fatigue damage accumulation in titanium alloy imi 834

183 Pages·2015·6.98 MB·English

FATIGUE DAMAGE ACCUMULATION IN
TITANIUM ALLOY IMI 834
By
Gavin James Baxter
,
\Jo~
Thesis submitted for the degree of Doctor of Philosophy
Department of Engineering Materials
University of Sheffield
May 1994
FATIGUE DAMAGE ACCUMULATION IN TITANIUM ALLOY
IMI 834
BY G. J. BAXTER
SUMMARY OF THESIS
As current aerospace materials are subjected in service to
increasingly onerous conditions of stress and temperature, the hazard
of fatigue failure becomes more acute. Engineers utilise the
methodology of fracture mechanics to estimate fatigue crack growth
rates but fatigue crack initiation, which involves the interplay of
many microprocesses, is only investigated empirically. The aim of
this study was to investigate the fatigue damage accumulation
mechanisms in the titanium alloy IMI 834 in order to develop a
fundamental understanding of the controlling physical processes and
the micromechanisms which occur at the dislocation level.
Load controlled four point bend test specimens of IMI 834 were
cyclically fatigued to failure with an R ratio of 0.1 over a range of
maximum stress levels and the fatigue and fracture surfaces were
examined by optical and scanning electron microscopy. The
examination of cross-sectional foils prepared from the fatigue
surface enabled the fatigue damage to be examined in the T.E.N. as a
function of or ientation and depth below the specimen sur face. The
distribution, orientation and type of slip bands were identified in
the primary-a and the transformed-fJ grains, and their interaction
with secondary phases, precipitates and grain boundaries was
determined.
The results show that fatigue damage accumulation in INI 834 occurs
primarily on basal slip bands in the primary-a phase and on basal and
prismatic slip bands in the transformed-fJ phase. The segregation of
a-stabilising elements to the primary-a phase during alloy processing
allows the formation of an ordered phase which increases the
propensity for planar slip on the basal plane. A mechanism for
fatigue crack initiation along this plane is proposed. In addition,
the occurrence and identification of an interface phase is discussed
in the light of current theories regarding this phase.
VOLUME 1
CONTENTS
VOLUME 1
contents ( i)
Abbreviations (vii)
CHAPTER ONE INTRODUCTION (1)
1.1 FATIGUE LIFE PREDICTION (1)
1. 2 HIGH TEMPERATURE TITANIUM ALLOYS (2)
1.3 PROJECT AIMS (3)
CHAPTER TWO LITERATURE SURVEY (5)
2.1 INTRODUCTION (5)
2.2 GENERAL BACKGROUND TO TITANIUM ALLOYS (6)
2.2.1 Introduction (6)
2.2.2 Effect of Alloy Additions (7)
2.2.3 Classes of Titanium Alloys (9)
2.2.3.1 a+p Titanium Alloys (10)
2.2.3.2 Near-a Alloys (11)
2.2.4 Heat Treatment and Microstructure (13)
2.2.4.1 Solution Treatment (13)
2.2.4.2 Effect of Cooling Rate (14)
2.2.4.3 Ageing Treatment (16)
2.2.5 Precipitates Associated with Solutes (17)
2.2.5.1 Ordered Phase, a3 (17)
2.2.5.2 Silicide Precipitation (18)
2.2.6 The Interface Phase (IFP) (20)
2.2.6.1 The Monolithic fcc Phase (22)
2.2.6.2 The Striated Phase (23)
2.2.6.3 Effect of Foil Preparation (24)
i
2.3 MECHANICAL PROPERTIES (27)
2.3.1 Creep Resistance (27)
2.3.2 Ductility
(30)
2.3.3 Ultimate Tensile Strength
(31)
2.3.4 Fracture Toughness
(33)
2.4 CRYSTALLOGRAPHY AND DEFORMATION MODES
IN TITANIUM
(34)
2.4.1 von Mises criterion
(36)
2.4.2 The Critical Resolved Shear Stress (37)
2.4.3 Effect of Interstitial content
(38)
2.4.4 Aluminium Additions ( 41)
2.5 FATIGUE BEHAVIOUR (42)
2.5.1 Long Fatigue Crack Growth (43)
2.5.2 The Fatigue Limit (44)
2.5.3 Short Fatigue Crack Growth Rate ( 45)
2.5.4 Fatigue Crack Initiation (46)
2.5.5 Damage Accumulation in Copper (47)
2.5.6 Damage Accumulation in Titanium Alloys (49)
2.5.6.1 Commercially Pure Titanium (49)
2.5.6.2 Ti-6Al-4V (50)
2.5.6.3 High Temperature Alloys (51)
2.5.7 Introduction of a Dwell Period at
Maximum Load (54)
2.5.7.1 Microstructural Effects (55)
2.5.7.2 Texture Effects (56)
2.5.7.3 Temperature Effects (57)
2.5.7.4 Hydrogen Additions (58)
2.5.7.5 Proposed Models (59)
CHAPTER THREE EXPERIMENTAL PROCEDURE ( 61)
3.1 Material Preparation ( 61)
