Ion-beam-induced formation of a stable phase at the expense of an otherwise dominant metastable phase
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Ion-beam-induced formation of a stable phase at the expense of an otherwise dominant metastable phase E. H. Lee and L. K. Mansur Metals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831 (Received 20 January 1987; accepted 24 February 1987) Thermal aging and irradiation experiments were carried out in Fe-Ni-Cr base alloys containing Ti or Nb as well as low levels of oxygen, nitrogen, and carbon. After aging, fine dispersions of TiC and NbC were formed. However, after irradiation the Ti-containing alloy precipitated Ti(N,O) rather than TiC, while the Nb-containing alloy precipitated NbC. Analysis of the energetics of these reactions reveals that the Ti(N,O) phase is thermodynamically more stable than the TiC phase, while the NbC phase is more stable than the NbO phase. These results are therefore consistent with observed behavior. While it is well known that irradiation can drive a system away from equilibrium, the present results show that irradiation also can catalyze the formation of a more stable phase at the expense of a persistent metastable phase.
Phase relationship under irradiation can be altered drastically with respect to thermal behavior.1"3 Often the irradiated alloy is driven to form highly nonequilibrium phases that are not normally found in the alloy by such processes as radiation-enhanced diffusion,4'5 cascade resolution6"9 and mixing, 1011 and radiation-induced solute segregation.1213 On the other extreme, irradiation has also been found to accelerate the formation of prevalent thermal phases, often with some modification of the phase composition.14 Here we report a new type of observation, a radiation catalyzed reaction, wherein irradiation leads to the formation of a stable phase that is normally precluded under thermal conditions because of the presence of a competing metastable phase. The observation was made in thermal aging and irradiation experiments on two Fe~13Cr-15Ni base alloys designated B5 (0.2Ti-0.04C) and B13 (0.4Nb0.04C), both containing about 100 appm oxygen and nitrogen. Note that the compositions are given in weight percent, so that the atomic fractions of Ti and Nb are equivalent in the two alloys. The TiC and NbC are the major phases that occurred during thermal aging at 948 K in B5 and B13, respectively. During irradiation of solution-annealed alloys, however, while NbC persisted in B13, TiC formation in B5 was drastically reduced in favor of Ti(N,O) with concurrent formation of M23C6. Figure 1 illustrates the microstructures observed by TEM in each alloy after 6 months of thermal aging or 70 dpa irradiation dose with 4 MeV Ni ions, both treatments carried out at 948 K. J. Mater. Res. 2 (3), May/Jun 1987
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In the figure the phases have been identified. Distinguishing between TiC and Ti(N,O) phases was somewhat difficult because both have the same crystal structure (fee) and very similar lattice parameters. However, the morphologies and compositions are distinct. The TiC phase generally occurs as finely dispersed part
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