Microstructure,Grain Boundary Structure And Segregation In High Pressure Torsion Al-Mg Binary Alloys

The Al-Mg alloys are widely used in manufacturing aircraft,automobile and ship because of its high strength,good corrosion resistance and process ability.However,due to the poor aging effect of Al-Mg alloys,it is difficult to further improve strength by heat treatment but we can use plastic deformation to improve its mechanical properties.At present,most researchers have paid attention to the plastic deformation of Al-Mg alloys,especially the severe plastic deformation?SPD?.The alloy prepared by severe plastic deformation?SPD?has fine grains,high dislocations and a large number of high angle grain boundaries.Some researchers have obtained bimodal structure by controlling severe plastic deformation parameters and they found that the strength and plasticity of the alloy could be improved obviously.Recent studies pointe out that severe plastic deformation can induce alloy elements to gather at non-equilibrium grain boundaries and strengthen the alloy.It can be seen that severe plastic deformation has incomparable advantages over traditional deformation.However,there are still many problems to be further studied and clarified in the microstructure of severe plastic deformation and the relationship with properties.Therefore,it is of great significance to study the grain boundary structure and grain boundary segregation of the alloy processed by severe plastic deformation and its effect on the properties.In this paper,the as-cast Al-Mg binary alloys produced by Norwegian Aluminum Corporation were homogenized and processed by high pressure torsion.1)The microstructure parameters such as average grain size,micro-strain,dislocation density and lattice constant of HPT Al-Mg binary alloys were calculated by X-ray diffraction?XRD?pattern.The influence of Mg element content on these parameters was analyzed.The stacking fault energy of HPT Al-Mg binary alloy was calculated by formula and the quantitative relationship between the stacking fault energy and grain size was obtained based on the research of Mohamed.The dynamic recrystallization of HPT Al-Mg binary alloy was analyzed and discussed.2)The grain size and grain boundary distribution of HPT Al-Mg binary alloy?including high angle,small angle and coincidence site lattice grain boundaries?were studied by HRTEM and EBSD.At the same time,the distribution of Mg elements at the grain boundary of HPT Al-Mg binary alloy was analyzed by means of HAADF-STEM and EDX.3)By measuring the Vickers hardness of Al-Mg binary alloy before and after high pressure torsion,the effect of equivalent strain on hardness was analyzed.The relationship between hardness and strength was verified by linear fitting.Finally,the contributions of strengthening mechanisms to hardness of HPT Al-Mg binary alloy were discussed.The main conclusions include:?1?Related XRD results show that the content of Mg has a great effect on the microstructure parameters of HPT Al-Mg binary alloy.With the increasing of Mg,the average grain size decreases,the dislocation density and lattice constant increase.The lattice distortion is introduced into Al-Mg binary alloy by HPT treatment.?2?The stacking fault energy of HPT Al-Mg binary alloy is closely related to the content of Mg.The higher the Mg content is,the lower the stacking fault energy will be.The stacking fault energy of HPT Al-0.5Mg and HPT Al-4.1Mg alloys are calculated to be 132±20mJ/m² and 59±9mJ/m²,respectively.It is found that the mass fraction of Mg increases 3.6%,the stacking fault energy decreases 73mJ/m².?3?The stacking fault energy determines the grain size of Al-Mg binary alloy after deformation.In general,the smaller the fault energy is,the smaller the grain size will be.The linear fitting basing on Mohamed's research shows that the relationship between stacking fault energy and grain size of HPT Al-Mg binary alloys is approximately satisfied with d=3938×b??/Gb?^0.65.The dynamic recrystallization of HPT Al-Mg binary alloy is discussed based on the above results.?4?A small angle equilibrium grain boundary with misorientation of 7°is observed in HPT Al-Mg binary alloy by HRTEM.The dislocation density on?11^—1?surface is about 1.7×10¹⁷.At the same time,a non-equilibrium grain boundary with misorientation of 21.5°which has a large number of dislocations on the?111?and?11^—1?surface is observed under the[1^—10]crystal band axis.?5?The results of EBSD show that the grain size will become smaller and the microstructure will be more uniform when the content of Mg increases which may also lead to new generation of high angle grain boundaries.For example,the average grain size of HPT Al-0.5Mg alloy is 87.21 nm,in which the grains smaller than100nm are about 83%and the fraction of large angle grain boundaries are 78.9%.The average grain size of HPT Al-4.1Mg alloy is 35.66 nm,in which the grains smaller than 100nm are about 99.73%and the fraction of large angle grain boundaries are95%.?6?The results of HAADF-STEM and EDX show that there is obvious Mg segregation in HPT Al-Mg binary alloy.The atomic fraction of Mg in HPT Al-0.5Mg and Al-4.1Mg alloy are 0.55%and 4.53%,respectively.The atomic fraction of Mg at the corresponding grain boundary are 2.77%and 10.07%,which are 2.22%and5.54%higher than that of the matrix.Based on the experimental results,the causes of grain boundary segregation in HPT Al-Mg alloy are analyzed.?7?The measurements of hardness show that the hardness distribution of HPT Al-Mg binary alloy are related to the equivalent strain.In the center of the sample,the hardness is lower because of the lower equivalent and the hardness increases with the increase of the radius.The results of linear fitting show that the Vickers hardness HV and yield strength?_y of HPT Al-Mg binary alloy satisfy with HV=0.298?_y which conforms to the basic law of HV?0.3?_y.?8?The hardness of Al-Mg binary alloy increases remarkably after high pressure torsion.For example,the hardness of Al-0.5Mg?original state?is only 51kg/mm²,but the hardness increases to 119.751kg/mm² after high pressure torsion.In the same way,the hardness of Al-1Mg,Al-2.5Mg,Al-4.1 increase nearly 72.6kg/mm²,82.6 kg/mm²and 124.4kg/mm² respectively,compare with pre-deformation samples.It can be seen that the higher the content of Mg is,the higher the hardness will be.?9?A variety of strengthening mechanisms contribute to the hardness of HPT Al-Mg binary alloy.When the content of Mg is low,fine grain strengthening is dominant,but solid solution strengthening and dislocation strengthening contribute little to hardness.With the increase of Mg,the effects of three strengthening mechanisms are enhanced,in which dislocation strengthening is the largest.The total contributions of three strengthening mechanisms to hardness are about 50%⁶0%.?10?The contribution of grain boundary segregation to hardness is calculated by the formula.The results show that in HPT Al-0.5Mg alloy,the contribution of Mg-Mg cluster to hardness is 65.15 Kg/mm²,nearly accounting for 54%of the total hardness.This indicates that grain boundary segregation has an significant effect on strengthen.