aniruddh das | odisseus workshop 2015| 24.03.15 fracture behavior and mechanisms of ods steels

Aniruddh Das | ODISSEUS Workshop 2015| 24.03.15

Fracture behavior and mechanisms of ODS steels

Member of the Helmholtz Association


Content

• History of ODS alloys• Motivation• Fracture toughness analysis methods• Preliminary results• Conclusion



History of ODS alloys



Need for advanced materials

• Use at high temperatures higher efficiency• Good mechanical properties at high temperature• Simplification of design• Reduced activation and swelling

𝜂=1−𝑇𝑐𝑜𝑙𝑑

𝑇 h𝑜𝑡

Thermal efficiency of sodium fast reactors



Gen IV ReactorsYear Reactor type Efficiency Material Temperature

1950-2014 Gen I-III Low (15-30%) Ferritic /Martensitic,

Austenitic

< 500°C

Year Reactor type Efficiency Material Temperatureafter 2014 Gen IV High (38%) ODS/NFA < 650°C

Advantages of Gen-IV reactor

• Better use of nuclear fuel• Faster decay of radioactive waste• Ability to consume nuclear waste for electricity• Improved operating safety



Candidate materials for high temperature nuclear application

Ni Superalloys Ferritic/Martensitic Steels

ODS steels

Temperature range < 1200°C < 600°C < 800°C

Tensile strength (at RT) 1000-1200 MPa 630-830 MPa 800-1200 MPa

Ductility (at RT) up to 6% about 20% up to 26%

Creep rupture strength (105h) 125 MPa (700°C) 50 MPa (650°C) 180 MPa (650°C)

DBTT Always ductile -100°C to 0°C -100°C to 0°C

Fracture toughness (RT) 70-90 MPa-m1/2 150-300 MPa-m1/2 75-150 MPa-m1/2

Activation Ni, Co 60Co Low Low

Irradiation swelling 5-10 % (100 dpa, 550-650°C)

Low Very low

High temperature corrosion Al Oxidation protection layer

Cr oxidation protection layer

Cr oxidation protection layer



Ferritic ODS steels: Nuclear applications

Alloy Fe Cr W Ti Mo Mn V Ta C Y2O3 Other

ODS Eurofer 97 Bal. 9 1.1 - - 0.4 0.2 0.12 0.11 0.3 0.03N

12Y1 Bal. 12.8 0.01 - 0.03 0.04 - 0.04 0.25 0.24Ni

12YWT Bal. 12 3 0.4 - - - - 0.25

14YWT Bal. 14 3 0.4 - - - - 0.3

MA 957 Bal. 14 - 0.9 0.3 - - 0.01 0.25

PM 2000 Bal. 20 - 0.5 - - - 0.01 0.5 5.5 Al

MA 956 Bal. 21.7 - 0.33 - 0.06 - - 0.03 0.3 5.7Al



History of ODS alloys

High strength Ti,Y,O nano particles

Stable creep Low fracture toughness

Chaouadi et al2010 Thak Sang Byun et al. 2010

Klueh et al. 2002

Byun et al. 2014

Klueh et al. 2002



Motivation



Why is fracture toughness of ODS important?

• Normal operation gives rise to stresses (pressure, vibrations, temperature) + presence of microcracks from manufacturing Fracture

• Accidental conditions can result in additional thermal shock

• Low fracture toughness is detrimental for machinability of ODS steels.

• Fracture toughness is a well-defined generally accepted quality parameter, which helps for qualitative ranking of materials and design of components.



Motivation

• No clear systematic fracture toughness data of ODS steels from RT to 800°C

• Ductile-to-brittle transition: Master Curve approach

• Ductile region : determination of JR Curves

• Understanding of fracture mechanisms from RT to 800°C

• Understanding the role of selected process parameters on fracture toughness

• Effect of bulk microstructure of ODS on fracture toughness

• Relation between fracture surfaces and fracture toughness

• Development of small specimen methods

• Development of high temperature testing



Small scale specimens

• Less amount of material required• More specimens irradiated• Less activity to handle

• Difficulty in COD measurement• Experimental setup challenging• Some validity criteria in ASTM

E1820 are not fulfilled

1T C(T) Inner Knife 0.16T C(T) Outer Knife 0.25T C(T) Front Face



Fracture toughness analysis methods



Fracture toughness analysis

• Multiple specimen method• Potential drop method• Unloading compliance method• Normalization method

Most common specimens

Analysis methods

Most common orientations




• Simple procedure

Multiple specimen method

• Single specimen method

• Convenient and time saving

Unloading Compliance (UC)

• Single specimen method

• Initial and final crack length required for JR curve

Normalization method

• Requires multiple specimen

• Time consuming

• Needs high accuracy in COD measurement

• High temperature COD measurement needs special arrangement

• Empirical method of estimation

• Needs to be combined with another method for validation



• Single specimen method• Only monotonic loading needed• Can be used at high temperatures

• Requires specimen isolation from machine• Has to be coupled with a COD

measurement device for J integral calculation

• Needs to be validated with other techniques such as Unloading Compliance

Advantages

Disadvantages


Potential drop method (PD):Approximate linear relation of crack extension with potential difference

0 2 4 61.0

1.2

1.4

1.6

1.8

2.0

PDeff Multiple specimen method

Pde

ff [V

]

da corrected [mm]

0 2 4 60

500

1000

1500

2000

UC

PD 0.2mm offset line

J [K

N/m

]

da [mm]



Preliminary Results



Our ODS materialSource Composition Production method

iFAM/HZDR (Ms Hilger)

KIT (LT & TL)

14Cr ODS bar (GETMAT)

ODS PM 2000 (Plansee)

