1. 11/22/ /22/2018
HOT MATERIALS
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2. Some material properties have a linear dependence on temperature while others may have an exponential relationship

3. Creep
At room temperature, most metals and ceramics deform in way that depends on stress but no on time.
As the temperature is raised, loads that are too small to give permanent deformation at room temperature cause materials to creep.
Creep is slow, continuous deformation of a material at elevated temperatures, ending in fracture

4. Specimen is loaded in tension or compression, usually at a constant load, inside a furnace that is maintained at a constant temperature

5. Steady-state creep rate
The constants ε0, σo, n, and Qc are experimentally found and vary from material to material

6. Design data based on creep is generally presented in a stress- rupture curve
This allows you to identify either the design stress or rupture life at a given temperature
Figure 13.5

7. The temperature at which a material starts to creep depends on its melting point
Polymers can start to creep at room temperature
Metals – 0.35Tm
Ceramics – 0.45Tm

8. Figure 13.7

9. At room temperature, material selection requires only a single- strength-density chart
For high-temperature design, charts are needed that account for temperature and an acceptable strain rate

10. Diffusion is the spontaneous intermixing of atoms over time – the rate of diffusion is expressed by Fick’s law:
D: diffusion constant
dc/dx: concentration gradient
In a crystalline solid,
two things are needed
for an atom to switch
sites:
Enough thermal energy
An adjacent vacancy
Figure 13.9

11. The mean distance that one type of atom
Figure 13.11
The mean distance that one type of atom
travels from diffusion is given by

12. Iron is hardened by introducing carbon through solid solution and precipitation strengthening
Carburizing is a type of surface engineering technique in which carbon diffuses into the surface of a metal, locally increasing the concentration to that of a harder alloy
Figure 13.12

13. Diffusion can cause creep as well as fracture due to creep by creating voids that nucleate on grain boundaries
Figure 13.16

14. Figure 13.20

15. Typical operating conditions of 650°C at 15 MPa
Figure 13.21
Typical operating conditions of 650°C at 15 MPa
Figure 13.22
Design
For a known design life, use the chart to find the stress below which fracture will not occur – then plug the stress value into the equation to find the minimum pipe thickness

16. Diffusional creep can then be slowed by increasing grain size
Figure 13.23
At typical stress and temperature levels, pure nickel would deform by power-law creep at an unacceptable level
The impact of strengthening mechanisms on MAR- M200 nickel alloy reduces this rate by a factor of 106
Diffusional creep can then be slowed by increasing grain size
Figure 13.24

17. SUMMARY All material properties depend on temperature
Some are not very sensitive (density, modulus)
Transport properties such as thermal conductivity are very sensitive
Transport properties depend on diffusion
Diffusion depends on vacancies and temperature
Higher temperature-- higher rate of diffusion
In diffusion, atoms move from place to place. It can occur in metals, ceramics, glasses, etc. - though details are different
Diffusion is important in processing of materials
Carburizing for example
Diffusion is the source of creep and creep fracture
Creep is the slow movement of materials over time
Hot bolts need to be periodically tightened, etc.