Hi all I am learning about Grain Size Hardening, and find it difficult to understand some concepts. With refer to the link: http://aluminium.matter.org.uk/content/html/eng/default.asp?catid=64&pageid=1000301064 What is that dislocation source, and why it emits dislocation when the stress is increasing? For my understanding, dislocation are some imperfections already exist in the metals,so I find it very strange to have something called "source" to continuousy produce some dislocations. Can anyone help answer what's S1 and S2 and why's that? Please.