The bauschinger effect, surface and interior stresses in cyclically strained 70-30 alpha brass single crystals

Abstract

Single crystals of 70-30 alpha brass, oriented for easy glide, were cyclically strained under total strain control Δε 1 2 = 0.15% . The peak stress, σ p, and the Bauschinger stress, σ B , defined as the 0.00005 strain offset stress in the reverse direction, were determined as the number of cycles, N, increased. σ p increased and σ B decreased with increasing N.This was explained by assuming that the surface flow stress, σ s , was lower than the interior flow stress, σ l . This difference in flow stress would produce residual surface and interior stresses on unloading to zero applied stress. These residual stresses increased during cycling because σ l increased more rapidly than σ s. These assumptions were tested by longitudinally electropolishing half the gage length and observing the bending, due to residual stresses, while the specimen was in the top grip. Bending was independent of where along the gage length polishing took place. The direction of bending agreed with the assumption of a lower surface flow stress. From the magnitude of the deflection, the elastic modulus and the volume fraction of surface and interior material, it was possible to calculate the residual stresses on the surface and the interior and the flow stress of the surface and interior. As dectropolishing of the entire gage length took place σ p decreased and σ B increased. No further changes occurred, when about 0.35 mm was removed from the diameter, and the values of σ p and σ B approached the values of the virgin single crystal after one cycle. The changes in σ p and σ B with surface removal along the entire gage length suggested that the surface was harder than the interior. To rationalize the two apparently contradictory behaviors it was assumed that the surface consisted of three different dislocation density areas: a high surface dislocation density region, followed by a larger low dislocation density region and at 150–175 μm below the surface the third region, a narrow highest dislocation density volume. This region would act as a barrier for dislocations to move out of or into the crystal. The entire surface region was assumed to have a lower average dislocation density than the interior, thus producing the lower flow stress in the surface region. Qualitative TEM observations revealed a higher dislocation density in the interior than in the surface. The presence of pile-ups near the surface was taken as partial support of the postulated high dislocation density region at 150–170 μm below the surface.