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HOT MATERIALS. Some material properties have a linear dependence on temperature while others may have an exponential relationship. Creep At room temperature, most metals and ceramics deform in way that depends on stress but no on time.
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Some material properties have a linear dependence on temperature while others may have an exponential relationship
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
Specimen is loaded in tension or compression, usually at a constant load, inside a furnace that is maintained at a constant temperature
Steady-state creep rate • The constants ε0, σo, n, and Qc are experimentally found and vary from material to material
Figure 13.5 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
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
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
Figure 13.9 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 13.11 The mean distance that one type of atom travels from diffusion is given by
Figure 13.12 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 13.16 Diffusion can cause creep as well as fracture due to creep by creating voids that nucleate on grain boundaries
Figure 13.21 Figure 13.22 Typical operating conditions of 650°C at 15 MPa 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
Figure 13.23 Figure 13.24 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
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.
