Broadening of diffraction lines is often due to the lattice distortions caused by lattice defects as dislocations. A flexible model has been devised for distorted (poly)crystalline materials in terms of lattice defects, their distributions and associated strain fields. Parameters of the model are the mean and variance of the distances between the defects, and the extent and the variance of the amplitudes of the distortion fields. From the model the diffraction-line profile and its Fourier transform can be calculated directly. The order-dependence of the shape and width of line profiles can be studied as a function of the model parameters mentioned. In contrast with the generally adopted view that the line broadening can be subdivided into order-independent ("size") broadening and order-dependent ("strain") broadening, it can be shown that the order-dependence is not that simple in general. Only if the strain fields around the defects are confined to regions small compared to the distances between the defects and the strains are sufficiently large, the line broadening becomes order-independent. In that extreme case it is allowed to subdivide the specimen into domains that can be considered to scatter incoherently with respect to each other, and, therefore, to speak of "size" broadening.
Line profiles simulated with this model are analysed by applying the Warren-Averbach analysis and the William-Hall analysis which are both frequently applied to analyse X-ray diffraction-line profiles. The values of the "size" and "strain" obtained are used to investigate the adequacy of these analyses. It is shown that only in limiting cases the results of these analyses can be related directly to the microstructure. Experimental line profiles taken from ball-milled tungsten powder are used to show that the line-profile simulations on the basis of the strain-field model pertain to realistic situations. It is concluded that, in principle, an interpretation of measured line broadening is possible directly in terms of the strain-field parameters used in the model.