Alain Reza Yavari

Unusual room-temperature compressive plasticity in nanocrystal-toughened bulk copper-zirconium glass

Cast Cu50Zr50 alloy rods with a diameter of 1 mm have been found to consist of a glassy phase containing fine crystalline particles with a size of about 5 nm. They have a glass transition temperature T g of 675 K, and a large supercooled-liquid region extending 57 K above T g. The rods exhibit a high yield strength of 1860 MPa and a Youngs modulus of 104 GPa. Because they contain a dispersion of embedded nanocrystals, the as-cast bulk metallic glass rods can sustain a compressive plastic strain at room temperature of more than 50%, an exceptional value which is explicable by compensation of any shear softening by nanocrystal coalescence and pinning of shear bands.

Direct observation of local atomic order in a metallic glass.

The determination of the atomic configuration of metallic glasses is a long-standing problem in materials science and solid-state physics. So far, only average structural information derived from diffraction and spectroscopic methods has been obtained. Although various atomic models have been proposed in the past fifty years, a direct observation of the local atomic structure in disordered materials has not been achieved. Here we report local atomic configurations of a metallic glass investigated by nanobeam electron diffraction combined with ab initio molecular dynamics simulation. Distinct diffraction patterns from individual atomic clusters and their assemblies, which have been theoretically predicted as short- and medium-range order, can be experimentally observed. This study provides compelling evidence of the local atomic order in the disordered material and has important implications in understanding the atomic mechanisms of metallic-glass formation and properties.

Geometric Frustration of Icosahedron in Metallic Glasses

Order, Order The structure of glassy materials, which are known to have short-range order but no long-range pattern, continues to be a puzzle. One current theory is that some glassy materials possess icosahedral ordering, a motif that cannot show translational periodicity. Hirata et al. (p. 376, published online 11 July) obtained diffraction patterns from subnanometer volumes in a metallic glass, which show some, but not all, of the expected features of an icosahedron. Simulations suggest that the patterns arise from icosahedrons distorted to include features of the face-centered cubic structure. This observation is different from the predictions of molecular dynamics simulations and provides pivotal information in understanding the competition between the formation of the globally inexpensive long-range order and the locally inexpensive short-range order. Small-volume regions in a bulk metallic glass show icosahedral ordering distorted by partial face-centered cubic symmetry. Icosahedral order has been suggested as the prevalent atomic motif of supercooled liquids and metallic glasses for more than half a century, because the icosahedron is highly close-packed but is difficult to grow, owing to structure frustration and the lack of translational periodicity. By means of angstrom-beam electron diffraction of single icosahedra, we report experimental observation of local icosahedral order in metallic glasses. All the detected icosahedra were found to be distorted with partially face-centered cubic symmetry, presenting compelling evidence on geometric frustration of local icosahedral order in metallic glasses.

Mechanical properties of Fe-based bulk glassy alloys in Fe-B-Si-Nb and Fe-Ga-P-C-B-Si systems

Mechanical properties of cast Fe-based bulk glassy alloy rods with compositions of (Fe 0 . 7 5 B 0 . 1 5 Si 0 . 1 ) 9 6 Nb 4 and Fe 7 7 Ga 3 P 9 . 5 C 4 B 4 Si 2 . 5 were examined by compression and Vickers hardness tests. The Youngs modulus (E), yield strength (σ y ), fracture strength (σ f ), elastic strain (∈ e ), fracture strain (∈ f ), and Vickers hardness (H v ) were 175 GPa, 3165 MPa, 3250 MPa, 1.8%, 2.2%, and 1060, respectively, for the former alloy and 182 GPa, 2980 MPa, 3160 MPa, 1.9%, 2.2%, and 870, respectively, for the latter alloy. The σ f /E and H v /3E were 0.019-0.017 and 0.020-0.016, respectively, for the alloys, in agreement with the previous data for a number of bulk glassy alloys. The agreement suggests that these Fe-based bulk glassy alloys have an elastic-plastic deformation mode. The syntheses of high-strength Fe-based bulk glassy alloys with distinct compressive plastic strain and elastic-plastic deformation mode are encouraging for future development of Fe-based bulk glassy alloys as structural and soft magnetic materials.

