Creep, Superalloys And Superplasticity.pdf

Creep and high temperature materials

Engineering structure used in ambient temperature

Design of engineering component • Based on mechanical strength (Tensile, creep, fatigue ………)

• Mechanical strength ability to carry load

• Load Steady load (creep) Unsteady load (fatigue)

• Strength of materials Basic structural sensitive Temperature sensitive

Atomic Model of Polycrystalline Material

50 µ m

• Single crystal with different orientations • Grain boundary has porous structure • Grain boundary – source of weakness at high temperature

Tensile Strength of materials Standard ASTM E8

Structural component used at temperatures < 0.4 Tm -- designed based on tensile strength

STRESS

UTS

Yield point

Fracture STRAIN Plastic deformation

Plastic Deformation in Crystal Schematic Force

Force

Force

Slip along slip plane by dislocation glide

Details of slip plane Force

Actual Force

300 µm Force

Increase in Stress on Continuation of Plastic Deformation (Cross Slip)

stress

Activation of dislocation source on increasing stress on other slip plane to cross slip the dislocation To continue the deformation

τ

Obstacle stress τ

Complex dislocation net work and grain boundary as obstacles to dislocation movement

Grain boundary

Dislocation net work 10 nm

Precipitates as obstacle to dislocation movement Precipitate

Dislocation

Precipitate dislocation interaction

10 nm

Concept of Creep Deformation Holding at a particular stress σ and at relatively high temperature (> 0.4 Tm)

STRESS

what happened to strain with time??

Fracture

strain εo

STRAIN

Deformation under the conditions

Constant (σ )

Stress Time Tempera ture

Constant (T) Time

Strain

εo•

Remained constant, increased or decreased ?? Time

Steam turbine used in power plant

Gas turbine used in Jet engines

The deformation may become so large that a component can no longer perform its function — for example creep of a turbine blade will cause the blade to contact the casing, resulting in the failure of the blade.

Aero Engine ( ≤ 1500 oC)

Definition of High-Temperature Alloys • "High" temperature is broadly defined as 300-1200°C. • Applications for high-temperature alloys are typically 600-1200°C range. • Able to maintain high strengths at high temperatures • Resistance to embrittlement (thermal stability) • Good resistance to creep and rupture at high temperatures • Good corrosion and oxidation resistance at high temperatures • 3 main classes of High Temperature Materials – Ni – Base (Nimonics, Hasteloy, Waspaloy) – Ni-Fe – Base (cheaper than Ni-base Inconel family IN600/718) – Co – Base (Stellites like KC20WN)

Service Environment and Property Requirements
Liquid Engines : Temp ~1400 °C, High Temp. Strength and Good erosion resistance
Thrust Chamber: Temp ~ 2000 °C, High Temperature Strength, Stability
Cryogenic Engine: Temp ~ –253 °C : Mechanical properties, Co-efficient of thermal expansion, toughness

High temperature stress rupture properties are special requirement

Uses for High-Temperature Alloys Aerospace Gas Turbine Engines Land-Based Power Generation Turbines High-Temperature Fasteners (Super alloy or TZM) Combustion Engine Exhaust Valves Space vehicles Rocket engines Submarines Nuclear reactors Hot Working Tooling and Dies

What Properties are Important? • • • • • • • •

Tensile Creep and Stress-Rupture Fatigue and Crack Growth Toughness Oxidation and Corrosion Resistance Wear/Erosion Physical (expansion, conductivity) Machinability

• Other properties may be critical depending on the application.

Selection of Materials for High Temperature Applications (Pure Metals) • Pure Metals or Dilute Alloys -Melting point limitation -Drastic Loss of Strength with increase in Temperature -Oxidation issues -Melting points of selected HT materials -Strength of pure metals as a function of Temperature -Copper alloys (OFHC Cu, Cu-0.8Cr, Cu-Cr-Zr-Ti, Cu-Ag-Zr) for Thrust Chamber Applications -Thermal Conductivity

Effect of high temperature in combination with stress • • • • • •

Atoms get mobility (diffusion-controlled process). This affects mechanical properties of materials. Greater mobility of dislocations (climb). Increased amount of vacancies. Deformation at grain boundaries. Metallurgical changes, i.e., phase transformation, precipitation, oxidation, recrystallisation. High temperature materials/alloys • Improved high temperature strength. • Good oxidation resistance.

