Characterization of plastic deformation mechanisms in ultrafine grained FCC metals using MEMS based nanomechanical testing methods

Abstract

A MEMS-based in-situ TEM nanomechanical testing technique was developed to characterize mechanisms of plastic deformation in ultrafine-grained FCC metals via transient mechanical tests. 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 models were used to rationalize and correct the stress-strain curves obtained with the MEMS device. This MEMS device was used to measure the signature parameters of rate-controlling mechanisms of plastic deformation, like true activation volume, of ultrafine-grained FCC microspecimens and the reliability of the measurements was quantified. In a separate study, MEMS-based microresonators were used to study the effects of an 850-nm-thick Au coating on very high cycle fatigue behavior of Ni microbeams under extreme stress gradients. FIB, SEM and EDS techniques were used to characterize cracking in the microbeams and models were developed to rationalize the observations. These studies have provided new insights towards our understanding of the mechanisms of plastic deformation in ultrafine-grained metals.