Microstructure of Nickel-Chromium Superalloys
Doctoral thesis, 1998
The microstructure of nickel-chromium superalloys has been investigated using several advanced microanalytical techniques; atom probe field ion microscopy (APFIM), transmission electron microscopy (TEM) together with energy dispersive X-ray analysis (EDX) and electron energy loss spectroscopy (EELS), secondary ion mass spectroscopy (SIMS) and optical microscopy (OM). The features primarily considered include precipitation, segregation and ordering. In particular the influence of carbon and boron on the microstructure after various heat treatments has been studied.
Model materials with major composition Ni-16Cr-9Fe (wt%) and Ni-30Cr-10Fe were analysed. Segregation of carbon, boron and nitrogen to grain boundaries was quantified using APFIM. It was found that the dominating intergranular precipitates in the low chromium material were Cr7C3 and Ni23B6. Moreover, Cr23C6 and Cr2(C,N) were sometimes present. In the high chromium material, Cr23C6 and Cr2B were dominant. It was proved that the concentration of boron in carbides as well as the concentration of carbon in borides was low. The precipitation process in the alloys was explained by considering the local chemistry at the grain boundaries.
A new statistical method for evaluation of APFIM data was developed. Using this method the elemental distribution, in terms of ordering or phase separation, was studied. In a commercial Ni-16Cr-15Mo-6Fe-4W alloy, short-range ordering was observed after solution annealing, whereas aging resulted in long-range ordering based on Ni2(Cr,Mo). In this phase chromium and molybdenum appeared to be inhomogeneously distributed. APFIM studies also proved existence of short-range ordering in Ni-16Cr-9Fe and Ni-30Cr-10Fe.