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Research Headlines - Motoarele cu ardere internă au un viitor luminos datorită aprinderii cu laser - [image: Image]Până să devină practice și tuturor accesibile mașinile electrice și alte inovații energetice, va mai fi larg folosit motorul cu ardere, rezul...5 years ago
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Research Headlines - Inteligentne miasta zorientowane na obywateli - [image: Image]Dzięki technologiom IT usługi miejskie mogą być bardziej wydajne, dostępne i przyjazne dla środowiska. By takie rozwiązania były efektywne, m...5 years ago
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Research Headlines - En route to safer, more reliable autonomous driving - [image: Image]The development of autonomous driving systems is currently a focus of research for the automotive industry. An EU-funded project has moved wo...5 years ago
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Instant Inflation Systems for Stand-Up Paddle Boards - Inflatable Stand-Up Paddles (SUP) have provided great flexibility to enthusiasts and allowed the sport to grow in popularity. However, manually or electr...7 years ago
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The List of Online and Free Access Journals about Metallurgy , Mining - The List of Online and Free Access Journals about Metallurgy and Mining RKOJ = Related Keywords of the Journal Open Mineral Processing Journal RKOJ: ...16 years ago
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Thursday, 9 January 2014
Energy materials to combat climate change'
Update on Energy Materials from The Royal Academy of Science (RSA)
The menu is as follows and all articles have been made available to all- Climate Change Oblige!?
So dig in and apply my friends.
The menu is as follows and all articles have been made available to all- Climate Change Oblige!?
So dig in and apply my friends.
Tuesday, 7 January 2014
Solar Energy Harvesting
In this post both the highly imaged approach originally from Science Daily for more general public reading is maintained and even copied unashamedly, referenced naturally, as well as references to full scientific publications, many of which are freely available probably due to the immensely important nature of this subject (solar energy harvesting)
"For solar panels, wringing every drop of energy from as many photons as possible is imperative. This goal has sent chemistry, materials science and electronic engineering researchers on a quest to boost the energy-absorption efficiency of photovoltaic devices, but existing techniques are now running up against limits set by the laws of physics." Ref. Science Daily Nov. 12, 2013 below.
"Now researchers from the University of Pennsylvania and Drexel University claim to have experimentally demonstrated this a new paradigm for solar cell construction which may ultimately lead to
-less expensive,
-easier to manufacture
and
-more efficient harvesting energy from the sun.
The study, published in the journal Nature, was led by professor Andrew M. Rappe and research specialist Ilya Grinberg of the Department of Chemistry in Pennsylvania Uni, Peter K. Davies, chairman of the Dept of Mat Sci and Eng,School of Engineering and Applied Science, and professor Jonathan E. Spanier, of Drexel's Department of Materials Science and Engineering.
Existing solar cells all work in the same fundamental way: they absorb light, which excites electrons and causes them to flow in a certain direction ie. electric current. But to establish a consistent direction of their movement, or polarity, solar cells need to be made of two materials. Once an excited electron crosses over the interface from the material that absorbs the light to the material that will conduct the current, it can't cross back, giving it a direction.
However, "there's a small category of materials, that when you shine light on them, the electron takes off in one particular direction without having to cross from one material to another," Rappe said. "We call this the 'bulk' photovoltaic effect, rather than the 'interface' effect that happens in existing solar cells. This phenomenon has been known since the 1970s, but we don't make solar cells this way because they have only been demonstrated with ultraviolet light, and most of the energy from the sun is in the visible and infrared spectrum."
Finding a material that exhibits the bulk photovoltaic effect for visible light would greatly simplify solar cell construction. Moreover, it would be a way around an inefficiency intrinsic to interfacial solar cells, known as the Shockley-Queisser limit, where some of the energy from photons is lost as electrons wait to make the jump from one material to the other.
"Think of photons coming from the sun as coins raining down on you, with the different frequencies of light being like pennies, nickels, dimes and so on. A quality of your light-absorbing material called its 'bandgap' determines the denominations you can catch," Rappe said. "The Shockley-Queisser limit says that whatever you catch is only as valuable as the lowest denomination your bandgap allows. If you pick a material with a bandgap that can catch dimes, you can catch dimes, quarters and silver dollars, but they'll all only be worth the energy equivalent of 10 cents when you catch them.
