| Quote: |
Ti15Cu15Ni and the newly developed Ti21Ni14Cu. Zr-Ti-rich side of the Zr-Ti-Ni(Cu) alloy system were investigated for brazing of titanium alloys. Low-melting ternary and quaternary eutectic alloys with melting temperatures below 800°C were discovered. Using eutectic as well as off-eutectic braze alloys, CP-Ti and Ti-6Al-4V alloys were successfully brazed at 830°C and 850°C. |
| Quote: |
Aluminum nitride is one of the few materials that is both a good thermal conductor and a good electrical insulator. It is also a high-temperature ceramic, that has a low thermal expansion coefficient, and low dielectric constant. It is also stable to molten metals such as aluminum, has good wear resistance, and good thermal shock resistance. Various preceramic oligomer and polymer routes to aluminum nitride have been investigated. For example, the reaction of LiAlH4 or AlH3 with ammonia initially yields Al(NH2)3, which loses ammonia and hydrogen during pyrolysis and leaves AlN contaminated by carbon from the initial reaction solvent. Fibers of aluminum nitride have been produced by the melt-spinning of ethyl-alazanes derived from the reactions of triethylaluminum and ammonia. The spinnable products have compositions such as [(EtAlNH)x(Et2AlNH2)y(Et3Al)z]n which probably consist of linked alazane rings and chain structures. Pyrolysis in ammonia gives aluminum nitride fibers. Interrante and coworkers reported techniques for the preparation of alloys of SiC and AlN by the copyrolysis of precursors to both silicon carbide and aluminum nitride. The source of the aluminum component was commercially available aluminum alkyls [such a trimethylaluminum] which, when treated with ammonia, initially give cyclic alazanes such as (CH3AlNH2)x, and cross-linked species CH3AlNH by pyrolytic loss of CH4. Further pyrolysis gives a high purity, oxygen- and carbon-free AlN in nearly quantitative yield. … Copyrolysis took place initially at 170 °C , but later at temperatures up to 350 °C under nitrogen, and then up to 2,000 °C to give a homogeneous Si/C/AlN ceramic. "Inorganic Polymers", James E. Mark, H. R. Allcock, Robert West, p330 |
| Quote: |
A process for synthesizing aluminium nitride fibres includes such technological steps as mixing microcrystalline aluminium silicate fibres in sillimanite structure with carbon black or graphite, heating to 1500-1750 deg.C in nitrogen atmosphere, keeping the temp for a certain time, and exhausting residual carbon at 550-650 deg.C, and features no strict requirement to purity and Al/Si ratio of raw material, simple process, good operation and repeatability, high output rate of AlN fibres up to 100% and low cost. Chinese Patent 98103408, year 1998 |
| Quote: |
Materials scientists in California have made a special metallic glass with a strength and toughness greater than any known material. “It has probably the best combination of strength and toughness that has ever been achieved,” said Robert O. Ritchie, a materials scientist at Lawrence Berkeley National Laboratory who is one of the authors of a paper describing the new glass. “It’s not the strongest material ever made, but it’s certainly one of the best with a combination of strength and toughness.” In other words, some tougher materials exist, but they are less strong; there are stronger materials, but they’re not as tough. To grasp this, you have to define the the difference between strength and toughness. Strength refers to how much force a material can take before it deforms. Toughness explains the energy required to fracture or break something; it describes an object’s ability to absorb energy. Most of the time, these qualities are mutually exclusive. “The holy grail is to get both those properties at the same time,” Ritchie said. If you do a Google search (admittedly not very scientific) for “world’s strongest material” and “world’s toughest material,” among the results graphene and spider silk tend to rank the highest, respectively. If you expand your efforts to search for materials that are both strong and exhibit fracture toughness, you start finding a variety mentioned, including silicon carbide (and several other engineering ceramics), Ni/Ti alloys (and other engineering metals) and metallic glasses. Engineering ceramics are hard to beat on the strength scale. They are scratch resistant and difficult to bend. However, they suffer from a tendency to brittleness. Some engineering metals tend to have higher numbers for combination of both strength and fracture toughness than engineering ceramics. However, even these metals’ toughness come with a price: a tendency to malleability. But now a group of researchers from California Institute of Technology , Lawrence Berkeley National Lab and University of California, Berkeley report in a new paper in Nature Materials that they have found a new composition for a highly damage-tolerant a palladium based metallic glass alloy that is tougher and stronger than Ni and Ti alloys. The alloy contains the noble metal palladium, with a small fraction of silver, and a mixture of other metalloids, and has shown itself in tests to have a combination of strength and toughness at a level that has not previously been seen in any other material. Furthermore, the researchers insights suggest even stronger, tougher materials may exist. "Our study demonstrates for the first time that this class of materials, the metallic glasses, has the capacity to become the toughest and strongest ever known," Demetriou says. Indeed, the researchers write in their paper, these materials allow for "pushing the envelope of damage tolerance accessible to a structural metal." The search for materials that suppress fractures while maintaining high strength can be difficult because these two properties are, generally speaking, mutually exclusive. Fracture-tough crystalline materials resist crack expansions because of plastic shielding (think of tiny areas of “shear” bands of the material sliding by each other) ahead of the crack. But, because this shielding doesn’t require much energy, its easy for it to occur, thus decreasing its overall strength. Conversely, noncrystalline (i.e., amorphous or glass) materials tend to strongly resist the development of openings, but lack the mobile microstructures to employ plastic shielding. So, once an opening does develop, a crack can readily expand to the point of failure (brittleness). As a result, scientists and engineers are faced with the dilemma of the trade-off between strength and toughness. Previously, metallic glass compositions had been investigated and found to have shear band-forming properties, but under strain a single shear band would often form and grow extensively, resulting in major material failure. That’s what is so important about this group’s work: They achieve higher strength and higher toughness, successfully dodging the trade-off, by using a composition and process that creates a glass capable of multiple microscale shear bands when subjected to stress. The important component of this new material is palladium. Researchers leverage the high bulk-to-shear stiffness ratio of palladium. ACerS Fellow Rob Ritchie, a professor at the university and materials scientist at LBL, explains, “Because of the high bulk-to-shear modulus ratio of the palladium containing material, the energy required to form shear bands is much lower than the energy required to turn these shear bands into cracks. The result is that glass undergoes extensive plasticity in response to stress, allowing it to bend rather than crack.” Besides palladium, the glass contains phosphorous, silicon, germanium and silver (Pd79Ag3.5P6Si9.5Ge2). Marios Demetriou, one of the paper’s coauthors, has been able to make rods of the glass with diameters of six millimeters. The researchers think this is only the beginning. Ritchie tells me that they know “this is a compositional thing, not a structural thing. We are looking at a lot of other compositions. We are fairly certain we didn’t just get lucky with our first composition. We are sure extensive plasticity can be induced in other metallic glasses and that even higher levels of damage resistance are accessible.” Previously, an amorphous alloy based on titanium and zirconium, also developed at the same university, was thought to be the strongest, achieving a yield strength of over 1723 MPa. Zr: 52.5 Ti: 5 Cu: 17.9 Ni: 14.6 Al:10 |