Defect in Crystal - Vacancy typically, crystals contain more vacancies at higher temperatures ..vacancies make it easier for atoms to move through the crystal structure; atoms next to a vacancy can jump into it; this process is diffusion

purple atoms have moved from original positions by jumping into adjacent vacancy. note that atoms and vacancies diffuse in opposite direction applied stress creates gradient in vacancy concentration; atoms migrate down gradient causing material to flow

Vacancy  −Q  nv = n. exp   RT 

Nv = No. of vacancies pr cm3 n = No. of atoms per cm3 Qv = energy required to produce one mole of vacancies in cal/mole or j/mole R = gas constant, T = temperature in K

Grain boundary sliding during creep causes (a) the creation of voids at an inclusion trapped at the grain boundary, and (b) the creation of a void at a triple point where three grains are in contact

Creep cavities formed at grain boundaries in an austentic stainless steel (x 500). (From ASM Handbook, Vol. 7, (1972) ASM International, Materials Park, OH 44073.)

Failure Analysis of a Pipe A titanium pipe used to transport a corrosive material at 400oC is found to fail after several months. How would you determine the cause for the failure?

Since a period of time at a high temperature was required before failure occurred, we might first suspect a creep or stress-corrosion mechanism for failure. Microscopic examination of the material near the fracture surface would be advisable. If many tiny, branched cracks leading away from the surface are noted, stress-corrosion is a strong possibility. However, if the grains near the fracture surface are elongated, with many voids between the grains, creep is a more likely culprit.

Evaluation of Creep Behavior  Creep test - Measures the resistance of a material to deformation and failure when subjected to a static load below the yield strength at an elevated temperature.  Climb - Movement of a dislocation perpendicular to its slip plane by the diffusion of atoms to or from the dislocation line.  Creep rate - The rate at which a material deforms when a stress is applied at a high temperature.  Rupture time - The time required for a specimen to fail by creep at a particular temperature and stress.

Slip of an edge dislocation

Dislocations can climb (a) when atoms leave the dislocation line to create interstitials or to fill vacancies or (b) when atoms are attached to the dislocation line by creating vacancies or eliminating interstitials

• Creep occurs when a metal is subjected to a constant tensile load at an elevated temperature. • Undergo a time-dependent increase in length. • Since materials have its own different melting point, each will creep when the homologous temperature > 0.5. • The creep test measure the dimensional changes which occur when subjected to high temperature. • The rupture test measures the effect of temperature on the longtime load bearing characteristics. • Homologous temperature = Testing temperature/Melting temperature

Creep Testing Machine Load application by lever

Standard: ASTM E 139 Stress variation: < 1 % Temperature variation: - up to 1000 oC ± 2 oC - above 1000 oC ± 3 oC

Creep curve under constant load/stress

Constant stress curve

A typical creep curve showing the strain produced as a function of time for a constant stress and temperature

Classical creep curve Creep strength - is the stress at a given temperature, which produces a steady-state creep rate

1) Primary creep provides decreasing creep rate. – strain hardening 2) Secondary creep gives the representing constant creep rate. – compensation of strain hardening by recovery process 3) Tertiary creep yields a rapid creep rate till failure. Associated with effective reduction of cross sectional area because of necking and void formation. Metallurgical changes such as coarsening of precipitates, recrystallisation and diffusional changes happens. For design purpose, Minimum creep rate – 0.0001% per hour or 1%/10000 hours

Cold work

Anneal

Recovery

Recrystallization

Grain growth Low angle grain boundaries

Effect of stress on creep curves at constant Temperature

Structural changes

1) Deformation by slip More slip systems operate at high temperature 2) Sub grain formation 3) Grain Boundary sliding Produced by shear process and promoted by increasing temperature/or decreasing strain rate. Results in grain boundary folding or grain boundary migration.

