[show abstract][hide abstract] ABSTRACT: Transformation-induced plasticity and significant work-hardening capability were verified to be an effective way to enhance the performance of materials. This work reports on a Zr49Co49Al2 alloy obtained by traditional water-cooled copper mold casting with high strength and large plasticity due to B2-to-B33 deformation-induced martensitic transformation. The B33 martensite with orthorhombic structure does not form spontaneously above room temperature in Zr–Co–(Al) alloys, except when it is induced by local stress concentration.
[show abstract][hide abstract] ABSTRACT: Tremendous research effort has been put into the study of plastic deformation mechanisms of metallic glasses (MGs) in an attempt to elucidate the origin of their high fracture strength. Little attention has, however, been paid to the elastic deformation of MGs. In this paper, a series of MGs with different yield strengths are studied, with a focus on the fine structural evolution (at the atomic level) during elastic deformation. Our results reveal that an atomic reorientation happens in the first nearest-neighbor shell due to elastic deformation. This reorientation subsequently leads to a drop in the local stress, which further results in a cooperative shift of surrounding atoms to counterbalance this change in local stress level. A concordant region is formed as a result. The relation between this concordant region and the yield strength is thoroughly discussed in terms of its size and the stress level in this region. It is proposed that this concordant region could be the missing part that bridges the macroscopic yield strength and the microscopic atomic structure.
[show abstract][hide abstract] ABSTRACT: The structural evolution of the Ti40Zr10Cu34Pd14Sn2 bulk metallic glass (BMG) upon was investigated by means of in situ high-energy x-ray diffraction. The position, width, and intensity of the first peak in diffraction patterns are fitted through Voigt function below 800 K. All the peak position, width, and intensity values show a nearly linear increase with the increasing temperature to the onset temperature of structural relaxation, Tr = 510 K. However, these values start to deviate from the linear behavior between Tr and Tg (the glass transition temperature). The changes in free volume and the coefficient of volume thermal expansion prove that the aforementioned phenomenon is closely related to the structural relaxation releasing excess free volume arrested during rapid quenching of the BMG. Above 800 K, three crystallization events are detected and the first exothermic event is due to the formation of metastable nanocrystals.
Journal of Materials Research. 11/2010; 25(12):2271 - 2277.
[show abstract][hide abstract] ABSTRACT: Gold monolayer protected clusters (MPCs) exhibit strong quantum confinement effects and size dependent electronic, optical and chemical properties. Chemical tuning of these properties can be achieved by established synthesis methods, providing an excellent system for the study of the relationship between chemical and electronic structure. In this paper, the first electronic spectra of individual Au MPCs (Au(25)) acquired by single electron tunneling force spectroscopy on non-conducting silicon dioxide surfaces are reported. A HOMO-LUMO (highest occupied molecular orbital-lowest unoccupied molecular orbital) energy gap is observed in the Au(25) spectra. Hysteretic charging of the particles is also observed while obtaining the energy spectra. The new single electron tunneling measurement methodology is described. A model explaining the measurements supports the existence of mid-HOMO-LUMO gap defect states.
[show abstract][hide abstract] ABSTRACT: Bulk metallic glasses (BMGs) generally fail in a brittle manner under uniaxial, quasistatic loading at room temperature. The lack of plastic strain is a consequence of shear softening, a phenomenon that originates from shear-induced dilation that causes plastic strain to be highly localized in shear bands. So far, significant tensile ductility has been reported only for microscopic samples of around 100 nm (ref. 4) as well as for high strain rates, and so far no mechanisms are known, which could lead to work hardening and ductility in quasistatic tension in macroscopic BMG samples. In the present work we developed CuZr-based BMGs, which polymorphically precipitate nanocrystals during tensile deformation and subsequently these nanocrystals undergo twinning. The formation of such structural heterogeneities hampers shear band generation and results in macroscopically detectable plastic strain and work hardening. The precipitation of nanocrystals and their subsequent twinning can be understood in terms of a deformation-induced softening of the instantaneous shear modulus. This unique deformation mechanism is believed to be not just limited to CuZr-based BMGs but also to promote ductility in other BMGs.
Nature Material 06/2010; 9(6):473-7. · 35.75 Impact Factor
[show abstract][hide abstract] ABSTRACT: In this paper, the superplastic forming behaviour of a Zr based metallic glass ribbon at different die diameters is examined. The wall thickness distribution and the relative bulging height values of the ribbon are experimentally determined, and the results are in reasonably good agreement with the finite element method (FEM) findings. The ribbon demonstrated early failure in both tensile tests and gas pressure forming. It is considered that the presence of air pockets in the as cast ribbon is one of the reasons for this. The FEM results also demonstrate that the air pockets may develop into localised regions which may eventually lead to failure of the ribbon. The differential scanning calorimeter curves of the as cast and deformed specimens show that the onset temperature of crystallisation is changed after deformation. This change is related to its change of microstructure. More work is needed to improve the formability of the ribbon and to study the post-deformed properties of the metallic glass.
