Phonon dispersion relation of metallic glasses

Experimental data on the phase sound speed of metallic glasses show anomalies in the terahertz range, reflecting an underlying complex behavior of their phonon dispersion spectrum not yet explained. We determine the phonon dispersion curve of metallic glasses by means of massive molecular dynamics simulations, allowing us to obtain the low-q region behavior with unprecedented detail. Results confirm that the sound speed is constant below the THz range, down to the macroscopic limit. On the contrary, a hardening of the sound speed, more notable in the transverse case, is found in the THz range. This behavior is modeled in terms of a relaxation model. The model gives quantitative agreement and allows us to determine a new threshold frequency ¿h, at the end of the boson-peak region. Above ¿h the shear modulus increases dramatically, reflecting the end of the amorphous-like acoustic propagation region characterized by the excess density of vibrational states.Peer ReviewedPostprint (author's final draft

Microstructural characterization and kinetics modelization of vermicular cast irons.

Several experimental techniques are used for phase identification and microstructure characterization of austempered vermicular cast irons (XRD, SEM, TEM and Mössbauer spectroscopy). Acicular structures were found to be composed by ferrite and austenite with average sizes compatible with those reported for bainitic ferrite in steels. An assessment of the free energy change involved in the austenite→bainite transformation indicated a plate-like nucleation shape for bainite with an average characteristic length close to the observed from statistical length distributions. The kinetics of the transformation was modelled in the Avrami framework. Both the diffusion controlled and the diffusionless growth hypothesis were considered in order to elucidate the mechanism underlying the austempering phase transformation

Phase-field modelling of microstructural evolution in primary crystallization

One of the main routes to obtain nanostructured materials is through the primary crystallization of metallic glasses. In such transformations, crystallites with a different composition than the amorphous precursor grow with a diffusion-controlled regime. Particle growth is slowed and eventually halted by the impingement between the concentration gradients of surrounding particles. Primary crystallization kinetics is not well described by the KJMA equation, and this fact was generally ascribed to both the soft-impingement effect and the non-random nucleation. However, recent phase-field simulations showed that the underlying physical reason is the change in the local diffusion properties of the amorphous precursor due to the variation of the composition during the transformation. The kinetics of primary crystallization is thus well described by considering a diffusion coefficient of the slowest diffusing species dependent on the local concentration. The nanostructure developed in such transformations is a key point to explain the macroscopic properties of these materials. In this work the grain size distributions obtained in realistic phase-field simulations of transformations with continuous nucleation and both constant and variable diffusion coefficient are presented. The obtained distributions are analyzed and the physical mechanisms responsible of their different features are recognized

Effect of Si and B on the electrochemical behavior of FeCoNiCr-based high-entropy amorphous alloys

The ability to produce high-entropy alloys with an amorphous structure, so-called high-entropy metallic glasses (HEMGs), offers the possibility to produce new compositions with good mechanical properties and resistance to corrosion. In this study, corrosion behavior was studied in two HEMGs, FeCoNiCrB and FeCoNiCr(BSi). In both cases, the total amount of metalloid atoms was kept constant at 20 at.%. The electrochemical behavior of these alloys was studied by means of linear polarization resistance (LPR) measurements and electrochemical impedance spectroscopy in a 3 wt.% NaCl solution. The effect of corrosion was characterized by using X-ray photoelectron spectroscopy (XPS) and the surface morphology was checked using a scanning electron microscope (SEM). The results show that samples with B but without Si exhibit better corrosion resistance due to its chemical homogeneity and lack of structural heterogeneity.Postprint (published version

Influence of carbon content on microstructure and properties of a steel matrix cermet

There is a marked correlation between the composition, microstructure and properties in TiCN-based cermets. In the case of using iron alloys as metallic matrix the carbon content is of particular significance, as not only influences the stoichiometry of ceramic phase but also induces phase transformations in the steel matrix. However, such influence has been less studied in steel matrix cermets than in conventional Ni or Co ones, so the aim of this work is to contribute to the study of the influence of carbon content on the microstructure and properties of a steel matrix cermet containing fixed quantities of alloying elements. Samples were prepared by powder metallurgy and characterized combining different techniques as Transmission Mössbauer spectroscopy (TMS), X Ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM), to explain differences found in hardness and toughness.Peer ReviewedPostprint (author's final draft

