Comparison of Hall–Petch Law with an Elastic Limit Identification Method Using Kinematic Field Measurements

Background

Plastic deformation in polycrystalline metals is highly heterogeneous due to the varied microstructure distribution. Although some traditional laws, such as the Hall–Petch law, describe the relationship between microstructure and yield stress, accurately predicting the initial yield stress (hence elastic limit) related to local plasticity activation remains challenging.

Objective

This study proposes a novel approach to identify local elastic limits using full-field strain measurements, avoiding complex constitutive models.

Methods

Full-field kinematic measurements were performed on the heat-treated polycrystalline 316L austenitic stainless steel. By examining the different mechanical responses during elastic and plastic deformation, the onset of plasticity activation for each grain is identified from its grain-average strain evolution, allowing further calculation of the grain-scale elastic limit.

Results

Strain field observations indicate early strain localizations, particularly at twin boundaries and triple junctions. Based on microstructures segmented by ordinary grain and twin boundaries, considering and not considering twins, two different local elastic limits are identified.

Conclusions

The average elastic limit for the case considering twins is closer to the value obtained from the macroscopic stress–strain curve. In addition, the statistical analysis of the classified grain sizes reveals a more pronounced Hall–Petch relationship when twins are considered. These results indicate the necessity of considering twins in identifying the local mechanical properties.