http://www.academia.edu/5908670/SRINIVASAN_ENGINEERING_COLLEGE_AE2354_HIGH_TEMPERATURE_MATERIALS_16_MARKS_UNIT-1_CREEP 

AE2354 HIGH TEMPERATURE MATERIALS16 MARKS

13 : 3, 4, 5, 6, 7, 8, 9, 11, 12, 13, 15(optional• Definition of Creep and Creep Curve : (13-3)• def. Creep is the time-dependent plastic strain at constant stress and temperatureCreep curve : Fig. 13-4• steady-state creep-rate (D s or simply D ) : Temperature and Stress Dependencies- Fig. 13-6 Fig. 13-8- total creep curve : = o + p + so = instantaneous strain at loading (elastic, anelastic and plastic)s = steady-state creep strain (constant-rate viscous creep ) = D stp = primary or transient creep : Andrade-flow (or 1/3 rd law) : t1/3primary or transient creep :• Andrade-flow (or 1/3 rd law) : p = t1/3 ⇔problem as t→0• Garofalo / Dorn Equation : p = t (1 - e-rt ) , r is related tosi>> (~1-20)Dorn ⇒ Both primary and steady-state follow similar kinetics- temperature compensated time (= t e- Qc/RT)- single universal curve with t replaced by or ÝstOr, creep strain - o = t (1 - e-D st ) + D st ⇔see Sherby-Dorn (Al), Murty (Zr)Sherby-Dorn -parameterCreep curves for Al at Sherby & Dorn (1956) A single curve demonstrating the(3,000 psi) and at three different temperatures validity of -parameterKL Murty MSE 450 page 2ÝstCreep data in Zircaloy at varied temperatures (°F)and stresses (ksi) fall into a single curvedemonstrating the validity of Dorn equation(Murty et al 1976) (K. L. Murty, M.S. Thesis, 1967)• Zener-Holloman : Z DeQ/ RT

• Stress Rupture Test : (13-4) vs tr• Representation of engineering creep / rupture data (13-12, 13-13)- Figs. 13-17, 13-18• Sherby-Dorn Parameter : PS-D = t e-Q/RT• Larson-Miller Parameter : PL-M = T (log t + C) Fig. 13-19-21

• Manson-Haferd Parameter : PM-H =T - Talog t - log ta--- these parameters are for a given stress and are functions of (Fig. 13-20) ---• Monkman-Grant :Cstr Eq. 13-24Demonstration of Monkman-GrantRelationship in Cu (Feltham and Meakin 1959)KL Murty MSE 450 page 3Creep Under Multiaxial Loading(text 14-14)Use Levy-Mises Equations in plasticityeff =12 (1-2)2 + (2-3)2 + (3-1)2and d1 =deffeff[1 - 12(2+3) ] ,since creep is plastic deformation 1/2 appears as in plasticity.Similarly, d2 and d3.Dividing by dt, get the corresponding creep-rates,D 1 =Ýeffeff[1 - 12(2+3) ], etc.One first determines the uniaxial creep-rate equation,D s = A n e-Q/RTand assume the same for effective strain-rate : D eff = A neff e-Q/RTso that Ý1 = A n-1eff e-Q/RT [1 - 12(2+3)] etc.Stress RelaxationAs noted in section 8-11, the stress relaxation occurs when the deformation is held

constant such as in bolt in flange where the constraint is that the total length of thesystem is fixed.t = E + creep = const. Here, E =E .Thusdtdt = 0 =1Eddt + D s Or,ddt = - E D s = - E A n @ fixed TIntegration from o to t gives,⌡ ⌠ifdn = - E A ⌡⌠otdt = - E A t[1 ( 1) 1 ]1/( 1)( ) −−−n noofinal AE n tor t0 1000 2000 3000 4000 500020406080100

Data from "HW #8-8"time, hrKL Murty MSE 450 page 4• Deformation / Creep Mechanisms :• Introduction - structural changes (13-5)- Slip (difficult to observe slip lines / folds etc are usually noted)SubgrainsGBS- excess (deformation induced) vacancies• Two important relationships :

