Mechanistic investigation on Ce addition in tuning recrystallization behavior and mechanical property of Mg alloy

Constructing bimodal grain structure is a promising approach to achieve the high strength-ductility syn-ergy in Mg alloy.Formation of bimodal grain is closely related to the dynamic and/or static recrystal-lization process,which has not been fully understood in the typical Mg-RE based alloy.In this work,it is claimed for the first time that the minor Ce addition(∼0.3 wt%)into Mg matrix significantly pro-motes the pyramidal<c+a>and non-basal<a>dislocations at the early stage of extrusion,which con-sequently enhances the formation of sub-grain boundaries via the movement and recovery of pyramidal II-type<c+a>dislocations.At this stage,fine sub-grain lamellae are widely observed predominantly due to the low migration rate of sub-grain boundary caused by the limited mobility of<c+a>dislocations.At the later stage,the sub-grains continuously transform into dynamic recrystallized(DRXed)grains that have10¯10Taylor axis and also strong fiber texture,indicating substantial activation of pyramidal II-type<c+a>dislocation.The low mobility of<c+a>dislocations,accompanied with the solute drag from grain boundary(GB)segregation and pinning from nano-phases,cause a sluggish DRX process and thus a bimodal microstructure with ultra-fined DRXed grains,∼0.51μm.The resultant texture hardening and grain refinement hardening effects,originated from bimodal microstructure,result in a yield strength of∼352 MPa,which is exceptional in Mg-Ce dilute alloy.This work clarifies the critical role of Ce addition in tuning recrystallization behavior and mechanical property of magnesium,and can also shed light on designing the other high-performance Mg alloys.

Two-stage Hall-Petch relationship in Cu with recrystallized structure

Although Cu was studied extensively,the Hall-Petch relationship was mainly reported in the coarsegrained regime.In this work,fully recrystallized Cu specimens with a wide grain size regime of 0.51–14.93μm manifest a two-stage Hall-Petch relationship.There is a critical grain size of 3μm that divides stagesⅠandⅡwhere the Hall-Petch slope k value are quite different.The stageⅡis supposed to be validified down to 100 nm at least by comparing with a Cu-Ag alloy.The critical grain size varies in different materials systems,and the underline mechanisms are discussed based on the dislocation glide modes.