Metallographic Study on the Structure Changes in Cu-3%Ti Alloy by Aging

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  • Cu-3%Ti合金の時効による組織変化
  • Cu-3パーセント Ti ゴウキン ノ ジコウ ニ ヨル ソシキ ヘンカ

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An age-hardenable Cu-Ti alloy (Cu Side) has been shown to precipitate pearlitic or widmanstätten β′-Cu3Ti (or Cu7Ti2) under different conditions of aging. Using a Cu-3 wt%Ti alloy, this paper deals with the changes in the shape of precipitates by aging in two different temperature ranges for the pearlitic precipitation and the widmanstätten precipitation after water-quenching from 900°C. (1) Isochronal aging for 13.5 min with a temperatures interval of 50°C in the temperature range between room-temperature and 800°C: The pearlite became visible above 400°C at grain boundaries where two neighbouring grains have a particular orientation relation with each other. Furthermore, the pearlite seemed to grow into one grain with the orientation affected by that of the other grain. In the temperature range between 650° and 750°C, the pearlite still remained, but different type precipitates (see Photo. 7(b), 8(b)(c)) were observed frequently at grain boundaries. These new type of precipitates could be interpreted as follows: Some of the pearlitic precipitates, whose habit plane coincided with {111} planes of surrounding super-saturated α-phase, grew into the α-phase. By the aging above 700°C, widmanstätten precipitates appeared in the super-saturated α-phase coexsistent with the pearlite. (2) Aging at 700°C after preaging at 550°C: When residual super-saturated α-phase remained after the pre-aging, widmanstätten precipitates appeared in the residual α-phase. On the other hand, pearlitic precipitates changed into straight rod-type precipitates (see Photo. 14) when the entire alloy had been changed into the pearlite by the pre-aging. However, the rod-type precipitates, retained traces of pearlitic precipitates and were different from widmanstätten precipitates.

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