A eutectic in La-rich La-Cu₀.₅Ni₀.₅-Al alloys was determined by studying the melting behaviors and the microstructure observations. The microstructures of the La-Cu-Ni-Al alloys prepared by Bridgman Solidification and copper mould casting were studied by using scanning electron microscope (SEM). The results show that La₆₆[Cu₀.₅Ni₀.₅]₂₀Al₁₄ alloy is very near to a pseudo-ternary eutectic. When the cooling rate is higher than 450 K/s, fully amorphous can be formed; when the cooling rate is within 15 K/s to 450 K/s, the alloy has a microstructure of dendrite plus amorphous, when the cooling rate is within 12 K/s to 1.5 K/s, the microstructures of the alloy are dendrite plus eutectic, and when the cooling rate is lower than 0.12 K/s, the morphology of the alloy is eutectic microstructure. The off eutectic alloy has better glass forming ability, the best glass forming alloy obtained at La₆₂[Cu₀.₅Ni₀.₅>]₂₄Al₁₄ along the composition line of La₈₆₋x[Cu₀.₅Ni₀.₅]xAl₁₄. It has a potential to form bulk metallic glassy rod samples with diameter larger than 12 mm. / Singapore-MIT Alliance (SMA)

The objective of this thesis is to study the fracture behavior of bulk metallic glasses. For this purpose, detailed finite element investigation of the mode I and mixed mode (I and II) stationary crack tip fields under plane strain, small scale yielding conditions is carried out. An implicit backward Euler finite element implementation of the Anand and Su constitutive model [Anand, L. and Su, C., 2005, J. Mech. Phys. Solids 53, 1362] is used in the simulations. The effects of internal friction (μ), strain softening, Poisson's ratio (ν) and elastic mode mixity (Me) on the near-tip stress and deformation fields are examined. The results show that under mode I loading, a higher μ leads to a larger normalized plastic zone size and higher plastic strain level near the notch tip, but causes a substantial decrease in the opening stress. The brittle crack trajectories and shear band patterns around the notch are also simulated. An increase in ν reduces the extent of plastic zone and plastic strain levels in front of the notch tip. The results from mixed mode simulations show that increase in the mode II component of loading dramatically increases the maximum plastic zone extent, lowers the stresses and significantly enhances the plastic strain levels near the notch tip. Higher μ causes the peak magnitudes of tensile tangential stress to decrease. The implications of the above results on the fracture response of bulk metallic glasses are discussed. The possible variations of fracture toughness with mode mixity predicted by employing two simple fracture criteria are examined. Finally, mixed mode (I and II) fracture experiments on a Zr-based bulk metallic glass are performed. It is found that the fracture toughness increases with Me and Jc under mode I is higher than that under mode II loading by a factor of 4. The operative failure mechanism and fracture process zone size are discerned based on observations of incipient crack growth and fractographs. Lastly, a fracture criterion is proposed which predicts the experimentally observed variation of fracture toughness with mode mixity.

Amorphous Mg-Cu-Y metallic glasses are established by density functional theory and simulated annealing method in this study. The mechanical properties of amorphous Mg-Cu-Y metallic glasses are investigated by molecular statics simulations for the nano-indentation and the nano-scratch process. In this study, some potential energy parameters are obtained by fitting for describing the Mg-Cu-Y system. The bulk modulus, the Young¡¦s modulus and X-ray structure of the Mg-Cu-Y system are calculated. Our results are within 10% error compared with experimental values, which prove the correctness of fitted potential parameters. For the cases of nanoindentations, the indentation force-displacement and the influenced depth are calculated. The mechanical properties are obtained are close to experimental results. The both ¡§slip vector¡¨ and Honeycutt-Andemen index (HA index) parameters are also used to study the deformation behavior and bond-type of a group of atoms. Our results indicate that the influenced depths can be affected by the tip indentation and the gather of copper atoms. The gather of copper atoms can provide the resistance and strengthen the mechanical properties of Mg-Cu-Y material. On the other hand, our results indicate that the amorphous structure of Mg-Cu-Y metallic glasses cannot be transferred to crystal structure during nano-indentation process by analysis of HA index. For the cases of nano-scratch, two different scratch depth (5Å and 15Å) are investigated to understand the understand the depth effect. the scratch force-displacement curve is also obtained. As the same with nano-indentation results, the scratch force will increase because the gather of copper atoms and provide the resistance.

The mechanical properties of the amorphous bulk metallic glassy (BMG) alloy, Mg58Cu31Y11, are examined by a non-traditional analytic method - nanoindentation scratch test. This thesis will discuss the influences of friction force, and fracture surface geometry on the BMG surface for load, depth of scratch, scratch velocity, and test temperature of the nano-scratch process. In this study, experimental factors, including load, depth of scratch, scratch velocity, and test temperature, are taken into consideration to investigate the effects of the friction force. And then, this research utilizes regression analysis to establish BMG machining experience formula. The significant parameters of the friction force on nano-scratch and the reliability of the prediction model are investigated by statistical software. According to the results, the friction force is nearly proportional to power of the load. The friction force exhibits a slightly dependence on the test temperature. Besides, the nano-scratch results show that the friction coefficient also increases as the load and test temperature increases. The results associated with the analysis of the variance can be practiced to assess the prominence among experimental factors. The analysis indicates that the load, test temperature play significant factors on the friction force. The results of the regression analysis using a statistical software can be applied to model the mathematical relationship between machining factors and friction force. It anticipates that the model is able to predict friction force over a wide variety of scratching conditions. The model is also proved in good agreement with experimental results.

