Tham khảo tài liệu 'modern physical metallurgy and materials engineering part 6', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | The characterization of materials 141 where t is the effective particle size. In practice this size is the region over which there is coherent diffraction and is usually defined by boundaries such as dislocation walls. It is possible to separate the two effects by plotting the experimentally measured broadening p cos 0 Ằ against sin Ớ À when the intercept gives a measure of t and the slope . Small-angle scattering The scattering of intensity into the low-angle region e 20 10 arises from the presence of inhomogeneities within the material being examined such as small clusters of solute atoms where these inhomogeneities have dimensions only 10 to 100 times the wavelength of the incident radiation. The origin of the scattering can be attributed to the differences in electron density between the heterogeneous regions and the surrounding matrix 1 so that precipitated particles afford the most common source of scattering other heterogeneities such as dislocations vacancies and cavities must also give rise to some small-angle scattering but the intensity of the scattered beam will be much weaker than this from precipitated particles. The experimental arrangement suitable for this type of study is shown in Figure . Interpretation of much of the small-angle scatter data is based on the approximate formula derived by Guinier I Mn2Ie exp -4 2e2R2 3À2 where M is the number of scattering aggregates or particles in the sample n represents the difference in number of electrons between the particle and an equal volume of the surrounding matrix R is the radius of gyration of the particle Ie is the intensity scattered by an electron e is the angle of scattering and À is the wavelength of X-rays. From this equation it can be seen that the intensity of small-angle scattering is zero if the inhomogeneity or cluster has an electron density equivalent to that of the surrounding matrix even if it has quite different crystal structure. On a plot of log101 as a function of
