Micromechanical Tension Testing of Additively Manufactured 17-4 PH Stainless Steel Specimens

1. Sample preparation for photolithography Cut a sample from the area of interest and polish it using a semi-automatic polishing machine. Use a slow dicing saw or a band saw to cut a section of ~6 mm from the area of interest to be studied. For this study, the material was cut from the gage section of an AM 17-4 PH fatigue specimen, as shown in Figure 1. Prepare the cut sample in a metallographic mount for polishing. Use a semi-automatic polisher to polish the sample to mirror-like surface (having a surface roughness on the order of 1 µm) starting from 400 grit abrasive paper and moving to 1 µm diamond particles. To ensure sufficient polish at each abrasion level and uniform surface abrasions, alternate the polishing direction by 90° following each grit level. Maintain a flat surface during polishing to avoid issues during a later spin coating process. Section the material into a thin disk. Protect the polished surface using an adhesive tape. Use a slow speed saw to align and cut a thin section (0.5-1 mm). ​NOTE: An even section will be important for the spin coating process. 2. Photolithography Clean the sample. Remove the protective adhesive tape from the polished surface and place the sample with the polished surface facing up in a beaker with acetone. Use an ultrasonic cleaner to clean the sample for 5 min. Use enough acetone to cover the sample. Remove the sample from the acetone and dry it using compressed air. Submerge the sample in isopropanol and use an ultrasonic cleaner to clean the sample for 5 min. Use enough isopropanol to cover the sample. Remove the sample from the container with isopropanol and dry the sample with compressed air. Place the sample in a holding container and perform an oxygen plasma cleaning for 1 min. Prepare the photoresist solution in advance. Using a mixer, mix 27.2 g (50 wt%) of liquid PGMEA and 25.1 g (50 wt%) of SU-8 3025 for 2 min. De-foam the mixture for 1 min. Perform the photo-resist patterning. Place the sample (polished side up) on the spin-coater. Use compressed air to remove any dust or particle on the surface of the sample. Apply photoresist on the sample and run the spin-coater using the parameters shown in Table 1. NOTE: The thickness of the resulting SU-8 photoresist used in this study was measured to be near 1.5 µm on average. Place the sample on a hot plate and heat at 65 °C for 5 min. Heat the sample at 95 °C for 10 min. Remove the sample from the hot plate and allow the sample to cool to room temperature. Using a photomask with an array of squares measuring 70 µm on each side, expose the sample for 10-15 s at a power density of ~75 mJ/cm2. Heat the sample to 65 °C for 5 min on a hotplate. Heat the sample to 95 °C for 10 min on a hotplate and then let the sample to cool to room temperature before continuing to the next step. Submerge the sample (with the pattern facing up) in a clean container with propylene glycol monomethyl ether acetate (PGMEA) and agitate it for 10 min. Use enough PGMEA to cover the sample. Remove the sample and splash with isopropanol before carefully drying with compressed air. ​NOTE: Figure 2 shows the final result of a patterned SU-8 on the sample. In Figure 2, there are locations on the steel surface having no photoresist (note the bottom-left specimen surface) likely due to uneven surface affecting the spin coat. For the purpose of this study (creating local micro-tensile specimens), it is considered a satisfactory pattern. 3. Wet-etching Prepare the AM 17-4PH stainless steel aqueous etchant9 shown in Table 2. Inside of a fume hood, place the sample in a beaker and place it on top of a hotplate at ~65-70 °C. Leave the sample on the hot plate for 5 min. With the sample on the hot plate, place a few drops of the prepared etchant so that the patterned surface is completely covered. Leave the etchant for 5 min. Remove the sample from the beaker and neutralize the etchant with water. ​NOTE: Figure 3 shows the resulting sample after etching. Note in Figure 3 that the remaining photoresist prevents the etchant from reacting the steel surface, creating localized platform areas of unremoved material. 