SUBJECT: Ph.D. Dissertation Defense
BY: Saurabh Gupta
TIME: Thursday, January 17, 2019, 2:00 p.m.
PLACE: MRDC Building, 4211
TITLE: Characterization of Plastic Deformation Mechanisms in Ultrafine Grained FCC Metals using MEMS based Nanomechanical Testing Methods
COMMITTEE: Dr. Olivier Pierron, Chair (ME)
Dr. Ting Zhu (ME)
Dr. Richard Neu (ME)
Dr. Josh Kacher (MSE)
Dr. Brad Boyce (Sandia National Labs)


MEMS-based nanomechanical testing methods were developed and used to characterize mechanisms of plasticity in ultrafine-grained FCC metals. Advances were made to an existing in-situ TEM nanomechanical tensile testing technique which uses a MEMS device that integrates a thermal actuator and two capacitive sensors to load and measure the uniaxial stress-strain response of a sample, respectively. Several characterization tools such as SEM along with Finite Element and analytical models were used to characterize the accuracy of measurements made with the MEMS device. Three significant improvements were made to the existing technique: (1) Electronic sensing scheme was modified to obtain independent measurements from the two capacitive sensors compared to the old scheme of measuring only the difference between the capacitive sensor readings. (2) The elastic compliance due to the epoxy used for clamping the microspecimens was measured and its viscoplastic behavior was characterized. Finite Element and analytical models along in-situ SEM characterization were used to establish that the plastic strain in the microspecimen can be accurately measured although there were compliance issues with obtaining elastic strains. (3) LabView code was developed to filter the electronic signals to maintain a low noise-to-signal ratio required for performing transient mechanical tests. Finally, issues related to drift in the TEM imaging while performing in-situ TEM experiments were mitigated. This in-house MEMS technique was found to be more reliable for transient mechanical testing compared to the commercial Push-to-Pull technique by Hysitron Inc. owing to the dynamic sensitivity of the capacitive sensors compared to the physical contact based load sensing scheme in the PTP. This enhanced MEMS technique was used to perform uniaxial tests including transient mechanical tests such as repeated stress relaxations on UFG 100-nm-thick Au and 200-nm-thick Al microspecimens inside the TEM. For both microspecimens the true activation volume was obtained between 5-10 b^3. However, GB dominated dislocation plasticity was observed in the Au microspecimens however grain growth along with grain rotation was observed in the Al microspecimens. The environment (vacuum vs air) was seen to have no effect on the activation volumes. Attempts were made to correlate the TEM observations to the signature parameters. This study has provided new insights towards our understanding of the mechanisms of plastic deformation in ultrafine-grained metals. The development of this quantitative MEMS based in-situ TEM tensile technique is a major step towards integrating our understanding from computer simulations, which have become powerful enough to simulate volumes as large as a cubic micrometer, to the in-situ TEM observations directly.