Novel insight into precipitation behavior of γ′ phase particles in Cu-15Ni-xAl alloys through Calphad method

gamma' phase (AlNi3) particles' precipitation behavior in the Cu-15Ni-xAl alloys (wt.%, x = 0, 1, 2, 3 and 4) ageing at different temperature was systematically discussed with the help of Calphad method. With fixed ageing temperature, when Al content increases from 1 wt% to 4 wt%, the gamma' phase particles transform from a few microns to dozens of nanometers in size. The increasing Al content was proven to raise the instability of single phase solid solution (FCC), which can be expressed by the increasing value of G(excess)(FCC) and will result in the increasing critical temperature for precipitation. Supersaturation at fixed ageing temperature is thus raised with increasing Al content, so does the nucleation driving force of gamma' phase. Simultaneously, the precipitation strengthening in Cu-15Ni-4Al firstly increases and then decrease with ageing temperature decreasing from 700 degrees C to 400 degrees C, which is due to the competition between increasing nucleation driving force and decreasing self-diffusion coefficient of element with decreasing temperature. The continuous and discontinuous precipitation behaviors of gamma' phase were also discussed, which will be respectively promoted and suppressed with decreasing ageing temperature due to the relatively high nucleation driving force. However, both continuous and discontinuous precipitation will be suppressed when the ageing temperature decreases to 400 degrees C due to the too low self-diffusion coefficient.