The Correlations between the Eutectoid Transformation, Order-Disorder Transformation, and Martensitic Transformation in Cu3Al Binary Alloys

In view of the fact that three kinds of phase transformations, i.e. aneutectoid decomposition β→ α+γ2, an ordering transformation β→ β1, and a martensitic transformation β1→ β' have been observed on isothemlal holding and cooling at different rates from the β region in Cu-Al binary alloys, the author has tried to obtain the T/T/T diagrams in order to show the correlations between these three transformations. In the case of tempering the β' martensite formed by water-quenching, the above three transformations have occurred in the reverse direction, and the T/T/T diagrams associated with the tempering process have also been determined. Specific heat meassurements and microscopic observations were carried out, and the X-ray diffraction pattern was used for phase identification during these heat-treatments. As the result of the experiments, it was verified that the growth of the ordered domains of the Cu3Al superlattice occurred below the critical temperature for ordering, Tc, when cooling from the β region, (therefore, the transformation β1→ α+ γ2 appeared instead of the eutectoid transformation β→ α+ γ2,) and then the β' martensite contained about 25% aluminium after air-cooling. On the other hand, in the case of tempering of quenched alloys the growth of the ordered domains occurred at temperatures above 400°C, and below 400°C the ordering did not occur, but the precipitation of α or γ2 appeared preferentially. (Received December 23, 1960)