Vaclav Ocelik

In-situ microscopy investigation of failure mechanisms in Al/SiCp metal matrix composite produced by laser embedding

Laser surface treatments are suitable techniques for improving the mechanical, tribological and chemical properties of metal surfaces. In the laser injection process the laser beam interacts primarily with the substrate and to a lesser extent with the particles, which are simultaneously injected into the melt pool produced by the absorbed laser power density. Metal Matrix composite (MMC) layers with interesting properties and very good connection to the metal substrate may be prepared by a selection of suitable combination of metallic substrate and ceramic particles. In this study Al/SiC MMC layers prepared by laser injection of SiC particles into Al substrate were mechanically tested to reveal the weakest structural components from a mechanical point of view. The objective is to obtain accurate information about the initiation and propagation of cracks in such heterogeneous structures by microscopic inspections of the sample surface during the loading. A combination of techniques was used to characterize the nucleation and progress of fracture: i.e., in-situ microscopy observations during testing and by conventional fractographic methods after failure.

New features of the low temperature ductile shear failure observed in bulk amorphous alloys

Fractographic studies of ductile shear failure under the uniaxial compression for rod–like samples of the Zr41.2Ti13.8Ni10Cu12.5Be22.5 and Cu50Zr35Ti8Hf5Ni2 bulk amorphous alloys at temperatures 300 and 77 K are presented. The mechanisms of shear deformation and failure appeared to have characteristics in common with other amorphous alloys prepared in the form of thin ribbons. However, there were a number of new fractographic features observed due to the bulk character of the samples and to the large supercooled liquid region of these alloys.

On the optimum resolution of transmission-electron backscattered diffraction (t-EBSD)

The work presented aims at determining the optimum physical resolution of the transmission-electron backscattered diffraction (t-EBSD) technique. The resolution depends critically on intrinsic factors such as the density, atomic number and thickness of the specimen but also on the extrinsic experimental set-up of the electron beam voltage, specimen tilt and detector position. In the present study, the so-called physical resolution of a typical t-EBSD set-up was determined with the use of Monte Carlo simulations and confronted to experimental findings. In the case of a thin Au film of 20 nm, the best resolution obtained was 9 nm whereas for a 100 nm Au film the best resolution was 66 nm. The precise dependence of resolution on thickness was found to vary differently depending on the specific elements involved. This means that the resolution of each specimen should be determined individually. Experimentally the median probe size of the t-EBSD for a 140 nm thick AuAg specimen was measured to be 87 nm. The first and third quartiles of the probe size measurements were found to be 60 nm and 118 nm. Simulation of this specimen resulted in a resolution of 94 nm which fits between these quartiles.

Possible local superplasticity of amorphous metallic alloys in the catastrophic shear band under low temperature ductile shear failure

The vein pattern (VP) formation on the failure surfaces is a characteristic feature of the low temperature ductile shear failure of amorphous alloys. Argon and Salama proposed the meniscus instability in the thin liquid-like layer (layer with a low viscosity) as a mechanism of the VP formation. The real instant plasticity of the material in the liquid-like layer can be estimated from the plastic deformation localized in single veins. In this work the authors have observed and estimated the magnitude of the plastic deformation in these protruding veins. This estimation has been realized by the fractographic analysis of fractured surfaces for various amorphous alloys. Failure surfaces after the low temperature (0.5--300 K) shear failure were observed using a TESLA BS-300 scanning electron microscope for various amorphous alloys deformed under uniaxial tension (Fe{sub 84}B{sub 16} and Fe{sub 80}B{sub 20} amorphous ribbons with the cross sections of 6.0 {times} 0.03 mm{sup 2}, Pd{sub 84.5}Si{sub 15.5} amorphous ribbon with cross section of 2 {times} 0.03 mm{sup 2}) and under uniaxial compression (Cu{sub 50}Zr{sub 35}Ti{sub 8}Hf{sub 5}Ni{sub 2} amorphous rod with 3 mm diameter).

