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

Abstract

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.