Creep in Metals Flashcards
(17 cards)
Intro to Creep in Metals
- Mechanical strength of metals decreases with increasing temperature and the properties become much more time dependent.
- Developments in high temperature alloys with improved high temperature strength and oxidation resistance have had to keep pace with these demands, and applications like rocket engines present greater problems.
- Metals subjected to a constant load at elevated temperatures will undergo ‘creep’, a time dependent increase in length. The terms ‘high’ and ‘low’ temperature in this context are relative to the absolute melting temperature of the metal.
- At homologous temperatures of more than 0.5, creep is of engineering significance.
Effects of High Temperature on Metals
- Alloys developed for successful use at high temperatures must cope with the following effects:
. Lower Strength
. Greater atomic and dislocation mobility, assisting dislocation climb & diffusion.
. Higher equilibrium concentration of vacancies.
. New deformation mechanisms, such as new slip systems or grain boundary sliding.
. Recrystallisation and grain growth.
. oxidation and interganular penetration.
High Temperature Mechanical Tests
.Different tests may be required to evaluate high temperature properties, based on the time scale of the service requirements.
These may include the following:
- High Temperature Temperature Tensile Test: Similar to a short term room temperature test, i.e. completed in a few minutes and producing stress versus strain curves at specific temperatures.
- Measures dimensional changes accurately at constant high temperature and constant load or stress.
- Stress Rupture Test: Measures time to failure at specified stress and temperature. Useful for applications where some strain can be tolerated but failure must be avoided, such as large furnace housings.
The Creep Curve
- Creep in metals is defined as time dependent plastic deformation at constant stress and temperature.
Primary Creep
- In which the creep resistance increases with strain leading to a decreasing creep strain rate.
Secondary Creep
- in which there is a balance between work hardening and recovery processes, leading to a minimum constant creep rate.
- The minimum secondary creep rate is of most interest to design engineers, since failure avoidance is required and in this region some predictability is posibble.
Tertiary Creep
in which there is an accelerating creep rate due to the accumulating damage, which leads to creep rupture, and which may only be seen at high temperatures and stresses and in constant load machines.
Engineering Creep Data
- The multitude of creep curves involving the variables of stress, strain, time and temperature can be presented in other forms which are more convenient for the design engineer, depending on the question for which answer is required.
Isometric stress-time curves
- These are obtained by plotting constant strain lines on creep curves at various stresses and constant temperature.
- The Isometric stress-time curves enables the time required to reach a particular strain to be read off for a specified stress and temperature.
Isochronous Stress-Temperature Cruves
- These are obtained by plotting constant time lines on curves and reading off the combination of stress and temperature required to produce a specified strain.
Mechanisms of Creep in metals
There are three basic mechanisms that can contribute to creep in metals, namely:
- dislocation slip and climb
- grain boundary sliding
- Diffusional flow
Dislocation slip and climb
- Dislocations are line defects that slip through a crystal lattice when a minimum shear stress is applied.
- An edge dislocation consists of an unfinished atomic plane, the edge of the plane being the line of 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.
Grain Boundary Sliding
- The onset of tertiary creep is a sign that structural damage has occurred in an alloy.
- Rounded and wedge shaped voids are seen mainly at the grade boundaries and when these coalesce creep rupture occurs.
- The mechanism of vid formation involves grain boundary sliding which occurs under the action of shear stresses acting on the boundaries.
Diffusional Flow
- The third distinct mechanism for creep is significant at low stress and high temperature. Under the driving force of the applied stress, atoms diffuses from the sides of the grains to the tops and bottoms.
- The grains becomes longer as the applied stress does work, and the process will be faster at high temperatures as there are more vacancies.
- For diffusion paths through the grains the atoms have a slower jump frequency, but more paths, and is called Nabarro-Herring creep.
Coble Creep
- Along the grain boundaries the jump frequency is higher, but fewer paths exist, and this mechanism is called Coble Creep.
Activation Energy for Creep
The activation energy for creep may be calculated by assuming equation
Creep Life Prediction
- Creep tests take a long time to perform making the generation of design data expensive and the lead time between developing a new alloy and it’s exploitation excessive.
- The fact that there is an Arrhenius relation between creep rate and temperature has led to a number of time-temperature parameters to be developed which enable extrapolation & prediction of creep rates or creep rupture times to longer times than have been measured. They also enable rating comparisons to be made between different materials.
- It is important that no structural changes occur in the region of extrapolation, but since these would occur at shorter times for higher temperatures it is safer to predict below the temperature for which data is known than above. One parameter used is the Larson-Miller Parameter.
This is derived by taking natural logs of the Arrhenius equation