On the determination of local residual stress gradients by the slit milling method

This paper concentrates on an extension of a rather new methodology to determine local residual gradients at an enhanced lateral resolution using the so-called slit milling technique. The method is based on stress relaxation by making a slit using a focused ion beam and the displacements are measured through digital image correlation so as to calculate the residual stress. Our novel approach consists of a multiple fitting procedure along the length of the slit instead of the commonly applied averaging method. The applicability of our approach is demonstrated when stress gradients exist. In accordance to the Saint-Venant principle in linear elasticity, longer slits are better than shorter slits because of end effects. The proof-of-principle is supported by measurements on steel under controlled bending and by finite element modeling.

Non-Newtonian plastic flow of a Ni-Si-B metallic glass at low stresses

The problem of the rheological behavior of metallic glasses (MGs) is quite important both from theoretical and practical viewpoints. Early experiments carried out on MGs at temperatures T > 300 K using low shear stress levels revealed plastic flow to be Newtonian while measurements at relative high shear stresses (more than 200 to 400 MPa, depending on temperature, thermal prehistory of samples and chemical composition) indicated a non-linear behavior with 1 < m < 12. Numerous investigations performed later both on as-cast and relaxed MGs of various chemical compositions using a number of testing methods (tensile creep, tensile and bend stress relaxation) showed that a transition from Newtonian behavior at low stresses to a non-linear flow at high stresses was observed. At present, such a situation is considered to be generally accepted. The authors performed precise creep measurements of a Ni-Si-B metallic glass. The results obtained indicate that plastic flow in this case at low tensile stress (12 {le} {sigma} {le} 307 MPa) is clearly non-Newtonian and, consequently, the viscosity is stress dependent.

Interfacial adhesion of laser clad functionally graded materials

Specially designed samples of laser clad AlSi40 functionally graded materials (FGM) are made for evaluating the interfacial adhesion. To obtain the interfacial bond strength notches are made right at the interface of the FGMs. In-situ microstructural observations during straining in a field-emission gun environmental scanning electron microscopy reveal different failure modes of the FGMs and substrate. Mapping of strain fields using digital imaging correlation shows a gradual transition of deformation over the interface region and softening effects in the heat-affected zones of the FGM tracks. The strengthening of the FGM is dominated by the size of the Al halos around the particles, in accordance with a dislocation pile-up model.

FRACTURE-TOUGHNESS OF SOME METALLIC GLASSES

The critical stress intensity factor of Fe40Ni40B20, Fe30Cr10Ni40B20, Ni80Si10B10 and Ni80Si5B15 metallic glass ribbons was measured. Stressing by ultrasound, a new method for preparation a sharp crack in a metallic glass, was used. Only shear rupture failures was investigated for all alloys.

Microstructure and Phase Formation in a Rapidly Solidified Laser-Deposited Ni-Cr-B-Si-C Hardfacing Alloy

In this study, microstructural evolutions and phase selection phenomena during laser deposition of a hardfacing Ni-Cr-B-Si-C alloy at different processing conditions are experimentally investigated. The results show that even minor variations in the thermal conditions during solidification can modify the type and morphology of the phases. Higher undercoolings obtained at faster cooling rates suppressed the primary borides and encouraged floret-shape mixtures of Ni and Cr5B3via a metastable reaction. Variations in the boride phases are discussed in terms of nucleation- and growth-controlled phase selection mechanisms. These selection processes also influenced the nature and proportion of the Ni-B-Si eutectics by changing the amount of the boron available for the final eutectic reactions. The results of this work emphasize the importance of controlling the cooling rate during deposition of these industrially important alloys using laser beam or other rapid solidification techniques.

Direct spectrum analysis of anelastic deformation response during structural relaxation of amorphous metals

Direct relaxation time spectrum analysis method has been successfully used to observe and to study changes in the kinetics of isothermal anelastic deformation response of soft magnetic metallic glass Fe/sub 40/Ni/sub 40/B/sub 20/ during structural relaxation. Computed relaxation time spectra contain three or four quite well separated peaks. >