FATIGUE CRACK PROPAGATION BEHAVIOR OF NEWLY FATIGUE CRACK PROPAGATION BEHAVIOR OF NEWLY DEVELOPED DEVELOPED ALLVACALLVAC®® 718PLUS 718PLUS™ ™ SUPERALLOYSUPERALLOY

Xingbo Liu1, Shalini Rangarajan1, Ever Barbero1, Keh-Minn Chang1, Wei-Di Cao2, Richard Kennedy2, Tadeu Carneiro3

1West Virginia University, Morgantown, WV 26505, USA2Allvac, An Allegheny Technologies Company, Monroe, NC 28110, USA

3Reference Metals Company, Bridgeville, PA 15017, USA

BACKGROUNDBACKGROUND• 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:

• 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 OBJECTIVESRESEARCH 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 DISCUSSIONRESULTS AND DISCUSSION

MICROSTRUCTURE OF 718PLUSMICROSTRUCTURE OF 718PLUS™™ • Main features of the microstructure of 718Plus™

• Strengthening phase γ' uniformly distributed in γ matrix• δ phase well distributed on grain boundaries

TENSILE AND STRESS RUPTURE TENSILE AND STRESS RUPTURE PROPERTIESPROPERTIES

• Yielding and tensile strength: 718Plus™ > Alloy 718 and Waspaloy

• Stress rupture properties: 718Plus™~ Waspaloy > 718

FATIGUE CRACK GROWTH WITHOUT HOLD FATIGUE CRACK GROWTH WITHOUT HOLD TIMETIME

• At 650 °C: 718Plus™ > Alloy 718, Waspaloy• 718Plus™ has better FCG resistance at 704 °C than Alloy 718 and Waspaloy at 650 °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

FATIGUE CRACK GROWTH WITH HOLD-FATIGUE CRACK GROWTH WITH HOLD-TIMETIME

• At 650 °C: Waspaloy, 718Plus™ > Alloy 718

• 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

CONCLUSIONSCONCLUSIONS• 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 WORKFUTURE WORK• Investigate effect of grain size and alloy chemistry on the hold-time FCG of 718Plus™ alloy• Improve 718Plus™ alloy’s hold-time FCG properties through thermo-mechanical treatment and microstructural optimization

Alloy / Element C Ni Cr Mo W Co Fe Nb Ti Al

718PLUS® 0.026 Bal. 17.47 2.7 1.03 9.15 9.86 5.43 0.7 1.46718 0.033 / 18.41 3.03 / / 17.94 4.98 0.91 0.56

Waspaloy 0.019 / 19.55 4.25 / 13.51 / / 2.95 1.37

Equilibrium phase diagram of 718Plus™ alloy

0

0.5

1

500 1000 1500Temperature (C)

We

igh

t F

rac

tio

n

DeltaFCCLiquidGamma'

40m

Microstucture of 718Plus™ alloy

800

1200

1600

0 250 500 750Test Temperature (C)

Str

engt

h (M

Pa)

718 PLUS - YS718 PLUS - UTSIn 718 - YSIn 718 - UTSWaspaloy - YSWaspaloy - UTS

15

20

25

30

35

0 200 400 600 800Test Temperature (C)

Elo

ngat

ion

(%)

718 PLUSIn 718Waspaloy

433.1

157.9

430.5

35.4 19.5 27.8

0

250

500

718 PLUS In 718 WASPALOY

Life, HoursElongation, %

1.E-07

1.E-06

1.E-05

1.E-04

10 100delta K MPa-m0.5

da

/dN

m/c

yc

le

718 Plus A1, 650C, 3s

718 L 650 3s

Waspalloy 650 3s

718 Plus A1, 703C, 3s

Fracture surface of 718Plus™ at 650 °C

Fracture surface of 718Plus™ at 704 °C

1.E-07

1.E-06

1.E-05

1.E-04

1.E-03

10 100delta K MPa-m0.5

da/

dN

m/c

ycle

718 Plus A1, 650C, 3+100s

718, 650C, 3+100s

Waspaloy, 650C, 3+100s

Fracture surface of 718Plus™ at 650 °C

Fracture surface of 718Plus™ at 704 °C