BMG (Bulk Metallic glasses)

Mode I fracture experiments were conducted on brittle bulk metallic glass (BMG) samples and the fracture surface features were analyzed
in detail to understand the underlying physical processes. Wallner lines, which result from the interaction between the propagating
crack front and shear waves emanating from a secondary source, were observed on the fracture surface and geometric analysis of them
indicates that the maximum crack velocity is 800 m s1, which corresponds to 0.32 times the shear wave speed. Fractography reveals
that the sharp crack nucleation at the notch tip occurs at the mid-section of the specimens with the observation of flat and half-pennyshaped
cracks. On this basis, we conclude that the crack initiation in brittle BMGs is stress-controlled and occurs through hydrostatic
stress-assisted cavity nucleation ahead of the notch tip. High magnification scanning electron and atomic force microscopies of the
dynamic crack growth regions reveal highly organized, nanoscale periodic patterns with a spacing of 79 nm. Juxtaposition of the crack
velocity with this spacing suggests that the crack takes 1010 s for peak-to-peak propagation. This, and the estimated adiabatic temperature
rise ahead of the propagating crack tip that suggests local softening, is utilized to critically discuss possible causes for the nanocorrugation
formation. Taylor’s fluid meniscus instability is unequivocally ruled out. Then, two other possible mechanisms, viz. (a) crack
tip blunting and resharpening through nanovoid nucleation and growth ahead of the crack tip and eventual coalescence, and (b) dynamic
oscillation of the crack in a thin slab of softened zone ahead of the crack-tip, are critically discussed

A new a biocompatible Ti 42 Zr 40 Ta 3 Si 15 (atomic %) porous bulk glassy alloy was produced by combination of rapid solidification and powder metallurgy techniques. Amorphous alloy ribbons were fabricated by melt spinning, i.e. extremely fast quenching the molten alloy with 10 6 K/s from T = 1973 K down to room temperature. The ribbons were then cryo-milled at liquid nitrogen temperature in order to produce powder , which was subsequently hot pressed. The resulting thick pellets have a porosity of about 14 vol%, a high compression strength of 337 MPa and a Young's modulus of about E = 52 GPa, values very close to those characteristic of cortical bone. Moreover, the morphology of the samples is very similar to that of cortical bone. The biocompatibility, which is due to the absence of any toxic element in the chemical composition, together with the suitable mechanical behavior, make these samples promising for orthopedic and dentistry applications. Statement of Significance Ti-based alloys are nowadays the standard solution for biomedical implants. However, both the conventional crystalline and amorphous alloys have higher rigidity as the human bone, leading to the damage of the bone at the interface, and contains harmful elements like vanadium, aluminum, nickel or beryllium. The hierarchical porous structures based on glassy alloys with biocompatible elements is a much better alternative. This work presents for the first time the manufacturing of such porous bodies starting from Ti-based amorphous alloy ribbons, which contains only non-harmful elements. The morphology and the compressive mechanical properties of these new products are analyzed in regard with those characteristic to the cortical bone.

A vital requirement for bulk-metallic glasses (BMGs) as structural materials is the attainment of both strength and toughness, yet invariably, as in most materials, these properties are mutually exclusive. However, by utilizing a hydrogen microalloying technology, involving alloying with a gas mixture of hydrogen/argon, we have converted "strong-yet-brittle" bulk-metallic glasses into "stronger-and-tough" ones. We combine experiments with molecular dynamics simulations to systematically analyze the atomic-scale details on how trace hydrogen additions can induce internal changes in the amorphous structure of Zr-Cu-based glassy alloys, with the aim of discerning the structural origin of the combined high strength, ductility and toughness of these materials. Our results, from both relaxation spectrum analysis and calculations of the atomic configurations, indicate that minor additions of hydrogen, instead of causing embrittlement, can have a positive influence on the mechanical properties of BMGs. Specifically , they generate more highly activated "soft spots" to promote multiple shear bands to enhance deformability, yet at the same time engender the formation of a more strengthened structural matrix to delay their initiation, a factor which can serve to elevate strength. Accordingly, our H-alloyed samples display a larger yield strength and fracture strain than the H-free ones. The current findings not only show how the strength-toughness trade-off can be successfully overcome in bulk-metallic glasses, but also we regard this understanding as a step forward to decoding the salient underlying mechanisms for the correlating structure, relaxation behavior and mechanical properties of these materials. Published by Elsevier Ltd on behalf of Acta Materialia Inc.

