Atomic structure of the amorphous state of TiC-modified Nd2Fe14B as revealed by positron annihilation spectroscopy

Positron annihilation spectroscopy utilizing the Doppler broadening of the annihilation radiation (DBAR) method was performed on stoichiometric Nd2Fe1 4B ribbons containing 0, 2, and 6 wt % Ti-C that were melt spun to the x-ra y amorphous state at a wheel speed of 40 m/s. DBAR data indicate that the a s-spun samples are very homogeneous. It is found that an increase in the co ncentration of alloying additions promotes an increase in the shape paramet er ("S parameter") of the symmetric Doppler-broadened positron-electron ann ihilation peak. This increase in the S parameter may be interpreted as an i ncrease in the amount of open volume in the quenched glass. The presence of increased open volume is concurrent with an increased stability of the gla ss against crystallization. However, at the present time it is not clear if the increased thermal stability is a direct product of the increased open volume, or if stereochemical considerations determine the stability of the glass. (C) 1999 American Institute of Physics. [S0021-8979(99)47908-3].