Characterizing heterogeneous intragranular deformations in polycrystalline solids using diffraction-based and mechanics-based metrics


Abstract

Deformations within and among crystals have been observed to be heterogeneous for most structural alloys whether the alloys are subjected to monotonic or cyclic loading. Over a material’s loading history, these intragrain deformations influence how failure mechanisms activate. A series of finite element simulations were conducted for a completely reversed loading cycle applied to a precipitation hardened copper alloy. The simulations were conducted using different hardening assumptions within a single crystal, elasto-viscoplastic constitutive model. The results were used to develop several intragrain heterogeneity metrics applicable to both measured and computed data. The computed metrics are shown to correlate strongly with the corresponding values derived from x-ray diffraction experiments. The intragrain heterogeneity metrics provide an effective tool to quantitatively measure and compare the influence of different constitutive models under the same loading conditions. This is demonstrated for the differences in deformation heterogeneity between isotropic and anisotropic hardening assumptions under cyclic loading.

Citation/Link Info

https://doi.org/10.1088/1361-651X/aa6dc5

Citation: Robert Carson et al 2017 Modelling Simul. Mater. Sci. Eng. 25 055008

Motivation

This work was strongly by a need to better understand how intragrain deformation evolves while cyclic loading is occurring. This is no easy task as seen in the below figure where we can see how heterogenous the stress state can be in not only the aggregate but also a grain. Nonetheless through a better understanding of how deformation evolves in the body, we can begin to better understand the mechanisms that lead to crack and void formation in a metal that ultimately lead to fatigue failure. However, the road to understanding these mechanisms requires a combined effort from both experimentalist and modelers.

Heterogeneous stress states
Heterogeneous stress states

My group uses a combined high energy x-ray diffraction (HEXD) experiments in conjunction with single crystal elasto-viscoplastic finite element simulations. The HEXD experiments allow us a nondestructive way to see how grain average lattice strains/stresses and lattice orientations evolve as a sample is being loaded. They also provide information about the spread of strain/stress and misorientation in a grain. However, current technology really limits our ability to obtain spatial intragrain information. This is where simulations come into play. They allow us to obtain a wealth of data about the system that we can then use to infer what the behaviors seen occurring in the experiments mean. Simulations do have limitations though due to the very nature in which the models they are based upon were created. These models often contain several assumptions about the physical system that might not be true. Therefore, the experiments can help guide us to creating better models if there was a simple way to link the results of the two. My work helps to overcome this barrier by finding and creating kinematic metrics that strongly correlate with new diffraction based metrics.

Awards

This article was chosen as the July 2017 cover article for MSMSE

Cover Article for MSMSE July 2017
Cover Article for MSMSE July 2017