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Title:Strain rate and temperature dependent mechanical behavior of nanocrystalline gold
Author(s):Karanjgaokar, Nikhil
Director of Research:Chasiotis, Ioannis
Doctoral Committee Chair(s):Polycarpou, Andreas A.
Doctoral Committee Member(s):Chasiotis, Ioannis; Lambros, John; Geubelle, Philippe H.; Beaudoin, Armand J.
Department / Program:Mechanical Sci & Engineering
Discipline:Mechanical Engineering
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):Thin film
Strain rate
Activation volume
Abstract:Nanocrystalline metal films are candidate materials for microelectronics and Microelectromechanical Systems (MEMS). The long term mechanical stability of metal films requires quantitative understanding of their thermo-mechanical behavior in the large range of operating strain rates and temperatures. This dissertation research studied (a) the role of thermally activated processes based on the strain rate and temperature dependent mechanical behavior of nanocrystalline Au thin films, and (b) deformation processes at nominally elastic loads that lead to creep strain over a moderate temperature range that is relevant to MEMS applications. The rate dependent mechanical behavior of nanocrystalline Au thin films was first investigated at room temperature and at strain rates between 10-6 to 20 s-1. The use of digital image correlation (DIC) facilitated repeatable and accurate measurements of full-field strain from free-standing nanocrystalline Au thin films. The experimental stress-strain curves were used to calculate activation volumes for two film thicknesses (0.85 μm, and 1.75 μm), which were 4.5b3 and 8.1b3, at strain rates smaller than 10-4 s-1 and 12.5b3 and 14.6b3 at strain rates higher than 10-4 s-1. The reduced activation volume and increased strain rate sensitivity at slow strain rates were attributed to grain boundary (GB) diffusional processes that result in creep strain. The room temperature strain rate results were augmented with microscale strain rate experiments at temperatures up to 110˚C. Two methods for heating free-standing microscale thin film specimens, namely uniform heating using a custom-built microheater and resistive (Joule) heating, were evaluated using a combination of full-field strain measurements by optical microscopy and full-field temperature measurements by infrared (IR) thermal imaging. It was shown for the first time that the Joule specimen heating method results in large underestimation of the inelastic material properties by about 50% due to the extended temperature gradient along the specimen gauge section that causes plastic strain localization. On the contrary, the microheater based uniform heating method results in uniform temperature and strain fields during tensile experiments and was more suited for experiments at elevated temperatures. This uniform heating method was applied to annealed Au films with average grain size of 64 nm and for strain rates 10-5 to 10 s-1, and temperatures 298-383 K. Activation volume calculations based on the combined temperature and strain rate experimental results pointed to two rate limiting mechanisms of inelastic deformation: Creep-driven and dislocation-mediated plasticity, with the transition occurring at increasing strain rates for increasing temperatures. The activation volume for the creep-dominated regime increased monotonically from 6.4b3 to 29.5b3 between 298 and 383 K, signifying GB diffusion processes and dislocation-mediated creep, respectively. The trends in the dislocation-mediated plasticity regime followed an abnormally decreasing trend in the activation volume values with temperature, which was explained by a model for thermally activated dislocation depinning. Furthermore, the experimental data allowed us to evaluate the hardening behavior for Au films and model it using a linear hardening law and exponential relationships for the state variable and the saturation stress. The creep response of nanocrystalline Au films with 40 nm grain size was also obtained experimentally in an effort to assess its contribution to the overall mechanical response under uniaxial tension. Unusually high primary creep rates (3.3 × 10-8 to 2.7 × 10-7 s-1) and steady state creep rates (5.5 × 10-9 - 1.1 × 10-8 s-1) were measured with the primary creep regime lasting up to 5-6 hr for some stress amplitudes. A non-linear model based on the kinetics of thermal activation was applied to model the creep behavior of Au films, which captured very well the effect of applied stress on primary and steady-state creep. Additionally, multi-cycle creep experiments were conducted on annealed nanocrystalline Au films at four different temperatures. The creep exponents indicated a change in the dominant mechanism for inelastic deformation from diffusion and GB sliding at room temperature to dislocation climb at 110°C. It was also shown that about 50% of the accumulated strain during each forward creep cycle was recovered at zero load. The amount of strain recovery was weakly influenced by temperature which implies a dominant role of backstress in the recovery process.
Issue Date:2013-05-24
Rights Information:Copyright 2013 Nikhil Karanjgaokar
Date Available in IDEALS:2013-05-24
Date Deposited:2013-05

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