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The elevated temperature mechanical response of a Zr-based bulk metallic glass (BMG) was examined using in situ microcompression in a scanning electron microscope at temperatures up to 387 °C. The flow stress was found to remain constant with temperature at ∼2 GPa below the glass transition temperature, T g . The magnitude of the stress drops/serrations in the stress–strain curve was found to increase with temperature. Above T g , plastic flow was observed to be homogeneous without any shear band formation.

The elevated temperature indentation response of a Zr-based bulk metallic glass (BMG) was examined using a novel in situ SEM indentation system modified with tip and sample heating. Hardness was found to linearly increase with temperature from 7 GPa at 25 °C to 7.6 GPa at 200 °C, while the modulus remained constant within measurement accuracy. Adhesion between indenter and the BMG was observed to increase with increasing temperature. Both the number and pointed morphology of the surface shear offset displacements were found to decrease with increasing temperature. Conversely, the magnitude of the load drops/serrations in the load displacement curve was found to increase dramatically with temperature. The transitions from smooth to serrated flow were observed to fall within the temperature–strain rate ranges observed by other researchers in uniaxial compression.

We demonstrate that irradiation may enhance the plasticity in metallic glasses by increasing the free-volume content via micropillar compression experiments on an ion-irradiated bulk metallic glass (BMG). Results show that irradiation decreases the flow stress and enhances the shear band formation by lowering the magnitude of stress serrations in plastic flow regime. These results highlight that amorphous alloys can mitigate the deleterious affects of severe ion irradiation as compared to their crystalline counterparts.

The deformation and fracture response of a bulk metallic glass (BMG) post-annealing above the glass transition temperature is examined. The toughness of the glass-matrix composite exhibits a sharp transition beyond a critical volume fraction of crystallization to values as low as that of brittle silicate glass. Instrumented indentation tests supplemented by impact tests were used to study this ductile to brittle transition exhibited by the partially crystallized samples. Indentation on the anneal-embrittled specimens shows lateral cracks in addition to cracks along the corners. The applicability of the Poisson's ratio-toughness correlation with respect to partially crystallized samples is also investigated.

Journal of Materials Research, 24 (8), ISSN:0884-2914, ISSN:2044-5326

An experimental study to ascertain the ductile-to-brittle transition (DBT) in a bulk metallic glass (BMG) was conducted. Results of the impact toughness tests conducted at various temperatures on as-cast and structurally relaxed Zr-based BMG show a sharp DBT. The DBT temperature was found to be sensitive to the free-volume content in the alloy. Possible factors that result in the DBT were critically examined. It was found that the postulate of a critical free volume required for the amorphous alloy to exhibit good toughness cannot rationalize the experimental trends. Likewise, the Poisson’s ratio–toughness correlations, which suggest a critical Poisson’s ratio above which all glasses are tough, were found not to hold good. Viscoplasticity theories, developed using the concept of shear transformation zones and which describe the temperature and strain rate dependence of the crack-tip plasticity in BMGs, appear to be capable of capturing the essence of the experiments. Our results highlight the need for a more generalized theory to understand the origins of toughness in BMGs.

Uniaxial compression experiments on 0.3, 1 and 3 mu m diameter micropillars of a Zr-based bulk metallic glass in as-cast, shot-peened and structurally relaxed conditions were conducted. Shear band formation and stable propagation is observed to be the plastic deformation mode in all cases, with no detectable difference in yield strength according to either size or condition. The limitations of uniaxial compression tests in assessing the influence of various material conditions on plasticity, when it is inhomogeneous in nature, are illustrated.

The dry sliding friction coefficient μ and the wear volume loss W, in a zirconium-based bulk metallic glass (BMG) under high-frequency (50 Hz) reciprocating conditions, were investigated with the objective of assessing the influence of free volume and crystallization on the wear behavior of amorphous metals. The BMG samples were annealed either below the glass transition temperature Tg to induce structural relaxation and hence reduce the free volume that controls plasticity through shear-band formation or above Tg to crystallize the amorphous BMG prior to wear testing. Results show that the wear behavior of both the as-cast and relaxed glasses was dominated by the oxidation of the surface layers. A sharp transition in the contact electrical resistance complemented by a marked increase in μ was noted. This was attributed to the formation of a thick tribo film with high oxygen concentration and its subsequent delamination. The μ values, before as well as after the transition, in the relaxed glasses were similar to those for the as-cast alloy. However, a gradual decrease in W with annealing temperature was observed. A good correlation between W and nanohardness was noted, implying that the intrinsic hardness in the BMGs controlled the wear rate.

An experimental investigation into the role of the excess free volume that is created during plastic deformation in strain softening of amorphous metals was conducted. A well-defined and large plastic zone was created through the spherical indentation of a bonded interface of a Zr-based bulk metallic glass (BMG). Elastic modulus and hardness mapping of the deformation zone was conducted through nanoindentation. Experimental observations show that the load, P, vs. depth of penetration, h, curves obtained from the deformation zone are decorated with discrete displacement jumps, which are otherwise absent in the undeformed material. The prior-deformed zone underneath the large indenter was also found to be softer than that far away from the indenter. A simple and approximate analysis shows that the strain softening of the BMG is related to the excess free volume that is created during prior deformation. Contrary to general expectation, differential scanning calorimetry of the deformed material indicates a reduced free volume. These results can be explained by postulating the formation of nanovoids due to the coalescence of the excess free volume. These nanovoids, in turn, lower the stress required for plastic deformation through shear bands, which leads to the observation of reduced hardness.

