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

Abstract

The present work has investigated whether the twin boundaries are to be included 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 8.4 to 176.0μm and the different ratio of annealing twins, were produced and pulled in tension at temperatures from 77 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. 0.1% proof stress including the twins for each specimen at room temperature conforms well to the Hall-Petch relation. The halves of intersecting 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 crystals and the polycrystalline specimens. The temperature dependence on the σ0/2 values for Cu-14.8at%Al alloys is similar to the variations for the CRSS of single crystals, especially at the region of low and elevated temperatures. From the Hall-Petch parameters at 293K, the unlocking stress of edge dislocations from the solute atoms is found to be 3.5 times larger than the CRSS of single crystals. This CRSS is thought to be the unlocking stress of screw dislocations which come out onto the crystal surface. It is considered that the role of the twin boundaries is almost equivalent to the grain boundaries, because the multiplicative dislocations pile up against the twin and grain boundaries during the small deformation.