Structure and mechanical properties of metallic nanoglasses

Metallic nanoglasses are a new class of amorphous materials with interesting magnetic and mechanical properties. They are characterized by interfacial regions with enhanced free volume compared to the core of the nanoparticles. Till now, nanoglasses are primarily synthesized by using thermal evaporation in inert gas condensation (IGC). However, due to the different vapour pressure of constituent elements and reproducibility issues in thermal evaporation, it is difficult/impossible to synthesize different glassy compositions. In this work, by using magnetron sputtering in IGC, Cu50Zr50, Cu60Zr40 and Pd84Si16 nanoglasses are produced with completely amorphous nature and good reproducibility. By varying several parameters, the yield of the sputtering process in IGC is optimized to make sufficient amount of material to obtain a nanoglass pellet. The influence of several processing parameters like inert gas pressure, sputtering power, the type of material etc., on the yield of the process are studied in the current work. The primary aim of the current work is to study the properties of the nanoglasses and compare them with conventional metallic glasses produced by melt-spinning and thus comment on the relation between the structure and properties of nanoglasses. Structural characterization of the metallic nanoglasses showed that the samples are amorphous in nature. Elemental segregation in the samples was studied by atom probe tomography and significant segregation was found in Cu-Zr alloys while very little chemical inhomogeneity was observed in Pd-Si nanoglasses. Crystallization temperature was higher in Cu-Zr nanoglasses than that in melt-spun ribbons while Pd-Si nanoglasses showed lower glass transition and crystallization temperature compared to melt-spun ribbons. Mechanical properties of the nanoglasses and melt-spun ribbons were tested by indentation and micropillar compression tests. Hardness and elastic modulus were found to be higher in Cu-Zr and lower in Pd-Si nanoglasses compared to their corresponding melt-spun ribbons. Deformation mode was also found to be different in Cu-Zr and Pd-Si nanoglasses. While Cu-Zr nanoglasses deformed homogenously without the formation of shear bands during indentation, Pd-Si alloys showed shear bands around the indents. Similar results were also observed in micropillar tests of Pd-Si and Cu-Zr nanoglasses. Cu-Zr nanoglasses showed less catastrophic deformation compared to the melt-spun ribbons while shear banding was observed in both Pd-Si nanoglasses and melt-spun ribbons. With the help of molecular dynamic simulations, the effect of topological structure at the interfacial regions was studied in Pd-Si metallic nanoglasses. Simulation results conveyed that the fraction of major Si polyhedra i.e. Si[0,3,6,0] played an important role in determining the shear band formation and consequently the ductility of glassy Pd-Si alloys. With the increase in the fraction of Si[0,3,6,0] in the interfacial regions of Pd-Si nanoglasses, the mode of deformation changed from homogenous to heterogeneous one. The importance of chemical inhomogeneity on the thermal and mechanical properties of nanoglasses was described in detail based on a segregation model. Finally, Pd80Si20 thin film nanoglasses synthesized by conventional magnetron sputtering were also studied in the current work. No elemental segregation was observed in thin films. Annealing the nanoglassy thin films did not lead to any change in the globular nanostructure even after crystallization. The mode of deformation was practically the same as that in the rapidly quenched ribbon. The reasons for similar behaviour of the thin films and melt-spun ribbons are discussed

Effect of Structural Relaxation on the Indentation Size Effect and Deformation Behavior of Cu–Zr–Based Nanoglasses

In this work, the deformation behavior of as-prepared (AP) and structurally relaxed (SR) Cu–Zr–based nanoglasses (NGs) are investigated using nano- and micro-indentation. The NGs are subjected to structural relaxation by annealing them close to the glass transition temperature without altering their amorphous nature. The indentation load, p, vs. displacement, h, curves of SR samples are characterized by discrete displacement bursts, while the AP samples do not show any of them, suggesting that annealing has caused a local change in the amorphous structure. In both the samples, hardness (at nano- and micro-indentation) decreases with increasing p, demonstrating the indentation size effect. The micro-indentation imprints of SR NGs show evidence of shear bands at the periphery, indicating a heterogeneous plastic flow, while AP NG does not display any shear bands. Interestingly, the shear band density decreases with p, highlighting the fact that plastic strain is accommodated entirely by the shear bands in the subsurface deformation zone. The results are explained by the differences in the amorphous structure of the two NGs

