Aluminum 5083 powder was mechanically milled under liquid nitrogen to achieve a nanocrystalline grain size in the range of 20 to 30 nm. The powder was subsequently sprayed using a nozzle designed with a validated numerical model for cold spray technology. The resulting coatings were evaluated using scanning electron microscopy (SEM), transmission electron microscopy (TEM), micro- and nanoindentation. The TEM analysis shows that the nanocrystalline grain structure of the cryomilled feedstock powder was retained after the cold spray process. A significant increase in hardness from 104 to 261±8 (HV300g) was observed when comparing the nanocrystalline coating with cast, cold-worked, Al 5083. The ability to use cold spray to produce nanocrystalline large deposits was also demonstrated in this work.

https://link.springer.com/content/pdf/10.1007%2Fs11661-005-0099-y.pdf

Cold spray deposition of nanocrystalline aluminum alloys

High temperature thermal and mechanical stability of high-strength nanotwinned Al alloys

The microstructure and aging behavior of a nanocrystalline Al-Cu-Mg-Fe-Ni-Sc alloy was studied. The nanocrystalline powders were produced by milling at liquid nitrogen temperature and then consolidated using spark plasma sintering (SPS). The microstructure after SPS consisted of a bimodal aluminum grain structure (coarse-grained and fine-grained regions), along with Al9FeNi and Al2CuMg particles dispersed throughout. The microstructure observed in the as-consolidated sample is rationalized on the basis of high current densities that are generated during sintering. Solution treatment and aging of the SPS Al-Cu-Mg-Fe-Ni-Sc sample resulted in softening instead of hardening. This observation can be explained by the reduced amount of Cu, Mg, and Si in solid solution available to form S′ Al2CuMg due to the precipitation of Al7FeCu2 and Si-rich particles, and by the fact that rodlike S′ Al2CuMg particles could only precipitate out in the coarse-grained regions, greatly decreasing their influence on the hardness. This lack of precipitation in the fine-grained region is argued to represent a new physical observation and is rationalized on the basis of physical and thermodynamic effects. The nanocrystalline SPS Al-Cu-Mg-Fe-Ni-Sc sample was also extremely thermally stable, retaining a fine-grained structure even after solution treatment at 530°C for 5 h. The observed thermal stability is rationalized on the basis of solute drag and Zener pinning caused by the impurities introduced during the cryomilling process.

Spark plasma sintering of a nanocrystalline Al-Cu-Mg-Fe-Ni-Sc alloy

High-energy ball milling was used to synthesize aluminum-based alloys containing amorphous and nanocrystalline phases to investigate the compositional effects of transition metals (TM) on the amorphization and crystallization processes of the ball-milled Al{sub 85}Y{sub 7}Fe{sub 5}TM{sub 3} alloys (TM = Ni, Co, Cu, and Fe) were investigated. The crystallization kinetics of the ball-milled Al-Y-Fe-TM nanocomposite powders were studied using differential scanning calorimetry (DSC). The DSC results of Al{sub 83}Y{sub 7}Fe{sub 5}Ni{sub 5} show that the crystallization temperature and the activation energy of crystallization are 668 K and 310 kJ/mol, respectively. In-situ high-temperature X-ray diffraction showed that the crystallization was a complex process involving growth of the nanocrystalline phase along with crystallization of the amorphous matrix phase.

Amorphization and crystallization processes of the ball-milled Al–Y–Fe–TM alloys (TM = Ni, Co, Cu, and Fe)

The second amorphous-to-crystalline phase transformation in Cu60Ti20Zr20 bulk metallic glass was investigated by differential scanning calorimetry and x-ray diffractometry. The difference of the Gibbs free energies between the amorphous phase and the crystalline products during the transformation is estimated to be about 2.46 kJ mol−1 at 753 K, much smaller than the 61 kJ mol−1 obtained assuming that it is a polymorphic transformation. It was revealed that the phase transformation occurs through a eutectic crystallization of Cu51Zr14 and Cu2TiZr, having an effective activation energy of the order of 400 kJ mol−1. The average Avrami exponent n is about 2.0, indicating that the crystallization is diffusion controlled.

Second amorphous-to-crystalline phase transformation in Cu(60)Ti(20)Zr(20) bulk metallic glass.

Crystallization from the amorphous state to metastable crystalline Al(6)Fe was induced by thermal annealing of mechanically alloyed Al(90)Fe(10) composite powders (where the composition is given in atomic per cent), which consisted of fcc Al nanocrystals in an amorphous matrix. The microstructural evolution and the transformation kinetics were characterized in detail. A thermodynamic analysis of the transformation was given, showing the nature of the polymorphous crystallization in the amorphous composite. The influence of the pre-existing crystals on crystallization was not morphologically distinctive. The observed linear kinetics law behaviour in combination with an Avrami exponent value of 2.7 +/- 0.1 suggest that the growth of a fixed nuclei population at a constant rate in three dimensions governs crystallization. The activation energy for crystallization was found to be close to the activation energy for the diffusion of Fe atoms in Al.

Amorphous-to-crystalline transformation induced by thermal annealing of a metastable Al90Fe10 composite

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Amorphous-to-crystalline transformation induced by thermal annealing of a metastable Al90Fe10 composite