Hall-Petch Relation and Twin Boundaries in Pure Copper and Cu-Al Alloys

Abstract

The present work has investigated whether the twin boundaries to be accounted or not in the Hall-Petch equation for polycrystalline metals. Pure copper, Cu-3.5at%Al, Cu-6.8at%Al and Cu-14.8at%Al alloys, which have the average grain sizes from 8.4μm to 176.0μm and different ratio of annealing twins, were produced and pulled in tension at the temperatures from 77K to 973K under a strain rate of 1.2 × 10-4s-1. The distribution of dislocations in the surface grains was also observed by using the etch pit technique. The 0.1% proof stress including twins for each specimen at room temperature well obeys to the Hall-Petch relation. The halves of frictional stress σ0 derived from the straight lines are approximately equal to the critical resolved shear stress of each single crystal, i. e. CRSS, so far reported. It is found that the proportional relation between the σ0/2 value and the square root of aluminum concentration is in good agreement for the single crystal and polycrystalline specimens. Temperature dependence on the σ0/2 values for Cu-14.8at%Al alloys is similar to the variations for CRSS of single crystal, especially at low and elevated temperature regions. The shear stress for unlocking of edge dislocations from solute atoms estimated from the Hall-Petch parameters at 293K is found to be 3.5 times larger than CRSS of single crystals, which is thought to be the unlocking stress of screw dislocations. It is considered that the role of twin boundaries is almost equivalent to the grain boundaries, because the multiplicated dislocations pile up against the twin and grain boundaries in the small deformation.