight Ti alloys are preferred for their good mechanical properties. For high temperatures instead, Ni-based superalloys exhibit outstanding properties up to very high temperatures despite a rather high material’s density. Research have focused on enhancing to the maximum the potential of materials in gas turbine engines. According to the application, the components experience various mechanical and environmental constraints. Special designs, manufacturing process, material compositions and protective coatings have been developed to push the limits of advanced materials. Nowadays, the attention is focused on innovative materials to replace the existing Ti and Ni based alloys leading to substantial benefits. Light weight composite materials in particular were found very attractive to replace some components’ Ti alloys. At higher temperatures, it is of great interest to replace Ni-based superalloys by materials with lower density and/or higher temperatures applications, which in turn would lead to substantial weight reduction and increase efficiency. At the highest temperatures range, in particular in the combustion chamber and high pressure turbine sections, ceramic based materials offer promising balance of properties. Research are dedicated to overcome the drawbacks of ceramics for such structural applications, and in particular their brittle fracture behavior, by addition of reinforcing fibers. At lower temperatures range, TiAl based intermetallics emerged as very promising materials at half the density of Ni-based superalloys. Significant weight reduction could be achieved by the introduction of TiAl based alloys for rotating components of the compressor and low pressure turbine. 2nd generation γ-TiAl alloys were lately introduced in GE’s GEnx and CFM’s LEAP engines. The present work concerns the fabrication by the additive manufacturing technique Electron Beam Melting of 3rd generation γ-TiAl alloys for high temperatures application in gas turbine aeroengines. EBM, building parts layer by layer according to CAD, offers many advantages compared to other manufacturing processes like casting and forging. Reported by Avio, 2nd generation γ-TiAl alloys have been successfully fabricated by EBM. To increase the material’s potential, the production of 3rd generation γ-TiAl alloys Ti-(45-46)Al-2Cr-8Nb was therefore studied. The optimization of the EBM parameters led to high homogeneity and very low post-processing residual porosity ≤ 1%. The fine equiaxed microstructure after EBM could be tailored towards the desired mechanical properties by simple heat treatment, from equiaxed to duplex to fully lamellar. In particular, a duplex microstructure composed by about 80 % lamellar grains pinned at grain boundaries by fine equiaxed grains was obtained after heat treatment slightly over the α transus temperature. The study showed that addition of a higher amount of Nb significantly increased the oxidation resistance of the material, thus increasing the application temperature range of these γ-TiAl alloys

Permissionless Consensus in the Resource Model

In the permissionless regime of distributed computing, participants may join and leave an internet-scale protocol execution at will. The permissionless regime poses challenges to the classical techniques used for consensus protocols, in which participants attempt to agree on a function of their inputs. For example, classical consensus techniques require bounding the numbers of honest and corrupt participants, and for honest participants to remain online throughout. Bitcoin\u27s introduction of Proof of Work enabled dynamic participation by shifting focus from the number of parties to the number of hash puzzles that parties collectively solve, and in turn, enforcing constraints on the blocks sent by honest parties. Other Bitcoin-inspired works have developed Proof of X (PoX) variants to remediate the shortcomings of Proof of Work. We propose a new abstraction called resources and argue that in practice, several PoX variants appear to implement resources. For every resource that a party obtains, it is permitted to send a special protocol message. We show that given few additional assumptions, resources are sufficient to achieve consensus in the permissionless regime, even in the presence of a full-information adversary that can choose which parties get resources and when they get them. In particular, it is not necessary to know a bound on the network delay, participants do not need clocks, and participants can join and leave the execution arbitrarily, even after sending only a single message. We require only a known upperbound on the rate at which resources enter the system, relative to the maximum network delay (without needing to know the network delay), and that over the long term, a majority of resources are acquired by honest participants. Our protocol for consensus in the permissionless model follows from a protocol for graph consensus, which we define as a generalization of blockchains. Our graph consensus works even when resources enter the system at high rates, but the required honest majority increases with the rate. We show how to modify the protocol slightly to achieve one-bit consensus. Finally, we show that for every graph consensus protocol that outputs a majority of honest vertices there exists a one-bit consensus protocol

THE FUTURE OF CRYPTOCURRENCY: EVALUATING ITS VIABILITY AS A SUSTAINABLE TECHNOLOGY AND ITS POTENTIAL IMPLICATIONS ON SOCIETY

50 pagesCryptocurrencies are an extremely new and constantly changing technology that has been gaining tremendous amounts of popularity in the last several years. The underlying technology behind cryptocurrencies, blockchain, acts as a decentralized and distributed ledger for which it holds transaction information associated with the users of the network. This is beneficial to those who use these currencies because it allows for the network to be decentralized, transparent, and immutable. While for many, these benefits are very compelling, some argue that cryptocurrencies are simply a fad and do not possess a strong enough foundation to last into the future as our society changes. The aim of this thesis is to take this question head on, and assess the long-term viability of cryptocurrency, based on the underlying technology, the direction the industry is heading, public opinion, and many other factors. Additionally, this paper will explore the potential implications that would occur in our society as a result of a more widespread adoption of cryptocurrency. Based on all these factors, one outcome is certain for cryptocurrency; the future is unclear. Cryptocurrency is an extremely innovative and important technology with many undeniable benefits that come along with the technology, such as security, transparency, lack of centralized authority, and many more. However, it is simply too new of a technology with many uncertainties to be able to determine with absolute certainty whether it will last into the future or not, as the answer to this question is extremely nuanced with many factors that make it impossible to determine a straightforward answer

