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Diffusion
Published in Gregory N. Haidemenopoulos, Physical Metallurgy, 2018
Creep. Diffusion plays an important role during the deformation of metals at high temperatures, i.e., during creep. It was shown (Chapter 3) that the plastic deformation of metals takes place by dislocation glide. At high temperatures dislocations acquire an extra degree of freedom and in addition to glide they can climb. Climb provides the dislocations the opportunity to change slip plane and continue their glide, if for some reason glide is impeded in the current slip plane. Climb takes place by diffusion of atoms towards or away from the dislocation. This creep mechanism is termed dislocation creep. At even higher temperatures, deformation takes place exclusively by diffusion. This creep deformation is termed diffusional flow and takes place by diffusion in the grain from compressive to tensile regions. The resulting mass transfer changes the shape of the grains and causes grain boundary sliding, a mechanism to be discussed in Chapter 9.
An investigation on creep deformation and mechanical properties of a polycrystalline Fe-based alloy: a molecular dynamics study
Published in Mechanics of Advanced Materials and Structures, 2023
Abeer Abdullah Al Anazi, Ghaidaa Raheem Lateef Al-Awsi, Suphatchakorn Limhengha, Andrés Alexis Ramírez-Coronel, Shafik- Shaker Shafik, Abbas F. Almulla
The role of alloying composition on the exponent n is represented in Figure 3c and d. Based on the results, the increase in both of Al and Cr content leads to the rise of exponent n and change of creep mechanism in the alloys. However, one should note that the compositional impact is insignificant. For example, the change of exponent n in the dislocation creep of samples (end of stage 3) in Figure 3a–d is 3.63, 3.12, 0.32 and 0.7, respectively, showing that the FeCrAl alloy is more sensitive to temperature compared to the other factors. With all these descriptions, ignoring the variations in temperature, grain size or alloying composition, one can see that the creep-rate/stress plots are separated in 3 distinguished regions with specific n values. At low stress values, the Coble deformation mechanism is dominant in the creep process. In this condition, the creep deformation is carried out through the vacancy diffusion so that the defects movement leads to a mass transfer in the microstructure [38, 40]. The stress exponent of Coble creep lies in the range of n = 1, which is consistent with creep deformation in low applied stresses (See Figure 3). With the rise of stress values, grain boundary (GB) sliding mechanism with n = 1.5-2.5 is prevailing in the microstructure. In this condition, the displacement of grains against each other occurs so that the crystals slide past one another without altering the crystal shape [41]. At high n value which is sign of high applied stress and temperature, the dislocation creep becomes the main mechanism for deformation in the microstructure. The dislocation creep is originated from the propagation of dislocations within a crystal lattice so that a linear lattice defect is produced in the atomic configuration [42].