Nanostructured steels are on the horizon.
Steel is known for its versatility, strength, and low cost. There are roughly 25,000 distinct alloys currently in its ranks. But this number may someday drastically expand thanks to recent developments in bulk-material nanotechnology and science, says Daniel Branagan, chief technical officer, at The Nanosteel Co., in Idaho Falls, Idaho. In the coming decade, nanostructured steels may be one of the key technology drivers in the 21 st century, playing a pivotal role in industrial, architectural, and civil design. The range of possible properties available via nanoscale science may include steels with the hardness of alumina ceramics and the strength of carbon-based fibers. Other potential attributes may be superior corrosion resistance over nickel-based superalloys, higher strength-to-weight ratios than titanium alloys, and better weldability than cobalt-based stellites.
Branagan and other researchers have focused tremendous effort over the last decade in the study and development of nanoscale composites, ceramics, polymers, and metals. The defining characteristic of this class of materials is their extremely tiny (10 to 100 nm) features (molecules, particles, and grains) that are assembled from relatively few atoms. In the field of material science, nanotechnology can be broken into two categories. The first, particulate materials nanotechnology, involves producing materials or particles on a dimensional length scale that is nanoscale. Nanoparticulate materials are used for applications as diverse as catalysts for chemical reactions, as pigments for paints, as UV absorption particles in lotions, and are the basis for future nanomachines in what has been termed the "Feynman Vision."
The other which is the basis for NanoSteel's technology is bulk materials nanotechnology and involves shrinking the microstructural (i.e., phase/grain size) scale down to the nanoscale regime.
Bulk-material nanotechnology is basically the examination of how complex structures (i.e., phase/grain size) can be built from the "bottom-up." The ability to coax atomic-level constituents ( elements or molecules) into self-assembling structures produces materials with revolutionary new properties. The key to improving material properties beyond what's currently possible comes via the simultaneous reduction in microstructural size and scale with simultaneous increases in microstructuraluniformity, says Branagan.
That's because the extremely small features of bulk nanomaterials are on the same scale as the critical size for physical phenomena that cause conventional materials to fail. Crack propagation through the macroscale or bulkmaterial features (1-mm-sized grains, for example) in conventionally formed materials is likely to be different from how cracks will grow in materials with nanograins. Fundamental electronic, magnetic, and chemical properties are also likely to differ in their classical behavior at the nanolevel because the laws of atomic (quantum) physics may instead govern them.
The buckyball is the most notable example of a nanoscale material. It consists of 60 carbon atoms (C60) chemically bonded together in a ball-shaped molecule (fullerene). Its discovery set off an explosion of research among materials scientists, chemists, biologist, and physicists. Soon, a family of related fullerenes including C70, C84, and C240 were developed, all with extraordinary chemical and physical properties. Adding potassium or cesium atoms, for example, into empty spaces within some fullerenes reportedly creates the best organic superconductors ever made. Likewise, carbon nanotubes reportedly are some of the strongest and stiffest materials ever made. They are also said to be thermally stable in vacuum up to 2,800C and can carry electric current a thousand times better than copper wire with twice the thermal conductivity of diamond.
THE BEGINNINGS OF NANOSCALE STEELS
In recent years nontraditional nanoscale "steels" have also made head way. The first advances have revolutionized the field of hard magnets. These high-energy-density permanent magnets are made by alloying the rare earth element neodymium (Nd) with iron (Fe) and boron (B). They make up about 70% of the multibillion-dollar magnet market that serve in automobiles, minimotors, generators, electronic products, MRI-medical equipment, speakers, and switches.
In 2000, Branagan reported in The Journal of Materials Science, the ability to further bolster the properties of the Nd2Fe14B magnets via microstructural engineering on the nanoscale level. The article, "Engineering Magnetic Nanocomposite Microstructures," describes how the team carefully selected and controlled crystal phases and grain growth of the magnet's nine alloying elements (Nd, Pr, Dy, Fe, Co, B, Ti, Zr, and C) to produce more uniform microstructures with fewer defects. Such microstructures are expected to dramatically reduce easy magnetization reversal and thus narrow the critical nucleation fields leading to higher coercivity and much squarer loop shapes.
