Mihai Stoica

Tensile fracture morphologies of bulk metallic glass

The fracture morphology plays a much important role not only for the failure analysis of materials but also for the clarification of their fracture mechanisms. However, quantitative analysis of the fracture morphology of bulk metallic glasses (BMGs) is still very lacking. In this study, both model and mechanical experiments were conducted to reveal the development of the fracture morphology and the effect of stress state. Inclined notch tensile specimens of a Zr-based BMG with different notch angles were designed to obtain various stress states. For the first time, some new parameters to describe the tensile fracture surfaces of the BMG specimens were proposed. Statistical analysis of these parameters shows that the stress state has a significant influence on the features of fracture surfaces. Through a discussion on the structure and heat evolution during tensile deformation, two damage mechanisms associated with thermal softening and weakening by defects are found to have a combined effect on the tensil...

Fracture mechanism of some brittle metallic glasses

A systematic study on the fracture surface of brittle Mg-, Fe-, and Co-based metallic glasses under compressive loading is approached and a fracture mechanism is proposed. Experimentally, the metallic glass samples are compressed into many small fragments, displaying an explosion fracture feature. Therefore, an energy equilibrium model is employed to describe the fracture processes of those brittle metallic glasses. Furthermore, some regular nanoscale steps, which were scarcely discovered, are found on the mirror region on their fracture surfaces. It is suggested that such nanoscale steps are associated with the energy distribution in metallic glasses and are created by the shear waves generated by the instability of crack propagation during the explosion rupture processes. Based on the comparison of experimental observations with numerical calculations, we recommend a novel model for interpreting the development of nanoscale steps on the dynamic fracture surfaces of these brittle metallic glasses, which appropriately describes the experimental findings.

Brittle-to-Ductile Transition in Metallic Glass Nanowires

When reducing the size of metallic glass samples down to the nanoscale regime, experimental studies on the plasticity under uniaxial tension show a wide range of failure modes ranging from brittle to ductile ones. Simulations on the deformation behavior of nanoscaled metallic glasses report an unusual extended strain softening and are not able to reproduce the brittle-like fracture deformation as found in experiments. Using large-scale molecular dynamics simulations we provide an atomistic understanding of the deformation mechanisms of metallic glass nanowires and differentiate the extrinsic size effects and aspect ratio contribution to plasticity. A model for predicting the critical nanowire aspect ratio for the ductile-to-brittle transition is developed. Furthermore, the structure of brittle nanowires can be tuned to a softer phase characterized by a defective short-range order and an excess free volume upon systematic structural rejuvenation, leading to enhanced tensile ductility. The presented results shed light on the fundamental deformation mechanisms of nanoscaled metallic glasses and demarcate ductile and catastrophic failure.

Hierarchical surface patterning of Ni- and Be-free Ti- and Zr-based bulk metallic glasses by thermoplastic net-shaping

In order to establish a strong cell-material interaction, the surface topography of the implant material plays an important role. This contribution aims to analyze the formation kinetics of nickel and beryllium-free Ti- and Zr-based Bulk Metallic Glasses (BMGs) with potential biomedical applications. The surface patterning of the BMGs is achieved by thermoplastic net-shaping (TPN) into anisotropically etched cavities of silicon chips. The forming kinetics of the BMG alloys is assessed by thermal and mechanical measurements to determine the most suitable processing temperature and time, and load applied. Array of pyramidal micropatterns with a tip resolution down to 50nm is achievable for the Zr-BMG, where the generated hierarchical features are crucial for surface functionalization, acting as topographic cues for cell attachment. The unique processability and intrinsic properties of this new class of amorphous alloys make them competitive with the conventional biomaterials.

