2Materials at High Temperature Microstructure Change – Stability of MaterialsGrain growthSecond-phase coarseningIncreasing vacancy densityMechanical Properties ChangeSofteningIncreasing of atoms mobilityIncreasing of dislocations mobility (climb)Additional slip systems
3Time-dependent Mechanical Behavior - CreepCreep: A time-dependent and permanent deformationof materials when subjected to a constant load at a hightemperature (> 0.4 Tm). Examples: turbine blades, steamgenerators.
5Creep CurveTypical creep curve under constant load
6Creep Curve 1. Instantaneous deformation, mainly elastic. 2. Primary/transient creep. Slope of strain vs. time decreases with time: work-hardening3. Secondary/steady-state creep. Rate of straining is constant: balance of work-hardening and recovery.4. Tertiary. Rapidly accelerating strain rate up to failure: formation of internal cracks, voids, grain boundary separation, necking, etc.
7Creep Curve – Constant Stress Comparison between constant load and constant stress
8Parameters of Creep Behavior The stage secondary/steady-state creep is of longest duration and the steady-state creep rate is the most important parameter of the creep behavior in long-life applications.Another parameter, especially important in short-life creep situations, is time to rupture, or the rupture lifetime, tr.
10Power-Law CreepBy plotting the log of the steady creep-rate ss, against log (stress, ), at constant T, in creep curve, we can establishss = BnWhere n, the creep exponent, usually lies between 3 and 8. This sort of creep is called “power-law” creep.
13Creep: Stress and Temperature Effects With increasing stress or temperature:The instantaneous strain increasesThe steady-state creep rate increasesThe time to rupture decreases
14Creep: Stress and Temperature Effects The stress/temperature dependence of the steady-state creep rate can be described bywhere Qc is the activation energy for creep, K2 is the creep resistant, and n is a material constant.(Remember the Arrhenius dependence on temperature forthermally activated processes that we discussed for diffusion?)
18Larson-Miller PlotExtrapolate low-temperature data from fast high-temperature tests
19Creep RelaxationCreep Relaxation: At constant displacement, stress relaxes with time.
20Creep Relaxation tot = el + cr (1) But el = /E (2) and (at constant temperature)cr = Bn (3)Since tot is constant, we can differentiate (1) with respect to time and substitute the other two equations into it give(4)
21Creep RelaxationIntegrating from = i at t = 0 to = at t = t gives(5)As the time going on, the initial elastic strain i/E is slowly replaced by creep strain, and the stress relaxes.
22Creep Damage & Creep Fracture Void Formation and Linkage
24Creep Damage & Creep Fracture Since the mechanism for void growth is the same as that for creep deformation (notably through diffusion), it follows that the time to failure, tf, will follow in accordance with:
25Creep Damage & Creep Fracture As a general rule:ss tf = CWhere C is a constant, roughly 0.1. So, knowing the creep rate, the life can be estimated.
27Creep Design In high-temperature design it is important to make sure: that the creep strain cr during the design life is acceptable;that the creep ductility fcr (strain to failure) is adequate to cope with the acceptable creep strain;that the time-to-failure, tf, at the design loads and temperatures is longer (by a suitable safety factor) than the design life.
28Creep Design Designing metals & ceramics to resist power-law creep Choose a material with a high melting pointMaximize obstructions to dislocation motion by alloying to give a solid solution and precipitates; the precipitates must be stable at the service temperatureChoose a solid with a large lattice resistance: this means covalent bonding.
29Creep Design Designing metals & ceramics to resist diffusional flow Choose a material with a high melting pointArrange that it has a large grain size, so that diffusion distances are long and GBs do not help diffusion muchArrange for precipitates at GBs to impede GB sliding.
33General Electric TF34 High Bypass Turbofan Engine Case Study – Turbine BladeGeneral Electric TF34 High Bypass Turbofan EngineFor (1) U.S. Navy Lockheed S-3A anti submarine warfare aircraft(2) U.S. Air Force Fairchild Republic A-10 close support aircraft.
35Case Study – Turbine Blade Alloy requirements for turbine blades(a)Resistance to creep(b)Resistance to high-temperature oxidation(c)Toughness(d)Thermal fatigue resistance(e)Thermal stability(f)Low density
37Turbine Blade Materials – Nickel-base Superalloys Microstructures of the alloy:Has as many atoms in solid solution as possible ( Co, W, Cr)(2) Forms stable, hard precipitates of compounds like Ni3Al, Ni3Ti, MoC, TaC to obstruct the dislocations(3) Forms a protective surface oxide film of Cr2O3 to protect the blade itself from attack by oxygen
38Turbine Blade Materials – Nickel-base Superalloys Microstructures of the alloy
39Development of Processing Turbine Blade –Development of ProcessingInvestment Casting of turbine blades
40Development of Processing Turbine Blade –Development of ProcessingDirectional Solidification (DS) of turbine blades