Fe14Cr1W0,4Ti0,3Si0,3Mn0,15NiFe14Cr1W0,4Ti0,3Si0,3Mn0,15Ni + 0,3 Y2O3

Fe14Cr1W0,4Ti0,3Si0,3Mn0,15Ni + 0,6 Y2O3

Fe 13Cr 1W 0,3Ti + 0,3Y2O3

Fe 13.98Cr, 1.03W, 0.39Ti,0.29Mn, 0.32Si, 0.17Ni + 0.3Y2O3

Fe 20Cr, 0.5Ti, 0.01C, 5.5Al + 0.5Y2O3

MA + SPS (1050°C + 247KN)

MA+HIP (1100°C/100MPa)+ HR(1100°C)

Hot extruded (1100°C) + Annealed (1050°C ,1.5 hrs)

Hot Rolled

0 100 200 300 400 500 600 700 800100

200

300

400

500

600

700

800

900

Str

eng

th /

MP

a

Temperature / °C

Rp0.2

Rm

0 200 400 600 800

18

20

22

24

26

28

30

32

34

36

Tota

l elo

ngat

ion

/ %

Temperature / °C

°C



Fracture surface macrostructure of ODS-KIT (LT 0.25T C(T)specimen)

RT-1 RT-2

100°C 200°C

L

ST

• Heat tinting and Liq. N2 fracture• Advanced cracks• Larger crack front at 200°C

T

S



Preliminary results

Fracture toughness of 0.25T C(T) specimen ODS KIT

Load drop

0.0 0.5 1.00

1000

2000

3000

4000

5000

ODS-KIT-01: 20 °C

ODS-KIT-02: 20 °C

ODS-KIT-03: 100 °C

ODS-KIT-04: 200 °C

Load

/ N

LLD / mm

Heintze, Cornelia

Bullets



Preliminary results

Fracture toughness of 0.25T C(T) specimen ODS KIT

0.0 0.5 1.0 1.50

100

200

300

400

ODS-KIT-1: J Q = 53.07 kN/m (20 °C)

ODS-KIT-2: J Q = 72.11 kN/m (20 °C)

ODS-KIT-3: J Q = 35.11 kN/m (100 °C)

ODS-KIT-4: J Q = 38.76 kN/m (200 °C)

0.2mm Offset line

J / K

N/m

D a / mm



ODS fracture surface microstructure at 20°C

Presence of sharp cracks4 Residual fracture

3 Fatigue post-cracking

2 Crack growth region

1 Fatigue pre-cracking

4

3

2

1

Dimple formation at micro level

T

S



Pre-fatigue cracks in ODS-KIT at 20°C

1

Presence of similar crack advancement in fatigue pre-crack region

T

S



ODS fracture surface microstructure at 200°C

T

S



Porosity in ODS KIT

As-hipped material

TS Plane

LS PlaneHot Rolling

• The pores are elongated after hot rolling• The pores are linearly arranged• More elongation in L than in T direction

L

T



Delamination

• Fracture between weak interfaces (matrix-

inclusions or matrix-pores)

• Higher plastic deformation in the S-direction

than in the T-direction

• Delamination perpendicular to notch base LT

• Delamination parallel to notch base LS

• Relaxation of triaxial towards biaxial tension

• Lowering the DBTT and reducing the upper-

shelf energy.

Chao et al 2013

Kimura et al. 2010



Delamination of ODS KIT (LT)L

ST• Delamination similar to crack divider geometry• Existing linearly arranged pores act as weak interfaces

along L and T directions• Energy absorption variation in crack front Non-

uniform crack front• Non-uniform crack front makes ASTM JIC evaluation

complicated

ODS-KIT cutting scheme

LT Orientation

RT 200°C



GETMAT ODS fracture surface microstructure (C-L direction at RT)



Porosity in GETMAT as-extruded microstructure (C-L direction)

2000X 5000X

L-R Plane

2000X 5000X

C-R Plane

L

CR

Extrusion direction



Conclusion



Conclusion

• Linear arrangement of existing pores in material lead to delamination and

advanced cracks

• Load drops due to advanced cracks?

• Fracture toughness decreases with increase in temperature

• More dimple formation and larger crack front at 200°C than at 100°C

• Production method critical for presence and arrangement of pores



Current challenges

Adaptation of testing methods required

• Limited ODS material : use of small specimen• Small specimen setup adaptation: 0.25T C(T) and 0.16T C(T) • Pre-cracking of ODS (GETMAT) difficult

• High temperature COD measurement (UC)• Mechanical clip on gauge (only up to 200°C)• LSE accuracy needs to be improved

• Potential drop method development• Isolation of specimen• Adaptation to small specimen

• Development of analysis methods• UC, Normalization and PD method• Modification for load line offset and outer knife measurement locations



Thank You!

Sources• Busby et al , Economic benefits of advanced materials in nuclear power systems (2009)• ASTM E1820 – 2013 Standard Test Method for Measurement of Fracture Toughness• T.L. Anderon, Fracture Mechanics fundamentals and applications• Amir Shirzadi and Susan Jackson, Structural alloys for power plants• Oh Young Jin, PhD thesis, Development of modified normalization method using blunt notch specimen for

J-R test of nuclear piping material• Chao et al, Notch Impact Behavior of Oxide-Dispersion-Strengthened (ODS) Fe20Cr5Al Alloys• Kimura et al. ,Delamination Toughening of Ultrafine Grain Structure Steels Processed through

Tempforming at Elevated Temperatures

aniruddh das | odisseus workshop 2015| 24.03.15 fracture behavior and mechanisms of ods steels

Documents

odisseus workshop

helmholtz association

mechanisms of ods steels

history of ods alloys

c fracture toughness

motivation slide

al slide

toughness of ods important