On the Nanocrystalline to Amorphous Phase Transition in Al Based Alloys during Ball Milling

Abstract

We have found that mechanical grinding of Al enriched alloys (Al > 80 at.%) containing late transition metals as solute components such as Ni and Co in the composition ranges of 6 to 9 at.% Ni and 3 to 5 at.% Co (with an atomic ratio Ni/Co equal to 2) leads to the formation of nanostructured equilibrium phases: fcc-Al solid solution and Al 9 (Co,Ni) 2 compound for Al-Ni-Co systems. Significant differences towards the amorphization reaction were observed in these alloys as result of addition of a few atomic percent of Zr or Ti. Upon disordering (Al 0.88 Ni 0.08 Co 0.04 ) 100-x (Zr/Ti) x for x = 1 to 5 at.% prealloyed powders, partial to complete amorphization occurs by the following sequence: fcc-Al + Al 9 (Co,Ni) 2 + Al 3 Zr / or Al 3 Ti --> fcc-Al + amorphous I --> amorphous II. While mechanical grinding is shown to amorphize Al 3 Zr which enters in equilibrium with the fcc-Al solid solution, amorphization of Al 3 Ti compound is not evident. However, increasing Zr or Ti content, the thermal stability of the amorphous phase increases without appreciable variation of the melting temperature of the quaternary alloys. Moreover, the amorphization reaction rate was found tow times larger for Ti than for Zr substitution. Presumably due to an increase of the reduction rate of the fcc-Al particles allowing a faster dissolution. The melting temperature, the crystallization temperature and enthalpy of the ternary and quaternary prealloyed powders were determined.