The effects of AB2 (A = Zr,Hf,Ti) additions on the free-iron content in ingots with the initial composition of Nd2.2Fe14B1.1

Additions of zirconium diboride (ZrB2), to small scale (similar to 1 g) arc-melted (AM) and larger scale (5 kg) book mould (BM) cast alloy with the composition Nd2.2Fe14B1.1 (near to stoichiometric Nd2Fe14B) have been shown to be a very effective means of eliminating the free-iron content, which is present as a result of the peritectic nature of the Nd2Fe14B phase. The effect of zirconium diboride in reducing free-iron content has been determined (1) magnetically by means of the Honda-Owen plots on the BM alloys and (2) by image analysis using scanning electron microscopy for both AM and BM alloys. Generally, for the BM alloys, both sets of measurements exhibited good agreement with a progressive reduction in the free-iron content with increasing additions of ZrB2. Without any subsequent homogenisation treatment, zero free-iron was achieved in the AM and BM cast condition, after an addition of just 1 mol% of ZrB2. Closely similar behaviour was observed with up to 1 mol% additions of the other Group 4 diborides, TiB2 and HfB2. High resolution scanning electron microscopy indicated that the diborides were embedded largely within the areas of Nd-rich material.