MIT Researchers Develop Stable Nanocrystalline Metal Alloys with Exceptional Strength
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- Published on Sunday, 06 January 2013 11:53
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Most metals are made of crystals which are orderly arrays of molecules forming a perfectly repeating pattern. In many cases the material is made of tiny crystals packed closely together, rather than one large one, and for many purposes making the crystals as small as possible provides significant advantages in properties and performance. However, such materials are often unstable as the crystals tend to merge and grow larger if subjected to heat or stress.
Researchers at the Massachusetts Institute of Technology (MIT) Department of Materials Science and Engineering (DEMSE) in Cambridge, Mass., have been undertaking work funded by the U.S. Army Research Office to find a way to avoid that problem. The result of this research are alloys that form extremely tiny grains - called nanocrystals - that are only a few billionths of a meter across, which retain their nanocrystalline structure and high strength even in the face of high heat. Such materials hold great promise for high-strength structural materials, among other potential uses.
The research was undertaken by graduate student Tongjai Chookajorn, who guided the effort to design and synthesize a new class of tungsten alloys with stable nanocrystalline structures. Her fellow DMSE graduate student, Heather Murdoch, came up with the theoretical method for finding suitable combinations of metals and the proportions of each that would yield stable alloys.
Chookajorn then successfully synthesized and tested the material and demonstrated that it does, in fact, have the stability and properties that Murdoch’s theory predicted. They, along with their advisor Professor Christopher Schuh, department head of DMSE, co-authored a paper outlining their results in a recent issue of Science (Aug.24, 2012).
“For decades, researchers and the metals industry have tried to create alloys with ever-smaller crystalline grains, but nature does not like to do that. Nature tends to find low-energy states, and bigger crystals usually have lower energy,” stated Prof Schuh.
Looking for pairings with the potential to form stable nanocrystals, Murdoch studied many combinations of metals that are not found together naturally and have not been produced in the lab. “The conventional metallurgical approach to designing an alloy doesn’t think about grain boundaries,” Schuh explains, “but rather focuses on whether the different metals can be made to mix together or not. It’s the grain boundaries that are crucial for creating stable nanocrystals. So Murdoch came up with a way of incorporating these grain boundary conditions into the team’s calculations.”
One of the nanocrystalline alloys developed and tested at MIT is a combination of tungsten and titanium. This alloy is exceptionally strong and could find applications in protection from impacts, guarding industrial or military machinery or for use in vehicular or personal armor. Other nanocrystalline materials designed using these methods could have additional important qualities, such as exceptional resistance to corrosion, the team says.
But finding materials that will remain stable with such tiny crystal grains, out of the nearly infinite number of possible combinations and proportions of the dozens of metallic elements, would be nearly impossible through trial and error. “We can calculate, for hundreds of alloys, which ones work, and which don’t,” Murdoch stated.
The key to designing nanocrystalline alloys, they found, is “finding the systems where, when you add an alloying element, it goes to the grain boundaries and stabilizes them,” Prof Schuh says, rather than distributing uniformly through the material. Under classical metallurgical theory, such a selective arrangement of materials is not expected to occur.
The tungsten-titanium material that Chookajorn synthesized, which has grains just 20 nanometers across, remained stable for a full week at a temperature of 1,100 C - a temperature consistent with processing techniques such as sintering, where powdered metal is consolidated in a mold and sintered to produce a solid shape.
Julia Weertman, a materials science professor at Northwestern University, stated “this work represents a significant advancement toward the goal of creating nanocrystalline alloys that are usable at elevated temperatures.” She added that “Schuh and his students, using thermodynamic considerations, derived a method to choose alloys that will remain stable at high temperatures. This research opens up the use of microstructurally stable nanocrystalline alloys in high temperature applications, such as engines for aircraft or power generation.”
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The Stem Solution
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- Published on Sunday, 06 January 2013 11:35
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A small shop committed to labor-saving automation added a nine-axis turning center to machine mountain bike stems. The value of this complex machine is that it delivers a simple process. The learning curve was worth it, the owners say.
For Straitline Components in Sidney, British Columbia, the difference between downhill and cross-country mountain bike racing was the difference between milling and turning. Introducing a new bike stem for cross-country riding required this small shop and bike component maker to invest in a nine-axis multitasking turning center with two workpiece spindles and the capability for five-axis milling. Company co-owner D.J. Paulson says a major part of the investment has been the learning curve he has gone through to become proficient with the machine and apply it effectively to Straitline’s new part.
It was worth it, he says. While Straitline certainly could have machined the part using its existing equipment—turning it on a CNC lathe and milling it on a machining center—producing it this way would have meant making and using specialty fixtures, which would add setup time and also add to the danger of setup-related error.
More crucially, co-owner Dennis Paulson says, “We would also have had to make fixtures just to prototype the part.”
The problem in this last point is that the part’s design is still being refined, even today. It is being improved for functionality, economy and manufacturability. While the new turning center is complex, that very complexity enables the machine to deliver simple processes. Because it permits single-setup processing with no need for special fixtures, the machine makes it possible to keep improving the part’s design well into production.
