Influences of Granular Constraints and Surface Effects on the Heterogeneity of Elastic, Superelastic, and Plastic Responses of Polycrystalline Shape Memory Alloys

TL;DR

This study combines experiments and simulations to investigate how microstructural constraints and surface effects influence the heterogeneous elastic, superelastic, and plastic responses of polycrystalline shape memory alloys, revealing the dominant role of grain boundary constraints.

Contribution

It introduces a combined experimental and modeling approach to quantify the effects of microstructural constraints on deformation heterogeneity in SMAs.

Findings

Interior grains carry more stress and deform less than surface grains.

Regions near grain boundaries show larger stress variations.

Intragranular heterogeneity is mainly driven by neighboring grain constraints.

Abstract

Deformation heterogeneities within microstructures of polycrystalline shape memory alloys (SMAs) during superelastic stressing are studied using both experiments and simulations. In situ X-ray diffraction, specifically the far-field high energy diffraction microscopy (ff-HEDM) technique was used to non-destructively measure the grain-averaged statistics of position, crystal orientation, elastic strain tensor and volume for hundreds of austenite grains in a superelastically loaded nickel-titanium (NiTi) SMA. This experimental data were also used to create a synthetic microstructure within a finite element model. The development of intragranular stresses were then simulated during tensile loading of the model using anisotropic elasticity. Driving forces for phase transformation and slip were calculated from these stresses. The grain-average responses of individual austenite crystals examined before and after multiple stress-induced transformation events showed that grains at the specimen interior carry more stress and plastically deform less than the surface grains as the superelastic response shakes down. Examination of the heterogeneity within individual grains showed that regions near grain boundaries exhibit larger stress variation compared to the grain interiors. This intragranular heterogeneity is more strongly driven by the constraints of neighboring grains than the initial stress state and orientation of the individual grains.