Investigation Of Hall-Petch Coefficient And Portevin-Le Chatelier Effect In Ultrafine-Grained Al And Al-Mg Alloys

Al-Mg alloys have been widely used in high-tech fields such as aerospace,shipbuilding and automobile manufacturing due to their excellent corrosion resistance,good formability,good welding performance and good mechanical properties.In recent years,with the presence of harsh environments and conditions in practice,Al-Mg alloys with better overall performance are required.As such,ultrafine-grained(UFG)Al-Mg alloys with excellent comprehensive properties have received much more attention.However,after processing the alloys plastically,the deformation bands are usually present on the surfaces of the alloy products,leading to surface roughness.This will impact surface quality and plastic processing ability of the alloys,which,in turn,will greatly limit the alloys' industrial applications.The above mentioned deformation bands are associated with the Portevin-Le Chatelier effect.Based on the characteristics of Al-Mg alloys and UFG metallic materials,UFG Al and Al-Mg alloys(Mg element content of 2.5,5 and 7.5 at.%)were prepared by a combination of mechanical alloying(MA)and hot extrusion.Both the as-extruded and annealed extruded samples were used to study the micro structures,strengthening mechanisms,Hall-Petch slope and Portevin-Le Chatelier(PLC)effect of UFG Al and Al-Mg alloys.The investigation of microstructures and strengthening mechanisms of the extruded UFG A1 and Al-Mg samples showed that in addition to the high dislocation density,most of the grains in the samples are slightly elongated along the extrusion direction and their average grain sizes decrease as the solute Mg concentration in the samples increases.The partitioning of the Mg element in Al-Mg alloys is observed.Partial Mg element is dissolved in the Al lattice,and the remaining is involved in the formation of MgO nonoparticles.The main strengthening mechanisms of UFG Al-Mg alloy are grain boundary strengthening,dislocation strengthening,solid solution strengthening and dispersion strengthening.The former two strengthening mechanisms play major contributions to the yield strength of the alloys.The Hall-Petch coefficients(k)of the annealed UFG Al and Al-Mg alloys were investigated.The results show that the k values increase with increasing the Mg content,resulting in the obtained k values here deviating from the Hall-Petch relationship of pure Al.Combining with the correlative micro structural characterization,the coefficient k in Al-Mg alloys is found to be related to the solute Mg clusters around pre-existing dislocations and Mg atoms dissolved in Al lattice,which increase the critical shear stress needed for initiating a dislocation source,and thus increases the difficulty ofthe operation of dislocation sources at grain boundaries or within adjacent grains.As a result,the k value is enhanced in the alloy samples.In addition,in-situ formed MgO nonoparticles in Al-Mg alloys can further increase the critical shear stress required to initiate the dislocation source by hindering the motion of dislocations.Therefore,compared with Al-Mg solid solution alloys without MgO nonoparticles reported in literature,Al-Mg alloys containing in-situ formed MgO nanoparticles in the present study have a higher Hall-Petch coefficient k value.The strain rate sensitivity of the flow stress and PLC effect of the as-extruded UFG Al and Al-Mg samples were investigated.The strain rate sensitivity of the flow stress of the samples decreases with the increase of the solute Mg content.The negative strain rate sensitivity corresponds to the dynamic strain aging,which generates a serrated stress-strain curve.The PLC effect is closely related to the solute Mg concentration.Based on the results of this study,the critical solute Mg concentration necessary responsible for the occurrence of the PLC effect is in the range from 2.9 to 5.2 at.%.