X-RAY-DIFFRACTION STUDY ON THE AMORPHIZATION OF THE PD80SI20 POWDER MIXTURE BY MECHANICAL ALLOYING

X-ray diffraction line profile analysis has been applied to investigat e the early stages of powder amorphization by mechanical alloying(MA) in the Pd80Si20(at.%) system. In particular, Fourier coefficients anal ysis has been performed to determine, from the large and progressive l ine broadening, the structural evolution of Pd grains in terms of aver age size (D) and lattice distortions (G2). A comparison with pure pall adium powder samples, milled under the same conditions has been used t o analyse better the response of palladium under ball milling. MA indu ces, on pure palladium, effects which are 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]] almost-equal-to 2 2/23. [D[100]] almost-equal-to 13/14(nm). From the unequal peak broade ning along the [100] and [111] crystallographic directions, a lattice distortion parameter G2 = [epsilon2(L)]L almost-equal-to 2 x 10(-4) (n m) (L is a length in the physical space perpendicular to the diffracti ng planes and [epsilon 2] is the mean square microstrain) and a deform ation and twin faults probability of 1.5alpha + beta almost-equal-to 0 .01/0.015 have been estimated. Accounting for planar defects the avera ge grain size is increased to about 35/40 (nm) in both [111] and [100] directions. The microstructure evolution of the Pd80Si20 sample can b e 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] almost-equal-to 3/5(nm), before promoting solid state amorphization. The lattice distortion pa rameter approaches, after a steep increase, the values measured for pu re palladium, so that the crystalline structure destabilization seems to be achieved, in this system, by grain boundary energy storage.