Diffraction from Layer Faults in bcc and fcc Structures

Abstract

Calculations, using the Warren-Mering method, of the broadening and shift of the x-ray diffraction peaks in powder patterns have been performed for the situation in which an interplanar spacing change occurs at the fault plane. Deformation-spacing faults on the {211} layers of the bcc lattice and the {111} layers of the fcc lattice, and spacing faults are discussed in detail. The principal effect of the spacing change is to introduce additional broadening and peak shifts, the magnitude (and direction, in the case of the peak shifts) of which depends on the reflection. In bcc structures the effect is particularly striking because no peak shifts are expected in the presence of stacking faults whose interplanar spacing is unchanged. Measurements were made on three cold-worked bcc materials: Ta, Nb, and a stainless steel martensite. No measurable peak shifts were found in Ta. In Nb and in the martensite, however, small shifts were observed and were in directions predicted from the theory; the shifts could therefore be interpreted and indicated an expansion of the layer spacing at the fault.