Relationship between indentation plastic zone size and residual stresses in plastically deformed Fe

Indentation induced plastic zone size was studied using electron channelling contrast imaging in polycrystalline Fe. Nanoindentation tests were carried out in samples that were compressed to different degrees of thickness reduction. Both strain hardening and residual stresses induced by the compression contributed to significant hardness elevations at all levels of thickness reduction. By comparing between the un-deformed and pre-deformed samples, texture and strain hardening effect were found to contribute to the differences between stresses at micro and macro scales. A correction for texture effects applied to average micro-stresses brought their values close to the macro-stresses. For each sample, a linear relationship was found between the cube of the plastic zone size measured and the inverse of the hardness. The slope of the linear trend line was shown to increase with increased compressive macro-stress level as determined by X-ray diffraction (XRD). This newly discovered empirical relationship demonstrated the potential of predicting macro-scale residual stress levels from nanoindentation experiments.