The precipitation kinetics in a Cu-4 mass% Ti alloy was studied using SEM, TEM, XRD and Vickers hardness. A Cu-4 mass% Ti alloy was prepared, homogenized, solution treated and then aged at 673, 773 and 873 K for times between 0.6 to 720 ks. The XRD and TEM results indicated that the phase decomposition occurred by spinodal decomposition during the early stages of aging. The growth kinetics of composition modulation wavelength is very slow at the early stages of aging. The precipitation of metaestable 0 (Cu 4 Ti) preceded to that of the equilibrium phase phase (Cu 3 Ti), which formed through cellular precipitation. The coarsening process of 0 phase followed the LSW theory for diffusioncontrolled growth. The activation energy for this coarsening process was determined to be about 190 AE 10 kJÁmol À1 . The discontinuous precipitation of phase has an activation energy of about 207 kJÁmol À1 and an exponent time of about one. The highest hardness and fastest transformation kinetics occurred at aging temperatures of 673 and 873 K, respectively.

The coarsening process of the NiAl ordered b9 precipitates was studied in the isothermally aged Fe-10 wt-%Ni-15 wt-%Al and Fe-10 wt-%Ni-15 wt-%Al-1 wt-%Cu alloys. The aging treatments at 750, 850 and 950uC caused the precipitation of the b9phase in a ferritic matrix. In general, the coarsening process of Cu containing alloy was slower with an activation energy of 248 kJ mol 21 compared to 194 kJ mol 21 for the other alloy. However, the coarsening kinetics for the former alloy was faster at 950uC. The precipitation hardening response was also better in the Cu containing alloy, with a slower overaging stage. The coarsening kinetics followed the behaviour predicted by the modified Lifshitz-Slyozov-Wagner theories for coarsening controlled by volume diffusion.

This work analyzed experimental and numerically the growth kinetics of β´precipitation of a Cu4wt.%Ti alloy after aging at 400, 500 and 600 °C for times from 0.0166 h to 200 h. Results indicated that the precipitation process is almost controlled by nucleation and growth during aging at 400 °C, originating a slow growth kinetics of precipitation. In contrast, the coarsening of precipitates dominates the precipitation process during aging at 500 and 600 °C. The interfacial energy of interface between the a matrix phase and β´ precipitates was determined to be about 0.1135, 0.0980 and 0.0725 Jm -2 for aging at 400, 500 and 600 °C, respectively. These values suggest a coherent interface which is in agreement with the flat faces of β´ cuboid precipitates. Calculated Time-Temperature-Precipitation diagram for the β´ precipitation indicated good agreement with experimental results. Precipitation hardening was higher for the slower growth kinetics of precipitation.

No abstract

No abstract