"If you set your limit too high, you might get more value per photon but catch fewer photons overall and come out worse than if you picked a lower denomination," he said. "Setting your bandgap to catch only silver dollars is like only being able to catch UV light. Setting it to catch quarters is like moving down into the visible spectrum. Your yield is better even though you're losing most of the energy from the UV you do get."
NB.
Starting more than five years ago, the team began theoretical work, plotting the properties of hypothetical new compounds that would have a mix of these traits. Each compound began with a "parent" material that would impart the final material with the polar aspect of the bulk photovoltaic effect. To the parent, a material that would lower the compound's bandgap would be added in different percentages. These two materials would be ground into fine powders, mixed together and then heated in an oven until they reacted together. The resulting crystal would ideally have the structure of the parent but with elements from the second material in key locations, enabling it to absorb visible light.
"The design challenge," Davies said, "was to identify materials that could retain their polar properties while simultaneously absorbing visible light.
The theoretical calculations pointed to new families of materials where this often mutually exclusive combination of properties could in fact be stabilized." This structure is something known as a perovskite crystal.
Most light absorbing materials have a symmetrical crystal structure, meaning their atoms are arranged in repeating patterns up, down, left, right, front and back. This quality makes those materials non-polar; all directions "look" the same from the perspective of an electron, so there is no overall direction for them to flow.
A perovskite crystal has the same cubic lattice of metal atoms, but inside of each cube is an octahedron of oxygen atoms, and inside each octahedron is another kind of metal atom. The relationship between these two metallic elements can make them move off center, giving directionality to the structure and making it polar.
"All of the good polar, or ferroelectric, materials have this crystal structure," Rappe said. "It seems very complicated, but it happens all of the time in nature when you have a material with two metals and oxygen. It's not something we had to architect ourselves."
After several failed attempts to physically produce the specific perovskite crystals they had theorized, the researchers had success with a combination of potassium niobate, the parent, polar material, and barium nickel niobate, which contributes to the final product's bandgap.
The researchers used X-ray crystallography and Raman scattering spectroscopy to ensure they had produced the crystal structure and symmetry they intended. They also investigated its switchable polarity and bandgap, showing that they could indeed produce a bulk photovoltaic effect with visible light, opening the possibility of breaking the Shockley-Queisser limit.
Moreover, the ability to tune the final product's bandgap via the percentage of barium nickel niobate adds another potential advantage over interfacial solar cells.
"The parent's bandgap is in the UV range," Spanier said, "but adding just 10 percent of the barium nickel niobate moves the bandgap into the visible range and close to the desired value for efficient solar energy conversion. So that's a viable material to begin with, and the bandgap also proceeds to vary through the visible range as we add more, which is another very useful trait."
Another way to get around the inefficiency imposed by the Shockley-Queisser limit in interfacial solar cells is to effectively stack several solar cells with different bandgaps on top of one another. These multi-junction solar cells have a top layer with a high bandgap, which catches the most valuable photons and lets the less valuable ones pass through. Successive layers have lower and lower bandgaps, getting the most energy out of each photon, but adding to the overall complexity and cost of the solar cell.
"The family of materials we've made with the bulk photovoltaic effect goes through the entire solar spectrum," Rappe said. "So we could grow one material but gently change the composition as we're growing, resulting in a single material that performs like a multi-junction solar cell."
"This family of materials." Spanier said, "is all the more remarkable because it is composed of inexpensive, non-toxic and earth-abundant elements, unlike compound semiconductor materials currently used in efficient thin-film solar cell technology."
The research was supported by the Energy Commercialization Institute of Ben Franklin Technology Partners, the Department of Energy's Office of Basic Sciences, the Army Research Office, the American Society for Engineering Education, the Office of Naval Research and the National Science Foundation.
Gaoyang Gou of Chemistry; D. Vincent West, David Stein and Liyan Wu of Materials Science and Engineering; and Maria Torres, Andrew Akbashev, Guannan Chen and Eric Gallo of Drexel, also contributed to the study.
