Deformation behavior of metallic glass composites reinforced with shape memory nanowires studied via molecular dynamics simulations

Molecular dynamics simulations indicate that the deformation behavior and mechanism of Cu64Zr36 composite structures reinforced with B2 CuZr nanowires are strongly influenced by the martensitic phase transformation and distribution of these crystalline precipitates. When nanowires are distributed in the glassy matrix along the deformation direction, a two-steps stress-induced martensitic phase transformation is observed. Since the martensitic transformation is driven by the elastic energy release, the strain localization behavior in the glassy matrix is strongly affected. Therefore, the composite materials reinforced with a crystalline phase, which shows stress-induced martensitic transformation, represent a route for controlling the properties of glassy materials. The authors acknowledge the financial support of the European Research Council under the ERC Advanced Grant INTELHYB (Grant No. ERC-2013-ADG-340025) and the German Science Foundation (DFG) under the Leibniz Program (Grant No. EC 111/26-1). A DAAD-PPP travel grant is also acknowledged. Computing time was made available at ZIH TU Dresden and IFW Dresden as well as by CSC Julich. The authors acknowledge Dr. Simon Pauly and Sergio Scudino for fruitful discussions

Ductile bulk metallic glass by controlling structural heterogeneities

A prerequisite to utilize the full potential of structural heterogeneities for improving the room-temperature plastic deformation of bulk metallic glasses (BMGs) is to understand their interaction with the mechanism of shear band formation and propagation. This task requires the ability to artificially create heterogeneous microstructures with controlled morphology and orientation. Here, we analyze the effect of the designed heterogeneities generated by imprinting on the tensile mechanical behavior of the Zr52.5Ti5Cu18Ni14.5Al10 BMG by using experimental and computational methods. The imprinted material is elastically heterogeneous and displays anisotropic mechanical properties: strength and ductility increase with increasing the loading angle between imprints and tensile direction. This behavior occurs through shear band branching and their progressive rotation. Molecular dynamics and finite element simulations indicate that shear band branching and rotation originates at the interface between the heterogeneities, where the characteristic atomistic mechanism responsible for shear banding in a homogeneous glass is perturbed

Ductile bulk metallic glass by controlling structural heterogeneities

A prerequisite to utilize the full potential of structural heterogeneities for improving the room-temperature plastic deformation of bulk metallic glasses (BMGs) is to understand their interaction with the mechanism of shear band formation and propagation. This task requires the ability to artificially create heterogeneous microstructures with controlled morphology and orientation. Here, we analyze the effect of the designed heterogeneities generated by imprinting on the tensile mechanical behavior of the ZrTiCuNiAl BMG by using experimental and computational methods. The imprinted material is elastically heterogeneous and displays anisotropic mechanical properties: strength and ductility increase with increasing the loading angle between imprints and tensile direction. This behavior occurs through shear band branching and their progressive rotation. Molecular dynamics and finite element simulations indicate that shear band branching and rotation originates at the interface between the heterogeneities, where the characteristic atomistic mechanism responsible for shear banding in a homogeneous glass is perturbed

High-resolution transmission electron microscopy investigation of diffusion in metallic glass multilayer films

Lack of plasticity is one of the main disadvantages of metallic glasses. One of the solutions to this problem can be composite materials. Diffusion bonding is promising for composite fabrication. In the present work the diffusion process in glassy multilayer films was investigated. A combination of advanced transmission electron microscopy (TEM)methods and precision sputtering techniques allows visualization and study of diffusion in amorphous metallic layers with high resolution. Multilayered films were obtained by radio frequency sputter deposition of Zr-Cu and Zr-Pd. The multilayers were annealed under a high vacuum (10 −5 Pa)for 1 and 5 h at 400 °C, that is, well below the crystallization temperatures but very close to the glass-transition temperatures of both types of the glassy layer. The structural evolution in the deposited films was investigated by high-resolution transmission electron microscopy. It was observed that, despite the big differences in the atomic mass and size, Pd and Cu have similar diffusion coefficients. Surprisingly, 1 h of annealing results in formation of metastable copper nanocrystals in the Zr-Cu layers which, however, disappear after 5 h of annealing. This effect may be connected with nanovoid formation under a complex stress state evolving upon annealing, and is related to the exceptionally slow relaxation of the glassy layers sealed with a Ta overlayer.The authors acknowledge the financial support through the European Research Council under the ERC Advanced Grants INTELHYB (grant ERC-2013-ADG-340025) and ExtendGlass (grant ERC-2015-AdG-695487), the German Science Foundation (DFG) under the grant SO 1518/1-1, and the Ministry of Education and Science of the Russian Federation in the framework of the ‘Increase Competitiveness’ program of NUST ‘MISiS’ (№ К2-2014-013 and К2-2017-089)

STZ-Vortex model: the key to understand STZ percolation and shear banding in metallic glasses

