Grain Boundary Reaction in a Cu-Ni-Mn Alloy

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  • Cu-Ni-Mn合金の粒界反応型析出
  • Cu Ni Mn ゴウキン ノ リュウカイ ハンノウガタ セキシュツ

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The characteristics of the grain boundary reaction in Cu-20 mass%Ni-20 mass%Mn and Cu-30 mass%Ni-30 mass%Mn alloys aged isothermally at temperatures of 523∼723 K after water quenching from 1173 K were investigated by optical and electron microscopic observations, hardness measurements and the X-ray diffraction method. Results obtained are as follows:<BR>(1) The incubation periods were not observed in the nodule growth curves, namely the nodule began to grow along with the hardening of the matrix from the early stage of aging. At the temperature below 623 K, the nodule engulfed all over the matrix after a long time aging, but on aging at the temperature above 673 K, the nodule did not engulf the matrix entirely owing to the influence of the intragranular precipitation. Moreover at the temperature above 723 K the nodule grew scarecely.<BR>(2) The microstructure in the nodule was not pearlitic. A light and shade structure with θ phases waved periodically was observed in the nodule at the early stage of aging and this structure grew with the progress of aging. Also the hardness of the nodule increased during aging, and decreased slowly after reaching its maximum value.<BR>(3) The lattice parameters of the α phase and the θ phase [(Ni·Cu)Mn] in the nodule changed with progress of aging.<BR>(4) The area fraction of the nodule, X, could be represented by Johnson-Mehl’s equation: X=1−exp(−btn), where t is the aging time and b and n are constants, and 0.7∼1.2 were obtained as the values of n.<BR>(5) The value of apparent activation energy for nodule growth was about 80 kJ/mol in Cu-20 mass%Ni-20 mass%Mn alloy or about 110 kJ/mol in Cu-30 mass%Ni-30 mass%Mn alloy.

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