Mass Transport Assisted Deformation

In chap. 7 we laid the foundations for the analysis of mass transport in solids. In the current section, our aim is to examine the ways in which mass transport can assist the deformation that occurs in a given material. As a preliminary, we remind the reader of one of the key features of the deformation mechanism map introduced in fig. 7.7. We refer to the fact that in many instances for stresses well below the putative yield stress, permanent deformation is still observed. Such deformation usually occurs at temperatures which are larger than, say, 0.37,  

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A second fundamental theme that will be taken up in this chapter in which there is an interplay between the various types of defects introduced earlier is that of mass transport assisted deformation. Our discussion will build on the analysis of diffusion at extended defects. We will begin with an examination of the phenomenology of creep. 

At lower stresses and high temperatures, diffusion-controlled mechanisms will be active. Atomic-level mass transport by vacancies and interstitials can cause deformation at the macroscopic level. Vacancies can assist the motion of dislocations by cross-slip processes. As vacancy mobility and/or the density increases, the phenomenon of grain boundary migration will be observed. 

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