b'Figure 4. Ph.D. student Caleb Clement and PI Janelle Wharry working on the FEI Helios dual-beam focused ion beam (FIB) /scanning electron microscopy (SEM) at PurdueWe have carried out TEM in situappears to occur at a lower stress tensile testing on unirradiatedthan in the unirradiated material. C37M on the three primary grainThis appearance of irradiation orientations: [100], [110], and [111].softening is inconsistent with the Two tensile tests were conductedirradiated microstructure evolu-from [100], two from [110], and onetion and is merely an artifact of from [111]. The TEM in situ testsspecimen size effects.show clear evidence of dislocationYield stress, ductility, and elastic slip in all three grain orientations.modulus values appear size-effected However, in the [111] test, evenin the tested specimens. However, though dislocation slip appears toa notable difference in yield stress occur, the slip does not ultimatelyis observed across the two grain cause failure of the tensile bar.orientations, again consistent with Instead, at a stress ~3.7 GPa, nano- models. The strain hardening coef-twin needles appear to form in theficient is determined to be 0.48 in specimen. With further strain, a[100] grains, and 0.68 in [110] grains. twin nucleates then widens. TheThese values are also consistent tensile bar ultimately fractureswith our deformation mechanism in the larger twin. We have alsoobservations, indicative that slip carried out TEM in situ tensileoccurs more readily in the [110] loading on the ion irradiated C37Mgrains. The stress-strain curve of the on [100], [110], and [111] grains. Allion irradiated TEM in situ tensile of these tensile tests have shownbar on the [111] grain orientation clear evidence of dislocation slip,shows considerable ductility, but a with no evidence of deformationsignificant number of load drops, twinning in the irradiated C37M.each of which correspond to clear Surprisingly, however, plastic yieldevidence of dislocation-mediated 2022|AFC ACCOMPLISHMENTS 37'