Strain relaxation processes have been investigated in chemically ordered FePd thin layers grown on Pd(001). Microtwins due to the pileup of a/6 <211> dislocations release most of the relaxation. A statistical analysis of scanning tunneling microscopy images provides a detailed understanding of the involved processes and indicates a linear dependence of the relaxation process upon the FePd thickness. Such an unusual behavior is confirmed by reflection high-energy electron diffraction and x-ray diffraction measurements. Through a modeling of the repulsion between the cores of the partial dislocations forming the microtwin, we put forward the mechanism explaining this discrepancy with the Matthews law.