TRICLINIC NI2AL PHASE IN 63.1 ATOMIC PERCENT NIAL

Transmission electron microscopy (TEM) studies on the 63.1 at pct NiAl specimens aged in the L1(0) phase indicate the presence of localized regions consisting of very high-density twins along with precipitates of Ni2Al stoichiometry. In regions representative of the 7R phase, it is observed that the ordered Ni2Al phase exists in lattice corresponde nce with the 7R phase. Based on the electron-diffraction results, the crystal structure of this Ni2Al phase is determined to be triclinic, b elonging to the space group P1BAR. This new Ni2Al phase alternatively may be viewed as a periodically microtwinned monoclinic Ni2Al phase, e xisting in coherence with the matrix L1(0) phase that is also periodic ally microtwinned.