FATIGUE CRACK PROPAGATION BEHAVIOR OF NEWLY DEVELOPED ALLVAC

FATIGUE CRACK PROPAGATION BEHAVIOR OF NEWLY DEVELOPED ALLVAC

FATIGUE CRACK PROPAGATION BEHAVIOR OF NEWLY

DEVELOPED ALLVAC 718PLUS SUPERALLOY
Xingbo Liu , Shalini Rangarajan , Ever Barbero , Keh-Minn Chang , Wei-Di Cao , Richard Kennedy , Tadeu Carneiro
1
West Virginia University, Morgantown, WV 26505, USA
2
Allvac, An Allegheny Technologies Company, Monroe, NC 28110, USA
3
Reference Metals Company, Bridgeville, PA 15017, USA
1

1

BACKGROUND
Allvac 718Plus was developed to
Work at a higher temperature than Alloy 718
Possess comparable mechanical properties and
thermal stability to Waspaloy at 1300 F
Have processing characteristics similar to Alloy
718 and much better than Waspaloy
Have 20-25% lower cost than Waspaloy
Chemical composition of 718Plus, Alloy 718 and
Waspaloy:
Alloy / Element C

0.026
718PLUS
718
0.033
Waspaloy
0.019

Ni
Bal.
/
/

Cr
Mo
17.47 2.7
18.41 3.03
19.55 4.25

W
Co
Fe
Nb
1.03 9.15 9.86 5.43
/
/
17.94 4.98
/
13.51
/
/

Ti
0.7
0.91
2.95

1200

0

Al
1.46
0.56
1.37

Delta
FCC
Liquid
Gamma'
40m

1000
Temperature (C)

1500

Equilibrium phase diagram of
718Plus alloy

Microstucture of
718Plus alloy

718 PLUS
In 718
Waspaloy

30

250
500
Test Temperature (C)

FATIGUE CRACK GROWTH WITH HOLDTIME
At 650 C: Waspaloy, 718Plus > Alloy 718
1.E-03

1.E-05

25

718 Plus A1, 650C, 3+100s

1.E-06

20

718, 650C, 3+100s
Waspaloy, 650C, 3+100s

1.E-07
0

750

200
400
600
Test Temperature (C)

800

10

Stress rupture properties: 718Plus~ Waspaloy > 718
500

433.1

Life, Hours
Elongation, %

3

1.E-04

15

800

delta K MPa-m0.5

100

430.5

250
157.9
35.4

Fracture surface of
718Plus at 650 C

27.8

19.5

0
718 PLUS

In 718

WASPALOY

FATIGUE CRACK GROWTH WITHOUT HOLD
TIME
At 650 C: 718Plus > Alloy 718, Waspaloy
718Plus has better FCG resistance at 704 C than
Alloy 718 and Waspaloy at 650 C
1.E-04

718 Plus A1, 650C, 3s
718 L 650 3s
Waspalloy 650 3s

1.E-05

718 Plus A1, 703C, 3s

1.E-06

1.E-07
10

Fracture surface of
718Plus at 650 C

0
500

35

2

da/dN m/cycle

Strength (MPa)

718 PLUS - YS
718 PLUS - UTS
In 718 - YS
In 718 - UTS
Waspaloy - YS
Waspaloy - UTS

Elongation (%)

1600

1

0.5

2

TENSILE AND STRESS RUPTURE
PROPERTIES
Yielding and tensile strength: 718Plus > Alloy 718
and Waspaloy

Balance Al to Ti ratio with Nb to improve thermal
stability and mechanical properties at high temperature
Minor elements B and P added to maximize creep
resistance
Fe, Co and W balanced to improve strength, creep
resistance, thermal stability and processing qualities
with minimum increase in raw material costs
RESEARCH OBJECTIVES
Investigate the mechanical properties of Allvac
718Plus and compare with Alloy 718 and Waspaloy:
Tensile properties at 650 C and 704 C
Stress rupture properties at 704 C
Fatigue crack propagation at 650 C and 704 C
3 seconds triangle wave
Hold-time fatigue crack propagation
Trapezoid wave with 3 seconds + 100 seconds
loading at maximum stress
RESULTS AND DISCUSSION

MICROSTRUCTURE OF 718PLUS
Main features of the microstructure of 718Plus
Strengthening phase ' uniformly distributed in
matrix
phase well distributed on grain boundaries
Weight Fraction

1

da/dN m/cycle

1

delta K MPa-m

0.5

100

Fracture surface of
718Plus at 704 C

Mixture of transgranular and intergranular modes at
650 C
Intergranular mode predominant at 704 C
Clear border present between RT precrack and
fatigue at 650 C
Severe oxidation observed during crack growth at
704 C

Fracture surface of
718Plus at 704 C

Crack propagation at both 650 C and 704 C is
predominantly intergranular with a little transgranular
cleavage fraction
Rough fracture surface covered with lots of oxide
products
Oxidation of surface more severe at 704 C than at
650 C
CONCLUSIONS
The newly developed 718Plus alloy shows better
tensile and stress rupture properties for temperatures up
to 704 C than Alloy 718
Tensile and stress rupture properties of 718Plus
alloy are similar to that of Waspaloy for temperatures
up to 704 C
For fatigue crack growth without holding time, there

is no significant difference between 718Plus ,Alloy
718 and Waspaloy, but in comparison, 718Plus has
the best FCGR resistance and Alloy 718 has the lowest
718Plus shows comparable resistance to hold-time
FCG than that of Waspaloy and both alloys show better
FCG properties than that of Alloy 718
FUTURE WORK
Investigate effect of grain size and alloy chemistry on

the hold-time FCG of 718Plus alloy
Improve 718Plus alloys hold-time FCG properties
through thermo-mechanical treatment and
microstructural optimization

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