IMPROVEMENT OF STRENGTH AND FRACTURE-TOUGHNESS OF A SPRAY-DEPOSITED AL-CU-MG-AG-MN-TI-ZR ALLOY BY OPTIMIZED HEAT-TREATMENTS AND THERMOMECHANICAL TREATMENTS

Optimized heat treatments and thermomechanical treatments have been developed in order to improve the strength and toughness of a spray-deposited alloy Al-5wt.%Cu-0.4wt.%Mg-0.4wt.%Mg-0.4wt.%Ag-0.4wt.%Ti-0.15wt.%Zr-0.2wt.%Mn (i) a two-step aging heat treatment consisting of artificial underaging at an intermediate temperature (160 degrees C) followed by peak aging at 210 degrees C; (ii) thermomechanical treatments implying cold rolling (8%) before artificial aging to peak hardness at 160 degrees C. Compared with the conventionally peak-aged T6 temper, fracture toughness can be considerably increased by the two-step aging treatment, whereas the strength is slightly lowered. However, simultaneous improvement in both fracture toughness and yield strength results from the thermomechanical treatments; thus excellent combinations of fracture toughness and yield strength are achieved (R(P)0.2 = 532 Mpa and K-IC = 47 MPa m(1/2)). Microstructure investigation by transmission electron microscopy reveals that the Al-Cu-Mg-Ag-based alloy is strengthened by comparable amounts of precipitates of type Omega and theta'. However, in accordance with the different mechanical properties of the various tempers, evident quantitative and qualitative differences in the precipitation state are observed: compared with the original T6 temper, the total amount of Omega and theta' is decreased in the two-step-aged material, but both precipitates are much finer; the thermomechanically treated material shows the finest precipitate sizes altogether with the highest precipitate densities.