Investigation of short range structural order in Zr69.5Cu12Ni11Al7.5 and Zr41.5Ti41.5Ni17 glasses, using X ray absorption spectroscopy and ab initio molecular dynamics simulations

Short-range order has been investigated in Zr69.5Cu12Ni11Al7.5and Zr41.5Ti41.5Ni17metallic glasses using X-ray absorption spectroscopy andab initiomolecular dynamics simulations. While both of these alloys are good glass formers, there is a difference in their glass-forming abilities (Zr41.5Ti41.5Ni17&gt; Zr69.5Cu12Ni11Al7.5). This difference is explained by inciting the relative importance of strong chemical order, icosahedral content, cluster symmetry and configuration diversity.</jats:p

A study on synthesis, crystallization, and magnetic behavior of nanocrystalline Fe-based metallic glasses

In the present work, structure of the as-cast melt-spun ribbons, nonisothermal crystallization kinetics, and the effect of heat treatment on the magnetic properties have been studied. X-ray diffraction (XRD) and transmission electron microscopy (TEM) analyses have revealed the presence of amorphous and partly crystalline structures in the as-cast Fe<SUB>67</SUB>Co<SUB>18</SUB>Si<SUB>1</SUB>B<SUB>14</SUB> and Fe<SUB>57</SUB>Co<SUB>26</SUB>Cr<SUB>3</SUB>B<SUB>14</SUB>C<SUB>0.2</SUB> metallic-glass ribbons, respectively. The crystalline phase present in the as-cast Fe<SUB>57</SUB>Co<SUB>26</SUB>Cr<SUB>3</SUB>B<SUB>14</SUB>C<SUB>0.2 </SUB>metallic-glass was identified as α -Fe. Direct transformation from liquid to α -Fe has been analyzed from a thermodynamic and kinetics point of view. The differential scanning calorimetry (DSC) studies have shown two-stage crystallization behavior. The primary and secondary crystallization phases were identified as bcc-Fe(Co) and bct-(Fe,Co)<SUB>3</SUB>(Si,B), respectively. Kissinger and Gao et al. methods were employed for nonisothermal crystallization kinetic studies. The activation-energy values obtained by the two models were in good agreement. The nucleation and growth morphologies of crystalline phases have been explained on the basis of the Avrami exponent, which were found to be consistent with the observed microstructures. The magnetic properties of as-cast amorphous ribbons showed low coercivity, and this has been attributed to averaging of magnetocrystalline anisotropy over grains coupled within an exchange length, i.e., based on a random anisotropy model. The influence of microstructure on magnetic properties was studied by crystallizing the amorphous phase at 400°C for 3 hours. The saturation magnetization and coercivity had increased after crystallization for both alloys

Textural and microstructural evolutions during deformation and annealing of Nb-1% Zr-0.1% C (wt%) alloy

Evolution of texture and microstructure in the Nb-1% Zr-0.1% C (wt%) alloy has been studied as a function of deformation and annealing treatments. The Nb alloy was deformed by rolling at room temperature up to 40%, 60% and 80% thickness reduction. Samples reduced to 60% thickness were annealed at 1300 degrees C for different soaking periods (0, 0.5, 1, 1.5, 2, 2.5, 3, 4, 5, 6 and 7 h). All the samples were characterised using X-ray diffraction (XRD), electron back scattered diffraction (EBSD) and analytical transmission electron microscopy (ATEM). Texture results have shown that deformed and annealed samples exhibited the development of typical bcc texture i.e., alpha and gamma-fibre texture. With increasing percentage of deformation ( >40%), gamma-fibre becomes the dominant texture. Samples deformed up to 80% have predominantly {111} texture. Rolling textures were simulated using Taylor models. The pancake relaxed constraint Taylor model, consideration of {110) type of slip systems exhibited good agreement with experimentally observed rolling texture. In the case of annealed samples, with increasing soaking time, volume fraction of alpha-fibre remained constant and the gamma-fibre volume fraction decreased up to 0.5 h soaking time, followed by it being constant. Samples annealed at 1300 degrees C for 7 h showed {100} orientation texture. ATEM results showed that with increasing annealing time at 1300 degrees C, Nb/Zr ratio in the carbide precipitates decreases. (C) 2013 Elsevier Ltd. All rights reserved

Stabilization of coherent precipitates in nanoscale thin films

<div><p>Coherent precipitates, on growth beyond a critical size (), become semi-coherent by the presence of interfacial misfit edge dislocations. In the case of precipitation in ‘small’ domains (matrix), the strain energy of the precipitate is altered with respect to its value in bulk domains, due to a lesser volume of strained material and domain deformations, resulting from the proximity of free surfaces. This results in a change in the value of critical size for the coherent to semi-coherent transition of the interface. In the current work, the Cu-2wt.%Fe system is used as model system to study the coherent to semi-coherent transition of precipitates. In this system, spherical Fe-2wt.%Cu precipitates form, which become cuboidal on growth beyond the critical size. Transmission electron microscopy is used to show experimentally for the first time that a coherent precipitate can be stable beyond , in nanoscale free-standing thin films (i.e. critical size for coherent to semi-coherent transition for thin films () > ). Electron energy loss spectroscopy has been used to determine the thickness of the sample. In parallel, finite element simulations are used to compute the critical size in representative domains and to comprehend the energetic basis for the observed phenomenon. Eigenstrains are used to simulate the precipitation process and the stress state due to an interfacial misfit dislocation in the finite element model.</p></div

Effect of structural defects, surface irregularities, and quenched-in defects on corrosion of Zr-based metallic glasses

X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM) revealed a fully amorphous structure for as-cast bulk metallic glass (BMG) and melt-spun ribbons of Zr<SUB>52</SUB>Ti<SUB>6</SUB>Al<SUB>10</SUB>Cu<SUB>18</SUB>Ni<SUB>14</SUB> prepared by copper mold casting and the melt-spinning technique, respectively. For melt-spun ribbons, the wheel-side surface contained a higher concentration of quenched-in defects (air pockets), whereas surface irregularities were noticed in the air side. Fluctuation microscopy indicated more medium-range order for the as-cast structure of BMG than for the melt-spun ribbon, whereas in the BMG, the medium-range order was located more in the central region than in the periphery. Macro- and microcell electrochemical experiments of the BMG along the cross section and macrocell experiments of each side of the ribbons in dilute acidic chloride environments indicated that the corrosion resistance of the BMG decreased with the increase in the medium-range order; the medium-range order, however, was less deleterious than either the surface irregularities or the quenched-in defects from the viewpoint of corrosion. A surface film formed on the metallic glasses in a dilute acidic chloride environment mainly consisted of oxide of zirconium, in which zirconium was present in the Zr<SUP>+3</SUP>, Zr<SUP>+2</SUP>, and Zr<SUP>+1 </SUP>state in the BMG and in the air-side and wheel-side surfaces, respectively