Direct Formation of Hard-Magnetic Tetrataenite in Bulk Alloy Castings.

Currently, predominant high-performance permanent magnets contain rare-earth elements. In the search for rare-earth-free alternates, body-centered tetragonal Fe-Ni is notable. The ordering to form this phase from the usual cubic close-packed Fe-Ni is understood to be possible only below a critical temperature, commonly accepted to be 593 K. The ordering is first demonstrated by using neutron irradiation to accelerate atomic diffusion. The tetragonal phase, designated as the mineral tetrataenite, is found in Fe-based meteorites, its formation attributed to ultra-slow cooling. Despite many attempts with diverse approaches, bulk synthesis of tetrataenite has not been reported. Here it is shown that with appropriate alloy compositions, bulk synthesis of tetrataenite is possible, even in conventional casting at cooling rates 11-15 orders of magnitude higher than in meteorites. The barrier to obtaining tetrataenite (slow ordering from cubic close-packed to body-centered tetragonal) is circumvented, opening a processing window for potential rare-earth-free permanent magnets. The formation of tetrataenite on industrially practicable timescales also throws into question the interpretation of its formation in meteorites and their associated cooling rates.

Structure-dynamics relationships in cryogenically deformed bulk metallic glass.

The atomistic mechanisms occurring during the processes of aging and rejuvenation in glassy materials involve very small structural rearrangements that are extremely difficult to capture experimentally. Here we use in-situ X-ray diffraction to investigate the structural rearrangements during annealing from 77 K up to the crystallization temperature in Cu44Zr44Al8Hf2Co2 bulk metallic glass rejuvenated by high pressure torsion performed at cryogenic temperatures and at room temperature. Using a measure of the configurational entropy calculated from the X-ray pair correlation function, the structural footprint of the deformation-induced rejuvenation in bulk metallic glass is revealed. With synchrotron radiation, temperature and time resolutions comparable to calorimetric experiments are possible. This opens hitherto unavailable experimental possibilities allowing to unambiguously correlate changes in atomic configuration and structure to calorimetrically observed signals and can attribute those to changes of the dynamic and vibrational relaxations (α-, ß- and γ-transition) in glassy materials. The results suggest that the structural footprint of the ß-transition is related to entropic relaxation with characteristics of a first-order transition. Dynamic mechanical analysis data shows that in the range of the ß-transition, non-reversible structural rearrangements are preferentially activated. The low-temperature γ-transition is mostly triggering reversible deformations and shows a change of slope in the entropic footprint suggesting second-order characteristics.

Fast and direct determination of fragility in metallic glasses using chip calorimetry.

We directly determine the thermodynamic fragility index of two metallic glasses (Mg65Cu25Gd10 and Au49Cu26.9Si16.3Ag5.5Pd2.3) from fictive temperature shifts induced by a variation of the quenching rate using fast differential scanning calorimetry (FDSC). Recent chip calorimeters are able to achieve the cooling rates necessary to perform such an evaluation. For the Mg65Cu25Gd10 and Au49Cu26.9Si16.3Ag5.5Pd2.3 metallic glasses studied, we find very good agreement of the kinetic fragility index with literature data obtained by conventional calorimetry and rheology. The thermodynamic fragility indices are m=25.5 ± 1.5 for Mg65Cu25Gd10 and m=50.3 ± 2.3 for Au49Cu26.9Si16.3Ag5.5Pd2.3, respectively. The FDSC method discussed here allows for fast and reliable determination of the kinetic fragility of metallic glasses, and is thus competitive with high-frequency methods.

Formation of nanostructured metallic glass thin films upon sputtering.

Morphology evolution of the multicomponent metallic glass film obtained by radio frequency (RF) magnetron sputtering was investigated in the present work. Two modes of metallic glass sputtering were distinguished: smooth film mode and clustered film mode. The sputtering parameters, which have the most influence on the sputtering modes, were determined. As a result, amorphous Ni-Nb thin films with a smooth surface and nanoglassy structure were deposited on silica float glass and Si substrates. The phase composition of the target appeared to have a significant influence on the chemical composition of the deposited amorphous thin film. The differences in charge transport and nanomechanical properties between the smooth and nanoglassy Ni-Nb film were also determined.

Nanostructured Zr-Pd metallic glass thin film for biochemical applications.

Zr-Pd metallic glassy thin films with a hierarchical nano-scale structure, produced by magnetron sputtering of the Zr and Pd powder mixture, demonstrate a unique combination of physical and biochemical properties. Thermal stability of the nano-structured glassy samples, their resistance to oxidation in dry air and phase transformation behavior are discussed in the present work. These binary alloy samples also show exceptionally high corrosion resistance and spontaneous passivation in a simulated body fluid. Experiments on the catalytic activity and biocompatibility of this nanostructured metallic glass indicate that this is a very suitable material for biochemical applications. Compared to the multicomponent alloys studied earlier this binary alloy has much simpler chemical composition, which makes preparation of the sample with defined stoichiometry easier, especially when the elements have different sputtering rates.