Microstructures and coarsening behavior of silicon particles in P/M Al-Si-Cu-Mg alloys containing Fe and Ni.

The microstructures and coarsening behavior of silicon particles in Al–Si–Cu–Mg alloys containing 5 mass% of Fe or 7.5 mass% of Ni produced by a powder metallurgy technique were investigated by means of optical and transmission electron microscopy. The effects of Fe or Ni on the coarsening of silicon particles have been examined for specimens annealed at 723, 778 and 793 K in relation to hardness and proof stress changes. Through TEM observations β–AlFeSi, β'–AlFeSi, Al16(MnFe)4Si3, AlCu2Fe7 and Al3Fe phases are found to be present in the Fe containing alloy, while Al3Ni and Al3Ni2 are detected in the Ni containing alloy. The mechanism of coarsening of silicon particles was discussed on the basis of the Lifshitz-Slyosov-Wagner theory (LSW theory). The activation energy for the coarsening of silicon particles in the alloys containing Fe or Ni is larger than that in the Fe-free or Ni-free alloy. The size distribution of Si particle is better described by the log-normal distribution, which is different from that expected from the LSW theory. The intermetallic compounds containing Fe or Ni effectively retard the coarsening of silicon particles, as well as increase the proof stress of the alloys.