Stability of glassy state in Zr-based glassy alloys correlated with nano icosahedral phase formation

摘要

Stability of glassy state in Zr-based glassy alloys correlated with nano icosahedral phase formation. We investigated the stability of the glassy state by examination of transformation behaviour and structural analysis in Zr-based binary and ternary glassy alloys. It is well known that the melt-spun Zr-Cu binary alloys have a distinct glass transition prior to crystallization in a wide composition region of 20 to 70 at% Cu, while no supercooled liquid state appears in the melt-spun Zr-Ni binary alloys. It is expected that the local structure of the disordered state in the two alloy systems is different. The icosahedral quasicrystalline phase (I-phase) is formed as a primary phase in the Zr70Cu30 binary glassy alloy by substitution of Pd for Cu. The minimum Pd concentration for I-phase formation is 0.5 at%. In contrast, the primary phase is a tetragonal Zr2Ni phase in a Zr70Ni30 amorphous alloy by substitution of Pd by less than 10 at% for the Ni element. The radial distribution function (RDF) of the Zr70Cu30 glassy alloy is different from that of the Zr70Ni30 amorphous alloy. The coordination numbers around Zr in the melt-spun Zr-70(Cu or Ni)(30) binary alloys are 12.4 and 10.7, respectively, indicating the possibility of existence of the icosahedral local atomic configuration in the Zr70Cu30 glassy alloy. It is also realized that a tetragonal Zr2Ni-like local atomic configuration is formed in the Zr70Ni30 amorphous alloy. Considering that the I-phase is also formed in the Zr(60)Ni(25)AI(15) glassy alloy by addition of 3 at% Pd, where the distinct glass transition is observed, the appearance of the supercooled liquid region is attributed to the existence of the icosahedral local atomic configurations consisting of Zr-Cu or Zr-Ni-Al. These results show that the icosahedral short- and medium-range orders exist in the supercooled liquid as well as in the glassy phase and they stabilize the glassy state in the Zr-based alloys. [References: 23]