Structural insights into metal-metalloid glasses from mass spectrometry

Despite being studied for nearly 50 years, smallest chemically stable moieties in the metallic glass (MG) could not be found experimentally. Herein, we demonstrate a novel experimental approach based on electrochemical etching of amorphous alloys in inert solvent (acetonitrile) in the presence of a high voltage (1 kV) followed by detection of the ions using electrolytic spray ionization mass spectrometry (ESI MS). The experiment shows stable signals corresponding to Pd, PdSi and PdSi$_{2}$ ions, which emerges due to the electrochemical etching of the Pd$_{80}$Si$_{20}$ metallic glass electrode. These fragments are observed from the controlled dissolution of the Pd$_{80}$Si$_{20}$ melt-spun ribbon (MSR) electrode. Annealed electrode releases different fragments in the same experimental condition. These specific species are expected to be the smallest and most stable chemical units from the metallic glass which survived the chemical dissolution and complexation (with acetonitrile) process. Theoretically, these units can be produced from the cluster based models for the MG. Similar treatment on Pd$_{40}$Ni$_{40}$P$_{20}$ MSR resulted several complex peaks consisting of Pd, Ni and P in various combinations suggesting this can be adopted for any metal-metalloid glass

Effect of Structural Relaxation on the Indentation Size Effect and Deformation Behavior of Cu–Zr–Based Nanoglasses

In this work, the deformation behavior of as-prepared (AP) and structurally relaxed (SR) Cu–Zr–based nanoglasses (NGs) are investigated using nano- and micro-indentation. The NGs are subjected to structural relaxation by annealing them close to the glass transition temperature without altering their amorphous nature. The indentation load, p, vs. displacement, h, curves of SR samples are characterized by discrete displacement bursts, while the AP samples do not show any of them, suggesting that annealing has caused a local change in the amorphous structure. In both the samples, hardness (at nano- and micro-indentation) decreases with increasing p, demonstrating the indentation size effect. The micro-indentation imprints of SR NGs show evidence of shear bands at the periphery, indicating a heterogeneous plastic flow, while AP NG does not display any shear bands. Interestingly, the shear band density decreases with p, highlighting the fact that plastic strain is accommodated entirely by the shear bands in the subsurface deformation zone. The results are explained by the differences in the amorphous structure of the two NGs

Controlling shear band instability by nanoscale heterogeneities in metallic nanoglasses

Strain localization during plastic deformation drastically reduces the shear band stability in metallic glasses, ultimately leading to catastrophic failure. Therefore, improving the plasticity of metallic glasses has been a long-standing goal for several decades. In this regard, nanoglass, a novel type of metallic glass, has been proposed to exhibit differences in short and medium range order at the interfacial regions, which could promote the formation of shear transformation zones. In the present work, by introducing heterogeneities at the nanoscale, both crystalline and amorphous, significant improvements in plasticity are realized in micro-compression tests. Both amorphous and crystalline dispersions resulted in smaller strain bursts during plastic deformation. The yield strength is found to increase significantly in Cu–Zr nanoglasses compared to the corresponding conventional metallic glasses. The reasons for the mechanical behavior and the importance of nanoscale dispersions to tailor the properties is discussed in detail

Dislocations in Grain Boundary Regions: The Origin of Heterogeneous Microstrains in Nanocrystalline Materials