A VOICE OF THEIR OWN: MUSIC AND SOCIAL COHESION IN TURKISH ALEVI LIFE

This dissertation examines the role that music has played in the expression of identity and revitalization of culture of the Alevis in Turkey, since the start of their sociocultural revival movement in the late 1980s. Music is central to Alevi claims of ethnic and religious difference—singing and playing the bağlama (Turkish folk lute) constitutes an expressive practice in worship and everyday life. Based on research conducted from 2012 to 2014, I investigate and present Alevi music through the lens of discourses on the construction of identity as a social and musical process. Alevi musicians perform a revived repertoire of the ritual music and folk songs of Anatolian bards and dervish-lodge poets that developed over several centuries. Contemporary media and performance contexts have blurred former distinctions between sacred and secular, yet have provided new avenues to build community in an urban setting. I compare music performances in the worship services of urban and small-town areas, and other community events such as devotional meetings, concerts, clubs, and broadcast and social media to illustrate the ways that participation—both performing and listening—reinforces identity and solidarity. I also examine the influence of these different contexts on performers’ musical choices, and the power of music to evoke a range of responses and emotional feelings in the participants. Through my investigation I argue that the Alevi music repertoire is not only a cultural practice but also a symbol of power and collective action in their struggle for human rights and self-determination. As Alevis have faced a redefined Turkish nationalism that incorporates Sunni Muslim piety, this music has gained even greater potency in their resistance to misrecognition as a folkloric, rather than a living, tradition

Mammal use of underpasses to cross Route 606 in Guacimal, Costa Rica

Roads severely affect the health of ecosystems across the globe by fragmenting and diminishing habitats, reducing population connectivity, and increasing animal mortality. Wildlife underpasses allow for increased road permeability–the ability for animals to safely cross the road. Despite growing success in other regions, little is known about underpass usage in Central America. In this study, I monitored two dry circular culverts and two unfenced tunnels with barbed wire partially blocking their entrances on Route 606 in Guacimal, Costa Rica, from 14 November to 6 December 2021 using 15 camera traps to assess which species used them to cross. Twelve species used the culverts and tunnels for a total of 108 individual crossings. The tunnels were used, in descending order, by agouti (Dasyprocta punctata), common opossum (Didelphis marsupialis), dog (Canis familiaris), nine-banded armadillo (Dasyous novemcinctus), cat (Felis catus), Norway rat (Rattus norvegicus), ocelot (Leopardus pardalis), squirrel (Sciurus variegatoides), northern tamandua (Tamandua mexicana), and coati (Nasua narica). The circular tunnel, Tunnel 1, was used more frequently and by a greater diversity of species than observed in the square tunnel, Tunnel 2. The two smaller culverts were used by common opossum (Didelphis marsupialis), cat (Felis catus), rat opossum (Micoureus alstoni), and Watson’s climbing rat (Tylomus watsoni). Culvert 2 was used more frequently; however, Culvert 1 was used by a greater diversity of species. This study highlights wildlife underpasses as a critical strategy for biological conservation in Costa Rica through improved road safety and habitat connectivity

Electron backscattered diffraction to estimate residual stress levels of a superalloy produced by laser powder bed fusion and subsequent heat treatments

Metal Additive Manufacturing and Laser Powder Bed Fusion (LPBF), in particular, have come forth in recent years as an outstanding innovative manufacturing approach. The LPBF process is notably characterized by very high solidification and cooling rates, as well as repeated abrupt heating and cooling cycles, which generate the build-up of anisotropic microstructure and residual stresses. Post-processing stress-relieving heat treatments at elevated temperatures are often required in order to release some of these stresses. The effects of 1 h–hold heat treatments at different specific temperatures (solutionizing, annealing, stress-relieve and low-temperature stress-relieve) on residual stress levels together with microstructure characterization were therefore investigated for the popular Alloy 625 produced by LPBF. The build-up of residual stress is accommodated by the formation of dislocations that produce local crystallographic misorientation within grains. Electron backscattered diffraction (EBSD) was used to investigate local misorientation by means of orientation imaging, thereby assessing misorientation or strain levels, in turn representing residual stress levels within the material. The heavily constrained as-built material was found to experience full recrystallization of equiaxed grains after solutionizing at 1150◦ C, accompanied by significant drop of residual stress levels due to this grains reconfiguration. Heat treatments at lower temperatures however, even as high as the annealing temperature of 980◦ C, were found to be insufficient to promote recrystallization though effective to some extent to release residual stress through apparently dislocations recovery. Average misorientation data obtained by EBSD were found valuable to evaluate qualitatively residual stress levels. The effects of the different heat treatments are discussed and suggest that the peculiar microstructure of alloys produced by LPBF can possibly be transformed to suit specific applications

A geographic information system (GIS) assisted approach for assessing environmental equity in the EPA RCRA program's site inspection selection process

Thesis (M.C.P.)--Massachusetts Institute of Technology, Dept. of Urban Studies and Planning, 1993.Includes bibliographical references (leaves 74-78).by Michael G. Terner.M.C.P