Creep in metals

Question 1

Viscoelastic materials

Answer 1

• strength is very dependent on time factor at room temperature.
• at high temperature, time factor becomes more important
• metals subject to elevated temperatures will undergo creep with time

Question 2

Effect of high temperature on metals

Answer 2

• lower strength
• greater atomic and dislocation mobility
• higher equilibrium concentration of vacancies
• new deformation mechanisms
• recrystalisation and grain growth
• over-aging of age hardened alloys
• oxidation and inter granular penetration

Question 3

High temperature mechanical tests for evaluation

Answer 3

1. tensile test
   - produce stress vs strain curves at specific temperatures, provides u-seful data for short term applications.
2. creep test
   - measures dimensional changes accurately at constant high temperature and constant load.
   - useful for long term applications
3. stress rupture test
   - measures time to failure at specified stress and temp
   - useful for application where train can be tolerated but failure must be avoided.

Question 4

Stages of creep

Answer 4

1. Primary creep
   - creep resistance increases with strain leading to a decreasing creep strain rate
2. Secondary creep
   - in which there is a balance between work hardening and recovering, leading to a minimum constant creep rate
3. tertiary creep
   - in which there is an accelerating creep due to accumulating damage, which leads to a creep rupture, and which may only be seen at high temperatures and stresses in constant load machines.

Question 5

Mechanisms of creep in metals

Answer 5

1. Dislocation slip and climb
2. Grain boundary sliding
3. Diffusional flow

Question 6

Dislocation slip and climb

Answer 6

• Mechanism for creep
• Line defects that slip through a crystal lattice when a minimum shear stress is applied
• Initially slip along the closest packed planes as this requires the least energy.
• Characterized by a burgers vector at 90 degrees to the dislocation line.
• Screw dislocations have a burgers vector parallel to the dislocation line and can slip on any close packed plane containing both line and burgers vector

Question 7

Grain boundary sliding

Answer 7

• Mechanism for creep
• Onset of tertiary creep is a sign that structural damage has occurred in an alloy
• rounded and wedge shaped voids are seen mainly at grain boundaries, and when they coalesce creep rupture occurs
• mechanism of void formation involves grain boundary sliding which occurs from shear stresses acting on the boundaries.

Question 8

Diffusional flow

Answer 8

• Mechanism for creep
• significant at low stress and high temp
• atoms diffuse form the sides of the grains to the tops and bottoms
• grain becomes longer as the applied stress does work
• process faster at high temperatures as there are more vacancies

Question 9

Creep life prediction

Answer 9

• Creep tests take a long time and are expensive , making developing a new alloy excessive.
• Arrhenius relationship between creep rate and temp, meaning parameters have been developed to enable prediction of creep rates or creep rupture times.
• Larson-miller parameter used for predicting creep.
• Larson-miller not useful always, as it assumes worst case scenario, which would lead to making a system less efficient by being too safe.
• Miner’s law in fatigue can be adopted, stating that rupture occurs when the sum of all fractions in the rupture life becomes equal