Evolution of microstructure of nitrogen ion implanted iron at high temperatures

Abstract

Pure polycrystalline iron layers are implanted with nitrogen ions of 50 keV with 2, 4, 6, or 8 × 10 17 ions/cm 2 respectively at 100, 175 or 250°C and subsequently annealed. In all cases, the implantation temperature is always equal to the subsequent annealing temperature. The microstructure after implantation and versus duration of annealing is examined with in situ high-voltage electron microscopy and transmission high-energy electron diffraction. The refinement of structure analysis using electron diffraction data is described. The integrated intensities and interplanar spacings of the individual reflections are derived by a spectrum fitting procedure. Remaining overlapping peaks can be separated by spectrum fitting. The evolution of precipitate size and phase transformation is discussed in terms of growth and dissolution processes. Three different phase transition processes are observed during annealing. At 100 °C, the implantation-induced α′-martensite, at 175°C, the α′-Fe 16N 2 precipitates and at 250°C, the ϵ-iron nitride phase are observed to transform or dissolve with increasing annealing times. Hereafter, the microstructure is characterized by α″-Fe 16N 2, γ′-Fe 4N and ζ-Fe zN precipitates, respectively.