During this time Branagan turned his attention on the development of nanostructured steel coatings. The research was initially funded by the DOE's Idaho National Engineering and Environmental Laboratory (INEEL) and later funded by DARPA's Structural Amorphous Metals (SAM) program by Leo Christodoulou. Specific DoD interests include corrosion-resistant, reduced magnetic mass hull materials; moderate temperature, lightweight alloys for aircraft and rocket propulsion; and wearresistant machinery components for ground, marine, and air vehicles. From this body of research on nanostructured coatings, commercialization of Super Hard Steel (SHS) followed after the formation of the The NanoSteel Co.
SHS coatings can be applied onto a wide variety of metals using conventional thermal-spray technology (wire arc, plasma, and high-velocity oxygen fuel) or weld overlay hardfacing (MIG and PTA). SHS is said to be one of the hardest metallic materials yet made, tougher than the industry's most durable coating made from chrome/ tungsten carbide alloys. "The resulting nearly defect-free structure of SHS exhibits incredible bonds with metal surfaces," says Branagan. A 0.20-in.-thick SHS717 wire-arc-formed coating, for example, has bond strengths with carbon and stainless-steel substrates of 12 and 10 kpsi, respectively.
What's next for Branagan? Based on the successes he's seen in permanent magnets and SHS steels he says that there's no scientific reason that the SHS alloys couldn't be reconfigured to produce bulk or sheet-steel products.
BUILDING NANOSTRUCTURED STEELS
The conventional approach toward alloy design relies on alteration of the intrinsic properties of the various steel microstructural phases. Current practice is to manipulate a solid-state transformation called a eutectoid transformation by first heating the alloying elements into the single-phase, supersaturated solid solution (austenite) and then cooling or quenching it at various cooling rates. This results in the formation of multiphase structures (i.e., ferrite + cementite). A phase is a component part of a system (in this case an alloy) that is immiscible (incompatible) with other parts of the system. (A combination of a solid, a liquid, and a gas would be a three-phase system.)
A phase may contain several chemical (elemental) constituents, which may or may not be shared with other phases. Therefore, depending on how the steel is cooled, a wide variety of microconstituent microstructures pearlite, bainite, and martensite can be obtained with a broad range of properties. This manipulation of the eutectoid transformation gives rise to the vast array of engineering steels now available.
Conventional steels, however, possess only 10% of the theoretical strength suggested by bond energies (i.e., metallic, covalent, or ionic) of perfect crystals, says Branagan. "It wasn't until the advent of the electron microscope that researchers were able to shed light on this so-called strength of material paradox' in which the measured strength of materials was always far lower than predicted by theoretical calculations." With this new tool, researchers were able to see that metal-alloy microstructures aren't made of "perfect" crystals but instead contain large numbers of crystalline defects including dislocations, stacking faults, point defects, and grain boundaries, he says.
To overcome the strength of materials paradox, researchers first tried to eliminate all crystalline defects by forming fine metal filaments called whiskers from the metal's liquid and gas phases. Due to their small external size, some of the whiskers produced were nearly defect free. In the best cases, tensile strengths of iron were measured at 95% of theoretical, says Branagan.
Unfortunately, all attempts at transforming the whiskers into viable engineering steels failed. During subsequent processing (consolidation) thermodynamically stable defects automatically formed, producing microstructures similar to conventional steels and drastically reducing alloy strength.
But Branagan says he can increase the hardness and toughness of steel without corresponding losses in other properties such as ductility. "The ability to alter and control microstructural development during solidification is key to the development of favorable microstructures." To accomplish this, he manipulates the alloy elements of steel in such a way that they ignore their normal (polycrystalline) spatial orientation. Instead, he coaxes the atoms, through an almost analogous solid-state transformation, into arranging themselves randomly into a precursor in what's called a metallic glass. The technique is to supercool the molten alloy into a liquid with the viscosity of a solid.