A pioneer in-situ synchrotron X-ray nanodiffraction approach for characterization and visualization
of strain fields induced by nanoindentation in amorphous materials is introduced. In-situ nanoindentation
experiments were performed in transmission mode using a monochromatic and highly
focused sub-micron X-ray beam on 40 lm thick Zr-based bulk metallic glass under two loading
conditions. Spatially resolved X-ray diffraction scans in the deformed volume of Zr-based bulk metallic
glass covering an area of 4040 lm2 beneath the pyramidal indenter revealed twodimensional
map of elastic strains. The largest value of compressive elastic strain calculated from
diffraction data at 1N load was 0.65%. The region of high elastic compressive strains (<0.3%)
is located beneath the indenter tip and has radius of 7 lm

A B S T R A C T In-situ neutron-diffraction and temperature measurements were simultaneously applied to investigate low-cycle-fatigue behaviour of a nano-precipitate strengthened nickel-based su-peralloy. Two transitions in the temperature-evolution are observed subjected to cyclic loading. Two models are compared with the measured temperature evolution. One is based on bulk stress, and the other is based on lattice-strain evolution. The calculated thermoelastic responses in both models qualitatively agree with the measured bulk-temperature evolution for the first transition. The in-situ neutron-diffraction results reveal that the first transition is associated with the cyclic hardening/softening dislocation-structural transformation. However , the second transition, which is observed at larger number of fatigue cycles during the steady cycles, does not correlate with the dislocation evolution. A phenomenological model is applied to describe the second temperature-transition stages. The energy dissipation evo-lutions in the second fatigue stage indicate the initiation and the growth activities of fatigue microcrack. The data reported here may be useful for cohesive zone model. A = cross-sectional area of the current specimen a = length of a surface crack (or half of the length of an internal crack) α = linear thermal expansion coefficient C v = specific heat at a constant volume C e = electron-specific heat per volume c = electron velocity d 0 = reference peak position before the deformation. d hkl = peak positions of lattice-plane hkl dT = temperature difference measured from two thermocouples dX = length of the heat transfer between two thermocouples σ c = critical stress at crack tip for crack propagation σ m = residual stress introduced by plastic deformation Δσ elastic = stress change because of elastic deformation Δσ = stress changes between maximum tension and compression of the same fatigue cycle E 111 = matrix (111)-plane lattice modulus E = Young's modulus ε hkl = lattice strain of hkl plane ε loading 111 = lattice strain of 111 plane in the loading direction ε transverse 111 = lattice strain of 111 plane in the transverse direction ε 200 = lattice strain of 200 plane f surroundin = total elastic force f system U = internal energy components f system S = entropic energy components

While it is well recognized, albeit qualitatively, that shear band mediated plasticity ahead of crack or notch tips is the raison d'être for the high fracture toughness of 'ductile' bulk metallic glasses (BMGs), quantitative connection between those two material properties is yet to be established. In an attempt to study this, we examine if mode I fracture initiation toughness, KIc, of a number of BMGs can be related to the shear band number, Ni, which is a discretized measure of plasticity in MGs, around spherical indentation impressions that are made to a fracture mechanism based predetermined indentation strain. Results show that KIc scales with (Ni)3/2. Then, the relation between the shear band density in the notch tip plastic zone, Nn, and KIc is examined, which shows that a power law: KIc∝(Nn)1/2, captures the data reported in literature for a number of BMGs. This result confirms that it is indeed the notch tip plasticity that determines KIc of BMGs. The power law exponent of 0.5 is rationalized by recourse to elasto-plastic fracture mechanics. Possible connections between Ni and Nn, ways of enhancing the latter so as to increase KIc, and the central role played by the relative density of MGs in determining both elastic, plastic, and fracture responses are discussed.

The mode I fracture toughness, KIc, of ductile bulk metallic glasses (BMGs) exhibits a high degree of specimen-to-specimen variability. By conducting fracture experiments in modes I and II, we demonstrate that the observed high variability in mode I, vis-à-vis mode II, is a result of highly variable propensity for the conversion of shear bands into cracks in mode I whereas in mode II, crack growth direction is fixed. Thus, the measured variability in KIc is intrinsic to the nature of BMGs.

In the present study, shear band circles obtained from Vickers indentations (microindentations) on bulk metallic glasses (BMGs) are used to establish the analytical model. Observations of the shear band circle morphologies indicate that the radius ratio of any two adjacent shear band circles is approximately a constant value (C). The modified expanding cavity model developed for the mechanics of shear band is
then used to establish the relationship for the pop-ins formed during the loading process of nanoindentations and the shear bands. It was confirmed that the depths of pop-ins are a geometric series with
a common ratio of C. The experimental results also indicate that the depth ratio of any two adjacent pop-ins is approximately equal to the radius ratio of two adjacent shear band circles. The ratio of the
deformation zone to the contact radius induced by an indentation is a constant, which depends on the constraint factor of the material. The ratios predicted using the present model are consistent with
experimental results. After indentation, the shear bands form terraces of various heights. The microstructure and the cause of these terraces are discussed.