Dislocations Gliding And Thermal Stability Of Al-Mg Alloys Processed By High Pressure Torsion

Al-Mg alloys have widespread applications for their good corrosion resistance,excellent mechanical properties and good molding performance in the field of aerospace and transportation.With the development of technology and modern industry,the strength of Al-Mg alloys needs to be further improved.Severe plastic deformation(SPD)techniques are effective methods to refine the grains of alloys,in which the high pressure torsion(HPT)has the strongest ability of grain refinement and can refine the grains to nanometer scale.The strength and hardness of the alloys processed by HPT have been obviously improved due to the significant grain refinement and high density crystal defects in the matrix.However,due to the high enthalpy,ultrafine grains and nanocrystalline materials are prone to structural instability when heated,which will result in a significant change of physical and mechanical properties.Therefore,it is of great practical significance to study the microstructure and thermal stability of HPT Al-Mg alloys.In this paper,a commercial pure Al,three binary Al-Mg alloys and a commercial AA5182 alloy were processed by HPT.The microstructure and hardness of the alloys before and after HPT have been investigated by metallographic microscope,transmission electron microscopy(TEM),high-resolution TEM(HRTEM),X-ray diffraction(XRD)and micro-hardness tester.Dislocation gliding,dislocation locks and special nanostructures of HPT pure Al and Al-Mg alloys have been investigated by HRTEM.Thermal stability of HPT Al-Mg alloys has been investigated by isothermal annealing and isochronal annealing experiments.The main conclusions of this work are as follows:(1)The grain size of pure Al and Al-Mg alloys has been significantly refined by HPT.High density dislocations and stacking faults exist in the matrix frequently.The TEM characterization shows the grain size of HPT alloys decreased from 374 nm to 84 nm with the increase of Mg content from 0 to 4.1%.The dislocations in the HPT pure Al and Al-Mg alloys often tangle to form dislocation lines,dislocation walls and dislocation cells.With the increase of Mg content,the dislocation density of the HPT alloys increases gradually.The dislocation density in HPT pure Al and Al-Mg alloys is 1014m-2 evaluated by XRD and the local dislocation density is as high as 1017m-2 calculated by HRTEM.SFs with the width of 5~15 nm and the local density of 1015 m-2 are observed in the HPT Al-Mg alloys,demonstrating the presence of the formation mechanism that SFs are formed by partials emission from grain boundaries in the HPT Al-Mg alloys.(2)The hardness of HPT pure Al and Al-Mg alloys has been significantly improved,which is about 2 to 4 times higher than that of their undeformed specimens.With the increase of Mg content,the hardness of HPT alloys increases obviously.(3)Dislocations with a high local density exist on {001} planes of HPT pure Al and Al-Mg alloys,demonstrating the occurrence of glide on {001} planes.HRTEM results show that the 1/6<110> L-C locks with an angle of 70.53° and the 1/3<110> dislocation locks with an angle of 109.47° simultaneously exist in the HPT pure Al.The high density crystal defects introduced by HPT produce a severe misalignment in the crystal,which will force the dislocation locks to move along the <110> direction of the(001)cube plane and initiate the glide on {001} planes.In addition,high density hexagonal special nanostructures have been observed by HRTEM in the HPT AA5182 alloys.The formation process of these nanostructures is processed based on the molecular-dynamics simulation of YAMAKOV et al.(4)Isothermal annealing and isochronal annealing results show that temperature is the most important factor affecting the recrystallization and grain growth of HPT Al-Mg alloys.Annealing temperature and annealing time both have important decisive effect on the recovery process of HPT Al-Mg alloys.With the increase of Mg content,the thermal stability of HPT Al-Mg alloys reduces gradually.With the increase of annealing temperature,recovery,recrystallization and grain growth occur successively in the HPT Al-Mg alloys,resulting in the increase of grain size and decrease of hardness.With the annealing time extendes,the grain size of the alloys increases and the micro-hardness of the alloys decreases,but the rate of decrease in the hardness is gradually reduced.(5)During the process of HPT AA5182 alloy annealed at 195 °C for 30 min,annealing strengthening occurs and the hardness of HPT AA5182 alloy increases from 195 HV to 197 HV.Dislocation source-Limited strengthening is the mechanism of annealing strengthening.The occurrence of annealing strengthening is closely related to annealing temperature and strain accumulation.