The Woodruff School

SUMMARY

The underlying mechanisms of nanoscale plasticity could be governed by the phase transformation or nucleation-controlled plasticity. We study the plasticity mechanisms of martensitic phase transformation in NiTi shape memory alloys. NiTi usually exhibits various metastable phases. The formation of a variety of twin structures further complicates the study. The research here involves four thrusts focusing on different length and time scales: (I) Molecular statics and dynamics simulations are applied to study the nanotwin structures and temperature-driven B2 → B19′ phase transitions. (II) Molecular dynamics simulations are performed to explore the stress-driven martensitic phase transformation governing the pseudoelasticity and shape memory effects in NiTi nanopillars. (III) Monte Carlo simulations are conducted to characterize the temperature- driven B2 → B19 phase transition and the patterning of martensitic nanotwins in NiTi thin films. (IV) Phase field simulations are performed to predict the formation and evolution of complex martensitic microstructures, including the detailed analysis of twin compatibility under complex loading conditions. The above results not only provide new insights into the nanoscale martensitic phase transformation in NiTi, but also provide an effective modeling framework for studying the diffusionless phase transformation in large systems with atomic resolution. We also study the nucleation-controlled plastic deformation in metals. Our work focuses on understanding how dislocations and twins nucleate in single crystals. The research involves two thrusts: (I) Interatomic potential finite element method is applied to determine when, where and how dislocations nucleate during nanoindentation in metals such as Cu, Al and Ni. We explore the effects of indentation orientation on the characteristics of activated dislocation sources. (II) Atomistic reaction pathway modeling is performed to study the competition between dislocation nucleation and twinning in nanopillars under different strain rates, temperatures and loading modes. Results provide insight into the nanoscale mechanisms of plastic yielding, and are useful for guiding the nanomechanical experiments in the future.

SUBJECT:	Ph.D. Dissertation Defense

BY:	Yuan Zhong

TIME:	Monday, August 20, 2012, 3:30 p.m.

PLACE:	MARC Building, 114

TITLE:	Nanomechanics of plasticity in ultra-strength metals and shape memory alloys

COMMITTEE:	Dr. Ting Zhu, Chair (ME) Dr. Kenneth Gall (MSE) Dr. David McDowell (ME) Dr. Olivier Pierron (ME) Dr. Arash Yavari (CEE)

SUMMARY The underlying mechanisms of nanoscale plasticity could be governed by the phase transformation or nucleation-controlled plasticity. We study the plasticity mechanisms of martensitic phase transformation in NiTi shape memory alloys. NiTi usually exhibits various metastable phases. The formation of a variety of twin structures further complicates the study. The research here involves four thrusts focusing on different length and time scales: (I) Molecular statics and dynamics simulations are applied to study the nanotwin structures and temperature-driven B2 → B19′ phase transitions. (II) Molecular dynamics simulations are performed to explore the stress-driven martensitic phase transformation governing the pseudoelasticity and shape memory effects in NiTi nanopillars. (III) Monte Carlo simulations are conducted to characterize the temperature- driven B2 → B19 phase transition and the patterning of martensitic nanotwins in NiTi thin films. (IV) Phase field simulations are performed to predict the formation and evolution of complex martensitic microstructures, including the detailed analysis of twin compatibility under complex loading conditions. The above results not only provide new insights into the nanoscale martensitic phase transformation in NiTi, but also provide an effective modeling framework for studying the diffusionless phase transformation in large systems with atomic resolution. We also study the nucleation-controlled plastic deformation in metals. Our work focuses on understanding how dislocations and twins nucleate in single crystals. The research involves two thrusts: (I) Interatomic potential finite element method is applied to determine when, where and how dislocations nucleate during nanoindentation in metals such as Cu, Al and Ni. We explore the effects of indentation orientation on the characteristics of activated dislocation sources. (II) Atomistic reaction pathway modeling is performed to study the competition between dislocation nucleation and twinning in nanopillars under different strain rates, temperatures and loading modes. Results provide insight into the nanoscale mechanisms of plastic yielding, and are useful for guiding the nanomechanical experiments in the future.