Article in Nature entitled "Perovskite oxides for visible-light-absorbing ferroelectric & photovoltaic materials"
"For solar panels, wringing every drop of energy from as many photons as possible is imperative. This goal has sent chemistry, materials science and electronic engineering researchers on a quest to boost the energy-absorption efficiency of photovoltaic devices, but existing techniques are now running up against limits set by the laws of physics." Ref. Science Daily Nov. 12, 2013 below.
"Now researchers from the University of Pennsylvania and Drexel University claim to have experimentally demonstrated this a new paradigm for solar cell construction which may ultimately lead to
-less expensive,
-easier to manufacture
and
-more efficient harvesting energy from the sun.
The study, published in the journal Nature, was led by professor Andrew M. Rappe and research specialist Ilya Grinberg of the Department of Chemistry in Pennsylvania Uni, Peter K. Davies, chairman of the Dept of Mat Sci and Eng,School of Engineering and Applied Science, and professor Jonathan E. Spanier, of Drexel's Department of Materials Science and Engineering.
Existing solar cells all work in the same fundamental way: they absorb light, which excites electrons and causes them to flow in a certain direction ie. electric current. But to establish a consistent direction of their movement, or polarity, solar cells need to be made of two materials. Once an excited electron crosses over the interface from the material that absorbs the light to the material that will conduct the current, it can't cross back, giving it a direction.
However, "there's a small category of materials, that when you shine light on them, the electron takes off in one particular direction without having to cross from one material to another," Rappe said. "We call this the 'bulk' photovoltaic effect, rather than the 'interface' effect that happens in existing solar cells. This phenomenon has been known since the 1970s, but we don't make solar cells this way because they have only been demonstrated with ultraviolet light, and most of the energy from the sun is in the visible and infrared spectrum."
Finding a material that exhibits the bulk photovoltaic effect for visible light would greatly simplify solar cell construction. Moreover, it would be a way around an inefficiency intrinsic to interfacial solar cells, known as the Shockley-Queisser limit, where some of the energy from photons is lost as electrons wait to make the jump from one material to the other.
"Think of photons coming from the sun as coins raining down on you, with the different frequencies of light being like pennies, nickels, dimes and so on. A quality of your light-absorbing material called its 'bandgap' determines the denominations you can catch," Rappe said. "The Shockley-Queisser limit says that whatever you catch is only as valuable as the lowest denomination your bandgap allows. If you pick a material with a bandgap that can catch dimes, you can catch dimes, quarters and silver dollars, but they'll all only be worth the energy equivalent of 10 cents when you catch them.
"If you set your limit too high, you might get more value per photon but catch fewer photons overall and come out worse than if you picked a lower denomination," he said. "Setting your bandgap to catch only silver dollars is like only being able to catch UV light. Setting it to catch quarters is like moving down into the visible spectrum. Your yield is better even though you're losing most of the energy from the UV you do get."
NB.
SHOCKLEY-QUEISSER LIMIT
ref. intro: Wikipedia: Shockley-Queisser Limit
As no known materials exhibited the bulk photovoltaic effect for visible light, the research team turned to its materials science expertise to devise how a new one might be fashioned and its properties measured.
THE RACE WAS ON; cf. below for example
by et al. Above-bandgap voltages from ferroelectric photovoltaic devices. Nature Nanotechnol. 5, 143–147 (2010)
Starting more than five years ago, the team began theoretical work, plotting the properties of hypothetical new compounds that would have a mix of these traits. Each compound began with a "parent" material that would impart the final material with the polar aspect of the bulk photovoltaic effect. To the parent, a material that would lower the compound's bandgap would be added in different percentages. These two materials would be ground into fine powders, mixed together and then heated in an oven until they reacted together. The resulting crystal would ideally have the structure of the parent but with elements from the second material in key locations, enabling it to absorb visible light.
"The design challenge," Davies said, "was to identify materials that could retain their polar properties while simultaneously absorbing visible light.
The theoretical calculations pointed to new families of materials where this often mutually exclusive combination of properties could in fact be stabilized." This structure is something known as a perovskite crystal.
Most light absorbing materials have a symmetrical crystal structure, meaning their atoms are arranged in repeating patterns up, down, left, right, front and back. This quality makes those materials non-polar; all directions "look" the same from the perspective of an electron, so there is no overall direction for them to flow.
