Formation Process of α2 Phase in Cu–Al Alloys

The process of formation of α2 Cu–Al phase was investigated in the slow-cooled specimens decomposed into α+γ2 phases as well as the quenched ones of β1′ phase. The specimens aged at 350°C for various times were examined by means of X-ray diffraction and electron microscopy. The results obtained are as follows:(1) In the decomposed specimens the domains of α2 phase of the long period superlattice in x-, y- or z-orientation are randomly distributed in the matrix. They do not grow more than 1000Å in diameter after aging for about 60 days.(2) In the quenched specimens the α2 phase is formed coherently with the matrix. When aged for about 60 days, the quenched specimen contains the domains of α2 phase with a particular orientation and a diameter of several microns.(3) The antiphase boundaries in the β1 phase are retained until the matrix finishes transforming to the α2 phase through β1′ and X phases.(4) In the quenched specimens aged for a long time straight boundaries as well as curved ones between domains of different orientations are observed.(5) In the quenched specimens of Cu-23.1 at% Al, the γ2 phase never appears throughout the long aging at 350°C.