Abstract
When Al-bronze castings were cooled at very low cooling rate,as in marine propeller castings, they show a brittleness called “self-annealing phenomena of Al-bronze”. This phenomena was caused by β→α+γ2 transformation in its microstructure. Therefore, it was expected that even large. Al-bronze castings could be free from self-annealing, if β→α+γ2 transformation were depressed by any way such as rapid cooling or alloying elements. The present study was intended to investigate the effects of Ni, Fe and Mn addition on depressing β→α+γ2 transformation of Cu-Al base alloy containing β phase. This transformation is analogous to austenite-pearlite transformation in carbon steel,so that the effects of these additional elements on the transformation was studied by isothermal method. The ternary Cu-Al base alloys containing Ni, Fe or Mn were homogenized at 900°C×1 hr and transformed isothermally between 550° and 400°C. Their microstructures were examined and plotted in TTT diagrams. The effects of the additional elements on the critical cooling rate in the transformation of Cu-Al alloys were discussed by Grange and Kiefer’s assumption in the case of steel. The results obtained were as follows: (1) Adding Mn into Cu-Al alloys was effective to depress β→α+γ2 transformation, the more the Mn content was increased. (2) Adding Ni and Fe seperately was little effective to depress the transformation. (3) The following equation showing the relationship between the critical cooling rate in β→α+γ2 transformation and the nose location of the TTT diagram was experimentally obtaind. Rs=(Te-Tn)/Cxt_n (C \fallingdotseq1.1-1.5) Rs: critical cooling rate (°C/min), Te: Eutectoid temperature, Tn: Temperature of the nose, tn: Time of the nose, C: co-efficient\ oindent(4) It was suggested that if on adequate quantity of Mn is added Mn in to Cu-Al alloy were enough, β→α+γ2 transformation would be depressed and thereby large castings containing β phase will be free from self-annealing.
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