FIM-ATOM PROBE STUDY OF THE PRECIPITATION IN A Ti_46.3. Ni_53.7 ALLOY

Abstract
The precipitation process in a B2-TiNi-alloy has been studied during aging between 873-973 K for various aging times. Earlier reported results obtained with TEM/EDX-analyses showed the development of three different precipitates. The compositions, based on crystallographic calculations were found to be between 55-65 at % Ni. In the present study FIM/AP-analyses have been used to verify these compositional variations. At the early stages of precipitation only Ti₃Ni₄ is formed which could be analysed both by EDX as well as FIM/AP-analysis. The results of AP-analysis seem to be in better agreement with the calculated value of 57.1 at % Ni than the EDX results. The FIM images of the alloys show no crystallographic poles which is ascribed to the heavily twinned martensite transformation of the ordered B2-phase during cooling in the FIM. The Ti₃Ni₄-phase could be identified in the FIM images by the brightness and clear pole formation. The Ti₂Ni₃- and the Ti₅Ni₉-phase showed very brittle behaviour in the FIM, and AP-analyses showed better agreement with the calculated values than EDX, but can not be treated as conclusive yet.