Giant thermal expansion and α-precipitation pathways in Ti-alloys

Ti-alloys represent the principal structural materials in both aerospace development and metallic biomaterials. Key to optimizing their mechanical and functional behaviour is in-depth know-how of their phases and the complex interplay of diffusive vs. displacive phase transformations to permit the tailoring of intricate microstructures across a wide spectrum of configurations. Here, we report on structural changes and phase transformations of Ti–Nb alloys during heating by in situ synchrotron diffraction. These materials exhibit anisotropic thermal expansion yielding some of the largest linear expansion coefficients (+u2009163.9×10−6 to −95.1×10−6u2009°C−1) ever reported. Moreover, we describe two pathways leading to the precipitation of the α-phase mediated by diffusion-based orthorhombic structures, α″lean and α″iso. Via coupling the lattice parameters to composition both phases evolve into α through rejection of Nb. These findings have the potential to promote new microstructural design approaches for Ti–Nb alloys and β-stabilized Ti-alloys in general.Complex phase transformations in β-stabilised titanium alloys can dramatically change their α and β microstructures, providing tailorability for aerospace or biomaterial applications. Here the authors show that Ti-Nb alloys exhibit giant thermal expansions and identify two new pathways that lead to α phase formation.

Structural modification through pressurized sub-Tg annealing of metallic glasses

The atomic structure of metallic glasses (MGs) plays an important role in their physical and mechanical properties. Numerous molecular dynamics (MD) simulations have been performed to reveal the structure of MGs at the atomic scale. However, the cooling rates utilized in most of the MD simulations (usually on the order of 109–1012u2009K/s) are too high to allow the structure to relax into the actual structures. In this study, we performed long-term pressurized sub-Tg annealing for up to 1 μs using MD simulation to systematically study the structure evolution of Cu50Zr50 MG. We find that from relaxation to rejuvenation, structural excitation of MGs and transition during sub-Tg annealing depend on the level of hydrostatic pressure. At low hydrostatic pressures, up to 2u2009GPa in this alloy, the relaxation rate increases with the increasing pressure. The lowest equivalent cooling rate reaches 3.3u2009×u2009106u2009K/s in the sample annealed at 2u2009GPa hydrostatic pressure, which is in the order of the cooling rate in melt spinni...

Origin of large plasticity and multiscale effects in iron-based metallic glasses

The large plasticity observed in newly developed monolithic bulk metallic glasses under quasi-static compression raises a question about the contribution of atomic scale effects. Here, nanocrystals on the order of 1–1.5u2009nm in size are observed within an Fe-based bulk metallic glass using aberration-corrected high-resolution transmission electron microscopy (HRTEM). The accumulation of nanocrystals is linked to the presence of hard and soft zones, which is connected to the micro-scale hardness and elastic modulus confirmed by nanoindentation. Furthermore, we performed systematic simulations of HRTEM images at varying sample thicknesses, and established a theoretical model for the estimation of the shear transformation zone size. The findings suggest that the main mechanism behind the formation of softer regions are the homogenously dispersed nanocrystals, which are responsible for the start and stop mechanism of shear transformation zones and hence, play a key role in the enhancement of mechanical properties.Iron-based bulk metallic glasses are remarkably plastic, but the origin of their plasticity remains challenging to isolate. Here, the authors use high resolution microscopy to show that nanocrystals are dispersed within the glass and form hard and soft zones that are responsible for enhancing ductility.