ID Gripping
The machining operations on the second spindle were the area of difficulty, D.J. says. The stem is not just a hardware connector, but also part of the appearance of the bike, and a rider is likely to choose a stem largely for its aesthetic appeal. Straitline’s process involved machining the part’s finished OD surface in the first spindle. Subsequently clamping the OD in the second spindle might distort the part and would almost certainly mar the attractive surface.
But clamping from the inside did not prove rigid enough, D.J. says. One problem was the part’s slight conical ID taper, which required chuck fingers to have the same conical profile. Another was the tendency of the chuck fingers to deflect. This led him to create preloaded fingers that would deflect into alignment with the inner surface of the part. Even with these steps, the part kept slipping. “It was baffling,” he says.
Worst of all, the slip often was too subtle to produce a clear and obvious defect. Just a slight slip during machining would throw the handlebar bore and steering tube bar slightly out of perpendicularity—an error that still would have to be caught because it would affect the alignment of the handlebars.
The solution was discovered thousands of miles away, at the International Manufacturing Technology Show in Chicago this past September. On their final day at IMTS, the brothers were making a hurried trip through the Tooling & Workholding Pavilion when they found Carbinite Metal Coatings, a company that adds tungsten alloy coatings to workholding components to improve their grip. Soon after he returned home, D.J. sent the ID grippers to Carbinite to receive this coating. As soon as the grippers returned and he re-installed them on the chuck—aligning them with the use of a sample stem part—it became clear that the coated surface would solve the problem. D.J. tried to turn the stem on the grippers by torquing it with a long-handle wrench, only to move the part, chuck and spindle all at once.
The brothers found another stem machining process improvement at IMTS as well. A boring tool from Criterion Machine Works that can be held in a collet chuck in the milling spindle is precisely the right size for the bore machined in the second spindle. Implementing this tool saved cycle time and improved accuracy by allowing the shop to get away from helical milling for this bore.
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Late-season Migratory Bird Seasons Continue
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- Published on Sunday, 06 January 2013 09:40
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The 2012-2013 late migratory bird seasons continue through early 2013.
The 2012-13 migratory bird brochure is available at the S.C. Department of Natural Resources (DNR) website.
There is no open season on harlequin ducks.
Feb. 2-3 are Federal Youth Days. Only hunters 15 years of age or younger may hunt waterfowl (ducks and geese) on these days. The youth(s) must be accompanied by an adult of at least 18 years of age. The adult is not allowed to carry a gun or hunt, and does not have to be licensed. The regular duck season limits apply.
Shooting hours for late-season duck hunting are uniform statewide. Hunters should take notice that DNR law enforcement officers will pay special attention to "late shooting violations" throughout South Carolina. Check local newspapers for sunrise and sunset times. Any South Carolina hunter born after June 30, 1979, must complete an approved hunter education course to purchase a hunting license.
Federally approved nontoxic shot (such as steel, bismuth-tin, tungsten-iron or tungsten-polymer) is required for all waterfowl hunting. The possession of lead shot is prohibited for all waterfowl hunting statewide.
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Rwanda: High Quality Gold Spotted in Gicumbi
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- Published on Sunday, 06 January 2013 10:02
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Rwanda has for decades been portrayed as a tiny resource-poor country, despite lying in the Great Rift Valley well-known for its abundant mineral wealth.
News that there is gold in the Western and Northern provinces is well-known, but the quality and quantity is what played on as a mystery for a long time.
In September, the quality question was ticked off when Rogi Mining, the country's largest gold explorer, assessed the gold in Gicumbi, Northern Province to be of high quality.
When its quantity is also verified as large in the near future, Rwanda's mineral sector, which is largely made up of cassetirite, wolfram and coltan, will be largely boosted by the this precious metal.
Also importantly, the government is also seeking to increase production of other minerals including tantalite, tungsten, niobium and lithium.
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It’s The Season for Slippery Streets and Sidewalks
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- Published on Saturday, 05 January 2013 16:57
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It’s the season for slippery streets and sidewalks, when just the simple act of walking from Point A to Point B can be an exercise in treachery.
With that in mind, a Grand Forks-based company has a simple, lightweight solution in the form of its DueNorth All Purpose Traction Aids. Available in small, medium and large sizes for both men and women, the devices feature tungsten carbide spikes on the soles and slip over boots or shoes to provide protection against slipping and sliding on even the iciest of surfaces.
Made of a durable rubber that retains elasticity in all temperatures, the traction aids are available in “Everyday,” “All Purpose” and “All Purpose Oversized” styles. Whether it’s ice fishing, running, working at an outdoors construction site or just stepping out to get the mail, Sure Foot’s traction aids will eliminate the need for walking like a penguin when shuffling across icy surfaces.
“People are able to decide which product is better for their needs, depending on how they plan to use them,” Jon Larson, president of Sure Foot, said in a news release.
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