Mechanism of creep Dislocation glide  Involves dislocation moving along slip planes and overcoming barriers by thermal activation.  Occurs at high stress. Dislocation creep  Involves dislocation movement to overcome barriers by diffusion of vacancies or interstitials. Diffusion creep  Involves the flow of vacancies and interstitials through a crystal under the influence of applied stress. - Occurs at high temperature and relatively low stress Grain boundary sliding  Involves the sliding of grains past each other.

Creep deformation mechanism

STRESS

Stress constant

Tensile curve Yield point STRAIN

strain

Under creep condition Stress: constant Temperature: constant > 0.4 Tm

Glide plane

Obstacle

NOW stress is not sufficient Creep curve

for the dislocation to bypass the obstacle by initiating glide in another slip plane as in tensile test since stress is constant and is not sufficient

Creep Deformation Mechanisms High Temperature ( > 0.4 Tm) Dislocation can climb with the help of diffusion

Vacancy Dislocation Vacancy

Dislocation creep mechanism τ

Diffusion of atoms unlock dislocations from obstacles to glide them for plastic deformation

Climb

obstacle Glide plane

Glide plane

τ Climb

Glide

Glide

Climb

Climb obstacle

τ

Plastic deformation by diffusion of atom itself under stress Stress

Stress Applied stress creates a gradients in vacancy concentration so that atoms migrate down the gradient causing plastic deformation by diffusion itself

Diffusion creep mechanisms Diffusion Creep Biased movement of atom under stress

vacancy

Atom

Flow of vacancies from grain boundaries experiencing tensile stresses to region of compressive stresses. Simultaneously there is a flow of atoms in the reverse direction – elongation of grain

vacancy

σ

Atoms move under stress through matrix

σ

Atom

σ vacancy

Atom

Nabarro-Herring Creep

Atoms move along the grain boundary

σ vacancy

Atom

Coble Creep

Creep Deformation Mechanism Map

Typical uses of deformation mechanism map  Location of testing domains (Creep, tensile tests)  Rate controlling mechanism (Strengthening methods)

Mechanisms of Creep • High rates of diffusion permit reshaping of crystals to relieve stress • Diffusion significant at both grain boundaries and in the bulk • High energy and weak bonds allow dislocations to “climb” around structures that pin them at lower temperature

Creep and stress rupture Parameters

Creep

Stress rupture

load

Low

high

Creep rate

Minimum

High

Test period

2000-10000 h

1000 h

Total strain

0.5%

50%

Creep in a filament bulb

• Sagging due to creep deformation • The adjacent turns of the coil touch one another, causing an electrical short and local overheating, which quickly leads to failure of the filament

Some practical examples • In steam turbine power plants, steam pipes carry superheated vapour under high temperature 565.5°C and high pressure often at 24.131 MPa or greater. • In a jet engine temperatures may reach to ~1000°C, which may initiate creep deformation in a weak zone. For these reasons, it is crucial for public and operational safety to understand creep deformation behavior of engineering materials.

Threshold for Creep The Critical Temperature for Creep is 40% of the Melting Temperature. If T > 0.40 TM  Creep Is Likely

Example Will Lead Creep at Room Temperature? TM = 327 °C = 600 °K TROOM = 23 °C = 296 °K 100 x 296 / 600 =49.3 %  Will Creep

Creep in Ice How cold does your freezer need to be to avoid creep in the ice cubes? 0C = 273 K 0.4 * 273 = 109.2K = -163.8 C!

