Surface nanocrystallization and surface alloying of aluminum alloys

Haiwei Chang (2010). Surface nanocrystallization and surface alloying of aluminum alloys PhD Thesis, School of Mechanical and Mining Engineering, The University of Queensland.

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Author Haiwei Chang
Thesis Title Surface nanocrystallization and surface alloying of aluminum alloys
School, Centre or Institute School of Mechanical and Mining Engineering
Institution The University of Queensland
Publication date 2010-06
Thesis type PhD Thesis
Supervisor MIngxing Zhang
Patrick M. Kelly
Total pages 240
Total colour pages 45
Total black and white pages 195
Subjects 09 Engineering
Abstract/Summary Surface nanocrystallization (SNC) not only can provide an alternative approach to improving the surface durability of Al alloys, it also can activate the surface to promote the subsequent surface alloying treatment. The aim of the present thesis is to investigate the nanocrystallization mechanism of aluminum alloys, the thermal stability of the nanostructured surface layer and its affected zone, the effect of the nanocrystalline surface layer on the surface alloying process of aluminum and on atom diffusion in the alloys. The nanostructured surface layer of aluminum alloys, including pure Al, Al-Si alloys, was produced through surface mechanical attrition treatment (SMAT). Microstructure analysis of the surface layer conducted using optical microscopy, X-ray diffraction (XRD) and transmission electron microscopy (TEM) has indicated that the formation of the nanocrystalline structure is through formation of dense dislocation walls (DDWs) and dislocation tangles (DTs) in original coarse grains and in the refined cells (under further straining) resulting from dislocation slip. In addition to the amount of strain that governs the grain refining process during SMAT, strain rate also plays a key role in nanocrystallization. It is hypothesized that there is a critical strain rate for a given material. Nanocrystallization can only occur when the strain rate is greater than this critical strain rate. In Al-Si alloys, the mechanism of grain refinement of the Al matrix is similar to that in pure Al. The breakage of Si particles is attributed to the stress concentration at the interface between the Si and Al matrix, which dramatically reduces the size of the stabilized Al grains under the same SMAT conditions. The thermal stability of the nanostructured layer of pure aluminum and Al-Si alloy was investigated through annealing the surface nanocrystalline samples at various temperatures for various times. Microstructure evolution showed that the topmost nanocrystalline layer has much higher thermal stability than the sub-micron grained layer. Increasing annealing duration will decrease the thermal stability of fine grains within both the nanocrystalline layer and the sub-micron grained layer. Measurement of the misorientation between the adjacent grains using both convergent beam Kikuchi line diffraction technique (CBKLDP) and the selected area electron diffraction patterns (SAED) has shown that the “real” nanocrystalline grains containing high angle grain boundaries are subject to high thermal stability, while the grains, even having nanocrystalline features, such as continuous electron diffraction rings, correspond to low thermal stability, if they are separated by low angle grain boundaries. Zn and Cu coatings were deposited on the Al and A356 alloy samples with and without SMAT treatment using kinetic metallization (KM) technology, in order to investigate the atom diffusion behaviour in the nanocrystalline surface layer. For Zn diffusion in pure Al and the A356 alloy, nanocrystallization of both pure Al and the A356 alloy can remarkably promote atom diffusion only when the nanostructure is stable. Within the stable temperature range, the diffusion coefficient of Zn in Al increases significantly compared to the diffusion in coarse grains. In the case of Cu diffusion, nanocrystalline structure produced by SMAT has no significant improvement in its diffusion behaviour in Al alloys substrate. However, the SMATed surface can significantly decrease the temperature for the formation of the intermetallic between Cu and the SMATed Al surface. To stabilize the nanostructured surface layer, Sc was added in the pure Al substrate. The thermal stability and the alloying behaviour after the addition of 0.5 wt.% Sc were investigated. It is found that the nanocrystalline grains in the Al-0.5Sc alloy have higher thermal stability than those in pure Al because of the precipitation of an Al3Sc intermetallic compound along the Al grain boundaries, which acts as an obstacle to grain growth. After post SMAT annealing at 370 ºC for 24 hours, nanometer scale grains can still be observed in this type of alloy. Zn diffusivity in such stabilized nanocrystalline grains is effectively increased and a thicker Zn alloyed layer can be produced through diffusion treatment at 300 C. However, when the diffusion treatment temperature is over 350 ºC, although the nanocrystalline grains are retained, the high atom mobility of Zn at such high temperature dominates the effect of nanocrystallization on the diffusivity of Zn in the alloy. The effect of a nanocrystalline surface layer on the packed powder diffusion coating (PPDC) technique has been investigated by applying the technique to the SMATed samples. The nanostructured surface layer promotes the formation of a surface alloyed layer and thicker coatings can be obtained, compared with the samples without SMAT. In addition, the lowest PPDC temperature on pure aluminum was reduced from 600 C in coarse Al samples to 560 C in SMATed samples. A practical, low-cost and environment friendly Al nitriding method was developed by using a powder mixture of 95 wt.% Al2O3 and 5 wt.% Mg to purify the industrial purity nitrogen. With this method, over 100 m thick AlN layers can be produced on pure Al, and over 20 m thick AlN layers formed on wrought Al6061 and Al2024 alloys. In Al-Si based alloys, a thin Fe coating is deposited on the Al alloy substrate through KM spray. Then nitriding treatment leads to the formation of a Fe nitride layer after nitriding treatment at 520 C for 24 hours. The coating not only has high hardness, but also has sufficient bonding strength with the Al substrate.
Keyword surface nanocrystallization, surface alloying, aluminum alloys, thermal stability of nanocrystalline grains, surface mechanical attrition treatment (SMAT), packed powder diffusion coating (PPDC), kinetic metallization (KM), nitriding of aluminum
Additional Notes 53-54,57,59,62,72,79-80,87,90,97-99,112,114,121,124-125,135-143,161-165,184-185,187,194-197,204-205,207-208,210,212

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Created: Fri, 29 Oct 2010, 16:04:51 EST by Mr Haiwei Chang on behalf of Library - Information Access Service