Preparation And Properties Of Ultrafine-grained/Nanocrystalline Pure Aluminum Thick Films

Recently, the research of ultrafine-grained (UFG)/ nanocrystalline (NC) bulk metals is the interested research area of materials scientists, especially on the relationship between their properties and microstructure. It has been shown that metals with submicron-grained structure can be produced by many methods. However, our understanding about the characteristics of mechanical behaviors of these materials is still limited, including ultrafine-grained/ nanocrystalline pure aluminum (FCC). According to the literature, many metals exhibit quite different mechanical properties as the grain size decreases to submicro range. As one of the methods, magnetron sputtering has many advantages to produce ultrafine-grained/ nanocrystalline pure aluminum, but it is also a challenge.In this study, pure aluminum thick films with ultrafine-grained/ nanocrystalline had been prepared successfully by DC magnetron sputtering after optimizing the sputtering parameters. The influences of sputtering parameters on grain size and grain size on the mechanical properties were studied in this work. The negative deviation in Hall-Petch slope was observed when the grain size d<65nm. The expressions of hardness and modulus as a function of grain size had also been obtained and fitted by wavelet analysis. These formulas not only establish but also fill the gaps in functional relationship between grain size and mechanical properties of pure aluminum in submicro or nanoscale. Furthermore, the application of pure aluminum thick films was also explored on magnesium alloys.The surface and cross-section morphologies of the films were examined by scanning electron microscope (SEM) and atomic force microscope (AFM). The energy spectrum analysis (EDS) and X-ray photoelectron spectroscopy (XPS) were also applied to investigate the component of the Al films. The grain size and structure of UFG/NC pure Al thick films with different sputtered parameters were analyzed by X-ray diffraction (XRD). Nanoindentation and nanoscratch tests were conducted to investigate their micro-mechanical properties, including hardness, modulus, thickness and critical load. Moreover, potentiodynamical polarization test were carried out in 3.5% sodium chloride (NaCl) solutions to study the corrosion resistance performances of AZ31B magnesium alloys deposited UFG/NC pure Al thick films.The results indicate that the grain size and thickness of pure aluminum films were 49~229nm and 8~25μm respectively. The aluminum thick films were polycrystalline structure and there were no obvious defects on the surface. For the grain size of Al thick film ranging from 49 to 92nm, the hardness was much higher than that of coarse grain aluminum target and rolling Al foil, ranging from 1.38 to 2.88GPa. The UFG/NC pure Al thick films also have quite different modulus than those of coarse grain.As the grain size d>~64nm (d-1/2<~0.125nm-1/2), the hardness increased with grain size decreasing. The Hall-Petch slope showed?positive deviation, but differed from that extended from coarse grains, here k=76.75GPa?nm1/2. However, A negative deviation in Hall-Petch slope occurred when grain size was smaller than ~64nm (d-1/2>~0.125nm-1/2), the hardness decreased with grain size decreasing. The critical grain size existed around 64nm。Compared to bare magnesium sample, the corrosion resistance of AZ31B magnesium alloys with the Al films was improved obviously.