4. Focused Ion Beam milling of specimen geometry Prepare the sample for the FIB-milling process. Place the sample in a container with isopropanol. Use an ultrasonic cleaner to clean the sample for 5 min. Use enough isopropanol to cover the sample. Remove and dry the sample with compressed air. Using a conductive adhesive, mount the sample on a stub compatible with the nanoindentation device to be used during later testing. Drill a hole in a 45° SEM mounting stub and use a carbon tape to place the indenter stub and specimen on a 45° SEM stub, as shown in Figure 4. NOTE: This step is intended to reduce direct contact with the sample once the micro tensile specimen is fabricated, decreasing the chance of damaging the sample. Place the sample in an SEM and identify an etched square to perform the FIB milling. NOTE: For this study, remaining material squares ~9 µm in height or larger were desired due to the chosen specimen geometry. Orient the chosen FIB location at the top of the SEM stub to avoid contact issues during alignment in the SEM. Perform FIB milling. NOTE: A SEM operated at 30 kV was used in this study. Although a specific procedure cannot be outlined, as it requires adjustment based on specific equipment, milling from outside to inside is a good practice to avoid material re-deposition within the specimen location. Additionally, it is good practice to use maximum energy to remove bulk material, but reduce the FIB energy while approaching the final specimen dimensions. Use the maximum power (20 mA, 30 kV) to remove any undesired bulk material from the remaining etched platform as shown in Figure 5. Use lower power (7 mA, 30 kV) or (5 mA, 30 kV) to make a rectangle with slightly larger dimensions than needed for the final specimen geometry (see Figure 6). With even lower power (1 mA, 30 kV) or (0.5 mA, 30 kV), perform cross section cuts near to the final micro-tensile specimen dimensions. NOTE: Following this FIB step (shown in Figure 7), the sample should have the required outer dimensions but should be missing the dog-bone shape profile. Rotate the sample at 180°. Using low power (0.5 mA, 30 kV) or (0.3 mA, 30 kV), perform the final FIB milling step to create the specimen geometry desired. Create and use bitmap to control the FIB intensity and location for the repeatability in the creation of final geometry for multiple specimens. ​NOTE: Figure 8 shows an SEM image of the resulting micro-tensile specimen fabricated from the steps described in sections 4.2.1 through 4.2.5. Dimensions of the tensile specimen are shown in Figure 9. 5. Grip fabrication Make alignment marks on the nanoindentation tip to be used for tensile testing. Mount the tip on the desired nanoindentation transducer. Using a laser scribe, make two alignment marks near the tip, as shown in Figure 10, to allow for proper tip orientation prior to fabrication of the tensile grip through FIB milling. Use a circular notch and linescribe as two alignment sources as the tip rotates during fabrication of the tensile grip geometry. FIB-mill the nanoindentation tip to make the tension grip. Place the marked tip on a SEM stub and align the markings as shown in Figure 10. Using the FIB, reduce the width of the indenter tip as shown in Figure 11A. NOTE: Reducing the indenter tip width is helpful in the maneuverability and clearance of the final tensile grip during tension testing. Remove the indenter tip from the SEM, use the alignment marks to rotate the tip at 90°. Use the FIB as shown in Figure 11B to reduce the thickness of the indenter tip. Remove the indenter tip from the SEM. Use the alignment marks back to 0° (front view) and create the final tensile grip geometry with the FIB as shown in Figure 11C. To reduce re-deposition of the removed material during the FIB process, remove the narrow tensile grip area before removing the wider grip area. 6. Micro-tensile test Mount the specimen and indenter tip on the nanoindenter device. Install the nanoindentation machine in the SEM following the manufacturer's recommendations. To ensure adequate imaging during in situ testing, avoid significant machine tilt. NOTE: For this test, a tilt of 5° was used. Excessive tilting will result in a perspective view and make it difficult to align the tensile grip with the test sample. To prevent an unexpected event during the tensile testing, perform the desired displacement-based tensile loading protocol in air, away from the sample. NOTE: This air displacement test will preserve the fabricated tensile grip in the event of unexpected displacements during the protocol. With caution, slowly approach the tip to the sample's surface. Move and align the tensile grip with the test sample, as shown in Figure 12. Perform the tensile test. NOTE: The test performed in this study considered a displacement-controlled protocol at a rate of 0.004 µm/s (resulting in an applied strain rate of 0.001 µm/ µm/s for the 4 µm tall specimen), a maximum displacement of 2.5 µm, and a returning rate of 0.050 µm/s. To perform the tensile test in the transducer used for this test, a negative displacement indentation (-2.5 µm) and negative rate (-0.004 µm/s) was used.