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Crack Path Mechanisms in Ti-1100 Near a-Widmanstatten Microstructure
Ronald Foerch, Alec Madsen, Hamouda Ghonem Mechanics of Materials Research Group
Department of Mechanical Engineering, University of Rhode Island, Kingston, 02881, USA
Controlling Parameters & Materials
Crack Path – Widmanstatten Microstructure (593°C)
Correlation of Crack Path and Slip Band Density
Examination of the Limiting Boundary Concept
Roles of Boundaries on Altering Crack Growth Path
Acknowledgements
The authors acknowledge the TIMET Corporation for supplying all the material required
for this study. Financial support was provided by Air Force Office of Scientific Research
Reference:
Microstructure and Fatigue Crack Growth Mechanisms in High Temperature
Titanium Alloys, H. Ghonem, Int. J. Fatigue, 32, pp. 1448-1460, 2010
Crack Path Mechanisms in Near-a Ti Widmanstatten
Multi-Scale Modeling of Time-Dependent Fatigue Crack-Tip Deformation and Fracture
Mechanisms
Microstructure Parameters: crystal structure and orientation, slip character, grain size, colony size, precipitate statistics , grain boundary morphology
Loading Parameters: loading frequency, temperature (520-800°C), environment-O2 (air and vacuum)
Titanium Alloys
IMI834, duplex microstructure (650, 700°C)
Timetal-21S, metastable b-titanium (538, 593, 650°C)
Timetal-1100, near-a widmanstatten microstructure (593°C)Ti6242
a/b fully lamellar (520, 593°C)
Precipitate Hardened Nickel-based Superalloys
IN718, supersolvus microstructure, cast (650°C)
N18, subsolvus microstructure, P/M (650, 700°C)
ME3, supersolvus microstructure, P/M (650,704,760,800°C)
IN100, subsolvus microstructure, P/M (650, 700°C)
Ti Al Sn Zr Mo Si Fe C O
Bal 6.0 2.9 4.2 0.39 0.41 0.017 0.022 0.078
b Forged - solution annealed above b transus at 1093°C
b / air cooled - stabilization (8 hours/593°C) / air cooled
prior b
grains and
a colonies
a needles
Ti-1100
a Colony Size
Colony Size (m)
0 40 80 120 160
Fre
quency (
Counts
)
0
10
20
30
Grain Size: 570m / Colony Size: 35m
Ti-1100 593oC
K (MPam)
20 30 40 50607010
da/d
N (
mm
/cycle
)
10-6
10-5
10-4
10-3
10-2
10-1
10 Hz 23°C
10 Hz 593°C
0.5 Hz 593°C
0.05 Hz 593°C
0.01 Hz 593°C
0.005 Hz 593°C
10s-300s-10s 593°C
Low
Frequency
High
Frequency
High Frequency
Room Temp
100 m
500 m
Frequency (Hz)
0.001 0.01 0.1 1 10 100
da/d
N (
mm
/cycle
)
@ 2
0 M
Pa
m
10-4
10-3
10-2
intergranular transgranular
Transitional
Frequency
0.005 Hz, Intergranular
10 Hz, Transgranular
Fracture mode is identified in terms of
transgranular / intergranular transitional frequency
Ti-1100/ LCF-593oC
10-2 sec-1 / 2.2% 100 sec-1 / 2.2%
slip/boundary intersection
50m 50m 100m
10-6 sec-1 / 1.8%
slip extended within the grain slip/boundary intersection
Experimental observations:
Higher frequency transgranular crack growth path
smaller slip band spacing - slip intersects colony boundary
Lower frequency intergranular crack growth path
larger slip band spacing - slip is uninhibited by colony boundaries
Frequency (Hz)
0.001 0.01 0.1 1 10 100
da/d
N (
mm
/cycle
)
@ 2
0 M
Pa
m
10-4
10-3
10-2
593°C
K=25 MPam
Frequency (Hz)
0.001 0.01 0.1 1 10 100
Su
rfa
ce P
lastic Z
on
e S
ize (
m
)
10
100
1000
10000
Slip
Lin
e S
pa
cin
g (
m)
0.1
1
10
PZS 593°C Vacuum
Slip Line Spacing
intergranular transgranular
Surface Slip Traces
(Vacuum, 593°C, 24 MPa√m)
20 m 25 m
250 m 100 m
10 Hz
0.5 Hz
0.05 Hz 10s-300s-10s
Transgranular
Intergranular
Fracture mode is correlated with slip band spacing (SBS)
Condition for intergranular fracture: SBS > 5m
transitional frequency
Fracture Mechanisms in Ti-1100 (593°C)
a colony
a colony
a
boundary
Pileup
colony
boundary
No
Passing
Slip Extension Blocking - high frequency loading
Frequency independent deformation
Small PZS
Large slip band density
Slip/colony boundary intersection
Transgranular fracture
Slip Transfer - low frequency loading
Time dependent deformation
Large PZS
Low slip band density
Slip transfer across colony boundary
Intergranular fracture
Passing
dislocations
Colony
boundary
Crack tip fracture models based on significance of slip/boundary interactions
Ti-1100593°C
Stress (MPa)
200 300 400 500
Str
ain
Rate
(s-1
)
10-8
10-7
10-6
unaged
aged 1000 hrs
Strain
0.00 0.04 0.08 0.12
Stre
ss (M
Pa)
400
450
500
550
600
650
700unaged
aged
Ti-1100Tensile Tests
593°C, Air
Unaged Peak Aged (593°C/10000min)
Over Aged (593°C/60000min) Peak Aged
(593°C/10000min)
200 nm 400 nm
50 nm 400 nm
Precipitation of silicides (TiZr)6Si3 along the a
lamellae in Ti-1100 as a function of aging time
Roles of Boundaries on Altering Crack Growth Path
Ti-1100 593°C Air
K (MPam)
20 30 40 50 60 7010
da
/dN
(m
m/c
ycle
)
10-5
10-4
10-3
10-2
Unaged 100s-100s
Unaged 10s-300s-10s
Unaged 10s-10s
Aged 10s-10s
Aged 10s-150s-10s
Aged 10s-300s-10s
Low Frequency
(Unaged)
High Frequency
(Unaged, Aged)
Low Frequency
(Aged)
500m
500m
10s-300s-10s
Mixed Mode
(Aged)
10s-300s-10s
Intergranular
(Unaged)
K = 28 MPa√m
K = 26 MPa√m
K = 28 MPa√m
K = 26 MPa√m
Boundaries define CG mechanisms in basket weave titanium alloys – (significance of colony size)
Crack Tip
Grain Boundary a Colony
a Colony
Boundary
Slip Blocking Mechanism
Transgranular fracture
(High f )
Slip transfer mechanism
Intergranular fracture
(Low f )
Fracture mode (crack path) in near-a Ti Widmanstatten is a loading frequency (f) dependent
Transitional frequency ( ft ) defines the transgranular / intergranular fracture mode transition
Slip band spacing is a function of the loading frequency ( f )
f > ft – high slip band density, slip is blocked by colony boundary transgranular (quazi cleavage) fracture
f < ft – low slip band density, slip transfer across colony boundary intergranular fracture
