I found shift of some my peaks by o.5 to 1 2theta value in my bulk sample. The sample is prepared by cold crucible casting process and is textured. The structure is martensite and definitely has residual stress due to rapid quenching. Sample was annealed for long tie at low T, but still peak shift is apparent. Is it due to tensile strain? If so, what is the explanation of strain effect after long hour annealing?
I suspect the peak shift you have observed is unlikely to be strain, since you have limited depth of penetration, and Bragg-Brentano optics suggests your diffraction vector is normal to the surface (i.e., the near-surface strains in this direction should be low). I may be mistaken though.
Possibly changes in composition (that is my bet), or even alignment/sample displacement errors? How flat is your sample surface, and how accurately are you able to place it on the focussing circle?
The presence of internal stresses causes a shift of a diffraction peak. A bigger shift of 2theta(hkl) is observed for higher 2theta scattering angle.The sensitivity of the method can be derived from Bragg law. With 2theta < 90°, the shift is very low. Instrumental errors can be seen with a stress analysis with a residual stress free reference powders. The temperature used for the thermal treatment may be too low to 'remove' residual stresses and should be reviewed.
For near-surface - the stress perpendicular to the surface is zero. The strain in this same direction (which is what you can measure with B-B x-rays) is indeed non-zero, deriving from a Poisson effect of the non-zero in-plane stresses. But this is usually pretty small, and what you nearly always want is the in-plane stresses/strains, which are larger. These can be found by tilting - e.g., the sin^2 psi method, which is described in pretty much any basic XRD text. Be careful as this technique is very sensitive to alignment errors, and to sample surface preparation (if any).
The peak shift is said to be large, but large with respect to what?
My sample size: 2 mm*0.1mm wire/cylinder.
@Edward: My sample is quite flat, I polish it before XRD. Peak shift is large with respect to general idea of shift. I may be wrong, but I suspect may be some residual stress can cause the shift.
Residual stresses certainly can do this (even the stresses from polishing). I am just cautioning you that there are a lot of geometric errors that can fool you, even when you are careful.
- Badges
- Science method
- Similar topics
- Methodology
- Crystallography
- X-ray Diffraction