Materials Science and Technology 01/2010; 26(2):247-252. · 0.75 Impact Factor
[show abstract][hide abstract] ABSTRACT: (Cu0.5Zr0.5)100−xAlx (x = 5, 6, 8) bulk metallic glasses (BMGs) were deformed in tension. Besides ductility up to 0.5%, the material shows work-hardening behaviour. Both effects are attributed to the deformation-induced precipitation of B2 CuZr nanocrystals and the formation of twins in the nanocrystals larger than 20 nm. The precipitation of the nanocrystals alters the stress field in the matrix and is expected to retard shear band propagation, which in turn allows stresses in the nanocrystals to rise. This stress build-up is more severe in the larger grains and might be responsible for the subsequent twinning. Both deformation-induced nanocrystallization and twinning consume energy and avoid crack formation and with it premature failure.
[show abstract][hide abstract] ABSTRACT: Under stress, bulk metallic glasses irreversibly deform through shear banding processes that manifest as serrated flow behavior. These serration events exhibit a shock-and-aftershock, earthquake-like behavior. Statistical analysis shows that the shear avalanches can self-organize to a critical state (SOC). In analogy to the smooth macroscopic-scale crystalline plasticity that arises from the spatio-temporal averages of disruptive earthquake-like events at the nanometer scale, shear avalanches in glassy metals are another model system that can be used to study SOC behavior. With our understanding of SOC behavior, we further demonstrate how to enhance the plasticity of glassy (brittle) materials. It is expected that the findings can be extended to other glassy or brittle materials.
[show abstract][hide abstract] ABSTRACT: The deformation behaviour and structural evolution of bulk metallic glass sheets were studied under biaxial tension in supercooled liquid state. The sheets were gas pressure formed into hemispherical and semi-ellipsoid domes using dies with aspect ratios of 1:1 and 3:2, respectively, at the optimum temperature of 676 K. The structural evolution of the metallic glass sheets was examined after deformation. The stress distributions of the bulging sheets are calculated employing a finite element model. It was found that the stress state of biaxial tension accelerates nanocrystallization, which leads to an increase in the hardness of metallic glass. The gas pressure applied, bulging time, and temperature are the principal factors involved in the formation of products.
[show abstract][hide abstract] ABSTRACT: The structural behaviour of Cu50Zr50 and Cu65Zr35 metallic glasses under uniaxial tensile stress was investigated in situ by high-energy X-ray synchrotron diffraction. The components of the elastic strain tensor were determined from both the change of positions of first maximum of the structure factor in reciprocal space as well as from the maxima of the atomic pair correlation function in real space. The atomic scale strain agrees with the macroscopic strain values. The topological and chemical short-range order of the Cu–Zr glasses changes upon loading. The number density of Cu–Zr and Zr–Zr nearest neighbour atomic pair becomes oriented along the loading direction whereas the partial nearest neighbour distances are only weakly influenced.
[show abstract][hide abstract] ABSTRACT: In this work, we report that Zr50Cu42Al8 bulk metallic glass (BMG) exhibits excellent glass forming ability and mechanical properties. Zr50Cu42Al8 glassy rods with a diameter of 3 mm were prepared using conventional copper mould suction casting. The glassy rod exhibits a modulus of about 115 GPa and a fracture strength of about 2 GPa, and, as compared with other large-scale BMGs, it has excellent room-temperature plasticity of up to 20% under compression. The fracture mechanism of the rod was investigated by microstructural investigations, and it was found that the large plasticity of the as-cast rod is closely related to the in situ formation of nano-crystalline particles embedded in the amorphous matrix.
Journal of Physics D Applied Physics 10/2008; 41(22):225410. · 2.53 Impact Factor
[show abstract][hide abstract] ABSTRACT: The instability of the crack tip in brittle Mg-based bulk metallic glass (BMG) is studied. The formation of various fractographic surfaces of the BMG is associated with the instability of the fluid meniscus, which is due to viscous fluid matter being present on the fracture process zone. Depending on the values of the wavelength of the initial perturbation of the fluid meniscus and the local stress intensity factor, different fracture surface profiles, i.e. a dimple-like structure, a periodic corrugation pattern and a pure mirror zone are formed. The fractogaphic evolution is significantly affected by the applied stress. A decreased fracture surface roughness is observed under a low applied stress. An increased fracture surface roughness, which has frequently been reported by other researchers, is also observed in the present studies under a high applied stress. Unique fractographic features are attributed to the non-linear hyperelastic stiffening (or less softening) mechanism.
[show abstract][hide abstract] ABSTRACT: We report that bulk metallic glasses (BMGs) with large plasticity can be obtained in conventional brittle BMGs by a shrink-fit metal sleeve. The mechanical performance especially the plasticity in the Zr41.2Ti13.8Cu12.5Ni10Be22.5 BMG with a shrink-fit copper sleeve is much enhanced. The approach results in the formation of the highly dense and frequent interacting and arresting events of shear bands and is the origin of the observed large global plasticity. The results present another simple step toward toughening the inherently brittle BMGs.
Journal of Materials Research. 08/2007; 22(09):2384 - 2388.
[show abstract][hide abstract] ABSTRACT: Out-of-plane, nanoscale periodic corrugations are observed in the dynamic fracture surface of brittle bulk metallic glasses with fracture toughness approaching that of silica glasses. A model based on the meniscus instability and plastic zone theory is used to explain such dynamic crack instability. The results indicate that the local softening mechanism in the fracture is an essential ingredient for controlling the formation of the unique corrugations, and might provide a new insight into the origin of fracture surface roughening in brittle materials.