Analysis of the interpretation of yielding and strengthening behavior in small-size samples

In recent literature related to mechanical testing of small-volume metal specimens, plastic strain bursts during apparent elastic loading have been reported for materials commonly known to exhibit smooth yielding. Interpretation of the observed plastic yielding effects in these tests have ignored a significant part of the actual experimental findings, and produced conclusions regarding dislocation structure that these tests do not unequivocally support. It is pointed out that the heterogeneity of dislocation debris left in the microstructure does not clearly represent deformation resistance, nor give a clear indication of strain within the specimen. The measured high rate of strain hardening and stability of plastic flow in the small-volume specimens are dependent on stress-state, and are shown to be not correlated with dislocation debris or dislocation-starvation concepts; rather the reason for the observed strengthening is related to the resistance to slip propagation through the specimen surface to form surface steps, viewed as an atomic-scale shear fracture process, elaborated in a companion paper. In a compression test the surface is closed under high compressive stress, thus repeatedly blocking the process, but in a tension test the surface separates more easily, causing loss of plastic stability. (C) 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.