A perovskite crystal has the same cubic lattice of metal atoms, but inside of each cube is an octahedron of oxygen atoms, and inside each octahedron is another kind of metal atom. The relationship between these two metallic elements can make them move off center, giving directionality to the structure and making it polar.
"All of the good polar, or ferroelectric, materials have this crystal structure," Rappe said. "It seems very complicated, but it happens all of the time in nature when you have a material with two metals and oxygen. It's not something we had to architect ourselves."
After several failed attempts to physically produce the specific perovskite crystals they had theorized, the researchers had success with a combination of potassium niobate, the parent, polar material, and barium nickel niobate, which contributes to the final product's bandgap.
The researchers used X-ray crystallography and Raman scattering spectroscopy to ensure they had produced the crystal structure and symmetry they intended. They also investigated its switchable polarity and bandgap, showing that they could indeed produce a bulk photovoltaic effect with visible light, opening the possibility of breaking the Shockley-Queisser limit.
Moreover, the ability to tune the final product's bandgap via the percentage of barium nickel niobate adds another potential advantage over interfacial solar cells.
"The parent's bandgap is in the UV range," Spanier said, "but adding just 10 percent of the barium nickel niobate moves the bandgap into the visible range and close to the desired value for efficient solar energy conversion. So that's a viable material to begin with, and the bandgap also proceeds to vary through the visible range as we add more, which is another very useful trait."
Another way to get around the inefficiency imposed by the Shockley-Queisser limit in interfacial solar cells is to effectively stack several solar cells with different bandgaps on top of one another. These multi-junction solar cells have a top layer with a high bandgap, which catches the most valuable photons and lets the less valuable ones pass through. Successive layers have lower and lower bandgaps, getting the most energy out of each photon, but adding to the overall complexity and cost of the solar cell.
"The family of materials we've made with the bulk photovoltaic effect goes through the entire solar spectrum," Rappe said. "So we could grow one material but gently change the composition as we're growing, resulting in a single material that performs like a multi-junction solar cell."
"This family of materials." Spanier said, "is all the more remarkable because it is composed of inexpensive, non-toxic and earth-abundant elements, unlike compound semiconductor materials currently used in efficient thin-film solar cell technology."
The research was supported by the Energy Commercialization Institute of Ben Franklin Technology Partners, the Department of Energy's Office of Basic Sciences, the Army Research Office, the American Society for Engineering Education, the Office of Naval Research and the National Science Foundation.
Gaoyang Gou of Chemistry; D. Vincent West, David Stein and Liyan Wu of Materials Science and Engineering; and Maria Torres, Andrew Akbashev, Guannan Chen and Eric Gallo of Drexel, also contributed to the study.
Article in Nature entitled "Perovskite oxides for visible-light-absorbing ferroelectric & photovoltaic materials"
Monday, 6 January 2014
IBM Research - Almaden Science & Technology_well worth a visit
Projects of Interest to Materials Scientists Technologists & Engineers are Listed below:
MORE:
IBM RESEARCH Almaden Science & Technology
I found their site of special interest for their choice of "Smarter Planet Projects",
over and above my materials science & technology orientation. Materials Research,Magnetism, Nano-science & technology.
MORE:
IBM RESEARCH Almaden Science & Technology
Friday, 3 January 2014
Fuel Cell Startup Looking To Grow
Fuel Cell Startup Looking To Grow
GLASTONBURY — Three years after Proton Energy became a public company, a co-founder who was essentially No. 2 there quit to get a Ph.D. from the University of Connecticut in materials science.
"Everybody told me I was crazy at the time," said Trent Molter, now CEO of Sustainable Innovations, a company that builds on fuel cell technology similar to that used at Proton.
With his proceeds from selling Proton stock, he said, "I could afford not to work for a few years." As he began writing his dissertation in 2007, he founded Sustainable Innovations in his garage.
"I'd gotten a lot of offers, but I knew I didn't want to go work for somebody," Molter said. "I like the process of starting something."
Molter said starting a company that seeks to develop an expandable electro-chemical architecture, similar to a fuel cell, only took what he calls "pocket change," but it did mean going three more years without a salary, "and even then it was a significantly reduced salary."