Creep Fracture – rupture life

Cavity

Cavity nucleation at grain boundary particle

σ

Slip bands

Crack

50 µm

• Cavity nucleates at particles on grain boundary due to

grain boundary sliding • Under stress nucleated cavity grow by diffusion of atoms from cavity surface to grain boundary

CREEP CAVITY GROWTH – IMPURITY EFFECT Creep cavity growth model Creep cavities

Stress

Grain boundary diffusion Surface Surface diffusion diffusion Grain boundaries atom

Cavity growth involves • Cavity surface diffusion (DCS) • Grain boundary diffusion (DGB) • Segregation of S, O, As, Sb etc.on cavity surface ↑ both DCS and DGB • Segregation of B segregation on cavity surface ↓ both DCS and DGB

600 h

900 h

1200 h

1500 h

1800 h

The progress of creep cavitation in 347BCe steel, creep tested at 78 MPa and 1023 K, showing the nucleation of r-type creep cavities and their growth. The lines shown were inscribed on the specimen surface by focused ion beam to study grain boundary sliding

Fracture mode

Strength of GB = grain at the equicohesive temperature (ECT). Below ECT small grain sized material is stronger due to high density of grain boundaries to improve strength. Above ECT large grain sized material is stronger due to less tendency for grain boundary sliding. Note: Single crystal structure is therefore appreciable for high temperature applications, i.e., nickel base alloy single crystal turbine blade.

Fracture mode Transgranular fracture Slip planes are weaker than grain boundaries

Intergranular fracture Grain boundaries are weaker than slip planes.

Note: at T just below Trecrys, ductility drops due to grain boundary sliding – leads to intergranular failure.

Intergranular fracture

How Do We Deal With Creep ? •

Reduce the effect of grain boundaries Use Single Crystals

• • •

Promote a mechanism to Inhibit Slip Change Materials Change Operating Conditions

Design to Resist Creep • Select high melting point material to increase elastic constant and to decrease self diffusion • Increase grain size (Single crystal) to avoid creep cavitation • Directional solidification to suppress creep cavitation • Restriction of grain boundary sliding by precipitation on the grain boundary or making the grain boundary corrugated. • Precipitation hardening to obstruct dislocation glide • Composite reinforcement to restrict mass flux /dislocation movement • Internal cooling to reduce temperature • Thermal barrier coating to reduce temperature • Metals with low stacking fault energy has good creep resistance (resistance to cross slip)

Summary • Strength of materials Structure, temperature and defect sensitive • Creep Plastic deformation under constant load and temperature • Two creep deformation mechanisms Dislocation creep ( dislocation climb + glide) Diffusion creep Nabarro-Herring creep (matrix diffusion) Coble creep (grain boundary diffusion) • Creep fracture Nucleation, growth and coalescence of creep cavities (Rupture life, tr)

High temperature alloys • High temperature alloys are complex in their microstructures to obtain the required properties at service temperatures. • High melting point alloys normally has high creep resistance. – NI (1453C), Co (1495C), Fe (1539C)

• Metals with high stacking fault energy is easy for slip and prone to creep. • Fine precipitates having high thermal stability are necessary for high creep resistance (prevent grain growth). – Ex: (1) Nickel base alloy containing fine precipitates of intermetallic compounds Ni3Al, Ni3Ti or Ni3(Al,Ti), – (2) Creep resistance steels containing fine carbides VC, TiC, NbC, Mo2C or Cr23C6.

Superalloy • A superalloy is a metallic alloy which can be used at high temperatures, often in excess of 0.7 of the absolute melting temperature. • Creep and oxidation resistance are the prime design criteria. • Superalloys can be based on nickel, nickel-iron or cobalt based – Nickel based alloys have highest strength at elevaed temperatues, followed by Ni-Fe and Co based alloys – All alloys have FCC structure (aids forgeability)

Superalloys • Combination of high strength, good fatigue and creep properties, good corrosion resistance – Metallurgical stability • Used in hot portion of jet engines such as blades, vanes, combustion chambers, etc.. • Strengthening Precipitation hardening - Ni3(Ti, Al), Co3(Al,Ti) Solid solution hardening (Fe, Cr, Co, Mo, W, Ti, Al etc) Carbide hardening γ’ - - Ni3(Ti, Al) – FCC ordered phase – low mismatch – high coherency between γ matrix and γ’ precipitates (spherical particles) – low coarsening rate- long term stability – Degree of ordering increases with temperature – – – –

