Effect Of Grain Size On The Mechanical Behavior And Deformation Mechanisms Of Mg-3Gd

As the lightest structural metallic materials,magnesium(Mg)alloys have great potential applications in automotive,aerospace,medical and electronic industries.However,the low strength and poor ductility limit their engineering applications.Therefore,it is of great academic values and engineering importance to study the effects of the micro structure and grain size on the mechanical behavior and deformation mechanisms and thus the strengthening and toughening mechanisms of Mg alloys.In this thesis,an Mg-3Gd(wt.%)alloy was used as starting material and by applying accumulative roll-bonding(ARB)and annealing treatment,samples with three different characteristic structures were produced:nanostructured,micro-nano heterogeneous lamella structured,and micro structured.There samples of different initial structures were tensile tested to evaluate the mechanical behavior.Electron backscatter diffraction and transmission electron microscopy were used to characterize the microstructures for the samples before tensile testing and the samples tensile deformed to different strains.The effects of the micro structure grain size on the mechanical behavior and deformation mechanisms were studied;and the strengthening mechanisms were analyzed,and the relationship between the strength and structural parameters was established.The main conclusions are summarized as follows:?Mg-3Gd sheet samples with nanoscale structures were produced by ARB processing up to four-cycle.Three characteristic microstructures were identified and characterized as:cell blocks,twin blocks and nanograins,all with boundary spacings greatly refined to 100-200 nm.ARB 1 cycled sample showed a refined structure with a high fraction of twin structure.While ARB 2-4 cycled samples showed similar nanostructures and boundary spacings.These structures illustrate that the Mg-3Gd alloy has a high efficiency in structural refinement but with heterogeneous structural subdivision,which is quite different from the FCC and BCC metals.?The micro structures of annealed ARB samples can be divided into three categories:(i)190? annealed samples still showed a similar nanostructure to the deformed samples without apparent coarsening;(?)partial recrystallization took place in the 290? annealed samples,resulting in a heterogeneous lamella structure composed of ultrafine-grained lamellae and recrystallized fine grain lamellae;(?)samples annealed at temperatures of 310-550? exhibited fully recrystallized grain structures with average grain sizes ranging from 3 ?m to 114 ?m and with weak basal textures.? ARB samples showed a high strength and a low ductility.An apparent hardening response was observed after annealing for 1h at temperatures up to 250?,with a peak hardening at 190?.Clear evidence of Gd segregation at grain boundaries,twin boundaries,stacking faults and dislocations was identified in the annealed samples,which was considered to be the main reason for the observed hardening.Annealing in this temperature range led to increase in both the strength and ductility.Activity of non-basal slips and intense interactions between dislocations and Gd atoms were observed in the annealed and tensile tested samples,which were responsible for the strength improvement.The increase in ductility was considered to be a result of the inhibition of premature failure by Gd segregation at twin boundaries and the activity of non-basal slips.? Heterogeneous lamella structured samples showed a combination of high strength,similar to the that of ARB deformed sample,and good ductility,larger than that of the coarse grained sample.During tensile testing,high dense geometrically necessary dislocations were generated at the interfaces between heterogeneous lamellae to accommodate the incompatibility of deformation.These dislocations were piled up against the interfaces resulting in strong backstresses and strain gradients,which cause the enhanced strength.Meanwhile,the high strength(flow stress)promoted the activation of<c+a>dislocations in fine recrystallized grained lamellae;and the stress concentration in the ultrafine grained lamellae were thus relaxed.Consequently,the considerable work hardening rate and good ductility were achieved in the heterogeneous lamella structured samples? The mechanical behavior and deformation mechanisms of fully recrystallized samples with average grain sizes ranging from 3 ?m to 114 ?m were significantly affected by two transitions:(?)a transition in the mechanical behavior from continuous flow to discontinuous flow associated with a yield point phenomenon;(?)a transition in the deformation mechanisms from<a>slip and twinning to<a>and<c+a>slip.Both transitions occurred with decreasing grain size.In parallel,there was a consistent and concise trend that both the strength and ductility increase with decreasing grain size,which was contradict to classic strength-ductility trend in FCC/BCC metals.The appearance of a yield point phenomenon at the smallest grain size(3.3 ?m)had a significant effect on both strength and ductility,illustrated by an increase in boundary(Hall-Petch)strengthening and an increase in the total elongation to 36%.Based on quantification of structural parameters,strengthening mechanisms were discussed and a quantitative strength-structure model was proposed.