UNIVERSITY OF CALIFORNIA, SAN DIEGO Phase Transformations in Nickel-rich Nickel-Titanium Alloys: Influence of Strain-rate, Temperature, Thermomechanical Treatment and Nickel Composition on the Shape Memory and Superelastic Characteristics A dissertation submitted in partial satisfaction of the requirements for the degree Doctor of Philosophy in Materials Science and Engineering by Raghavendra R. Adharapurapu Committee in charge: Professor Kenneth S. Vecchio, Chair Professor Prabhakar R. Bandaru Professor Sungho Jin Professor John B. Kosmatka Professor Robert O. Ritchie 2007 Copyright Raghavendra R. Adharapurapu, 2007 All rights reserved The dissertation of Raghavendra R. Adharapurapu is approved, and it is acceptable in quality and form for publication on microfilm: _______________________________________________ _______________________________________________ _______________________________________________ _______________________________________________ _______________________________________________ Chair University of California, San Diego 2007 iii DEDICATION Dedicated to My Parents, who taught me how to speak My Guru, who taught me how to be silent iv TABLE OF CONTENTS Signature Page...............................................................................................................iii Dedication......................................................................................................................iv Table of Contents ...........................................................................................................v List of Figures...............................................................................................................xii List of Tables.............................................................................................................xxiii Acknowledgements ...................................................................................................xxiv Curriculum Vitae......................................................................................................xxvii Abstract......................................................................................................................xxxi 1 INTRODUCTION ................................................................................................1 1.1 General introduction.......................................................................................1 1.2 Motivation and Purpose of the study..............................................................3 1.3 Objectives.....................................................................................................11 1.4 Significance of the study ..............................................................................13 1.5 Relevance to practical applications ..............................................................16 1.6 Arrangement of the thesis.............................................................................17 1.7 Figures ..........................................................................................................19 1.8 References ....................................................................................................20 2 BACKGROUND.................................................................................................28 2.1 Basic Phenomenon .......................................................................................28 2.1.1 What is unique about NiTi alloy?.........................................................28 2.1.2 Brief History.........................................................................................29 2.1.3 Thermoelasticity and Martensitic Transformations..............................37 v 2.1.4 Martensitic transformation in Ni-Ti alloys...........................................48 2.1.4.1 Crystal Structures of the austenite and the martensite phases..........48 2.1.4.2 Twinning...........................................................................................50 2.1.4.3 Twinning and Twins in NiTi ............................................................56 2.1.4.4 Classification of twin types ..............................................................60 2.1.4.5 Austenite-martensite interface..........................................................61 2.1.5 Effect of stress on the martensitic transformation................................68 2.1.6 Pseudoelasticity and Superelasticity.....................................................75 2.1.7 Shape memory effect: Origin and mechanisms....................................78 2.1.8 One-way, Two-way and All-round shape memory effect ....................80 2.1.9 Rubber-like effect and Pseudoelasticity ...............................................84 2.2 R-phase Transition........................................................................................86 2.3 Physical Metallurgy and Processing of NiTi alloys .....................................89 2.3.1 Phase diagram.......................................................................................89 2.3.2 Precipitation in NiTi alloys ..................................................................91 2.3.3 Crystal structure of different precipitates.............................................92 2.3.4 Processing of Ni-Ti alloys: Effect of Ni composition..........................95 2.3.5 Tailoring transformation temperatures: Effect of various parameters102 2.3.6 Multiple-stage Transformations (MST) .............................................104 2.4 Deformation behavior of NiTi alloys .........................................................111 2.4.1 Effect of temperature..........................................................................111 2.4.2 Effect of thermomechanical treatment ...............................................115 2.5 Summary of Objectives ..............................................................................119 vi 2.6 Figures ........................................................................................................119 2.7 References ..................................................................................................152 3 EVOLUTION OF MULTIPLE-STAGE TRANSFORMATIONS (MST) IN NICKEL-RICH 50.8-NiTi: EFFECTS OF AGING AND COOLING RATE ON THE SHAPE MEMORY CHARACTERISTICS .................................................178 3.1 Abstract.......................................................................................................178 3.2 Introduction ................................................................................................179 3.3 Experimental...............................................................................................189 3.3.1 Materials and microstructure..............................................................189 3.3.2 Texture Measurements .......................................................................190 3.3.3 Experimental Procedure ....................................................................193 3.4 Results and Discussion...............................................................................194 3.4.1 Phase Transformations: DSC Results.................................................194 3.4.1.1 As-Received: Transformation sequence.........................................194 3.4.1.2 Effect of aging Temperature (Aged+WQ): Evolution of MST......197 3.4.1.3 Effect of cooling rate......................................................................209 3.4.2 Stress-strain Response of NiTi ...........................................................215 3.4.2.1 As-received: Stress-Strain Behavior...............................................215 3.4.2.2 Effect of Aging Temperature: Aged+WQ......................................216 3.4.2.3 Effect of Cooling rate .....................................................................219 3.4.3 Discussion...........................................................................................220 3.5 Conclusions ................................................................................................224 3.6 Acknowledgements ....................................................................................225 vii 3.7 Figures ........................................................................................................226 3.8 References ..................................................................................................240 4 HIGH STRAIN-RATE RESPONSE OF NiTi SHAPE MEMORY ALLOY: A SYSTEMATIC INVESTIGATION OF TEMPERATURE EFFECTS ON TENSION-COMPRESSION ASYMMETRY .......................................................248 4.1 Abstract.......................................................................................................248 4.2 Introduction ................................................................................................249 4.3 Experimental Procedure.............................................................................253 4.3.1 Materials and Specimen .....................................................................253 4.3.2 Testing temperature............................................................................254 4.3.3 Quasi-static and high strain tests ........................................................254 4.3.4 High strain rate compression with pulse shaping...............................255 4.4 Results and Discussion...............................................................................258 4.4.1 Stress-strain response of NiTi at 20OC: Effect of strain rate..............258