NEEP 541 – Creep Fall 2002 Jake Blanchard

Outline Creep

Creep Creep=time dependent deformation under an applied load Generally occurs at high temperature (thermal creep) Irradiation can cause it at low temperature (irradiation creep) Exceptions: lead and glass creep at low temperature Temperature is measured relative to the melting temperature

Constant Stress Strain rate is constant Weight falls at constant rate

Constant Strain Fix total strain

Creep Rupture Defined as failure of a specimen that has been subjected to stresses below the yield stress The key parameter is the time to failure Primary variables are stress and temperature Tertiary creep is associated with both necking and grain boundary void formation, leading to rupture

Creep Tests Apply load and measure deformation as a function of time primary secondary tertiary Creep strain time

Thermal Creep Mechanisms Dislocation glide=dislocations overcome barriers by thermal activation Dislocation creep=movement of dislocations by diffusion of vacancies and interstitials Diffusion creep=flow of point defects by stress induced diffusion Grain boundary sliding=sliding of grains past each other

Radiation Effects Temperatures are irradiation temperatures

Radiation Effects Radiation lowers thermal creep rate due to production of defects which impede dislocation motion Radiation also lowers rupture time due to plastic instability

Plastic Instability Stability point determined by true stress-true strain curve Material is unstable when Radiation reduces strain at instability

Effect of neutron fluence on rupture life

Two Different Fracture Mechanisms Transgranular=voids grow at inclusions, coalesce, and lead to fracture of grains Intergranular=voids grow at triple points, leading to grain separation Triple point

Crack Initiation Wedge cracks initiate when the applied stress exceeds some critical stress Griffith criterion determines critical stress Griffith criterion=change in elastic strain energy during crack growth must balance energy required to create new crack surface

Model Problem for Crack Initiation Crack length=2c 2c

Crack Initiation Model Hardening reduces surface energy, thereby reducing fracture stress Therefore, depleted zones reduce fracture stress

Crack Growth After initiation, crack width becomes important Olander shows (d=grain size): Ductility can be improved by increasing surface energy or decreasing grain size Failure strain

Grain Boundary Voids Voids grow on grain boundaries at stresses below crack initiation stress Growth mechanism is the absorption of vacancies Voids coalesce, leading to intergranular fracture

Number of voids per grain boundary area Model Prediction Number of voids per grain boundary area Ductility reduced by increasing grain size or increasing number of voids on grain boundary.

Helium Embrittlement Helium is produced by transmutation reactions Thermal neutrons: 10B+n -> 7Li+4He 58Ni+n -> 56Fe+4He Fast Neutrons Many (n,) reactions

He Production Cross Sections (mb) Material LWR Fusion Mo .046 4.5 Nb .026 2.4 V .06 5.2 Al 0.28 32.5

Embrittlement of Austenitic Steels At low temperatures, ductility is reduced due to plastic instability As temperature increases, point defect mobility anneals out larger defects and ductility increases At high temperatures, ductility is reduced due to He embrittlement

Embrittlement of Ferritic Steels Less embrittlement due to: Reduced Ni content High diffusion rates, reducing stress concentrations at grain boundaries Defect mobility leads to recrystallization (grain growth) and recovery (softening due to annealing of dislocation network) Problem with ferritic steels is drop in DBTT

Cottrell-Petch Theory Compare yield stress to fracture stress For very small grains, fracture stress is higher than yield Situation reverses for large grains Irradiation changes point at which reversal occurs yield friction source fracture

Effect of Grain Size on Yield and Fracture Stresses

Effect of T on Yield and Fracture Stresses

Effect of fast neutron fluence on NDT