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

Supersaturated hcp alpha-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 650degreesC 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 xi phase, has a monoclinic Bravais lattice. Its lattice parameters are a = 0.32 nm, b = 0.60 nm, c = 0.56 nm, alpha = beta = 90degrees and gamma = 121.5degrees. Its orientation relationship with the alpha matrix is (0001)(alpha)//(020)(xi) and [01 (1) over bar0](alpha)//[001](xi) and the average habit plane of lenticularly shaped precipitates is (01 (1) over bar2)(alpha). Determination of the structure and other aspects of the phase transformation are discussed.