Malleable hypoeutectic Zr–Ni–Cu–Al bulk glassy alloys with tensile plastic elongation at room temperature

A new bulk glassy alloy (BGA) showing macroscopic tensile plastic elongation at room temperature has been developed in the hypoeutectic Zr–Ni–Cu–Al alloy system. The hypoeutectic Zr–Ni–Cu–Al BGA shows a high Poissons ratio, a low Youngs modulus, and is highly malleable in compression. It exhibits a Poissons ratio of 0.39, a Youngs modulus of 73 GPa, and a distinct tensile plastic elongation of about 1.7% at room temperature with necking due to the operation of many shear bands. The tensile plastic deformability seems to originate from modifications of the glass structure with increasing the number of Zr–Zr atomic pairs in the hypoeutectic composition.

Shear band melting and serrated flow in metallic glasses

Scanning electron microscopy observations of shear steps on Zr-based bulk metallic glasses show direct evidence of shear band melting due to heat generated by elastic energy release. The estimated range of attained temperatures and the observed morphologies are consistent with shear steps forming at a subsonic speed limited by a required redistribution of local microscopic stresses. The calculations indicate that a 0.2μm layer melts in the vicinity of a shear band forming a 1μm shear step. The plastic part of the stress strain curve is serrated but a majority of shear events are not associated to serrations.

Nanostructured Bulk Al90Fe5Nd5 Prepared by Cold Consolidation of Gas Atomised Powder Using Severe Plastic Deformation

Abstract We have used severe plastic deformation (high-pressure torsion straining) to achieve cold consolidation of gas-atomised Al 90 Fe 5 Nd 5 powders into bulk nanostructured disks with a dispersion of fcc-aluminium crystallites within the glassy matrix with full density and hardness in excess of 3 GPa. The micro- and nanostructure of the resulting disks is presented.

Tensile deformation accommodation in microscopic metallic glasses via subnanocluster reconstructions

We present results on the structure and the atomistic mechanisms for tensile deformation accommodation of the Cu46Zr54 microscopic metallic glass. At equilibrium, 23% of the atoms belong to tiny Cu-centered icosahedral clusters (Cu-ICO) and approximately 41% Zr centered slightly larger ICO-like clusters. Under deformation, the number of Cu-ICOs remains dynamically constant until yielding through a continuous cluster destruction-recreation process. Plastic deformation occurs homogeneously and is locally accommodated through the formation of rhombic dodecahedral clusters with significant (∼2%) atomic density drop. These findings explain very recent experimental results demonstrating the fundamental differences of plastic deformation mechanisms between bulk metallic and microscopic glasses.

Structural evolution and metastable phase detection in MgH2–5%NbH nanocomposite during in-situ H-desorption in a synchrotron beam

Abstract Hydrogen sorption in magnesium and magnesium hydrides has been shown to accelerate in sub-micron structures generated by mechanical attrition. It further accelerates when transition metal nanoparticle hydrides such as Nb are added. These hydrides are reported to trigger the desorption during heating. Metastable orthorhombic γ-MgH 2 also forms during attrition and has been shown to desorb first during heating. In this work, using a high brilliance high energy beam for diffraction in transmission, we find that indeed the desorption sequence during heating of mechanically milled MgH 2 –5%NbH nanocomposite proceeds through at least two major steps. First the desorption of the metastable orthorhombic γ-MgH 2 into hexagonal Mg triggers the decomposition of part of the tetragonal β-MgH 2 and the orthorhombic β-NbH. The second stage involving the desorption of hydrogen from the remaining β-MgH 2 appears to be linked to a NbH x metastable phase previously reported to be NbH 0.6 . The present results however, using more extensive diffraction data, indicate that this metastable phase is a supersaturated Nb solid solution as seen likely from the phase diagram.

New ternary Fe-based bulk metallic glass with high boron content

To satisfy thermodynamic and kinetic requirements, Fe-based alloys capable of forming bulk metallic glasses often contain five or more elements. Usually, such compositions are of the type transition-metals/metalloids, with metalloid content around 20 atomic%. Starting from known Fe-based compositions used to make melt-spun glassy ribbons, purifying the master alloy by fluxing with B2O3 and using copper mould casting, a ternary Fe66Nb4B30 bulk metallic glass was obtained. To our knowledge this is the first Fe-based fully amorphous bulk metallic glass with just three atomic constituents. The alloy is ferromagnetic with Curie temperature T c = 646 K, glass transition temperature T g = 845 K, crystallization temperature T x = 876 K, liquidus temperature T liq = 1451 K and having a mechanical strength of 4 GPa.