In the first six years, Sustainable Innovations hasn't had any sales. It has operated on research grants from the federal government, and has also received tiny grants from Connecticut and New York, where an Israeli company it is collaborating with — ICL Industrial Products — has its U.S. headquarters.
When the research dollars weren't enough to cover rent and payroll in early years, "a lot of the deficit came out of my pocket," Molter said.
The company employs 12, and is about make offers to a few more engineers. For the first time, it's beginning to search for production workers.
Sustainable Innovations and ICL Industrial Products were awarded a $900,000 grant late last year from the BIRD, which will pay half the cost of developing a regenerative fuel cell for energy storage. BIRD, which stands for the U.S.-Israel Binational Industrial Research and Development, pays for research projects conducted by Israeli and U.S. firms.
Anantha Desikan, president of ICL-IP America, said while the multinational company works with university researchers and "we at least talk to a lot of tech startups," the investment of time and money they have made in Sustainable Innovations is unique.
"I think Trent has a track record of taking a technology from concept to commercialization from his prior role at Proton power, and that kind of stood out to us," he said. He said this new startup is very professional, and while refining the technology always takes longer than you expect, he sees progress.
"They have the right technical skills," he said. "They're willing to listen, collaborate, they have a real team approach."
The federal research dollars and this foundation money means that Molter has been able to find enough money to hire other technical staff and business development help without giving away stakes in the company.
REFS
Fatigue of Nickel-Based Superalloys: Part Two
Fatigue of Nickel-Based Superalloys: Part One
A critical property of nickel-based superalloys is their resistance to fatigue-crack propagation, particularly at service temperatures.
Many nickel-based superalloys are subject to formation of cracks or incipient cracks, either in fabrication or in use, and that the cracks can actually propagate or grow while under stress during the use in structures such as gas turbines and jet engines.
Many nickel-based superalloys are subject to formation of cracks or incipient cracks, either in fabrication or in use, and that the cracks can actually propagate or grow while under stress during the use in structures such as gas turbines and jet engines.
Nickel-based superalloys are widely used in turbines for both aerospace and land-based power-generation applications, due to their exceptional elevated-temperature strength, high resistance to creep, oxidation, and corrosion, and good fracture toughness. However, a critical property of these alloys is their resistance to fatigue-crack propagation, particularly at service temperatures.
In engine applications, there are often two components to this problem:
low-cycle fatigue, which results from relatively large cycles associated with the stopping and starting of the turbine, and
high-cycle fatigue (HCF), associated with vibrational loading during service.
high-cycle fatigue (HCF), associated with vibrational loading during service.
Fatigue results in rapid, and often unpredictable, failures due to the propagation of fatigue cracks in blade and disk components under high-frequency loading, where the cracking initiates from small defects, in many instances resulting from fretting or foreign-object damage. Due to the high vibrational frequencies involved, even cracks growing at slow per-cycle velocities can propagate to failure in short time periods, possibly within a single flight segment. Consequently, HCF-critical turbine-engine components must be operated below the fatigue-crack initiation or growth thresholds, such that cracking cannot occur within ~109 cycles.
Nickel-based superalloys have been used extensively in jet engines, in land based gas turbines and other machinery where they must retain high strength and other desirable physical properties at elevated temperatures of 1000°F (540°C) or more. Many of these alloys contain a γ’ precipitate in varying volume percentages. The γ’ precipitate contributes to the high performance properties of such alloys at their elevated use temperatures.
A problem which has been recognized to a greater and greater degree with many nickel-based superalloys is that they are subject to formation of cracks or incipient cracks, either in fabrication or in use, and that the cracks can actually propagate or grow while under stress as during use of the alloys in such structures as gas turbines and jet engines. The propagation or enlargement of cracks can lead to part fracture or other failure and the consequences of failures of the moving mechanical part due to crack formation and propagation can be particularly hazardous.
A principal finding of a NASA sponsored study was that the rate of propagation based on fatigue phenomena or in other words, the rate of fatigue crack propagation (FCP), was not uniform for all stresses applied nor to all manners of applications of stress. More importantly, the finding was that fatigue crack propagation actually varied with the frequency of the application of stress to the part where the stress was applied in a manner to enlarge the crack.
