Microstructure,Solute Redistribution And Thermal Stability Of HPT Al-Mg Alloys

Severe plastic deformation?SPD?,as the most promising method for industrial production of nanostructured materials,has become a research hotspot to experts over the years.In this paper,commercial pure aluminum?CP-Al?and Al-Mg alloys are used as experimental raw materials to study the effect of high-pressure torsion?HPT?deformation on the microstructure and properties of materials.The grain size,micro-strain,dislocation density and micromorphology of HPT samples were investigated by X-ray diffraction?XRD?and transmission electron microscope?TEM?.High-angle annular dark-field scanning transmission electron microscopy?HAADF-STEM?and atom probe tomography?APT?were used to analyze the redistribution of solute atoms in Al-8Mg after HPT deformation.Combining with microhardness test of HPT samples,the hardening mechanism of HPT samples were discussed.Annealed samples were investigated by XRD,TEM and microhardness test to analyze the thermal stability of HPT samples.The main conclusions obtained are:?1?As the Mg content in alloys increases from 0 wt.%to 8 wt.%,XRD results show that the average grain size of HPT alloys decreases from 211.1 nm to 43.4 nm and the dislocation density increases from 5.6×10¹33 to 1.32×10¹⁵ m^-2.It shows that the increase of Mg content promotes the dislocation accumulation and grain refinement in Al-Mg alloys during the HPT process.?2?Some results obtained by HRTEM:1)High-density stacking faults?SFs?can be seen near GB of a 300 nm grain in HPT CP-Al,indicating the deformation mechanism of full dislocation slip and SFs or twin formed by partial emission from GBs can occur in ultrafine-grains at the same time.2)The Lomer-Cottrell?L-C?lock formed by three SFs in HPT samples are more stable than the conventional L-C lock formed by two SFs and has greater contribution to the strength of HPT samples as it can pin more than four dislocations.3)A micro-twin with 4{111}atomic plane thickness formed by the twining mechanism of stacking fault overlap were observed in HPT CP-Al.4)The ?3{112}incoherent twin boundaries?ITBs?in HPT Al-4.1Mg alloy are composed of the periodic arrangement of three{111}plane structural units formed by b2:b1:b3 partial dislocation groups.The migration is completed through the"move-drag"mechanism of partial dislocations.?3?High-density dislocations and vacancies will be produced by HPT deformation in alloys.These vacancies and dislocations will drag Mg atoms to GBs and cause solute segregation at GBs.APT and EDX line scan results show the different segregation behavior of Mg at different GBs.At the same time,the APT results show that inhomogenized Mg distribution along the same GB.These results indicate that the distribution of Mg along GBs and its vicinity is affected by the inhomogeneous stress field of intrinsic dislocations at GBs.?4?There are interconnected channels between adjacent Mg-rich regions.The number density of clusters of 0.5-2 nm is 1.2×10²⁵ m^-3 and are often distributed near GBs or Mg-rich regions.These phenomena indicate that Mg-rich regions in HPT Al-8.0Mg alloy are in the state of growth or development.?5?Microhardness test results show that the addition of Mg content will magnify the hardening effect of HPT on alloys.By calculating contribution values of traditional hardening mechanisms to the hardness increment of HPT samples,it is found that the contribution of the traditional hardening mechanism to the hardness increment are66.7%?56.4%?69.1%in HPT Al-1.0Mg,Al-4.1Mg and Al-8.0Mg.It indicates that other hardening mechanisms including segregation hardening is very important in HPT Al-Mg alloys.?6?High Mg content can improve the thermal stability of HPT Al-Mg alloys.The unstable temperatures of HPT Al-1.0Mg and Al-8.0Mg are 195°C and 240°C,respectively.When the annealing temperature reaches the unstable temperature,HPT Al-Mg alloys will show obvious softening and grain coarsening behavior.