High throughput exploration of process-property linkages in Al-6061 using instrumented spherical microindentation and microstructurally graded samples

Abstract

Recent spherical nanoindentation protocols have proven robust at capturing the local elastic-plastic response of polycrystalline metal samples at length scales much smaller than the grain size. In this work, we extend these protocols to length scales that include multiple grains to recover microindentation stress-strain curves. These new protocols are first established in this paper and then demonstrated for Al-6061 by comparing the measured indentation stress-strain curves with the corresponding measurements from uniaxial tension tests. More specifically, the scaling factors between the uniaxial yield strength and the indentation yield strength was determined to be about 1.9, which is significantly lower than the value of 2.8 used commonly in literature. The reasons for this difference are discussed. Second, the benefits of these new protocols in facilitating high throughput exploration of process-property relationships are demonstrated through a simple case study.

Mechanical characterization of Ti-6Al-4V titanium alloy at multiple length scales using spherical indentation stress-strain measurements

“…In other words, the average of multiple nanoindentation measurements made on randomly selected single grains or phases reasonably represents the bulk indentation strength (i.e., the bulk strength is a homogenized response of the constituents). Recent work has shown that the indentation yield strength using the protocols followed in this work is approximately 2 times higher than the uniaxial yield strength for isotropic materials [20,23]. The average indentation strength of the largest indenter size is approximately 1.9 times the reported tensile strength of 955 MPa for the same material [9], which is remarkably close given that the ratio is likely to be affected by the material anisotropy.…”

“…The protocols described above have been extensively validated in prior work with different indenter sizes on a broad range of material systems [23,25,32,34]. The validation has included comparisons with properties measured using alternate protocols, extensive imaging of the indenter shape, and comparisons between measurements and predictions from finite element simulations.…”

“…Additionally, it was argued that the new definition of indentation strain is (i) a more physically reasonable definition (allows interpretation as change in length over length), (ii) provides a more reasonable average strain of the heterogeneous strain field under the indenter, and (iii) yields a more reasonable plastic indentation stress-strain response. More recently, these protocols were also shown to provide reliable estimations of uniaxial mechanical responses [20,23] and anisotropic elastic constants for cubic crystals [24].…”

“…The protocols of Kalidindi and Pathak [18] are employed in this work, and recent work using these protocols to study the responses of individual grains in titanium [25] as well as microindentation stress-strain curves of polycrystalline volumes in Al-6061 [23] will be leveraged to achieve our goal.…”

High Throughput Assays for Additively Manufactured Ti-Ni Alloys Based on Compositional Gradients and Spherical Indentation

“…The main issues in this correlation are indeed related to the differences in the probe volumes and the inherent anisotropy of material response at the probed length scales. At this time, these correlations are most mature at the two ends of the spectrum of applications: (i) the indented volume is very large compared to the representative volume element of the material microstructure, and allows idealization of the material response in the indentation zone as a homogeneous isotropic medium [13,57] and (ii) the indented volume is a single phase crystalline region (e.g., a region within a grain in a single-phase polycrystalline sample) [7,13,32,58]. Clearly, the indentations in the sample studied here do not fall into either of these idealized conditions.…”