Dissemination of IT for the Promotion of Materials Science (DoITPoMS)


Homologous Temperature

The diffusivity of a material depends on how easily atoms can move to a vacant site on the crystal lattice (see more here). This becomes easier near the material’s melting point and hence it is the materials temperature relative to its own melting point (Tm) – its homologous temperature (T/Tm) – that influences its diffusivity. Rates of N-H creep and dislocation creep are both controlled by bulk diffusion and hence reduced when the diffusivity of a material is lower. The importance of the idea that it is homologous, not absolute temperature, which controls creep rate in the creeping coil experiment, in which lead undergoes easily perceptible deformation via creep at room temperature - 300K represents a significant homologous temperature of ~0.5 for Lead, but an insignificant ~0.2 for Nickel.