Key to these improved properties, says Branagan, is that the metallic glass eliminates entire classes of crystalline defects including one-dimensional dislocations and two-dimensional grain and phase boundaries that are responsible for degrading the steel's mechanical and physical properties. Unfortunately, the glass is not a defect-free material. It contains a large fraction of free volume defects so the full strength of the iron atomic bond is not realized. In the case of SHS, for example, strength (and corresponding hardness) is on the order of 40 to 45% of theoretical.
The alloy atoms in the metallic glass precursor (i.e., analogous in many ways to austenite at high temperatures) settle into a nearly random (amorphous) arrangement with no crystal boundaries, says Branagan. Final development of the microstructure comes with devitrification (crystallization). "Through the devitrification process the metallic glass transforms into multiple solid phases and depending on the specific composition, crystallization temperature is usually in the range of 500 to 650C. The total heat or enthalpy of the glass-to-crystalline transformation varies from -75 to -200 J/gm," says Branagan.
"Because the glass-forming steels commonly melt at 1,000 to 1,230C, this means the glass devitrification occurs at low fractions of the melting temperature (~0.5 Tm) where atomic diffusion is limited and the driving force for crystallization, due to the metastable nature of the glass state, is extremely high. Thus, during devitrification there's a very high nucleation frequency giving crystalline grains little time to grow before impingement between neighboring grains. This results in the formation of extremely fine nanoscale phases with grains on the order of 10 to 100 nm."
"There are several unique advantages of the devitrification transformation," he continues. Unlike conventional steel processing, where solubilities of alloying elements are limited, solubilities in metallic glass can be drastically widened, which allows the use of the elemental constituents in new ways. Because of the nonequilibrium nature and liquidlike structure of the metallic glass, atoms that are normally incompatible can be brought into close contact. For example, consider a conventional alloy made from Nd and Fe. The maximum solubility of Nd in the austenite phase at elevated temperature is only 4% by weight and zero at room temperature. "However," says Branagan, "in a steel glass, it is possible to dissolve up to 30% by weight Nd in the glass and maintain this solubility at room temperature."
When alloying elements are added to the base material, they interact in several ways, says Branagan. They may not react at all, forming immiscible (incompatible) liquids. They may also selectively dissolve in one of the phases in the base alloy, react with constituents of the base alloy to form new microstructural phases, or react with each other forming separate phases. This makes the glass-devitrification transformation incredibly complex. "And even when the composition is fixed," says Branagan, "even more complexity is obtained by varying the transformation pathway of the glass-devitrification transformation." The thermal history of the transformation and glass relaxation, recovery, crystallization, and recrystallization phenomena are all important factors resulting in microstructural development.
"By manipulating these effects, however," says Branagan, "the microstructures can be engineered in a variety of ways including the average phase size, causing precipitation in the glass or in the nanocomposite, and even forming anisotropic or isotropic microstructures."
NOT QUITE READY FOR PRIME TIME
There are currently only a handful of nanostructured steels commercially available, says Branagan, but headway is being made. NanoSteel is using a process called Rapid Alloy Design and Commercialization (RADAC) to determine key structures and property relationships. The technique incorporates basic theories on alloy design and physical mechanistic models to first develop, produce, and experiment on "ideal" microstructures. "We then try and match the processing characteristics of the experimental alloy to specific large scale processing methods such as casting, extrusion, or powder metallurgy," says Branagan. "Our hope is to develop nanostructures in the commercial process while maintaining the targeted material property improvements."
For example, says Branagan, "Using RADAC we've been able to produce nanostructured steel ribbons that are stronger than conventional steels at room temperature with measured tensile strengths over 4 GPa at 20C and strength levels on the order of 1.8 GPa at 750C. "However, the real promise of the bulk materials nanotechnology approach is to separate out the physical mechanisms governing strength and hardness from those responsible for toughness and ductility." The idea is to optimize strength and hardness qualities independently from those for toughness and ductility.
