Search

The grain size refinement of metallic materials to the nanometer scale produces interesting properties compared to the coarse-grained counterparts. Their mechanical behavior, however, cannot be explained by the classical deformation mechanisms. Using molecular dynamics simulations, the present work examines the influence of grain size on the deformation mechanisms and mechanical properties of nanocrystalline nickel. Samples with grain sizes from 3.2 to 24.1 nm were created using the Voronoi tessellation method and simulated in tensile and relaxation tests. The yield and ultimate tensile stresses follow an inverse Hall-Petch relationship for grain sizes below ca. 20 nm. For samples within the conventional Hall-Petch regime, no perfect dislocations were observed. Nonetheless, a few extended dislocations were nucleated from triple junctions, suggesting that the suppression of conventional slip mechanism is not uniquely responsible for the inverse Hall-Petch behavior. For samples respecting the inverse Hall-Petch regime, the high number of triple junctions and grain boundaries allowed grain rotation, grain boundary sliding, and diffusion-like behavior that act as competitive deformation mechanisms. For all samples, the atomic configuration analysis showed that Shockley partial dislocations are nucleated at grain boundaries, crossing the grain before being absorbed in opposite grain boundaries, leaving behind stacking faults. Interestingly, the stress relaxation tests showed that the strain rate sensitivity decreases with grain size for a specific grain size range, whereas for grains below approximately 10 nm, the strain rate sensitivity increases as observed experimentally. Repeated stress relaxation tests were also performed to obtain the effective activation volume parameter. However, the expected linear trend in pertinent plots required to obtain this parameter was not found.

Neste artigo, os comportamentos estrutural e mecânico da liga equiatômica de alta entropia Hf-Nb-Ta-Zr foram estudados mediante simulações computacionais de dinâmica molecular. As simulações foram realizadas no código livre LAMMPS, em um sistema composto de 154.000 átomos que interagiram sob o potencial Embedded Atom Method (EAM). O estudo focou na estrutura cúbica de corpo centrado (CCC). Esta estrutura apresenta a maior estabilidade estrutural ou a menor energia potencial a 0 K. A liga foi submetida a ensaios de nanoindentação utilizando um penetrador virtual esférico de diâmetro de 40 Å. Ensaiou-se à temperatura de 10 K para eliminar as contribuições térmicas dos átomos, em três planos cristalográficos (001), (011) e (111), e identificar efeitos de anisotropia. A evolução estrutural da liga foi analisada mediante as funções de distribuição radial parcial (PRDF), funções de distribuição radial total (TRDF) e também por difração de raios-X. As curvas de carregamento-descarregamento mostraram que a nanoindentação na superfície correspondente ao plano (011) requer a maior força de indentação, de cerca de 142 nN, enquanto que o plano (111) conduz à maior deformação elástica antes do início da deformação plástica.

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.

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.

The anelastic spectra of Cu39.5Zr51Al9.5, Cu54Zr40Al6 and Cu47.75Zr47.75Al4.5 bulk metallic glasses were obtained by Mechanical Spectroscopy Technique at different frequencies. Studies below room temperature show two principal mechanisms: one broad peak, similar to β relaxation, and another sharp peak, similar to α relaxation. This observation suggests that the interaction between amorphous structure and mechanical waves demonstrates that β ′ relaxation can be decomposed in two principal peaks, one due to the movement of flow units and another owing to the growth of molecular-like structures. The behaviour of hybridization among Al-Cu atoms tends to decrease the free space in the amorphous matrix, increasing internal friction. The mechanical excitation and interaction among nanostructures such as cluster type, supercluster and molecular-like structure, can lead to growth of microalloy which may give rise to the medium-range order structure as well as increasing the small nanocrystalline region scattered into the glassy structure. In addition, the effect of mechanical vibration and cryogenic temperature acting on the amorphous structure can promote interaction among nanostructures resulting in a barrier that tends to block the propagation of shear bands in the metallic glasses.

A homemade procedure for manufacturing of mechanical replacement parts was developed. The approach was applied to a guide ruler installed in machinery to produce steel rod frames. The guide ruler part, which is produced by the original manufacturer of the equipment, acts as an alignment guide of the steel wires. It ensures that the steel wires can be welded in the correct position so that the steel rod frames can be properly constructed. The guide ruler was selected due to its paramount importance for the proper functions of the equipment, the intensity of service loads, its high costs and poor availability. The methodology adopted for the homemade procedure was based on obtaining comparable chemical composition, microstructure and mechanical properties of the imported part. The main goal was to reduce the costs and delivery time for the acquisition of each spare part. The homemade procedure here used, however, is considered complete only after a comparison of the parts service performance and after an evaluation of the cost/benefit ratio. The results show that the studied part exhibited an increased economic efficiency in the equipment of about 83.0 percent, therefore its manufacturing in Brazil is viable.