3.1.1 Test Specimen Locations
( 61)
3.1.2 Specimen Polishing (63)
ii
3.2 Mechanical Testing (64)
3.2.1 Tensile Testing (64)
3.2.2 Four Point Bend Fatigue Testing (65)
3.3 Microstructural Examination (66)
3.3.1 Composition (66)
3.3.2 X-ray Diffraction (66)
3.3.3 Optical Microscopy (67)
3.3.4 Scanning Electron Microscopy (S.E.M.) (67)
3.3.5 Transmission Electron Microscopy (T.E.M.) (68)
3.3.5.1 Foil Preparation from Unfatigued
Material (68)
3.3.5.2 Back-thinned Foil Preparation (69)
3.3.5.3 Cross-sectional Foil Preparation (70)
3.3.5.4 Alternative Foil Preparation
Techniques (72)
CHAPTER FOUR MATERIAL CHARACTERISATION (73)
4.1 composition (73)
4.2 Optical Microscopy (73)
4.3 X-ray Diffraction (74)
4.3.1 Phase Identification (74)
4.3.2 Texture Determination (74)
4.4 Thin Foil Microstructure (75)
4.4.1 Grains (75)
Transformed-~
4.4.2 Primary-a Grains (77)
CHAPTER FIVE THE INTERFACE PHASE (79)
5.1 Cross-sectional Thin Foils (79)
5.2 Effect of Foil Preparation Techniques (80)
iii
5.3 Crystallography of the Phases
(82)
CHAPTER SIX FATIGUE DAMAGE
(85)
6.1 Tensile Tests
(85)
6.2 Fatigue Tests
(85)
6.3 Fatigue Surfaces
(86)
6.3.1 Slip Bands
(86)
6.3.2 Secondary Cracking
(87)
6.3.2.1 Secondary Crack Initiation
(87)
6.3.2.2 Secondary Crack Propagation
(88)
6.3.3 Fatal Fatigue Cracks
(89)
6.4 Fracture Surfaces (90)
6.4.1 Crack Origins (90)
6.4.2 Crack Propagation (91)
6.5 Thin Foil Observations (94)
6.5.1 Foil Preparation Results (94)
6.5.1.1 Back-thinned Foils (94)
6.5.1.2 Cross-sectional Thin Foils (94)
6.5.2 Slip in Primary-a Grains (95)
6.5.2.1 Slip in Back-thinned Foils (95)
6.5.2.2 Slip in Cross-sectional Thin Foils (97)
6.5.2.3 Grain Orientations (100)
6.5.3 Slip in Transformed-p Colonies (100)
6.5.4 Microcracking (101)
6.5.4.1 Microcracking in Transformed-p (102)
6.5.4.2 Microcracking in Primary-a (102)
6.5.5 Summary of Thin Foil Observations (104)
CHAPTER SEVEN DISCUSSION (107)
7.1 Observations on Unfatigued Material (107)
7.1.1 Composition
(107)
iv
7.1.2 Microstructure
(109)
7.1.2.1 Optical Microstructure (109)
7.1.2.2 Effect of Primary-a content (109)
7.1.2.3 X-ray Diffraction (110)
7.1.2.4 T.E.M. of Transformed-~ Grains (111)
7.1.2.5 T.E.M. of Primary-a Grains (113)
7.2 The Interface Phase (IFP) (115)
7.2.1 Foil Preparation Techniques (115)
7.2.2 Crystallography of the IFP (117)
7.3 Fatigue Damage (119)
7.3.1 Mechanical Testing (119)
7.3.1.1 Tensile Tests (119)
7.3.1.2 Fatigue Tests (120)
7.3.2 Fatigue Surfaces (121)
7.3.3 Fracture Surfaces (123)
7.4 T.E.M. Observations of Fatigued Material (126)
7.4.1 Slip Systems in Primary-a Grains (127)
7.4.1.1 Effect of Aluminium and Oxygen
Content (129)
7.4.1.2 Effect of Silicon Additions (132)
7.4.1.3 + A> Slip (133)
<~
7.4.1.4 Effect of CRSS and Grain orientation (134)
7.4.2 Slip Systems in Grains (137)
Transformed-~
7.4.3 Damage Accumulation and Microcracking (139)
7.4.3.1 Mechanisms of Fatigue Crack Initiation(141)
7.5 Fatigue Crack Initiation in IMI 834 (145)
CHAPTER EIGHT CONCLUSIONS (147)
CHAPTER NINE IMPLICATIONS AND FURTHER WORK (152)
CHAPTER TEN REFERENCES
(154)
v
Acknowledgements
APPENDIX 1 Burgers Vector Calculations
APPENDIX 2 Calculation of FlI\&x and Fllean
APPENDIX 3 Calculation of Dimple Depth
VOLUME 2
FIGURES
TABLES
vi
ABBREVIATIONS
a crack length
lattice parameter
a titanium alloy phase with hexagonal close packed
crystal structure
a2 ordered coherent phase Ti3AI
a' titanium martensite
ao initial defect size
af final crack size
AI- aluminium equivalent
at' atomic percent
b specimen breadth
Burgers vector
~
p titanium alloy phase with body centred cubic
crystal structure
bx microstructural barrier of magnitude x
B electron beam direction
b. cc body centred cubic
y strain rate
y intermetallic compound (eg. TiAl)
lattice parameter
&
CP commercially pure grade of titanium
CRSS critical resolved shear stress
d spacing of crystal planes
d specimen depth
AI< stress intensity factor range
Aa cyclic stress range (Nm-2)
E Young's modulus (GPa)
fcc face centred cubic
maximum force
Fllean mean force
minimum force
Fllin
reciprocal lattice vector
Sf
hcp hexagonal close packed
(hkl) Miller indices
(hkil) Miller-Bravais indices
IFP interface phase
vii