Transmission electron microscopy investigation of an ordered metastable phase in Zr-N alloys

Supersaturated hcp f-Zr(N) alloys containing 22-28 at.% N were prepared by nitriding sheets of Zr in an atmosphere of high-purity N 2 , followed by homogenization under high-purity Ar gas. Quenching and isothermal ageing of the alloys for various times between 500 and 650C resulted in precipitation of a metastable phase, rather than the equilibrium phase ZrN. This investigation focused on determining the structure, orientation relationship, habit plane, morphology, growth kinetics and atomic growth mechanism of this non-equilibrium precipitate using transmission electron microscopy imaging and diffraction techniques, high-resolution transmission electron microscopy, electron-energy-loss spectroscopy and various simulation programs. The precipitate, which was arbitrarily designated the ξ phase, has a monoclinic Bravais lattice. Its lattice parameters are a = 0.32 nm, b = 0.60 nm, c = 0.56 nm, f= g= 90 and n= 121.5. Its orientation relationship with the fmatrix is $$ (0001)_\alpha /\mskip-2/ (020)_\xi\quad \a\n\d\quad [01\bar 10] _\alpha /\mskip-2/ [001]_\xi, $$ and the average habit plane of lenticularly shaped precipitates is $(01\bar 12)_\alpha$ . Determination of the structure and other aspects of the phase transformation are discussed.