RESUMEN Los clásicos mecanismos de deformación basados en defectos de los policristales metálicos no son adecuados para describir el comportamiento mecánico de los materiales metálicos vítreos y nanocristalinos. Su complejidad inherente crea un verdadero desafío para comprender sus complicados fenómenos físicos. El uso de la dinámica molecular (DM) se torna interesante permitiendo evaluar la relación entre la estructura atómica con las propiedades mecánicas. En el presente trabajo, simulaciones de DM fueran utilizadas para estudiar los mecanismos de deformación a nivel de nanoescala de la aleación Al80Ti15Ni5. Los resultados revelaron una dependencia significativa entre el módulo de Young y la estructura atómica. El tipo de estructura a escala atómica, nanocristalina o amorfa, gobierna los mecanismos de la deformación. Para la aleación nanocristalina, el deslizamiento y la difusión del contorno de grano parecen ser los procesos dominantes en la deformación. Además, se observan emisiones de discordancias parciales en los contornos de grano. En el material amorfo, las zonas de transformación por cizallamiento comienzan a formarse en el régimen elástico y se convierten en bandas de cizallamiento actuando como los principales mecanismos en el proceso de deformación. Los resultados indican que las propiedades mecánicas de la estructura amorfa representan un caso límite inferior del nanocristal. Los módulos elásticos determinados en la aleación Al80Ti15Ni5 son muy bajos, por esta razón se evaluaron los efectos de los potenciales interatómicos unarios y ternarios para cada elemento.

ABSTRACT Classical deformation mechanisms based on crystalline defects of metallic polycrystals are not entirely suitable to describe the mechanical behavior of nanocrystalline and glassy materials. Their inherent complexity creates a real challenge to understand the acting physical phenomena. Thus, the molecular dynamics approach becomes interesting because it allows evaluating the mechanical properties and its related atomic structure. To study the atomic structure's influence on the deformation mechanisms at the nanoscale level of the Al80Ti15Ni5 alloy, molecular dynamics simulations, and post-processing techniques were used in the present work. The results revealed a significant dependency between the Young modulus and the atomic structure. Moreover, the type of structure, i.e., nanocrystalline or amorphous, governs the deformation mechanism type. For the nanocrystalline alloy, grain boundary sliding and diffusion seem to be the dominant deformation processes followed by the less essential emissions of partial dislocations from the grain boundaries. Concerning the amorphous material, the shear transformation zones begin to form in the elastic regime evolving to shear bands, these being the main mechanisms involved in the deformation process. The results also indicate the amorphous structure as a lower limit-case of the nanocrystal. The Al80Ti15Ni5 elastic moduli values were below expectations; for this reason, the effects of unary and ternary interatomic potentials were evaluated for each element.

RESUMO A simulação em Dinâmica Molecular (MD) é uma poderosa ferramenta para estudo de ligas amorfas nanométricas. Neste artigo, a MD foi empregada para o estudo da liga Ni80Cu20. As simulações foram realizadas com o código livre LAMMPS, em um sistema contendo 2.000 átomos com interação atômica dada pelo potencial de Finnis-Sinclair (EAM-FS). O aquecimento foi realizado à taxa de 2 K/ps, entretanto, no resfriamento foram utilizadas as taxas de 40, 10, 4, 3, 2 e 1 K/ps, com a finalidade de se observar a evolução estrutural da liga. Taxas superiores a 3 K/ps conduzem à formação de estrutura amorfa. Por outro lado, taxas inferiores a esta permitem ao sistema formar uma estrutura cristalina Cúbica de Face Centrada (CFC). A estrutura da liga foi analisada empregando-se Funções de Distribuição Radial (RDF), os poliedros de Voronoi, e Common Neighbor Analysis (CNA). Além disto, foram realizados estudos de tratamento térmico tanto em modo dinâmico à taxa de resfriamento de 40 K/ps como isotérmico com a finalidade de observar a cristalização da liga amorfa. A curva tensão vs deformação indica que a presença de 20 % de nanocristais eleva a resistência mecânica sem afetar o módulo elástico. Com esta fração, que se forma por efeito do tratamento térmico, o limite de resistência é aumentado em mais de 30 %.

ABSTRACT Molecular Dynamics (MD) is a powerful tool for the study of nanometric amorphous alloys. In the present work, MD was used to investigate the Ni80Cu20 alloy. The simulations were performed by using the free code LAMMPS, for a system containing 2,000 atoms. The atomic interaction was governed by Finnis-Sinclair (EAM-FS) potential. The heating was controlled by a rate 2 K/ps, however, the cooling stage used the following rates 40, 10, 4, 3, 2 and 1 K/ps, in order to observe the structural evolution of the alloy. Rates higher than 3 K/ps produce an amorphous structure. On the other hand, lower rates allow the formation of Cubic Face Centered (CFC) crystals. The alloy structure was analyzed by Radial Distribution Function (RDF), Voronoi polyhedrons, and Common Neighbor Analysis (CNA). Moreover, studies on the heat treatment with cooling rate at 40 K/ps as well as isothermal condition were performed to know the crystallization process of glassy alloy. The stress-strain curve indicates the presence of 20 % of nanocrystal increases the mechanical resistance without jeopardizing the elastic modulus. This fraction, resulting of heat treatment, increases the ultimate tensile strength at least 30 %.