The variation of impact toughness, Γ, of a Zr41.2Ti13.75Cu12.5Ni10Be22.5 (Vitreloy-1) bulk metallic glass (BMG) within the temperature range of 123–423 K was evaluated by using an instrumented Charpy impact testing machine, in order to examine if the BMGs exhibit ductile-to-brittle transition (DBT) that is seen in rapidly quenched glasses. Results show an abrupt reduction in Γ when the testing temperature is lowered to below 150 K, implying that the BMGs are also prone to the DBT. Fractographic observations indicate a transition in the fracture mode; from ductile vein-like morphology above DBT to a cleavage-dominant fracture mode below it. Complimentary Vickers indentation measurements show no variation in hardness with temperature. However, the shear banded plastic regions that are typically seen around the indents were observed to be completely absent around the indents that were made at low temperatures, indicating that the inhomogeneous plasticity mediated by shear bands becomes inoperative below a critical temperature resulting in the DBT. This observation suggests that the minimum amount of free volume required for extensive plasticity (and hence high toughness) in metallic glasses is strongly dependent on the temperature. Testing of the structurally relaxed samples (through annealing at 530 K for 2.5 h that induces severe embrittlement at room temperature) at 423 K reveal almost complete recovery of Γ, supporting this hypothesis.

By designing advantageous cellular geometries and combining the material size effects at the nanometer scale, lightweight hybrid microarchitectured materials with tailored structural properties are achieved. Prior studies reported the mechanical properties of high strength cellular ceramic composites, obtained by atomic layer deposition. However, few studies have examined the properties of similar structures with metal coatings. To determine the mechanical performance of polymer cellular structures reinforced with a metal coating, 3D laser lithography and electroless deposition of an amorphous layer of nickel-boron (NiB) is used for the first time to produce metal/polymer hybrid structures. In this work, the mechanical response of microarchitectured structures is investigated with an emphasis on the effects of the architecture and the amorphous NiB thickness on their deformation mechanisms and energy absorption capability. Microcompression experiments show an enhancement of the mechanical properties with the NiB thickness, suggesting that the deformation mechanism and the buckling behavior are controlled by the brittle-to-ductile transition in the NiB layer. In addition, the energy absorption properties demonstrate the possibility of tuning the energy absorption efficiency with adequate designs. These findings suggest that microarchitectured metal/polymer hybrid structures are effective in producing materials with unique property combinations.

Nanoindentation experiments were conducted on a Ni+ ion-irradiated Zr-based bulk metallic glass (BMG). The irradiation was carried out using 2.5, 5, 10 and 15 MeV ions and a flux of similar to 10(16) ions/cm(2). Post mortem imaging of the indents reveals a transition in the deformation mechanism of the irradiated regions from heterogeneous shear banding to homogeneous flow. Additionally, the load-displacement curves exhibit a transition from serrated to continuous flow with increasing severity of irradiation damage. The stress-strain response obtained from micro-pillar compression experiments complements the indentation response exhibiting a decrease in the flow stress and an `apparent' strain hardening at the lowest irradiation damage investigated, which is not observed in the as-cast alloy. (C) 2011 Elsevier B.V. All rights reserved.

Structural relaxation behavior of a rapidly quenched (RQ) and a slowly cooled Pd40Cu30Ni10P20 metallic glass was investigated and compared. Differential scanning calorimetry was employed to monitor the relaxation enthalpies at the glass transition temperature, T g , and the Kolrausch–Williams–Watts (KWW) stretched exponential function was used to describe its variation with annealing time. It was found that the rate of enthalpy recovery is higher in the ribbon, implying that the bulk is more resistant to relaxation at low temperatures of annealing. This was attributed to the possibility of cooling rate affecting the locations where the glasses get trapped within the potential energy landscape. The RQ process traps a larger amount of free volume, resulting in higher fragility, and in turn relaxes at the slightest thermal excitation (annealing). The slowly cooled bulk metallic glass (BMG), on the other hand, entraps lower free volume and has more short-range ordering, hence requiring a large amount of perturbation to access lower energy basins.

The fatigue lives of a Zr-based bulk metallic glass (BMG) in the as-cast and shot-peened conditions were evaluated. Results suggest that shot peening does not cause significant enhancement of the fatigue performance of the BMG. Cracks were observed to nucleate in subsurface regions. This was attributed to the domination of the compressive residual stress field on the surface over deformation-induced plastic flow softening that otherwise leads to easy nucleation of shear bands which act like microcracks.

The constraint factor, C , given by the hardness to the yield strength ratio and an indirect measure of the pressure sensitivity of plastic flow in amorphous alloys, of a Zr-based bulk metallic glass was measured as a function of temperature. Results show that, contrary to expectations, C increases linearly from ∼3.2 to 3.5 within the 300–525 K range. Gradual transition in the shear band-dominated deformation mechanism was suggested as a possible reason for the observed increase in C .