4.25 Consider an aluminum polycrystal with a grain size of 10 µm. If a dislocation source at the center of a grain emits dislocations under an applied shear stress of 50 MPa that pile up at the grain boundaries, what is the stress experienced by a grain boundary? Take G = 26 GPa and b = 0.3 nm.

4.26 (a) Iron (r = 0.124 nm, G = 70 GPa) is being deformed to a shear strain of 0.3. Assuming a constant dislocation density equal to 1010 cm-2, what is the average distance each dislocation has to move?