From qualitative to quantitative description of the anomalous thermoelastic behavior of V, Nb, Ta, Pd and Pt.

To study the anomalous thermoelastic behavior of bcc V, Nb, Ta as well as fcc Pd and Pt a density functional theory (DFT) based model is used, which allows for the calculation of the elastic constant [Formula: see text] and [Formula: see text] as a function of temperature. Available experimental [Formula: see text] trends are correctly reproduced indicating that the electronic structure mechanisms enabling anomalous behavior are captured by the model. A DFT based correlative investigation between V, Nb, Ta, Pd and Pt with anomalous thermoelastic properties and Mo and Cu with ordinary behavior reveals a high density of states (DOS) at the Fermi level to be a necessary but not sufficient condition for an anomalous thermoelastic behavior. In addition, anomalous metals in contrast to ordinary metals reallocate electronic states in the vicinity of the Fermi level upon lattice distortion, causing an increase in bond strength as identified by crystal orbital Hamilton population (COHP) analysis. Hence, we have identified the combination of high DOS and electronic reallocation upon lattice distortion to be the physical origin for anomalous thermoelastic behavior in metals. The absence of an anomaly for [Formula: see text]-type distortion in V, Nb, Ta, Pd and Pt is suggested to be due to the less pronounced reallocation of states compared to [Formula: see text]-type distortion.

Thermal expansion of Pd-based metallic glasses by ab initio methods and high energy X-ray diffraction.

Metallic glasses are promising structural materials due to their unique properties. For structural applications and processing the coefficient of thermal expansion is an important design parameter. Here we demonstrate that predictions of the coefficient of thermal expansion for metallic glasses by density functional theory based ab initio calculations are efficient both with respect to time and resources. The coefficient of thermal expansion is predicted by an ab initio based method utilising the Debye-Grüneisen model for a Pd-based metallic glass, which exhibits a pronounced medium range order. The predictions are critically appraised by in situ synchrotron X-ray diffraction and excellent agreement is observed. Through this combined theoretical and experimental research strategy, we show the feasibility to predict the coefficient of thermal expansion from the ground state structure of a metallic glass until the onset of structural changes. Thereby, we provide a method to efficiently probe a potentially vast number of metallic glass alloying combinations regarding thermal expansion.

Ultra-stiff metallic glasses through bond energy density design.

The elastic properties of crystalline metals scale with their valence electron density. Similar observations have been made for metallic glasses. However, for metallic glasses where covalent bonding predominates, such as metalloid metallic glasses, this relationship appears to break down. At present, the reasons for this are not understood. Using high energy x-ray diffraction analysis of melt spun and thin film metallic glasses combined with density functional theory based molecular dynamics simulations, we show that the physical origin of the ultrahigh stiffness in both metalloid and non-metalloid metallic glasses is best understood in terms of the bond energy density. Using the bond energy density as novel materials design criterion for ultra-stiff metallic glasses, we are able to predict a Co33.0Ta3.5B63.5 short range ordered material by density functional theory based molecular dynamics simulations with a high bond energy density of 0.94 eV Å-3 and a bulk modulus of 263 GPa, which is 17% greater than the stiffest Co-B based metallic glasses reported in literature.

Topology and electronic structure of flexible (Nb,Ru)O2 thermoelectrics.

Using combinatorial reactive sputtering, we have synthesised Nb-Ru-O thin films on Kapton (polyimide) with the Ru/Nb ratio from 0.5 to 1.1 in a dioxide type of environment. Based on correlative analysis, including synchrotron diffraction experiments and density functional theory, the topology of these amorphous samples is characterised by short metal-oxygen bonds and very pronounced metal-metal interactions within the second coordination shell. We suggest that the role of Nb is within bond length reduction and promotion of quantum confinement, giving rise to an increase in the Seebeck coefficient. Furthermore, these Nb-Ru-O thin films are mechanically flexible as there are no crack formation and delamination upon bending or rolling. This may be rationalised as follows. Nb-Ru-O appears ductile due to low topological connectivity and forms strong bonds with Kapton.

Electronic hybridisation implications for the damage-tolerance of thin film metallic glasses.

A paramount challenge in materials science is to design damage-tolerant glasses. Poisson's ratio is commonly used as a criterion to gauge the brittle-ductile transition in glasses. However, our data, as well as results in the literature, are in conflict with the concept of Poisson's ratio serving as a universal parameter for fracture energy. Here, we identify the electronic structure fingerprint associated with damage tolerance in thin film metallic glasses. Our correlative theoretical and experimental data reveal that the fraction of bonds stemming from hybridised states compared to the overall bonding can be associated with damage tolerance in thin film metallic glasses.

Stiffness and toughness prediction of Co­Fe­Ta­B metallic glasses, alloyed with Y, Zr, Nb, Mo, Hf, W, C, N and O by ab initio molecular dynamics.

Ab initio molecular dynamics simulations are used to systematically explore the influence of alloying on the stiffness and plasticity of Co­Fe­Ta­B metallic glasses. The Co(43.5)Ta(6.1)B(50.4) metallic glass studied in this work, with a Young's modulus of 295 GPa, is the stiffest metallic glass known in literature. From the analysis of the density of the states it is suggested that the very large stiffness is due to strong covalent metal to boron bonding. Furthermore it has been observed that by alloying with Y, Zr, Nb, Mo, Hf, W, C, N and O the bulk to shear modulus ratio can be varied from 2.08 to 2.82. As noted by Lewandowski et al (2005 Phil. Mag. Lett.85 77) a brittle to plastic transition for metallic glasses can be identified in the range of 2.33 to 2.44. Hence, it is evident that the whole range from brittle to plastic behaviour can be covered,with the systems studied in this work. This evolution from brittle to plastic behaviour can be attributed to a change from predominately covalent to predominately metallic bond character.