The fast dynamics and localized nature of shear banding together with the disordered structure of metallic glasses make it difficult to gain an understanding of the mechanism of strain localization and shear band formation during deformation. The most widely accepted theory for the atomic-scale mechanisms of shear banding is related to the percolation of shear transformation zones (STZs). Recently we reassessed and clarified the atomistic details of STZs self-assembly mechanisms and provided a new two-unit model crucial in understanding STZs percolation and shear band formation. This manuscript attempts to provide a brief but up-to-date review on the STZ-Vortex mechanism and its applications in the areas of the shear band dynamics and, in general, in the atomistic description of deformation mechanisms in metallic glasses. The review also identifies a number of unsolved issues concerning deformation and relaxation mechanisms in metallic glasses where the STZ-Vortex model could be successfully applied

Crack-healing mechanisms in high-entropy alloys under ion irradiation

High-entropy alloys (HEAs) are potential candidates for advanced nuclear structural materials due to their impressive mechanical properties under extreme conditions. However, micro-cracks, the most common material damage, are introduced upon of the material synthesis and in service. In this work, an atomistic investigation of the crack-healing mechanisms of a FeCoCrNiAl0.5 HEA under ion irradiation is provided by molecular dynamics simulations. Quantitative analysis of the generation and recombination of point defects during the process of overlapping collision cascades is implemented to assess the crack-healing mechanisms in the HEA. The interstitial defects generated in the core of the cascade during the first collision event diffuse to the crack surface, resulting in crack-healing during subsequent recrystallization. In addition, the corresponding vacancies accumulate and forms large-size vacancy clusters that generate stacking faults and complex dislocation networks distributed around the location of the healed crack. With increasing the number of overlapping cascades the defects recombination rate increases and the phase stability is further improved. Crack-healing engineering in HEAs subjected to ion irradiation could pave the way towards designing advanced nuclear materials

Atomic-Level Processes of Shear Band Nucleation in Metallic Glasses

The ability to control the plastic deformation of amorphous metals is based on the capacity to influence the percolation of the shear transformation zones (STZs). Despite the recent research progress, the mechanism of STZ self-assembly has so far remained elusive. Here, we identify the structural perturbation generated by an STZ in the surrounding material and show how such a perturbation triggers the activation of the neighboring STZ. The mechanism is based on the autocatalytic generation of successive strong strain and rotation fields, leading to STZ percolation and, ultimately, to the formation of a shear band

An atomic-level perspective of shear band formation and interaction in monolithic metallic glasses

Understanding the relationship between nanoscale structural heterogeneities or elastic fluctuations and strain localization in monolithic metallic glasses remains a long-standing underlying issue. Here, an atomic-level investigation of the correlation between elastic and structural heterogeneities and the mechanisms of shear banding in CuZr metallic glass is conducted using molecular dynamics simulations. The shear band formation and propagation processes and the intersection mechanism of multiple shear bands are evaluated by means of local entropy-based structural identification and von Mises stress calculation. The shear band follows the path of lower order and high entropy while shear deflection and branching occur when approaching regions of low entropy. The local von Mises stress calculation allows predictions on the shear band direction and the propensity for activation and propagation prior to yielding and sheds light on shear band branching and multiplication processes

Aspect ratio-dependent nanoindentation behavior of Cu64Zr36 metallic glass nanopillars investigated by molecular dynamics simulations

In this paper, we study nanoindentation in Cu64Zr36 metallic glass (MG) nanopillars with different aspect ratios by molecular dynamics simulations. The activation of shear transformation zones (STZs) and the deformation behavior of MG pillars are discussed during nanoindentation loading and unloading processes. Buckling and serrated flow are the two types of deformation behaviors observed during nanoindentation. For large aspect ratio pillars, a sudden stress drop in the load–displacement curve is found that relates to the buckling process, while smaller aspect ratio pillars exhibit large stress fluctuations. The serrated flow is associated with STZ activation. STZs are locally activated, and their number gradually increases with increasing indentation depth during loading, whereas their number decreases during unloading. For pillars with a large aspect ratio, no new STZs are activated and their number decreases rapidly once the indenter has left the sample because of the buckling deformation. In contrast, new STZs are activated for pillars with smaller aspect ratio during the unloading process. Analysis of STZ activation and shear localization reveals an inhomogeneous deformation process and an increase in the degree of structural heterogeneity as the aspect ratio of the pillars increases for both loading and unloading stages. The present work provides an insight into the atomic-scale plastic deformation behavior of MG nanopillars during nanoindentation loading and unloading processes

Chemical bonding effects on the brittle-to-ductile transition in metallic glasses

The influence of composition and temperature on the tensile deformation behavior of amorphous PdSi metal-metalloid alloys is investigated using large-scale molecular dynamics simulations. A correlation between highly directional Si-Si bonds and the deformation mechanisms is revealed by a Crystal Orbital Hamilton Population analysis based on electronic structure calculations from density functional theory. A transition from cracking perpendicular to the loading direction to shear banding can be achieved by increasing the temperature or decreasing the amount of silicon. Sampling of the saddle points on the potential energy surface reveals that a high fraction of rigid covalent Si-Si bonds increases the energy barriers for atomic rearrangements. These thermally-activated atomic relaxation events change the stress and strain state in the elastic regime and are precursor of local plasticity. High activation energies impede both the stress and the strain redistribution and cause cleavage-like cracking due to a delay of the onset of plasticity