More surprising still was the magnitude of the finding from the NASA sponsored study that the application of stress of lower frequencies rather than at the higher frequencies previously employed in studies, actually increased the rate of crack propagation. In other words the NASA study verified that there was a time dependence in fatigue crack propagation. Further, the time dependence of fatigue crack propagation was found to depend not on frequency alone but on the time during which the member was held under stress, so-called hold-time.
To date, the grain-boundary engineering approach has been shown to be particularly successful in promoting fracture resistance in specific cases, notably in the context of intergranular stress-corrosion cracking and creep. However, its effect on the fatigue resistance has largely been unexplored.
According to one study, ambient temperature, smooth-bar, tension-tension fatigue lives for two γ / γ’ superalloys were reported to be increased by a factor of ~1.5 in an Fe-based alloy by increasing the fraction of special boundaries from 20 to 65 pct and by a factor of 3 in a Ni-based alloy by increasing this fraction from 9 to 49 pct, although no mechanistic explanation was presented.
Clearly, the effectiveness of grain-boundary engineering will depend upon the nature of the crack path, specifically, the preponderance of intergranular versus transgranular cracking. In light of this, the objectives of the studies was to investigate, the feasibility of using grain boundary engineering processing to promote resistance to fatigue-crack propagation, particularly at near-threshold levels, in a new polycrystalline nickel-based disk alloy, ME3 (Ni-Co-Cr Alloy). Specifically, the crack growth rates and threshold behavior of large (8 to 20 mm) through-thickness cracks were examined over a range of temperatures (25°C, 700°C, and 800°C) in order to enhance the incidence of intergranular crack growth, Figure 1.
Figure 1: Variation in fatigue-crack propagation behavior for small surface cracks in the grain-coarsened ME3, together with EBSD characterization of the path for small crack propagation. Random boundaries are shown as black lines twin boundaries are in red, other special boundaries are in yellow.
Fatigue of Nickel-Based Superalloys: Part Two
It has been discovered that it is feasible to construct parts of nickel based superalloys for use at high stress in turbines and aircraft engines with greatly reduced crack propagation rates and with good high temperature strength. The properties needed for moving parts of the engine are usually greater than those needed for static parts, although the sets of needed properties are different for the different components of an engine.
The development of the superalloy compositions and methods of their processing of this invention focuses on the fatigue property and addresses in particular the time dependence of crack growth. Crack growth, i.e., the crack propagation rate, in high-strength alloy bodies is known to depend upon the applied stress (ς) as well as the crack length (a). These two factors are combined by fracture mechanics to form one single crack growth driving force; namely, stress intensity factor K, which is proportional to ς√a.
The development of the superalloy compositions and methods of their processing of this invention focuses on the fatigue property and addresses in particular the time dependence of crack growth. Crack growth, i.e., the crack propagation rate, in high-strength alloy bodies is known to depend upon the applied stress (ς) as well as the crack length (a). These two factors are combined by fracture mechanics to form one single crack growth driving force; namely, stress intensity factor K, which is proportional to ς√a.
Under fatigue conditions, the stress intensity in a fatigue cycle may consist of two components, cyclic and static. The former represents the maximum variation of cyclic stress intensity (ΔK), i.e., the difference between Kmax and Kmin. At moderate temperatures, crack growth is determined primarily by the cyclic stress intensity (ΔK) until the static fracture toughness KIC is reached.
Crack growth rate is expressed mathematically as da/dN α(ΔK)n. N represents the number of cycles and n is material dependent. The cyclic frequency and the shape of the waveform are the important parameters determining the crack growth rate.
For a given cyclic stress intensity, a slower cyclic frequency can result in a faster crack growth rate. This undesirable time-dependent behavior of fatigue crack propagation can occur in most existing high strength superalloys. To add to the complexity of this time-dependence phenomenon, when the temperature is increased above some point, the crack can grow under static stress of some intensity K without any cyclic component being applied (i.e. ΔK=0).
The design objective is to make the value of da/dN as small and as free of time-dependency as possible. Components of stress intensity can interact with each other in some temperature range such that crack growth becomes a function of both cyclic and static stress intensities, i.e., both ΔK and K.