The 13th International Conference on Rapidly Quenched and Metastable Materials

The 13th International Conference on Rapidly Quenched and Metastable Materials (RQ13) took place in Dresden, Germany, 24–29 August 2008. It belongs to the triennial series of RQ meetings with a long tradition, starting in 1970 – Brela, 1975 – Boston, 1978 – Brighton, 1981 – Sendai, 1984 – Wurzburg, 1987 – Montreal, 1990 – Stockholm, 1993 – Sendai, 1996 – Bratislava, 1999 – Bangalore, 2002 – Oxford, 2005 – Jeju Island. RQ13 was hosted by the Leibniz Institute of Solid State and Materials Research, IFW Dresden. Research on rapidly quenched and metastable materials is stimulated by the high demand for new materials with unique mechanical, chemical and physical properties. Topics of RQ13 conference have fallen into three parts: synthesis and processing, materials and properties, and applications of rapidly quenched and metastable materials. These topics cover exiting developments from the traditional field of rapidly quenched metals to newly emerging areas such as bulk metallic glasses and nanostructured materials. As such, the presentations reported on recent experimental and theoretical achievements in the fields of metastable materials, quasicrystals, nanometer-scale materials, magnetic materials, metallic glasses, solid state reaction, undercooling and modeling. As in the previous proceedings (RQ12), the largest number of papers is dedicated to bulk metallic glasses and magnetic materials. With respect to property characterization and applications, there are great attempts for use and application of these materials, particularly for bulk metallic glasses, as well as for further design and optimization of properties. The RQ13 conference attracted a total of 381 abstracts submitted by scientists from 38 different countries. The conference included 8 plenary talks and 25 invited keynote talks. In addition, 163 regular oral contributions were presented and more than 180 posters were presented. It was a particular highlight of the conference that Dr Ho Sou Chen was awarded the RQ distinguished Fellowship Award for his pioneering research on rapidly quenched and metastable materials and his outstanding contributions to the expansion of the RQ community. It is our pleasure to thank the members of the International Advisory Committee for their valuable help, especially for proposals for plenary and keynote speakers. 129 abstracts were submitted for publication and 119 papers were accepted to be included in the Proceedings. All the papers were reviewed. We would like to express our thanks to all referees for their efficient and prompt efforts. We acknowledge particularly support from the German Society of Research (DFG), the European Office of Aerospace Research and Development, the Dresden University of Technology and the Leibniz Institute for Solid State and Materials Research in Dresden. Finally we are grateful for industrial support from ZOZ, Alfa Aeser, AXO Dresden, Edmund Buhler, Evico magnetics, Goodfellow, RSP and Light Metals Centre. The International Advisory Committee met during the conference and decided to convene the next conference in Brazil, in August 2011. We wish the organizers of RQ14 great success and look forward to meeting you in Brazil. Ludwig Schultz (FW Dresden) Jurgen Eckert (IFW Dresden) Livio Battezzati (Universita di Torino) Mihai Stoica* (IFW Dresden) Guest Editors Dresden, 5 December 2008 (*Corresponding author; e-mail address: m.stoica@ifw-dresden.de)

In Situ Synchrotron X-Ray Diffraction Characterization of Corrosion Products of a Ti-Based Metallic Glass for Implant Applications

Ti-based bulk metallic glasses are under consideration for implants due to their high yield strength and biocompatibility. In this work, in situ synchrotron X-ray diffraction (XRD) is used to investigate the corrosion products formed from corrosion of Ti40 Zr10 Cu34 Pd14 Sn2 bulk metallic glass in artificial corrosion pits in physiological saline (NaCl). It is found that Pd nanoparticles form in the interior of the pits during electrochemical dissolution. At a low pit growth potential, the change in lattice parameter of the Pd nanoparticles is consistent with the formation of palladium hydride. In addition, a salt layer very close to the dissolving interface is found to contain CuCl, PdCl2 , ZrOCl2 ∙8H2 O, Cu, Cu2 O, and several unidentified phases. The formation of Pd nanoparticles (16 ± 10 nm at 0.7 V vs Ag/AgCl) containing small amounts of the other alloying elements is confirmed by transmission electron microscopy. The addition of albumin and/or H2 O2 does not significantly influence the nature of the corrosion products. When considering the biological compatibility of the alloy, the biological reactivity of the corrosion products identified should be explored.

Crystallization Behavior of [(Fe 0.5 Co 0.5 ) 0.75 B 0.2 Si 0.05 ] 96 Nb 4 BMGs

The glassy state is retained at room temperature if the nucleation and growth is avoided. In simple words this requirement is traduced in the necessity to cool the melt as fast as possible. In the real life the maximum achievable cooling rates are limited.