Microstructure of super alloy

Superalloys γ’’-Ni3Nb found in iron-nickel based alloys Cr and Al – Good oxidation resistance Al2o3, Cr203 Larger Al and Ti – lowers the melting point of super alloys Large number of alloying elements – Forms TCP phases – needle like structure – embrittlement Metallic carbides (MC, M23C6, M6C, M7C3) from in both grain boundary and well as within grains - grain boundary carbides prevents grain boundary sliding and permits stress relaxation - carbides in the grain increases strength -can tie up with other elements that would otherwise promote instability during service - Proper heat treatment to be carried out to prevent Continuous network of carbides – easy fracture path

Material Distribution in a jet Engine

Material Distribution in a GE Engine

A turbine blade experiences

Mechanical forces Creep Fatigue Thermo-mechanical fatigue

High Temperature Environment Oxidation Hot corrosion

Effect of Alloying elements in Superalloys

Improper alloy control can result in undesirable phases such as σ, µ and Laves phases (AB2). (plate like/needle like morphology)

Manufacturing Wrought form Cast form P/M Large grained cast alloys – turbine blade applications Small grained forged alloys – Turbine disc applications Cast alloys – more alloy segregations and coarse grain size – better creep and stress rupture properties Wrought alloys – uniform with fine grain size – superior tensile and fatigue properties

Manufacturing methods of Superalloys

Ingot metallurgy route Triple melting – VIM + ESR + VAR VIM Desired alloy configuration, reduce dissolved gases, eliminate slag/dross coarse and non-uniform grains, shrinkage, alloy seggregation ESR Remove oxygen containing inclusions VAR reduce compositional segregation during ESR

Homogenization treatment and Hot working

Processing of Superalloys – Ingot Metallurgy route

Investment Casting Process for turbine blades

Processing of Superalloys – P/M route

Cast superalloys – Turbine blade

Creep/Stress rupture properties

980 deg C

Directional solidification

Single Crystal Turbine Blade How to achieve it?

Single crystal initiation

Single crystal solidification

Microstructure of Ni based Superalloy

Applications

Cast turbine airfoils and other high-integrity investment-cast gas turbine components

Machined flat disk for aircraft gas turbine

IN-718 in ISRO

Deep drawn IN718 shells for Ni-H2 cell case

superplasticity

S U P E R P L A S T I C I T Y

Superplasticity  Superplasticity is the ability to withstand very large deformation in tension without necking.  Give elongation > 1000%.  Materials with high strain rate sensitivity (m) at high temperature (T>0.5Tm) superplasticity  Materials characteristics: fine grain size (<10 μm) with the presence of second phase of similar strength to the matrix to inhibit grain growth and to avoid extensive internal cavity formation.  Grain boundary should be high angle and mobile to promote grain boundary sliding and to avoid the formation of local stress concentration respectively.

Two-Sheet SPF/DB Aircraft Door

Superplastically formed parts

Pressure vessels

references • • • •

Constitutive behavior of superplastic materials, International Journal of Non-Linear Mechanics Volume 37, Issue 3, April 2002, Pages 461–484 F.C. Campbell, Manufacturing Technology for Aerospace Structural materials George E Dieter, Mechanical Metallurgy Donald R Askeland, Essentials of Materials science and Engineering ASM, Heat Resistant Materials

•

Matthew J. Donachie, Stephen J. Donachie, Superllaoys, A technical guide

•

Classes of High-Temperature Alloys

Base Fe

Class

Super 12% Cr Martensitic Stainless Steels 20-23% Cr Austenitic Valve Steels

Fe-Ni

Age-Hardenable Superalloys

Ni

Age-Hardenable, Low-Expansion Superalloys

Co

Solid solution strengthened alloys