Orowan equation : D =bv and Taylor equation : =22G2b2• Thermally Activated Dislocation Glide (at low T and/or high strain-rates)D = A eBe-Qi/RT where Qi is the activation energy for the underlying mechanismsPeierls mechanism (bcc metals) Intersection mechanism (fcc and hcp metals)• Dislocation creep - (lattice) diffusion controlled glide and climb• Diffusion creep - (viscous creep mechanisms mainly due to point defects) - at lowstresses and high temperatures• Grain-Boundary Sliding - (GBS) - intermediate stresses in small grainedmaterials and ceramics (where matrix deformation is difficult)• Many different mechanisms may contribute and the total strain-rate :parallel mechanism series mechanisms(fastest controls / dominates) (slower controls / dominates)D =iΣ D i D = Σ− 1 1_i

Slip following creep deformation in -ironUncrept specimen Crept at 5500 psi to 21.5% strain(K.L. Murty, MS thesis, Cornell University, 1967)KL Murty MSE 450 page 5• Dislocation Creep :• Pure Metals / Class-M alloys: Experiments :D = A n e-Qc/RT,n ≈ 5, Qc ≈ QL (QD)(edge ⊥) glide - climb model Weertman-Climb model (Weertman Pill-Box Model)• sequential processesL = average distance a dislocation glides

tg = time for glide motionh = average distance a dislocation climbstc = time for climb⊥⊥⊥FR LhLomer-CottrellBarrier= strain during glide-climb event = g + c ≈ g = b Lt = time of glide-climb event = tg + tc ≈ tc =hvc , vc = climb velocity∴D =t =b Lh/vc = b (Lh) vcwhere vc ∝Cv e-Em/kT , Em = activation energy for vacancy migrationHere, Cv = C+v - C-v = CoveV/kT - Cove-V/kT = Cov 2 Sinh(VkT )∴D = b (Lh) vc = b (Lh) Cove-Em/kT 2 Sinh(VkT )At low stresses, Sinh() ≈so thatD = A1 b (Lh) Cove-Em/kT V

kTD = A1 b (Lh) DLVkT ≈ A2 (Lh) DLOr D A3D ⇔natural creep-lawWeertman:Lh∝1.5, D = A 4.5 D asexperimentally observed in AlIn general D = A(T) n Power-law- n is the stress exponent{f(xal structure, )}Garofalo Eqn.D = A D (sinhB)nalso known as Norton’s Equation(n is Norton index)At high stresses (≥ 10-3 E), Sinh(x) ≈ ex, D = AH eBD (Power-law breakdown)KL Murty MSE 450 page 6Experimental Observations - Dislocation CreepFig. 13-13 (Dieter) (Sherby)What happens if we keep decreasing the stress, say to a level at and below the FR?As is decreased ⇒ reach a point when ≤ FR ,dislocation density would become constant (independent of ): D∝- viscous creep known as Harper-Dorn creepHarper-Dorn creep occurs atE ≤ b ≈ 10-5 , o ≈ 106cm-2• H-D creep is observed in large grainedmaterials (metals, ceramics, etc.)D HD = AHD DLlnln

Characteristics of Climb Creep (Class-M) :• large primary creep regions• subgrain formation (∝1)• dislocation density ∝2• independent of grain sizeKL Murty MSE 450 page 7• Effects of Alloying : (class-A)• Solid-solution - decreases rate of glide _ glide controlled creep althoughannihilation due to climb still occurs (micro-creep / viscous glide creep)viscous glide controlled creep : (decreased creep-rates)D g = Ag Ds 3 , Ds is solute diffusion• little or no primary creep• no subgrain formation• ∝2• grain-size independent5311class-Mclass-Alog(stress)(Al)(Al-3Mg)• At low stresses (for large grain sizes), Harper-Dorn creep dominates⇒ what happens as grain size becomes small ⇐As grain-size decreases (and at low stresses) diffusion creep due to point defectsbecomes important : (due to migration of vacancies from tensile boundaries tocompressive boundaries)• Nabarro-Herring Creep (diffusion through the lattice) : D NH = ANH DLd2• Coble Creep (diffusion through grain-boundaries) : D Co = ACo Db