This paper presents the results of Molecular Dynamics (MD) simulations of Cu60.0Zr32.5Ti7.5 alloy through the open source code LAMMPS. Amorphous samples were produced by quenching the molten metal from 2300 to 300 K. The pair distribution functions of the liquid and solid were calculated. Moreover, the atomic short-range order at 800, 700 and 300 K was obtained by using the Voronoi tesselation method. Cu-centered icosahedral clusters were the prevailing configuration. The tensile stress-strain curve showed that the material present plastic deformation, however, shear bands were not observed in the MD simulation. The evolution of viscosity in temperatures higher than its glass transition temperature was also determined. Furthermore, the fragility of the alloy at glass temperature was evaluated.

A structural and mechanical characterization of pure aluminum and 2124 T6 aluminum alloy reinforced with quasicrystalline phases of composition Al65Cu20Fe15 and Al70.5Pd21Mn8.5 (%at.) were performed. The quasicrystalline phases were synthesized by arc melting and then milled to produce powder of the alloys, which were then mechanical mixed with the starting powders of aluminum and 2124 aluminum alloy. The composites were produced by hot extrusion of a mechanical mixture containing 20% (%wt.) of the reinforcing phases on the metallic matrix. The structural characterization of the composites was carried out by X-ray diffraction, scanning electron microscopy and transmission electron microscopy. Mechanical characterization was carried out by Vickers hardness measurements and torsion tests at room temperature. The pure aluminum/quasicrystal composite showed the presence of the same phases from the starting powder mixture while for the 2124 aluminum alloy/Al65Cu20Fe15 the quasicrystalline phase transformed to the tetragonal ω-Al7Cu2Fe during the solution heat treatment. Mechanical strength of the composites presented a substantial increase in comparison to the original matrix metal. While the equivalent ultimate tensile strength of the Al/quasicrystal composites reached values up to 215MPa and Vickers hardness up to 60HV, the 2124/quasicrystal composites reached values up to 670MPa and Vickers hardness up to 190HV.

A mechanical spectroscopy study of Cu-Zr-Al bulk metallic glasses, was performed with two types of equipment: a Kê-type inverted torsion pendulum and an acoustic elastometer, working in the frequency ranges of Hz and kHz, respectively, with a heating rate of 1 K/min. The analysis of the anelastic relaxation shows similar spectra for both types of equipment resulting in internal friction patterns that vary with temperature and are not reproducible at each thermal cycle. The normalized of the square of the frequency changes from the first to later measurement cycles. These results indicate that the specimens of Cu-Zr-Al alloys were changing by mechanical relaxation, owing to the motion of atoms or clusters in the glassy state and possible "defects" produced during the processing of alloys.

The effect of minor additions of Gd and Sm on the glass-forming ability (GFA) of Cu-Zr-Al alloys is investigated here. The rationale for these additions is the fact that the atomic size distribution can increase GFA by changing the topology of the alloy as a function of cluster stability, which is tied to the electronegativity and ionic and covalent nature of alloys. Ingots with nominal compositions of Cu40Zr49Al10.5Gd0.5, Cu40Zr49Al10.5Sm0.5 and Cu39Zr50Al9Gd2 were prepared by arc-melting and rapidly quenched ribbons were produced by the melt-spinning technique. Bulk samples with a thickness of up to 10 mm were also produced by casting, using a wedge-shaped copper mold. The samples were characterized by differential scanning calorimetry, X-ray diffractometry and scanning electron microscopy. The three compositions showed a fully amorphous structure in the ribbons and a predominantly homogeneous amorphous structure with a thickness of up to 10 mm, although some gadolinium oxide crystals as well as samarium compounds were found to be scattered in the amorphous matrix in 5-mm-thick samples. The amorphous phases in the alloys showed high thermal stability with a supercooled liquid region (ΔTx) of about 70 K.

A combination of an extension of the topological instability "λ criterion" and the "average electronegativity" has been recently reported in the literature to predict compositions with high glass-forming ability (GFA). In the present work, both criteria have been applied to select the Ni61.0Nb36.0B3 alloy with a high glass-forming ability. Ingots were prepared by arc-melting and were used to produce ribbons processed by the melt-spinning technique further characterized by differential scanning calorimetry (DSC), X-ray diffraction (XRD) and scanning electron microscopy (SEM). The Ni61.0Nb36.0B3 alloy revealed a complete amorphization and supercooled liquid region ΔTx = 68 K. In addition, wedge-shaped samples were prepared using copper mold casting in order to determine the critical thickness for amorphous formation. Scanning electron microscopy (SEM) revealed that fully amorphous samples could be obtained, reaching up to ~800 µm in thickness.