The resistivity and thermopower of two series of amorphous alloys have been measured between 4 and 300K. The alloys studied are MgZn and CuZr, both in the largest concentration range available as amorphous materials. The alloys were measured in both the 'as made' and 'relaxed' states, as well as some partial or totally crystallized samples. The simple Ziman theory was found at variance with the experimental results in both cases (even for MgZn, proven to be free-electron like as required by the theory). A simple two component model is proposed as an extreme simplification of the Faber-Ziman theory of liquid metallic alloys. The excellent agreement obtained indicates that metallic glasses should be considered as the alloys they really are. A full Faber-Ziman calculation is performed for CuZr and from this follows the conclusion that the term containing the energy dependence of the pseudo-potential (r), usually assumed to be small, is probably of comparable magnitude to that of the disorder scattering (q). The suggested correlations between the electron-phonon mass-enhancement parameter (lamda) (determined from superconductivity experiments) and the thermopowers are studied and both found not to be valid for CuZr amorphous alloys.

The validity of the theories of quantum corrections to the electrical conductivity, namely weak localization and enhanced electron-electron interaction, has been tested quantitatively in well characterized, free-electron-like Mg-Cu and Mg-Zn metallic glasses containing various amounts of Ag and Au through measurement of the electrical resistivity between 1.5K and 20K in magnetic fields up to 5.6T. It is found that the theories give an excellent description of the magnetoresistance at low fields, in both the weak and strong spin-orbit scattering limit but that at higher fields they break down. The electron spin-orbit scattering and dephasing rates have been deduced. Above 4K the dephasing rate is controlled by inelastic electron-phonon scattering, below 4K it saturates to a value consistent with a new model of dephasing of the quantum back scattering interference by ionic zero-point motion. The first direct measurement of the effect of superconductivity on the magnetoresistance in bulk amorphous metals is also presented. The temperature dependence of the resistivity between 1.5 and 6K is in qualitative but not quantitative agreement with the quantum correction theories.

In recent years, nanostructured and amorphous metals have become more prominent in scientific research. Their unique characteristics and their vastly superior mechanical properties have lead to emerging technical applications however the use of these materials is only in its infancy. To further growth the industrial applications for these advanced materials, joining processes capable of maintaining the unique aspects of the microstructure are required. / Joining of two beveled plates has been accomplished by use of the Cold Spray process. This process uses a converging diverging nozzle to accelerate micron sized powder towards a substrate. The strength of the resulting joint has been compared to freeforms made of powder of the same composition. No significant difference was observed in the mechanical properties between the freeforms and the weldments and examination of the fracture surface showed that the strength of the interface is higher than the strength of the freeform itself. / A weld repair experiment has also been performed where the ElectroSpark Deposition process (ESD) has successfully been used to transfer amorphous material from an electrode to an amorphous substrate without crystallizing either material. This result is of crucial importance as it signifies that these extremely expensive and heat sensitive materials can be repaired when in-service wear causes damage. This process, due to its inherently fast cooling rate, has also successfully been used to transform a crystalline AlCoCe alloy into an amorphous deposit atop both amorphous and crystalline substrates.

No description available.

A repetitive low-pressure die casting technique has been developed for casting high quality Mg6SCU2SY10 bulk metallic glass (BMG) samples. Using these as-cast samples, the thermal and mechanical behaviour of the BMG in the supercooled liquid (SCL) region was investigated principally by uniaxial tensile testing, differential scanning calorimetry (DSC), X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Using the die casting facility, charge temperature, injection pressure and injection velocity were found to be important parameters for controlling the length, porosity and degree of crystallinity in the as-cast samples. A processing map was generated which showed that a melt temperature and casting pressure in the range 560-580°C and 0.4-0.5 bar, respectively, generated the highest quality samples. The static crystallization behaviour of the as-cast BMG was investigated in detail. It was found that the kinetics of both anisothermal and isothermal crystallization were adequately represented by a number of transformation models. Using the Johnson-Mehl-Avrami-Kolmogorov (JMAK) relation in conjunction with isothermal DSC, the Avrami exponent was found to vary from 2.2 to 2.5 with increasing annealing temperature which implies that, at high annealing temperatures, nucleation occurs at a constant rate accompanied by diffusion-controlled growth of spherical grains. A comparable Avrami exponent of 2.34 was also calculated by anisothermal DSC using the Ozawa method. The tensile flow behaviour of the BMG was investigated over a range of strain rates (10.3to 10.1S•1) and deformation temperatures (150 to 170 QC) in the SCL region using standardized tensile test samples. It was found that, the flow stress increased rapidly to a maximum value followed by a decrease to a very low steady-state value. In the SCL region, the relationship between peak flow stress, strain rate and absolute deformation temperature was described adequately by the classic Sellars-Tegart constitutive relationship. There was also a good correlation between the Zener-Hollomon parameter, Z, and the flow characteristics of the BMG such as the transition from Newtonian to non-Newtonian flow at Z>1 031 S•1 and optimum superplasticity for Z-values in the range 5 x 1030 to 5 x 1031 S•1 where tensile elongations in excess of 1400% were achieved.

Thesis (Ph. D.)--Ohio State University, 2008. / Title from first page of PDF file. Includes bibliographical references (p. 142-155).