PRECIPITATION IN CU-RICH CU-FE-TI TERNARY ALLOYS - A CONTINUOUS PROCESS

Two dilute alloys with nominal compositions Cu-1.2%Fe2Ti and Cu-0.8%Fe Ti are studied in the temperature range 500 to 950-degrees-C by electr ical conductivity measurements and by transmission electron microscopy (TEM) and field ion microscopy (FIM). Under equilibrium conditions, t he two alloys are located in the fields (''Cu'' + Fe2TiHT) at 950-degr ees-C and (Cu + Fe2TiLT + FeTi) at 500 to 525-degrees-C. The solubilit y limit of Fe and Ti in Cu is very small (less than 0.5%) at 950-degre es-C and vanishes at 500 to 525-degrees-C. The transformation between the high temperature and the low temperature states by aging of quench ed alloys involves the partial dissolution of Fe2TiHT and the precipit ation of FeTi and Fe2TiLT This precipitation is shown to proceed by ra pid nucleation of coherent precipitates in localized areas, loss of co herency, and growth of incoherent precipitates. The temperature depend ence of the electrical conductivity variation during aging leads to ev aluate an activation energy of about 200 kJ mol-1 for the decompositio n of the Cu matrix. Such a value suggests that the limiting step of th e precipitation is either the Fe2TiHT dissolution or the growth of the FeTi and Fe2TiLT with a diffusion controlled rate.