Thermal behavior of Cu-Al alloys near the alpha-Cu-Al solubility limit

The thermal behavior of Cu-Al alloys with 17, 19 and 21 at.%Al was examined by differential thermal analysis (DTA), differential scanning calorimetry (DSC), X-ray diffractometry (XRD), optical microscopy (OM) and scanning ele ctron microscopy (SEM). The presence of the gamma phase (Al4Cu9) was clearl y detected for the Cu-19 at.%Al alloy and caused the alpha (2) phase disord ering process in two stages. The tendency to increase the alpha (2) dissolu tion precipitates with the increase in the Al content seems to be reverted for compositions at about 21 at.%Al and the heating/cooling ratio seems to influence the thermal response of this process. The presence of the endothe rmic peak corresponding to the beta (1)--> beta transformation depends on a n incomplete beta decomposition reaction. The variation of the heating rate showed that the beta (1)-->(alpha+gamma (1)) decomposition is the dominant reaction for alloys containing 19 and 21 at.%Al.