3‘ V ,n( ‘ ‘ .p, ‘ mm ‘ .4.‘ a ‘r_ '. m. .36.: 9-3 64%? a: < , am. 2: 9‘32: a, 1g. ,{i‘ifi‘fi’ ' in“! , . ‘ . : .C‘ . NH' ’.".'A'~ hi ‘1‘ . a @w . \ ‘yqfifl‘ '. Wu ”l- .I' "gay I" ‘ my?! ,, “o'f‘» w A. - 4am; . -a‘ . «w n’ U 25335334" ”4» mama: madam:-.m4" v '1 . +7 ’ fl 2'?! “flak r” . 31%. llllllllllllllllllllllUIHIHIllllWIHIIHIIlllllllllllllilll 293 01411 0435 THESOS K...»- This is to certify that the thesis entitled High Temperature Creep Deformation of Gamma—Based Titanium Aluminide presented by Randy S. Beals has been accepted towards fulfillment of the requirements for Wdegree in Material Science fiesta Major professor Date 3/19/95 0-7639 MS U is an Affirmative Action/Equal Opportunity Institution LlBRARY 5 Michigan Statei University PLACE N RETURN BOX to romovo this chockout from your rocord. TO AVOID FINES Mum on or bdoro duo duo. DATE DUE DATE DUE DATE DUE HIGH TEMPERATURE CREEP DEF ORMATION OF GAMMA-BASED TITANIUM ALUMINIDE By Randy S. Beals A THESIS Submitted to Michigan State University in partial fulfillment of the requirements for the degree of MASTERS OF SCIENCE Department of Materials Science and Mechanics 1995 r CSpon Complt ConCer Comb'u tion as [he Cre in Gide the allr ture. A temper 0n the ‘ Undem "HA1. ABSTRACT HIGH TEMPERATURE CREEP DEFORMATION OF GAMMA-BASED TITANIUM ALUMINIDE By Randy S. Beats As part of an ongoing research concerning creep deformation mechanism(s) responsible for the creep behavior of ingot investment cast near y—TrAl, a study has been completed which several creep studies where performed and some theoretical arguments concerning the behavior of TiAl where introduced. The theoretical arguments include the combining the composite rule-of-mixture and the power-law Mukerjee-Bird-Dom equa- tion as a mathematical creep deforrnation model. Also, a transitional interface between the gamma and alpha2 constituents is introduced. A literature review has been completed and the creep characteristics of different compositions was explored at different test conditions in order to list the trends that appear in the data. The creep experiments achieved where on the alloy Ti-48Al-2Cr-2Nb and the data obtained was compared to the data in the litera- ture. All the experimental data in the literature and from this study was normalized by the temperature dependence of the diffusively and the shear modulus. The temperature nor- ' malization of values allows the investigator a more accurate view of the stress dependence on the creep deformation. The creep characteristics where then analyzed to improve our understanding of the deformation mechanism(s) responsible for the creep behavior of TiAl. T V1. 5.. F 2 «I. L C (L 5.. LIST ll illll il‘l'il“ I NEN LIST OF FIGURES .............................................................................................. vi LIST OF TABLES ................................................................................................ xi CHAPTER ONE. INTRODUCTION ............................................................... 1 CHAPTER TWO. THEORY ............................................................................. 2 2.1. REVIEW OF CREEP .................................................................................... 2 2.1.1. The high temperature materials problem ........................................ 2 2.1.2. Stages of creep ................................................................................ 3 2.1.3. Temperature effects ......................................................................... 5 2.1.4. Stress effects .................................................................................... 7 2.1.5. Power law relation .......................................................................... 8 2.1.6. Transitions in stress exponent, n ..................................................... 8 2.1.7. Transitions in activation energy, Q ................................................. 9 2.2. CREEP DEFORMATION MECHANISMS .................................................. 11 2.2.1. Diffusion controlled ........................................................................ 12 (a) Nabarro-Hem'ng ...................................................................... 12 (b) Coble ...................................................................................... 13 2.2.2. Dislocation controlled .................................................................... 14 (a) Dislocation glide ..................................................................... 15 (b) Dislocation climb .................................................................... 15 (c) Dislocation glide limited by solute atmospheres ..................... 16 (d) Grain boundary sliding ............................................................ 16 (e) Deformation induced twinning ................................................ 17 2.3. CREEP EQUATIONS .................................................................................. 18 2.3.1. Power law equation ........................................................................ 19 2.3.2. Mukerjee-Dorn equation ................................................................ 19 2.3.3. Power law breakdown .................................................................... 2() 2.3.4. Composite modeling............................... ....................................... 20 CHAPTER THREE. TITANIUM ALUMINIDE TiAl ................................... 22 3.1 CRYSTAL LATTICE STURCI'URE ............................................................ 22 3.1.1. L10 ................................................................................................. 22 3.1.2. D019 .............................................................................................. 23 3.1.3. Differences between L10 and D019 .............................................. 24 3.2. MICROSTRUCTURE ................................................................................. 25 3.2.1. Lamellar structure ........................................................................ 27 3.2.2. Thermomechanical structure ....................................................... 28 (a) Near-gamma (NG) ................................................................ 28 (b) Duplex (DP) ......................................................................... 29 (c) Nearly lamellar (NL) ............................................................ 29 ((1) Fully lamellar (FL) ............................................................... 29 3.2.3. Alloying ........................................................................................ 30 (a) Ti-48Al—2Cr-2Nb ................................................................... 31 CHAPTER FOUR. LITERATURE SURVEY OF CREEP IN TiAl ............ 4.1. INTRODUCTION ........................................................................................ 33 4.1.1. Trade-off approach ........................................................................ 34 4.1.2. Comparison of creep resistance of different rnicrostructures ........ 34 iii 4.2. PROBLEMS IN UNDRSTANDING CREEP OF TiAl .............................. 35 4.2.1. Minimum creep rate ..................................................................... 35 4.2.2. No instantaneous strain ................................................................ 4.2.3. Dynamic recrystalization .............................................................. 4.3. DEFORMATION HISTORY INDEPENDENCE ....................................... 4.4. DIFFUSION CONTROLLED CREEP OF TIA] ........................................ 38 4.4.1. Self-diffusion of 'I'r-S4Al ............................................................. 38 4.4.2. Alloys containing Cr ..................................................................... 40 4.5. DISLOCATION CONTROLLED CREEP IN TrAl .................................... 41 4.5.1. Twinning ....................................................................................... '41 4.5.2. Dislocation glide-climb ................................................................. 4.6.3. Geometrical constant A ................................................................. 4.6. DEFORMATION MECHANISM TRANSITION ....................................... 46 4.6.1. Unique creep behavior of Ii-48A1-2Cr—2Nb ................................. 47 4.6.2. Aluminum content. ....................................................................... 49 4.7. COMPOSITE VIEW OF CREEP ............................................................... 52 4.8. SUMMARY OF CREEP RATES ............................................................... 55 CHAPTER FIVE. CREEP EXPERIMENTS ON Ti-48Al-2Cr-2Nb .......... 5.1. INTRODUCTION ...................................................................................... 58 5.2. MATERIALS .............................................................................................. 58 5.3. EXPERIMENTAL PROCEDURE .............................................................. 59 5.3.1. Open-air ........................................................................................ 60 5.3.2. Vacuum ......................................................................................... 60 iv 5.3.3. Testing techniques ........................................................................ 61 5.4. EXPERIMENTAL RESULTS ..................................................................... CHAPTER SIX. ANALYSIS ........................................................................... 85 6.1. INTRODUCTION ......................................................................................