Papers by Author: M. Rafiq

Abstract: Bulk metallic glasses (BMGs) are well known for their promising properties. Surface properties can be further improved by using certain techniques such as electron beam melting (EBM), laser beam melting (LBM), ion irradiation, ion implantation and neutron irradiation. BMGs especially Zr-based BMGs have numerous applications as structural materials. In this manuscript, the results are presented on microstructural investigations and phase formations in Zr-based BMGs modified by using above mentioned techniques. Microstructure was studied by scanning electron microscopy (SEM). Phase analysis was done by X-ray diffraction (XRD) and confirmed by energy dispersive spectroscopy (EDS). Vickers hardness was measured and correlated with the microstructure. The phases identified in Zr-Cu-Al-Ni alloy samples modified by EBM, LBM and ion irradiation are Ni-Zr, NiZr₂, CuZr₂, Cu₁₀Zr₇ and Al₂NiZr₆. ZrSi₂ phase was detected in Zr₅₅Cu₃₀Al₁₀Ni₅ and Zr₆₅Cu₁₇Ni₁₀Al₈ BMGs irradiated with Si⁺ (ions). About 20-35 % increase in hardness and elastic moduli was achieved by surface modification. Modifications of BMGs by electron and laser beams melted the materials surfaces while ion irradiation improved the mechanical properties of localized zones without melting.

Abstract: A quaternary Ni₈₆Cr₇Si₄Fe₃ amorphous alloy was synthesized by melt spinning technique. Surface modification was done by electron beam melting (EBM), neutron irradiation and γ-rays. Microstructure of as cast, annealed and modified samples was examined by scanning electron microscope. Crystallization behavior was studied by annealing the samples in vacuum at different temperatures in the range 773-1073 K. Techniques of X-ray diffraction (XRD), differential scanning calorimetry (DSC), scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) were used for characterization. Differential scanning calorimetry (DSC) was conducted at various heating rates in the range 10-40 K/min. Thermal parameters like glass transition temperature T_g, crystallization temperature T_x, supercooled liquid region ΔT_x and reduced glass transition temperature T_rg were measured. The Ni₈₆Cr₇Si₄Fe₃ alloy exhibits wide supercooled liquid region of 60 K indicating good thermal stability. The activation energy was calculated to be 160±4 kJ/mol using Kissinger and Ozawa equations respectively which indicates high resistance against crystallization. The XRD results of the samples annealed at 773 K, 923 K, 973 K and 1073 K/20 min show nucleation of Ni₂Cr₃ and NiCrFe crystalline phases. Vickers microhardness of the as cast ribbon was measured to be 680.. About 30-50 % increase in hardness was achieved by applying EBM technique.