Amorphization and crystallization behaviors of glassy Zr 70Pd 30 alloys prepared by different techniques

Abstract

A single phase of glassy Zr 70Pd 30 binary alloy has been synthesized by solid-state reaction (SSR) and rapid-solidification (RS) techniques. The SSR was performed in a room temperature high-energy ball mill, employing mechanical alloying (MA) and mechanical disordering (MD) methods. A single roller melt-spinning method was performed for the RS. The results show that the mode of amorphization and subsequent crystallization of the obtained glassy materials strongly depends on the employed way of fabrication. In the MA method, post-annealing the mechanically deformed Zr/Pd multilayered composite powders at 723 K leads to the formation of a heterogeneous glassy phase (thermally enhanced glass formation reaction), which its heat formation was measured directly and found to be −2.16 kJ/mol. Further MA time (259 ks) was necessary to obtain a homogeneous glassy phase (mechanically induced glass formation reaction). In the MD method, however, the crystalline-glassy phase transformation is taking place through a single stage by introducing numerous lattice imperfections to the stable crystalline powders and this leads to raising of the free energy from the most stable phase (tetragonal-Zr 2Pd) to a less stable phase (glassy) that is obtained after 216–259 ks of MD time. The similarity in the crystallization behavior between the two classes of MA and MD glassy powders, implying the formation of the same glassy phase. In melt-spinning method, the solidification of the molten state occurs so rapidly that a small volume fraction of nano-icosahedral potential clusters are precipitated when the atoms are frozen in their liquid configuration. In contrast to the SSR glassy material powders, the crystallization of the RS-Zr 70Pd 30 ribbon proceeds through two separate exothermic reactions. The first reaction is owing to the short-or medium-range order transformation and the formation of metastable i-phase; however, the second crystallization step refers to metastable-ordered phase transformation and formation of single tetragonal-Zr 2Pd material. We assume that if such clusters are formed during the MA and/or MD procedures, increasing the milling time can homogenize them.