X-ray diffraction (XRD) is a powerful technique for studying the structural properties of metals and alloys. Crystallite size, microstrain and dislocation density can be estimated from the XRD plot (I Vs 2θ). Crystallite size is a key parameter affecting the chemical and physical properties of strain free materials. Scherrer equation is widely used to estimate the crystallite size from XRD data.
When X-ray interact with crystalline material, diffraction occurs as per Bragg’s law:
nλ=2dsinθ
Here, n is the order of diffraction, λ is the wavelength of the X-ray, d is the distance between the crystal planes and θ is the angle of incidence.

For polycrystalline materials, both small crystallite size and microstrain contribute to the broadening of X-ray diffraction peaks. The crystallite size (D) can be estimated from the peak broadening using the Scherrer equation, after correcting for instrumental broadening:
D=Kλ / (βCosθ)

Here, K is the shape factor (0.9 for spherical particles), λ is the wavelength of the X-ray, β is the full-width half maxima (FWHM) of the diffraction peak in radians and θ is the Bragg angle.