Mechanical and surface properties of copper-based bulk metallic glasses
Student thesis: Master's Thesis
The effects of adding small amount of aluminum to the binary Cu50Zr50 bulk metallic glass (BMG) on the thermal and mechanical properties were investigated. The Al addition was limited to 3 ≤ x ≤ 10 at.% in order to form fully amorphous bulk samples. Glassy rods of 3 mm diameter of these alloys were prepared by copper mould suction-casting for investigation. The (Cu50Zr50)100-xAlx BMGs (x = 0 and 3 ≤ x ≤ 10 at.%) were characterized with differential scanning calorimetry (DSC), X-ray diffraction (XRD), Vickers microhardness test and nanoindentation respectively. The glass transition temperatures (Tg), crystallization temperatures (Tx) and super-cooled liquid regions (ΔTx) of the specimens increased with increasing Al content. The Tg increases from 673 to 704 K when Al content increases from 0 to 10 at.%, while the Tx increases from 716 to 766 K for Al content of 0 to 8 at.%, but decrease to 763 K for Al content of 10 at.%. The highest value of ΔTx, 73 K, is obtained when x = 8. The microhardness of the specimens also increases by 20% with increasing Al content. Room temperature nanoindentation was carried out on the cross section of the rods. The results showed that the nanohardness increases by 14% for Al content of 0 to 10 at.%. The room temperature creep behaviour of the BMGs was examined by nanoindentation. The creep rate, stress exponent and creep rate sensitivity were derived from the indentation creep curve. The extremely large stress exponent of the samples suggested that (Cu50Zr50)100-xAlx BMGs do not have obvious creep deformation at room temperature, where the stress exponent of binary Cu50Zr50 sample is 7.4, which is greatly different from that of Cu-Zr-Al BMG, this suggested that the cluster structure and local atom arrangements are changed by the addition of Al to the Cu-Zr system. Plasma Immersion ion implantation (PIII) offers a technique for metallic glasses to improve their surface properties. In the (Cu50Zr50)100-xAlx BMG system (x = 0 and 3 ≤ x ≤ 10 at.%), the best mechanical properties and relatively good thermal property was found at x = 8 in our studies. Therefore, the (Cu50Zr50)92Al8 metallic glass was chosen for surface modification experiment. The influences of nitrogen, oxygen and argon ion implantation on (Cu50Zr50)92Al8 BMG were investigated. Characterization of the sample surface was done by grazing incidence X-ray diffraction (GIXRD). The surfaces of implanted samples remain amorphous except that ZrO2 formed on the surface. Surface characterization of the implanted metallic glass by X-ray photoelectron spectroscopy (XPS) further confirmed the formation of ZrO2 on the topmost layer. After nitrogen ion implantation on the sample, the formation of a thin ZrN layer beneath the ZrO2 layer was observed. The formation of oxide and nitride were explained based on the physical properties such as atomic size, electronegativity and heat of formation of the constituents in metallic glasses. We observed copper segregation beneath the top surface after ion implantation induced by oxidation as well as different surface binding energy of the constituents. In oxygen ion implantation, due to the high concentration of Cu formed by segregation, Cu2O is formed beneath the layer of ZrO2. There is no obvious change after the argon ion implantation, except that a thin layer of ZrO2 is formed on the surface. The TEM image showed that the (Cu50Zr50)100-xAlx as-cast sample is fully amorphous. After oxygen ion implantation, tetragonal primitive ZrO2 phase is formed on the surface by the confirmation of TEM. The nanoindentation tests showed that the nanohardness increased by about 10% after nitrogen and oxygen PIII when compare to that of no treatment control sample. Our work also showed that the wettability of the sample surface was improved after ion implantation. The results elucidate that the surface properties of Cu-Zr-Al BMG are improved after PIII. This may be a potential versatile surface modification and providing a way of improving the biocompatibility of the BMG.
- Surfaces, Metallic glasses, Mechanical properties