Normally there is an inverse relationship between hardness and strength in conventional materials. "Towards this end, recent experiments have shown that superplasticity can be obtained with ultrahigh tensile elongation (230%) and a strain-rate sensitivity factor of 0.51 in a nanocomposite steel alloy produced from a metallic-glass precursor. "Because a steel metallic-glass precursor-must be produced, it might be assumed that rapid solidification is also a must and that the realm of nanostructured steels will never be possible with conventional metallurgical processes such as casting," says Branagan. "This analysis is partially correct as appropriate bulk glass steel compositions are not currently available. However, in certain zirconium-based and rare-earth aluminum alloys, bulk glass formation has been achieved with critical cooling rates for metallic-glass formation down in the 1-K/sec range. With this benchmark now established, it appears only a matter of time before bulk glass-forming steels will be developed as well."
In our steel products, adds Branagan, with commercial purity constituents, we have reduced critical cooling rates to as low as 250 K/sec. Other researchers at Oak Ridge, University of Virginia, and University of Wisconsin have gone even further in this area on a research scale generally using high-purity elements and carefully controlled casting studies.
DARPA's Structural Amorphous Metals (SAM) program, www.darpa.gov/dso/thrust/matdev/sam/index.html
The NanoSteel Co., (407) 838-1427, www.nanosteelco.com
Superhard steel coatings
To test the bond strength of SHS717 coatings with Vickers hardness (HV300) of 1,100 kg/mm2 test specimens were bent 180. The coating did not crack or delaminate which underscores not only the high bond strength but also the high resiliency of the coating.
Transmission electron microscope micrographs of SHS717 alloy which has been heat treated at 700C for 10 min. (Left) "Ideal" microstructure (average grain size 25 nm), (center) HVOF coating with average grain size of 50 nm, (right) wire-arc coating with average grain size of 80 nm.
Although the microstructures of Super Hard Steel (SHS) coatings are coarser than "ideal" (i.e., average grain size 25 nm) and additionally contain isolated larger scale regions formed during solidification, they can still be classified as nanoscale, says Michael Breitsameter, vice president NanoSteel Co., Idaho Falls, Idaho. "What's remarkable about the SHS coatings, however, is that they were produced in air using offtheshelf thermal-spray technology." Another advantage is that their feedstock material is specifically formulated for HVOF (high-velocity oxygen fuel), plasma, and wire arc, and is physically identical to conventional feedstock, he adds. "This eliminates the spraying and handling problems normally associated with nanoscale particulate materials."
Spray studies have shown the SHS feedstocks exhibit a wide operational window with great latitude and forgiveness to spray angle and distance, says Breitsameter. "This is important for parts with complicated shapes and for manual field applications." And while other ceramic-based coatings may be harder than those of SHS, they can only protect surfaces as long as they remain adhered to the part. The resiliency or damage tolerance of SHS717 coatings combined with high hardness gives SHS coatings extremely high bond strengths to a wide variety of metallic substrates.
For example, the bond strength of a SHS717 wire-arc coating was measured using ASTMC611 bond pull tests. The bond strength onto 1018 bond plugs was measured for coating thicknesses of 0.02, 0.04, 0.07, and 0.1 in. At the thickness extremes (0.02 and 0.1 in.) the bond strengths were 12 and 6.5 kpsi, respectively. "The bond strength of the 0.1 coating is higher than most conventional materials sprayed at 0.015 in. thickness," says Breitsameter. Remarkable, he continues, considering the fact that the coating was applied with no intermediate bond coat applied.
To further test bond strength, 180 bend tests were performed. "During the bend test," explains Breitsameter, "the outside of the coating is put in tension which can easily exceed the bond strength causing cracking or delamination." The SHS717 coatings did not crack or delaminate which underscores not only the high bond strength but also the high resiliency of the coating which is noteworthy considering it has a Vickers hardness (HV300) of 1,100 kg/mm2. Additionally, after heat treatment, hardness rose to 1,200 kg/mm2.
|of SHS717 coatings|
|Bond strength of SHS717 Coatings|
|HVOF, 0.04-in. thick, kpsi|
|Wire-arc, 0.02-in.-thick, kpsi|