Search

There is limited understanding of time-dependent plastic deformation behavior of amorphous alloys as a function of their structural state. Here, the creep behavior of Zr52.5Ti5Cu17.9Ni14.6Al10 bulk metallic glass was investigated in its as-cast and relaxed states using nanoindentation technique with applied load in the range of 500–1500 mN and temperature in the range of room temperature to 573 K. The creep displacement increased with increasing load and temperature since the creep process is thermally activated and diffusion rate is enhanced at elevated temperature and higher load. The creep strain rate sensitivity, which is a measure of the creep mechanism, increased with increase in temperature and decrease in applied load. This was attributed to the transition from localized to more homogeneous creep. Reduction in free volume for the relaxed alloy resulted in lower creep displacement and larger strain rate sensitivity compared to its as-cast counterpart. The results suggest that diffusion-based deformation dominate at higher temperature in contrast to shear transformation mediated plasticity at room temperature. The volume of shear transformation zone for the metallic glass was calculated using cooperative shearing model and correlated with the structural state for fundamental insights into the deformation process as a function of temperature and load.

Laser-material interaction for a Zirconium based bulk metallic glass was investigated. The change in glass transition, crystallization temperature and enthalpies along with microstructure of the amorphous alloy was explained by the non-isothermal history experienced during processing. Varying volume fractions of amorphous and crystalline phases were found through the cross-section. A finite element based multi-physics modeling approach was employed to predict the temperature history and associated cooling rates during laser processing. Maximum devitrification occurred in sub-surface regions that experienced suitable combination of thermodynamic and kinetic conditions during laser treatment. It was demonstrated that the location and length scale of crystallized zone can be controlled by tailoring the laser processing parameters.

The corrosion behavior of a Zirconium-based bulk metallic glass subjected to friction stir processing was evaluated. Processing was done at the tool rotational speeds of 300 rpm, 500 rpm and 900 rpm. Potentiodynamic polarization and electrochemical impedance spectroscopy studies were carried out in 0.1 M NaCl solution. A strong correlation was found between the corrosion rate and enthalpy of structural relaxation, which is a measure of free volume. Electrochemical impedance spectroscopy measurements showed lower polarization resistance for processed samples as compared to as-cast metallic glass which is consistent with the DC polarization results. Shear bands in processed specimens were found to be preferred nucleation sites for the formation of corrosion pits. In contrast, pitting was more stochastic for the as-cast metallic glass.

The containerless high-temperature high vacuum electrostatic levitation (ESL) technique was used for the determination of thermo-physical properties of the binary eutectic alloy, Ni 59.5 Nb 40.5 , and the ternary alloy Ni 60 Nb 34.8 Sn 5.2 . The thermo-physical properties measured were density, specific heat over hemispherical total emissivity, surface tension and viscosity. The density for both the melts was found to be higher than that predicted by the rule of mixtures. This was attributed to strong attractive interaction between the constituents. The temperature dependent specific heat capacity of the binary alloy was determined by fitting with TEMPUS results. The hemispherical total emissivity was estimated to be 0.27. Using the specific heat data and the hemispherical total emissivity, the enthalpy and entropy of fusion were calculated for the binary alloy. The viscosities for both the alloys were fit with the Vogel–Fulcher–Tammann equation to obtain the fragility parameter. The surface tension for the binary alloy showed an anomalous positive gradient while that of the ternary alloy showed a small negative gradient with temperature.

Gels in the GeO2PbO system were synthesized by the hydrolytic polycondensation reactions of germanium ethoxide with lead acetate. The kinetics of crystallization of gels, gel-derived glasses, and conventional glasses were studied by the nonisothermal method using differential scanning calorimetry. The crystallization peak temperatures, as a function of heating rates, were determined. The data were analyzed using the Avrami equation. The kinetic parameters, such as the activation energies and frequency factors of three types of non-crystalline solids having the same composition, were determined and examined to compare their crystallization behavior. The influence of the proportion of water added during the gelling process on the structure and crystallization behavior of the gels was studied.