d3Nabarro-Herring Creep vs Coble Creep :Coble creep for small grain sizes and atlow temperatureNH creep for larger grain sizes and athigh temperatures• at very large grain sizes, Harper-Dorncreep dominates3211log (grain-size)N-HCobleHarper-DornAt small grain-sizes, GBS dominates at intermediate stresses and temperatures :• D GBS = AGBS Db2d2 ⇔superplasticityKL Murty MSE 450 page 8• Effect of dispersoids : Dispersion Strengthening / Precipitate Hardening- recall Orowan Bowing• at high temperatures, climb of dislocation loops around the precipitatescontrols creep ⇒ D ppt = Appt D 8 - 20Rules for Increasing Creep Resistance• Large Grain Size(directionally solidified superalloys)• Low Stacking Fault Energy(Cu vs Cu-Al alloys)• Solid Solution Alloying(Al vs Al-Mg alloys)• Dispersion Strengthening(Ni vs TD-Ni)Formability Improvement• Small (stable) Equiaxed Grain Size(superplasticity)• Strengthen Matrix

(i.e., increase GBS - ceramics)• Stoichiometry(especially Ceramics)KL Murty MSE 450 page 9Summary of Creep Mechanisms: D t = D N-H + D Coble + D H-D +D GBS +11 1− _c _g

Dorn Equation :nEADEbkT _ Mechanism D n AClimb of edge dislocations DL 5 6x107(Pure Metals and class-M alloys) (n function of Xal structure & *Low-temperature climb D⊥7 2x108Viscous glide (Class-I alloys - microcreep) Ds 3 6Nabarro-Herring DL 1 14 (bd)2Coble Db 1 100 (bd)3Harper-Dorn DL 1 3x10-10GBS (superplasticity) Db 2 200 (bd)2DL = lattice diffusivity; Ds = solute diffusivity; D⊥= core diffusivity;Db = Grain-Boundary Diffusivity; b = Burgers vector; d = grain size;= subgrain size = 10Gb

and = G2b2 where G is the shear modulus*n increases with decreasing (stacking-fault energy)KL Murty MSE 450 page 10Deformation Mechanism Maps• Visual picture of the domains (, T) where various mechanisms dominateAshby-MapLead pipes on a 75-year-old building in southern EnglandThe creep-induced curvature of these pipes is typicalof Victorian lead water piping. (Frost and Ashby)Other examples :• W filament (light bulbs)• turbind blade {Ni-based alloy DS by Ni3(Ti,Al)}

KL Murty MSE 450 page 11

UNIT-1 CREEP 1.Explain in detail about the factors influencing functional life of components at elevatedtemperatures. (√)

2.List out the metallurgical factors influencing various stages of creep using a suitablediagram, explain these stages. (√)

3.What are the structural changes involved during creep? Explain in detail.

4.Briefly explain the three steps of creep with a creep curve. (√)

5.Discuss the mechanism of creep deformation briefly with necessary sketches.

6.It is well known that at high tempe rature materials are subjected to creep corrosion andmicro structural changes. Analyse each of the things in limiting functional life oncomponents.

7.Write technical notes on various creep mechanisms.

8.It is well known that the functional life time of material put into service at hightemperature are subjected to three dimensional damages by way of creep, corrosion andmicro structural changes. For each type write on the various damage mechanisms and themethod to combat them.

9.It is well known that to predict long term properties accelerated short term tests are to becarried out and extrapolating procedure by parametric approach is to be followed.Analyse how various parameters are evolved based on the material behaviour and how tochoose the best parameters for Master curve.

10.Write the constitutive equations for plastic and creep deformations from high stress andlow temperature to low stress and high temperature and comment on the mechanism ofdeformation for each equation.

UNIT-2DESIGN FOR CREEP RESISTANCE

1.Analyse time hardening and strain hardening mathematically and pictorially and provethat for gradual loading strain hardening reduces strain hardening. 2.It is Obvious that as the material creeps it proceeds from primary stage into a steady stateand accelerates into the tertiary stage with a end result of fracture.(i) For creep ductile failure or rupture the area of cross section that supportsloadreduces to ze ro. Analyse how to compute time to rupture based on initialconditions.(ii) (√)

(ii) For creep brittle fracture void nucleate and grow form the onset of steadystate and in tertiary state voids coalesce into cracks which propagates toproduce fracture. This can be modelled by defining a damage parameter wwhich changes from zero at virgin state to unity at rupture .Based on thismodel compute time for brittle fracture. 3.Analyse hidden variables or internal variables on the primary and steady state creepdeformation.4. Derive expression for Creep ductile and creep brittle fractures .Also comment on theinstantaneous fractures in both cases.5. Analyse time hardening strain hardening pictorially and mathematically and prove that atconstant stress the strain hardening reduces to time hardening.6. Based on steady state creep rate derive expressions for creep ductile and brittle fracture.7. (i) How will you obtain creep resistance through strain hardening?(ii)Explain stress rupture test at elevated temperature.8. What are the approaches there for design under conditions of creep-fatigue interaction?Explain aii with neat sketches.9. Derive the various methods adopted in representing rupture life of creep(√).10. (i)Briefly explain the influence of brittle and ductile materials on creep. (√) (ii) Discuss the conditions favourable for creep cavitation.

