X-ray diffraction study on the amorphization of the Pd₈₀Si₂₀ powder mixture by mechanical alloying

Abstract

X-ray diffraction line profile analysis has been applied to investigate the early stages of powder amorphization by mechanical alloying (MA) in the Pd₈₀Si₂₀ (at.%) system. In particular, Fourier coefficients analysis has been performed to determine, from the large and progressive line broadening, the structural evolution of Pd grains in terms of average size (D) and lattice distortions (G ²). A comparison with pure palladium powder samples, milled under the same conditions has been used to analyse better the response of palladium under ball milling. MA induces, on pure palladium, effects 0are similar to conventional cold working. A minimum average size is soon reached after 1 h of MA which depends on the crystallographic direction: <D _[111]> ≈ 22/23, <D _[100]> ≈ 13/14 (nm). From the unequal peak broadening along the [100] and [111] crystallographic directions, a lattice distortion parameter G ² = <ϵ²(L>L≈ 2 × 10⁻⁴(nm) (L is a length in the physical space perpendicular to the diffracting planes and <ϵ²> is the mean square microstrain) and a deformation and twin faults probability of 1.5α+β ≈ 0.01/0.015 have been estimated. Accounting for planar defects the average grain size is increased to about 35/40 (nm) in both [111] and [100] directions. The microstructure evolution of the Pd₈₀Si₂₀ sample can be interpreted as a two-step process. At the beginning, up to 1 h of MA, the behaviour is similar to pure palladium. Thereafter MA induces a further particle size refinement to about <D>≈ 3/5(nm), before promoting solid state amorphization. The lattice distortion parameter approaches, after a steep increase, the values measured for pure palladium, so that the crystalline structure destabilization seems to be achieved, in this system, by grain boundary energy storage.