## 2 Materials and Methods
### Microindentation Stress-Strain (MSS) Protocols  
Samples for MSS were prepared using standard metallography protocols. Each machined MSS sample was mounted using a cold mounting epoxy process. The cold mounting epoxy process involved mixing 4 parts Buehler EpoxiCure 2 Epoxy Resin with 1 part Buehler EpoxyCure 2 Epoxy Hardener. Water lubricated grinding was initiated with 800-grit sandpaper and progressed through 1000-, 1200-, 2400-, and 4000-grit sandpapers prior to polishing. After grinding, the samples were subsequently polished on a 3 \(\upmu\)m and 1 \(\upmu\)m polishing pad using appropriate diamond pastes and alcohol-based lubricant. Next, a polishing pad with 0.04 \(\upmu\)m colloidal silica polishing suspension was applied. Polishing was completed by vibropolishing the samples for 24 h using a colloidal silica suspension at 30 RPM. Between each polishing step, the samples were rinsed with soapy water and ethanol to remove polishing remnants from the previous step. Figure 1: The cut plan used to extract small punch test (SPT) and microindentation stress-strain (MSS) samples from the ends of the tensile bars after uniaxial tensile test (UTT). MSS testing was carried out using a Zwick-Roell ZHU2.5 Zwick Hardness Tester. A Brinell spherical indenter tip with a 6.35 mm radius was utilized and produced a residual indentation radius of approximately 180 \(\upmu\)m depending on the sample. Sample grain sizes varied between 30 \(\upmu\)m and 50 \(\upmu\)m, so the residual indent covered no less than 10 grains. An array of 6 to 12 indentations was performed while maintaining a 2 mm distance between the indentation center points and from the edge of the sample. This ensured a center-to-center distance between indentations of approximately 10 times the residual indent radius; this exceeded the separation requirements described in ASTM-E10-12 to prevent interactions between the indentations or with the edge of the sample. ASTM-E10-12 [23] states that a minimum center-to-center spacing of six times the residual indentation diameter shall be used. [PERSON] et al. [19] followed this criterion for the separation distance between indentations to produce results from their indentation experiments performed on aluminum alloys that were consistent with tensile stress-strain data. The averaged measured response from the multiple MSS tests on a sample are assumed to reflect the bulk response of the material. Figure 2 depicts schematically the indenter-sample geometry under load and provides a visual representation of variables discussed in the following paragraphs. A typical measured load-displacement curve produced using the MSS protocols is shown in Figure 3a. Figure 2: Indentation schematic showing the variables involved in indentation stress-strain protocols. The indentation stress definition is based on the load applied over the projected area of contact, \(\pi a^{2}\), where \(a\) is the contact radius. The indentation strain definition is based on the dimensions of the primary indentation zone, which describes a cylindrical domain of height \(2.4a\), and radius \(a\) being compressed by the total indenter displacement, \(h_{t}\) which is the sum of the sample elastic displacement, \(h_{e}\), and the residual displacement, \(h_{r}\). As mentioned earlier, each unloading segment (indicated in Figure 3a) was analyzed to produce one datapoint on the indentation stress-strain curve. Only a small number of unloading segments are shown in Figure 3a for clarity. In the results presented in this work, ~20-25 unloading segments were utilized in the extraction of each indentation stress-strain curve. The elastic loading and all of the unloading segments were subsequently analyzed using [PERSON]'s theory for frictionless elastic contact between two quadratic surfaces. [PERSON] et al. [14] demonstrated and validated this method by testing both aluminum and tungsten samples and directly comparing the results to those obtained using a continuous stiffness measurement.