Corrosion resistance of crystalline and amorphous CuZr alloys in NaCl aqueous environment and effect of corrosion inhibitors

CuZr alloys are the basis of a family of metallic glasses with large glass forming ability and remarkable mechanical properties. The corrosion response of prepared crystalline and amorphous CuxZr100-x alloys (x = 40, 50, 64 at%), as well as bare Cu and Zr, in a severe corrosive environment, was tested. The alloys were immersed in 3 wt% NaCl aqueous solution. With the aim to increase the resistance of copper as less corrosion alloy component, nine imidazole-based compounds with different functional groups were tested as potential corrosion inhibitors. Potentiodynamic polarization measurements, electrochemical impedance spectroscopy, and long-term immersion tests followed by X-ray photoelectron spectroscopy and microscopy analysis were carried out. Overall, all the tested amorphous alloys exhibit a much better corrosion resistance than their crystalline counterparts in the presence and absence of inhibitors. The main factor controlling the corrosion resistance of the alloys appears to be the Zr-rich (or at least equiatomic) amorphous structure, the effect of the inhibitors being secondary. Results therefore show a complex relationship between inhibitor performance, microstructure and composition of CuZr alloys.Peer ReviewedPostprint (published version

Effect of minor Co additions on the crystallization and magnetic properties of Fe(Co)NbBCu alloys

The effect of partial replacement of Fe by Co (up to 8 %) on thermal stability, structure and magnetic properties of FeNbBCu alloys has been explored in this paper. The results indicate that Co reduces the stability against crystallization of the amorphous alloy and stabilizes the nanocrystalline phase prior to the further precipitation of metastable boride phases. Transmission Mössbauer spectroscopy reveals differences in the hyperfine interactions between the alloys: Co raises the mean hyperfine field of the amorphous state and differences in the nanocrystalline bcc-Fe(Co) environments between the alloys occur depending on the amount of Co near-neighbours in the bcc-Fe structure. The addition of Co has also a notable effect on the magnetic properties of both amorphous and nanocrystalline alloys increasing the Curie temperature, which shows a linear dependence with the Co composition, and the saturation polarization.Peer ReviewedPostprint (author’s final draft

Fe in P-doped basaltic melts: a Mössbauer spectroscopy study

The addition of P to a basalt causes a change in the environment of Fe. At low P contents iron is located in the glass structure and the Fe3+/Fe2+ ratio is about 80/20. Further addition of P causes a collapse of the glass structure related to a phase separation that triggers the crystallization of a more reduced iron-containing phosphate phase with a crystalline structure similar to that of stanfieldite, and Mössbauer parameters similar to those of the alluaudite group, together with the formation of a Fe3+-Fe2+ disordered phase. Their presence increases the viscosity of the melt.Peer ReviewedPostprint (published version

Structure, mechanical properties and nanocrystallization of (FeCoCrNi)-(B,Si) high-entropy metallic glasses

New high-entropy metallic glasses were produced by adding B and Si to the Fe25Co25Cr25Ni25 high entropy alloy. Extremely hard materials were obtained by inducing structural relaxation and nanocrystallization in the original amorphous metals. In this work we study the effects of the composition and annealing treatments on the structure and the mechanical properties. The changes in structure during heating treatments were determined by in situ synchrotron X-ray diffraction and the mechanical behavior was studied by nanoindentation. The me- chanical properties of the as-quenched glasses were tuned through the annealing treatments, obtaining amor- phous/nanocrystalline composites appropriate for highly demanding, wear resistance applications. The relative amount of B and Si is shown to determine the main nanocrystalline phase, FCC or BCC, which has a key influence on the mechanical properties.Peer ReviewedPostprint (published version

Structural evolution of metallic glasses during annealing through in-situ Synchrotron X-ray diffract

In this work we study the structural evolution of Al and Fe based metallic glass compositions. The samples were obtained as ribbons by melt-spinning, their glass stability and crystallization were analyzed by calorimetry and dilatometry, structural changes were followed by in-situ synchrotron X-ray diffraction during annealing throughout glass transition and crystallization. The synchrotron results are compared with calorimetric and dilatometric measurements and the structural changes occurred during annealing are determined and described for each alloy