Phase evolution during high energy ball milling of immiscible Nb-Zr alloys

Mechanical alloying (MA), a solid-state processing technique used extensively to synthesize metastable phases, was employed to synthesize solid solution and amorphous phases in Nb-rich Nb-Zr powder blends. These metastable phases could be synthesized by MA, under different processing conditions, even though the heat of mixing between Nb and Zr is positive, which makes alloying them difficult. The effect of alloy composition, milling time, and the ball-to-powder weight ratio (BPR) were varied and their effect studied on phase evolution and microstructure in the milled powders. The composition of the milled powders was varied starting from a low value of 5 to about 50 at.% Zr. At a Zr content below 40 at.%, amorphization was achieved at a higher BPR of 30: 1, i.e. more milling energy. The formation of an amorphous phase at Zr contents lower than 40% was achieved for the first time in this work and confirmed using TEM. However, this amorphous phase crystallized rapidly on continued milling (mechanical crystallization) to form an FCC phase. Additionally, milling of powders with low Zr contents primarily resulted in the formation of Nb-based Nb-Zr solid solutions. (C) 2014 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.