In one literature Warren suggested to evaluate strain from Warren-Averbach plots or Williamson-Hall plots by 200 and 400 reflections. Do we require to average all the planar strains or to calculate values for parallel planes?
you should adapt your topic section to material science, stress, strain, materials characterization, steel, strain analysis etc... I am not sure what you actually want to measure. Strain is not equal to strain...
Williamson-Hall Plot and Warren-Averbach method are widely used methods for separating the effects of size and strain broadening of powder diffraction peak.
You can find details about these methods in the following literature:
http://www.sciencedirect.com/science/article/pii/S1293255810004607
http://www.jtaphys.com/content/6/1/6
http://pd.chem.ucl.ac.uk/pdnn/peaks/sizedet.htm
For Scherrer equation, crystallite size was calculated based on the measurement of a(hkl) peak using the following equation:
L =Kλ / Bsize cos θ
where L is crystallite size, K is a dimensionless shape factor (0.9), Bsize is line broadening at half of the maximum intensity (FWHM) in radian, λ is the X-ray wavelength for example for Cu Kα radiation (1.5406 Å) and θ is Bragg angle in degree.
Meanwhile, Williamson–Hall plot was used to estimate the crystallite size and lattice strain of the samples using the following formalism:
Btot = Bstrain + Bsize = 4Cε tanθ + Kλ/ L cosθ
where Cɛ is the lattice strain, Βsize is the particle size broadening, Βstrain is the strain broadening, L is the crystallite size, K is a dimensionless shape factor (0.9), λ is the X-ray wavelength for Cu Kα radiation (1.5406 Å) and θ is Bragg angle in degree.
Then Eq. 2 is multiplied by cosθ to yield:
Btot cosθ = 4Cε sinθ + Kλ/L
Hence, by plotting the graph of Βtot cosθ against 4 sinθ, the lattice strain, Cɛ of the sample can be obtained from the slope (gradient) while the crystallite size can be estimated from the intercept (Kλ/L).
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