Following the documentation of this unusual degree of increased fatigue crack propagation at lower stress frequencies there was some belief in the industry that this phenomena represented an ultimate limitation on the ability of the nickel based superalloys to be employed in the stress bearing parts of the turbines and aircraft engines and that all design effort had to be made to design around this problem.
However, it has been discovered that it is feasible to construct parts of nickel based superalloys for use at high stress in turbines and aircraft engines with greatly reduced crack propagation rates and with good high temperature strength. It is known that the most demanding sets of properties for superalloys are those which are needed in connection with jet engine construction. The properties needed for moving parts of the engine are usually greater than those needed for static parts, although the sets of needed properties are different for the different components of an engine.
Nickel-base superalloys, strengthened by a high volume fraction of Ni3Al precipitates, have been the undisputed choice for turbine discs in gas turbines as they exhibit the best available combination of elevated temperature tensile strength and resistance to low cycle fatigue (LCF), which is essential for a disc alloy. Alloy 720LI is a wrought nickel-base superalloy developed for disc application and exhibit superior elevated temperature tensile strength and LCF properties. It is distinct because of its chemistry, especially Ti, Al and interstitial C and B contents, its processing and heat treatment. However, literature available in open domain to develop an understanding of these properties in alloy 720LI is rather limited.
The effect of temperature and strain rate on monotonic tensile properties were assessed at different temperature in the range of 25–750°C (0.67 Tm) at a strain rate of 10-4 s-1 and strain rate effects were explored in detail at 25, 400, 650 and 750°C at different strain rates between 10-5 s-1 and 10-1 s-1. Yield and ultimate tensile strength of the alloy remains unaffected by temperature till about 600°C (0.58Tm) and 500°C (0.51Tm), respectively, beyond which both decreased drastically. Negligible strain rate sensitivity exhibited by the alloy at 25 and 400°C indicated that flow stress is a strong function of strain hardening rather than strain rate hardening. However at 650 and 750°C, especially at low strain rates, strain rate sensitivity is relatively high.
The cast nickel-based superalloy Inconel 792-5A is used for the gas-turbine integral wheels of auxiliary power units in the aircraft industry. As well known, turbine wheels are subjected to repeated elastic-plastic straining as a result of heating and cooling during the start-up and shut-down periods. Consequently, low cycle fatigue is an important consideration in the design of the components, and the cyclic stress-strain and fatigue-life data are needed up to the working temperature of 900°C.
The fatigue behaviour of Inconel 792-5A (12.28 Cr; 8.87 Co; 3.98 Ti; 3.36 Al; 4.12 Ta; 4.1 W; 1.81 Mo; 0.1 Nb; 0.16 Fe; 0.031 Zr; 0.078 C; 0.015 B, all in wt %) has been reported only scarcely. Strain localization is one of the most important stages during the fatigue damage of crystalline materials. It is closely connected to crack nucleation and manifests itself in specific changes to the internal structure and in the formation of a characteristic surface relief.
Slip bands parallel to the active slip plane are formed and slip markings originate in the vicinity of the intersection of slip bands with the free surface. Slip bands and slip markings have been reported for many materials, including nickel-based superalloy single crystals, and polycrystals.
The investigation of the dislocation structure in superalloy polycrystals at room and at high temperatures indicated planar slip bands parallel to {111} slip planes and cutting of the strengthening particles. Surface slip markings were observed in Inconel 713 LC at room and at high temperature. The effect of slip bands on the cyclic stress-strain response in polycrystalline superalloys has not been studied systematically. The fragmentation and shearing of γ’ particles were considered to be the reason for the observed cyclic softening at room temperature.
Because some sets of properties are not attainable in cast alloy materials, a solution is sometimes to apply powder metallurgy techniques. However, one of the limitations for the use of powder metallurgy techniques in preparing moving parts for jet engines is the issue of the powder purity.
If the powder contains impurities such as a speck of ceramic or oxide the place where that speck occurs in the moving part becomes a latent weak spot where a crack may initiate. Such a weak spot is in essence a latent crack. The possible presence of such latent cracks makes the problems of reducing and inhibiting the crack propagation rate all the more important. It is possible to inhibit crack propagation both by the control of the composition of alloys and by the methods of preparation of such metal alloys.
REFERENCES:
Fatigue of Nickel-Based Superalloys: Part One
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