A model of grain refinement and strengthening of Al alloys due to cold severe plastic deformation

Abstract

This paper presents a model which quantitatively predicts grain refinement and strength/hardness of Al alloys after very high levels of cold deformation through processes including cold rolling, equal channel angular pressing (ECAP), multiple forging (MF), accumulative rolling bonding (ARB) and embossing. The model deals with materials in which plastic deformation is exclusively due to dislocation movement, which is in good approximation the case for aluminium alloys. In the early stages of deformation, the generated dislocations are stored in grains and contribute to overall strength. With increase in strain, excess dislocations form and/or move to new cell walls/grain boundaries and grains are refined. We examine this model using both our own data as well as the data in the literature. It is shown that grain size and strength/hardness are predicted to a good accuracy.

Hardening of pure metals by high-pressure torsion: A physically based model employing volume-averaged defect evolutions

“…This means in practice that we will follow classical approaches to strengthening (e.g. [30,31,32,33]) and recovery [34]; and include recent insights in the relation between dislocation generation and grain size [19,29]. We aim to show that these can explain published data on material dependence of hardening and grain size for HPT processed pure metals.…”

“…We define L gen as the total cumulative dislocation linelength generated during the deformation processing. We consider that dislocation are either retained in the grains or subsumed in grain boundaries (existing or new ones) [19,37] or annihilated within the grain [40]. We will consider that the annihilation can be described through a temperature and material dependent annihilation fraction, f an , i.e.…”

“…where L ig is the total dislocation linelength of dislocations stored in the grain and L gb is the total dislocation linelength of dislocations that have moved to grain boundaries and 4 have become part of the grain boundary (they are 'subsumed' in the grain boundaries [19,29]). We will consider that the effective width of grain boundaries can be neglected (this is further discussed in section 5) and hence…”

“…Following extensive work in the field by a range of researchers, extensive data on the hardness and microstructure of HPT processed metals and alloys is now available (e.g. [4,5,6,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22]). Data on hardness increase due to HPT have been published for over 20 pure metals, and several of the factors influencing the hardness of HPT processed pure metals have been investigated in some detail by Edelati and Horita [4,5].…”

“…To fully exploit the capabilities of SPD we need quantitative models that are able to predict the microstructure development during the process and predict resulting mechanical properties based on processing and materials parameters [19,25,26,27,28,29].…”