Static mean-square displacement as an indicator of the crystal-to-amorphous transformation

Abstract

Catastrophic amorphization of crystalline alloys supersaturated beyond a critical concentration has been postulated by Fecht, Desré and Johnson [Philos. Mag. B59 (1989) 577]. NbPd is a good candidate system for testing their ideas because of its plunging T 0 line on the Nb-rich side of the phase diagram. Signs of instability in bcc NbPd solutions were looked for by elastic neutron diffraction measurements of atomic mean-square displacements (MSDs) between 12 K and room temperature. Thermally induced MSD values have been estimated from low-temperature heat capacity measurements and subtracted from the total MSD. The remaining static MSD increases rapidly with Pd concentration and is much larger than expected from the size mismatch of Nb and Pd atoms. At 42 at.% Pd, the rms static disorder reaches approximately half the value at which the Lindemann criterion predicts that the lattice should melt. Supersaturation also causes the Debye temperature to decrease, suggesting that an elastic modulus softens. In light of these results, the usefulness of static atomic disorder as a measure of the stability of a crystal against amorphization is discussed.