A simulacao em Dinâmica Molecular (MD) e uma poderosa ferramenta para estudo de ligas amorfas nanometricas. Neste artigo, a MD foi empregada para o estudo da liga Ni80Cu20. As simulacoes foram realizadas com o codigo livre LAMMPS, em um sistema contendo 2.000 atomos com interacao atomica dada pelo potencial de Finnis-Sinclair (EAM-FS). O aquecimento foi realizado a 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 evolucao estrutural da liga. Taxas superiores a 3 K/ps conduzem a formacao de estrutura amorfa. Por outro lado, taxas inferiores a esta permitem ao sistema formar uma estrutura cristalina Cubica de Face Centrada (CFC). A estrutura da liga foi analisada empregando-se Funcoes de Distribuicao Radial (RDF), os poliedros de Voronoi, e Common Neighbor Analysis (CNA). Alem disto, foram realizados estudos de tratamento termico, na liga amorfa obtida a taxa de resfriamento de 40 K/os, em modo isotermico com a finalidade de observar a cristalizacao da liga amorfa. A curva tensao vs deformacao indica que a presenca de 20 % de nanocristais eleva a resistencia mecânica sem afetar o modulo elastico. Com esta fracao, que se forma por efeito do tratamento termico, o limite de resistencia e aumentado em mais de 30 %. Palavras-chave: Ligas amorfas, Cristalizacao, Analise de Voronoi, Comportamento mecânico.

A mechanical spectroscopy study of Cu54Zr40Al6 bulk metallic glasses composites was carried out in the kHz and MHz frequency ranges, by means of flexural and ultrasonic methods, respectively, in the temperature interval 150–300 K. In internal friction and attenuation curves at low temperature were observed peaks which were associated with distortions in the configuration of atomic clusters, which absorbed different quantities of energy due to short and medium order rearrangements. Changes within the clusters or atomic jumps between clusters occurring in the specimen induced the onset of polyamorphic peaks, since electronic interactions and bonding changed abruptly.

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.

In this study the Fe43.2Co28.8B19.2Si4.8Nb4 (at.%) alloy was processed by spray forming in an industrial facility with the aim of investigating the formation of amorphous phases. The thicker layers (5–15 mm) presented partial/fully crystalline microstructure with onset of crystallization of the amorphous phase at T x = 595 °C. On the other hand, thinner layers (below 2.5 mm) presented fully amorphous structure, and glassy behaviour with glass transition (T g ) at 555 °C. The thermal stability and the crystallization kinetics of the alloy were studied using the isothermal DSC curves, measured at different heating rates and temperatures. Despite its good glass-forming ability (GFA) and high stability against crystallizations, its incubation time for crystallization is almost zero. In order to rule-out the complete crystallization mechanism, in situ temperature resolved studies using TEM were done. In addition, the amorphous part of the deposit was heat treated at various temperatures/times and the results were compared with the in situ crystallization behavior observed by TEM.

This paper describes the mechanical properties under nanoindentation of a new glassy alloy with a nominal composition of Ni60Nb37B3, in the form of melt-spun ribbons and 1-mm-thick copper mold-cast sheets. The alloy composition was designed based on the synergy between the topological instability criterion and the difference in electronegativity among the elements. X-ray diffraction and scanning electron microscopy analyses confirmed that both ribbon and sheet samples possess totally amorphous structures with relatively high thermal stability (supercooled liquid region of about 60 K), as evaluated by differential scanning calorimetry (DSC). Nanoindentation tests revealed that the hardness of this alloy, about 15 GPa, is among the highest reported for metallic glasses. The elastic modulus of the cast sheet is higher and its hardness is similar to that of the ribbon. This correlates well with the different amounts of frozen-in free volume in both types of samples detected by DSC.

The composition of a glass former alloy and the cooling rate are both extremely important to determine the maximum glassy or critical thickness of a sample under a specific processing technique. This work compares the suction casting and the centrifugal casting techniques for processing two Cu–Zr–Al bulk metallic glasses where the main characteristics evaluated were the mold-filling ability, the average time per cycle, the oxygen contamination and the maximum thickness on which the glassy phase was obtained. The Cu–Zr–Al system was chosen due to its high glass forming ability, which is extensively reported in the literature. The wedge shape sample was chosen because it allows the evaluation of different thicknesses (hence cooling rates) at the same sample. Results revealed that the copper mold centrifugal casting had a higher oxygen contamination, whilst the mold-filling ability was similar in both processes. Also, for the centrifugal casting technique, the average time per cycle was higher and the maximum thickness in which glassy phase could be obtained was lower. A brief analysis considering the oxygen contamination on these alloys is also presented here, revealing the extreme importance of controlling this parameter.