UNIT-3FRACTURE1. Derive an expression for cleavage I failure based on Orowan and Griffith Theory andcomment how the various terms in these equations are responsible for strengthening andtoughening the materials.2. Derive an expression for cleavage II failure and ductile- brittle transition based onMicro void (√)formation by plastic deformation and comment on the various metallurgicalparameters affecting ductile-brittle transition.3. Analyse technically the following fractures (4X4=16)(i) Trans Granular Ductile fracture.(ii) (√)

Inter granular creep fracture.(iii) (√)

Pure diffusional Fracture.(iv) (√)

Rupture.4. Draw fracture maps for a bcc material or an oxide and explain various regimes.5. What are the approaches there for design under conditions of creep-fatigueinteraction? Explain all with neat sketches.6. Discuss fractures at elevated temperature with fracture mechanism map.7. Draw the fracture maps.(i) (√)

For pure aluminium and commercially pure aluminium and explain theabsence of intergranular creep failure field and large transgranular regime forpure aluminium.(ii) For nickel, a nickel base super alloy and thoria dispe rsed nickel and explainthe shrink and disappearance of some regimes.(iii) For a bcc or a ceramic and mark all the regimes of fracture.8. Briefly explain the theory of Ductile to Brittle transition with neat diagram. (√)9. Derive Orowon and Griffith theory of cleavage I failure.10. What are the various types of fracture? (√)Classify the fracture on the basis of type ofloading extent of deformation and appearance

  UNIT-4:OXIDATION AND HOT CORROSION1. Compare the parameters in controlling aqueous and dry corrosion. Discuss the kineticlaws of high temperature corrosion with special effect on the doping on the corrosionkinetics.2. Analyse the hot corrosion mechanisms involved in basic and acidic fluxing.(i) Analyse how doping of a n-type and p-type semiconductor oxides influencecorrosion kinetics.(ii) Analyse various kinetic laws of oxidation. (√)3. What are the kinematics principles in the Oxidation? Discuss in detail.4. Describe the different metal-gas reactions and their (√)remedies(i) It is well known that for corrosion resistance at high temperature there is aneed for a formation of a adherent oxide scale to prevent the diffusion ofdeleterious species such as

oxygen and sulphur through the scale and into thealloy. This is nearly impossible since the oxide scales are non-stoichiometricand defective such as n-type or a p-type oxide. Analyse how alloying ordoping can influence the defect population and corrosion in defect structures.(ii) Analyse the various stages of hot corrosion with respect to exposure timevarious chemical equations and degradation mechanisms.5. Fluxing in hot corrosion in loss of protective oxide due to dissolution. Analyse thevarious models for basic and acidic fluxing and effect of electrochemical polarisation onfluxing of various oxides with the help of phase stability diagrams6. Explain the various stages of hot corrosion with mechanisms and super alloys.7. Discuss the kinetic laws of oxidation and the fundamentals behind various method ofcombating corrosion (√)

 

 UNIT-5:SUPER ALLOYS AND OTHER MATERIALS1. In developing a super alloy for high temperature application how alloy chemistry can be judiciously employed to produce resistance for creep, corrosion and micro structuralinstability.2. It is also possible to produce resistance to the above three dimensional damage bydifferent processing routes. Analyse please.3. Write a technical note on the evolution of supe r alloys.4. Analyse how the processing parameters can influence the mechanical properties of superalloys at high temperatures.5.

Briefly explain the major phases of nickel-base alloys(√)6. Explain strengthening of cobalt-base super alloys. (√)7. Draw fracture maps for pure Ni chrome a solid solution and a precipitation hardenednickel base super alloy and explain how the boundaries shifts due to alloying and precipitation.