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Rapid Evaluation of Cyclic Performance Using Small-Volume Metal Samples Through Spherical Indentation

Committee Members:

Surya R. Kalidindi, MSE/ME/CSE (advisor)

David McDowell, MSE/ME

Richard Neu, MSE/ME

Aaron Stebner, MSE/ME

Reji John, Air Force Research Lab

Rapid Evaluation of Cyclic Performance Using Small-Volume Metal Samples Through Spherical Indentation

Abstract: 

Cyclic testing allows the collection of basic mechanical information needed to assess a material’s response in cyclic loading and is essential to understanding a material's in-service behavior. Traditionally, this information has been collected through standardized cyclic tension-compression tests; however, conventional testing is costly both in terms of time and money. The conventional testing methods also require large sample volumes, which may not always be available. These drawbacks become particularly evident in materials innovation efforts. This is because of the extremely high number of combinations of material compositions and thermo-mechanical processing histories evaluated in such efforts. More specifically, the material candidate pool is practically endless; thus, evaluations through conventional testing are infeasible.

This thesis develops and implements high-throughput indentation protocols which require small sample volumes to rapidly characterize the cyclic behaviors of metals through extraction of intrinsic material properties. First, novel spherical microindentation protocols are developed on wrought Ti-6Al-4V (Ti64) samples. In this study, repeatable load-displacement loops are successfully converted into hysteresis stress-strain loops. Hysteresis energy density plots allowed the indentation stress at which significant plastic deformation began to be pinpointed. Peak indentation stress and strain values are used to create a cyclic stress-strain curve which showed great agreement to that from conventional uniaxial cyclic tests. This study established the validity of the protocols for isotropic and rather homogeneous materials. Second, the cyclic spherical microindentation protocols were extended to the more complex microstructures of laser powder bed fusion processed Inconel 718 (IN718) samples. Such a study captured changes in the cyclic response as a function of post-processing heat treatments and anisotropy of the as-printed samples. In the two studies described above, observations between cyclic microindentation and cyclic uniaxial stress tests are possible due to the extraction of indentation samples from the same sample block material as those from the conventional cyclic tests. Finally, cyclic spherical nanoindentation protocols are developed and demonstrated on titanium alloy Ti-6Al-2Sn-4Zr-2Mo (Ti6242) which has a bimodal primary-alpha and basketweave microstructure. These cyclic nanoindentation protocols evaluated the cyclic behavior as a function of grain orientation of the primary-alpha phase and morphology between the globular primary-alpha and dual-phase alpha-beta basketweave grains. Due to the small length scales of this study, unique features tied to dislocation motion were captured in the load-displacement and stress-strain loops. Outcomes from this work contribute to material design rules and provide reliable mechanical data useful for refining crystal plasticity models. These case studies critically evaluate the effectiveness of the newly developed high-throughput cyclic response screening methods. It is seen that the protocols have a large applicability across multiple materials and length scales and contribute to expediting materials design, innovation, and characterization of advanced materials.