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.

The thermal crystallization of Al-based metallic glasses can be described in association with the topological instability λ criterion. In the present work, we report on the crystallization behavior and glass forming ability of Al-rich, Al–Ni–Sm alloys, designed with compositions corresponding to the same topological instability condition of λ ≈ 0.1. Amorphous melt-spun alloys were prepared with the following compositions, varying the ratio of Ni and Sm elements: Al 87.5 Ni 4 Sm 8.5 , Al 83.5 Ni 10 Sm 6.5 , Al 80.5 Ni 14.5 Sm 5 and Al 76.5 Ni 20.5 Sm 3 . The glass forming ability of each alloy composition was evaluated based on the thermal parameters obtained from DSC runs and on X-ray diffraction patterns. Better glass forming ability was observed in compositions whose Sm content was increased and Ni content reduced. Thermal crystallization of the alloys with low Sm content showed only one crystallization peak and no glass transition event. In alloys with higher rare-earth content, a glass transition event was clearly detected before the crystallization event. The results are interpreted considering the different types and proportions of Sm–Al and Ni–Al clusters that can be formed in the alloys along the λ ≈ 0.1 line. They also emphasize the relevance of these different types of clusters in the amorphous phase in defining the stability of the glass and the types of thermal crystallization.

This study consists of the characterization of the anelastic properties of a Bulk Metallic Glasses (BMG) by mechanical spectroscopy, which can be defined as an energy absorption technique. The equipment used was the acoustic elastometer system, the anelastic relaxation spectra were carried out with a heating rate of 1 K/min and vacuum better than 10-5 torr, in the temperature range of 300 K to 640 K. The amorphous sample studied, with nominal composition of Cu53.5Zr42Al4.5, was processed by skull push-pull casting technique in a rectangular cavity cooper mould. Differential scanning calorimeter (DSC) curves have evidenced the amorphous structure although the X-ray diffraction (XDR) pattern has indicated a heterogeneous microstructure with amorphous matrix and some metaestable nanocrystalline phases which have not been identified yet. The dynamical elastic modulus of this alloy (between 54 GPa and 58 GPa at room temperature) and internal friction patterns as temperature function implied an increase of the crystalline phase during the measurements. This effect was confirmed with new X-ray diffraction measurements after the internal friction experiments.

A thermodynamic approach to predict bulk glass-forming compositions in binary metallic systems was recently proposed. In this approach, the parameter γ * = Δ H amor /(Δ H inter − Δ H amor ) indicates the glass-forming ability (GFA) from the standpoint of the driving force to form different competing phases, and Δ H amor and Δ H inter are the enthalpies for glass and intermetallic formation, respectively. Good glass-forming compositions should have a large negative enthalpy for glass formation and a very small difference for intermetallic formation, thus making the glassy phase easily reachable even under low cooling rates. The γ * parameter showed a good correlation with GFA experimental data in the Ni–Nb binary system. In this work, a simple extension of the γ * parameter is applied in the ternary Al–Ni–Y system. The calculated γ * isocontours in the ternary diagram are compared with experimental results of glass formation in that system. Despite some misfitting, the best glass formers are found quite close to the highest γ * values, leading to the conclusion that this thermodynamic approach can be extended to ternary systems, serving as a useful tool for the development of new glass-forming compositions. Finally the thermodynamic approach is compared with the topological instability criteria used to predict the thermal behavior of glassy Al alloys.

A combination of two criteria, which have been recently reported in the literature to predict compositions with high glass-forming ability, has been applied in the present work for the Ni–Ti system. The first criterion is an extension of the topological instability ‘λ criterion’, used here to indicate (and avoid) the composition fields associated with the crystallization of the first phase (solid solution or intermetallic compound) to appear during crystallization. The second, the thermodynamic criterion, is based on the calculation of a parameter (γ∗) that relates the enthalpies of all the competing phases including the amorphous one. A combination of these two criteria for the binary Ni–Ti system indicated the existence of only a narrow composition interval for glass formation. Alloy compositions in this interval were prepared by melt-spinning and the glass-forming ability (GFA) of the as-cast ribbons was evaluated by X-ray diffraction and differential scanning calorimetry. The results indicated that the two criteria are complementary, since compositions with high GFA are expected to have a high γ∗ parameter and also correspond to intervals where the λ parameter is greater than 0.1.

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.

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.