Whole Powder Pattern Modelling: theory and application

In close analogy to the structural refinement provided by the Rietveld method, Whole Powder Pattern Modelling (WPPM) is proposed as a general technique for microstructure refinement. WPPM consists in a simulta- neous modelling of all peak profiles in the diffraction pattern from (single or multi-phase) polycrystalline materials, without using arbitrary profile func- tions. The entire diffraction pattern is modelled directly in terms of physi- cal parameters describing the main microstructural features influencing peak profile width, shape and position, also considering instrumental effects and background. Main parameters are lattice parameters and defect content, expressed in terms of dislocation density, effective outer cut-off radius, dislocation char- acter (edge/screw), contrast factor, twin and deformation fault probabilities, together with mean and variance of a grain size distribution. Different models for lattice distortions and grain size and shape can be tested, to simulate or to model diffraction data from several systems of practical interest, including highly deformed metals and finely dispersed crystalline powders. A further development is envisaged in the direction of the Rietveld method, whose basic algorithm can be easily incorporated in a WPPM ap- proach. In fact, besides the per se utility as an analytical technique for materi- als science studies, theWPPMapproach can improve the quality of structural refinements since it provides a better control of the peak profiles in terms of physically measurable quantities. Besides discussing the theoretical basis of WPPM, the present work illustrates the application to a typical case of study, a series of ball milled Ni powder samples. Results are successfully compared with information obtained from TEM pictures and literature data.