Nanocrystalline materials reveal excellent mechanical properties but the mechanism by which they deform is still debated. X-ray line broadening indicates the presence of large heterogeneous strains even when the average grain size is smaller than 10 nm. Although the primary sources of heterogeneous strains are dislocations, their direct observation in nanocrystalline materials is challenging. In order to identify the source of heterogeneous strains in nanocrystalline materials, we prepared Pd-10 pct Au specimens by inert gas condensation and applied high-pressure torsion (HPT) up to γ ≅ 21. High-resolution transmission electron microscopy (HRTEM) and molecular dynamic (MD) simulations are used to investigate the dislocation structure in the grain interiors and in the grain boundary (GB) regions in the as-prepared and HPT-deformed specimens. Our results show that most of the GBs contain lattice dislocations with high densities. The average dislocation densities determined by HRTEM and MD simulation are in good correlation with the values provided by X-ray line profile analysis. Strain distribution determined by MD simulation is shown to follow the Krivoglaz–Wilkens strain function of dislocations. Experiments, MD simulations, and theoretical analysis all prove that the sources of strain broadening in X-ray diffraction of nanocrystalline materials are lattice dislocations in the GB region. The results are discussed in terms of misfit dislocations emanating in the GB regions reducing elastic strain compatibility. The results provide fundamental new insight for understanding the role of GBs in plastic deformation in both nanograin and coarse grain materials of any grain size

Precipitation kinetics in Al-Si-Mg/TiB<SUB>2</SUB> in-situ composites

Al-7Si-0.3Mg-TiB2 in-situ composites were made by the salt-metal reaction i.e., the reaction of K2TiF6 and KBF4 salts with the molten alloy. The kinetics of the formation of intermediate metastable precipitates in the process of Mg2Si formation in Al-7Si-0.3Mg-TiB2 in-situ composites with three different amounts of TiB2 particles (2.5, 5 and 10 wt.%) were studied using differential scanning calorimeter (DSC) and also compared with the Al-7Si-0.3Mg base alloy. Kissinger analysis of non-isothermal DSC scans at various heating rates was carried out to evaluate the activation energies associated with the precipitation processes. The metastable precipitates were characterized by taking the solutionized samples to their respective DSC peak temperatures at a particular heating rate and the samples were then observed under a transmission electron microscopy. It was found that there is a decrease in the activation energies of the GP zones with increase in TiB2 content

Microstructural and mechanical characterization of two aluminium based in situ composite foams

In situ composites are multiphase materials where the reinforcing phase is synthesized within the matrix during composite fabrication. The present paper deals with the processing, microstructural and mechanical characterization of Al–7Si–0.3Mg–10TiB₂ and Al–4Cu–10TiB₂ foams. Composite foams with very low relative density (&#961;_r = 0.17–0.37) and foams containing uniform cell sizes were successfully processed. Since the TiB₂ particle sizes are less than 2 &#956;m and have a good wetting behaviour, TiB₂ can be very good foam stabilizers. Microstructural characterization of the cell walls showed significant grain refinement since TiB₂ is a grain refiner. Elemental mapping clearly showed TiB₂ particles at inter dendritic boundaries. Compression testing of the processed foams showed some interesting features. Stress–strain curve showed a lot of serrations which indicated brittle fracture of the cell walls and edges. Hence, it is observed that a balance should be attained between the grain refinement of &#945;-Al grains and the amount of TiB₂ particles to obtain desirable mechanical properties. Energy absorbed by the processed foams was calculated and they were observed to be close to that of the commercially available ALPORAS foams

Structure and Properties of Nanoglasses

Nanoglasses represent a novel structural modification of amorphous materials, exhibiting properties and structural details that are markedly different from those observed in metallic glasses prepared by rapid quenching. In this review, the synthesis method and the techniques used for charactering the structure of nanoglasses are described together with our current understanding of their salient microstructural features. It is believed that the structure of nanoglasses consists of two distinct amorphous regions give rise to mechanical, thermal, and magnetic properties that are significantly different from those observed in rapidly quenched (RQ) metallic glasses. Nanoglasses, therefore, constitute a distinct new class of amorphous materials and thus opening up new opportunities for their potential use in a number of structural and functional applications