Containerless undercooling and solidification of bulk metastable Nb3Ge alloys

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Abstract
A containerless low‐gravity environment, produced within a 32‐m drop tube has been used to undercool and solidify Nb(1−x) Gex alloys for 0.13⩽x⩽0.27 with sample diameters in the range of 2–4 mm. Even for modest undercoolings of 100 K, evidence is found for the formation of metastable Nb3Ge. For major undercoolings in the range of 300–500 K, the x‐ray diffraction studies on these samples revealed the A‐15 structure with lattice constants consistent with the values obtained on chemical vapor deposited films. The instabilities of Nb3Ge at temperatures near the solidification temperature are revealed by quenching the samples in oil. The Tc widths vary from broad, similar to splat‐solidified specimens, to narrow, similar to thin‐film values, depending upon the high temperature solid state quenching rates in oil. The importance of maintaining a stoichiometric composition near 25 at. % Ge and the requirements for longer processing times are also revealed in the experiment.