Reversible phase changes of Ni-rich Al-Co-Ni decagonal quasicrystals studied by means of transmission electron microscopy and thermal analysis

In the stability region of the Ni-rich Al-Co-Ni decagonal quasicrystals, phase relationships among three different modulations, i.e. the so-called Ni-rich basic, S1-state and Type-I phases, and their phase changes have been thoroughly investigated by various transmission electron diffraction and imaging techniques in combination with thermal analysis. Electron diffraction experiments have clearly indicated that the S1-state phase formed in the Al70Co12Ni18 alloy, which is stable at 900 deg. C, transforms into the Type-I phase by annealing at 650 deg. C and that the Ni-rich basic phase formed in the Al71.5Co9Ni19.5 alloy, which is stable at 900 deg. C, transforms into the S1-state phase by annealing at 650 deg. C. In addition, differential scanning calorimetry measurements have evidenced for the first time that both phase changes occur reversibly at certain temperatures ranged between 700 and 800 deg. C. The structural features of the modulations mentioned above have been distinguished and uniquely characterized by making the best use of high-angle annular detector dark-field scanning transmission electron microscopy as well as high-resolution transmission electron microscopy