Microstructural investigation of four kinds of γ′–Fe₄N nitrides in ion-nitrided pure iron

Conventional and high-resolution electron microscopies were used to investigate the microstructure and precipitation mechanism of four kinds of γ′–Fe₄N nitrides, formed at the surface during nitriding of polycrystalline pure iron sheet. γ′ nitrides in the top columnar compound layer were thin plates (3–12 nm), which precipitated from ε–Fe₂₋₃N grains with orientation relationship of (111)γ′//(0001)ε and [011]γ′//[1210]ε. Those in the transition compound layer remained as equiaxed grains (1–1.5 μm), which grew from the α–Fe substrate during nitriding. In the diffusion layer, striated γ′ nitrides nucleated from α″–Fe₁₆N₂ nitrides with orientation relationship of (100)γ′// (110)α″ and [011]γ′//[111]α″ while unstriated γ′ nitrides precipitated from the α–Fe substrate with the orientation relationship of (100)γ′//(110)α and [011]γ′//[111]α.