Microstructure,Mechanical Properties And Thermal Stability Of Pure Al And Al-Mg Alloys Processed By High Pressure Torsion

5000 series aluminum alloys play an important role in fields of aerospace,pressure vessels and daily necessities because of its good corrosion resistance,weldability and machinability.Strength of traditional 5000 series aluminum alloy has been unable to meet the new demand with continuous progress of science and technology and improvement of human life and industrial production.Ultrafine/nano-grained materials have excellent physical and chemical properties and mechanical properties compared with coarse-grained materials.Severe plastic deformation is a widely used way to produce ultrafine/nano-grained materials and high pressure torsion?HPT?has the best grain refinement ability in severe plastic deformation.HPT can directly reduce grain size of circular metal material to nano scale by making shear deformation of circular specimen at higher pressure.HPT can improve strength and hardness of materials through microstructure evolution such as dislocation multiplication.However,increase of dislocation density will make ultrafine/nano-grained materials extremely unstable and face the defects of microstructure evolution and disappearance of excellent properties when temperature increases.Therefore it is important to research the microstructure,mechanical properties and thermal stability of HPT pure Al and Al-Mg alloys.In this paper,commercial pure Al,Al-Mg alloys?Mg elements are 0.5%,1%,2.5%and 5%,respectively?and a commercial AA5182 aluminum alloy?Mg element content is 4.1%?were processed by HPT.Microstructure of pure Al and Al-Mg alloys were analyzed by means of optical microscope?OM?,X-ray diffraction?XRD?and transmission electron microscope?TEM?.Mechanical properties of HPT Al and Al-Mg alloys were investigated through tensile test and microhardness test.Thermal stability of HPT pure aluminum and Al-Mg alloys were analyzed by X ray diffraction,transmission electron microscope and differential scanning calorimetry?DSC?.The main conclusions of this work are as follow:?1?HPT can significantly refine pure Al and Al-Mg alloys and improve their hardness.Average grain sizes of HPT pure Al and Al-5.0Mg alloys are 164 nm and 46nm,respectively.Dislocation density of HPT pure Al and Al-5.0Mg alloys are 0.16 x10¹⁴ m^-2 and 4.46 x 10¹⁴ m^-2,respectively.Hardness of specimens after HPT is 2.35,3.12,3.40 and 2.79 times higher than that of undeformed specimens,respectively.Hardness of Al-5.0Mg alloy will increase from 72 HV before HPT to 201 HV after HPT.Microhardness of HPT Al-Mg alloy will increase with continuous increase of Mg content.?2?HPT can cause more defects such as dislocation,non-equilibrium grain boundary and dislocation cell in pure Al and Al-Mg alloys.This will greatly facilitate decomposition of larger grains.It will eventually achieve transformation from small angle grain boundaries to large angle grain boundaries when orientation difference between dislocation cells and subgrains keep increasing.?3?HPT can greatly enhance strength of pure Al and Al-Mg alloys.Yield strength and tensile strength of pure Al increased from 45 MPa and 75 MPa to 145 MPa and200 MPa before and after HPT,respectively.Yield strength and tensile strength of AA5182 alloy increased from 115 MPa and 255 MPa to 690 MPa and 800 MPa before and after HPT,respectively.Elongation decreased with increase of Mg content.?4?Stacking faults of HPT pure Al,Al-0.5Mg,Al-1.0Mg,Al-2.5Mg,AA5182 and Al-5.0Mg alloys are 103,101,96,72,42 and 37 mJ/m^-2 with increase of Mg content,respectively.Stacking fault density increases with the increase of Mg element addition.Nano stacking fault is strengthened to improve the mechanical properties on the basis of grain refinement.?5?It is found that HPT pure Al,Al-1.0Mg and Al-5.0Mg alloys will undergo process of recovery,recrystallization,grain growth and secondary recrystallization during heating process through the DSC analysis.The onset time of exothermic peak will advance with increase of Mg content.It shows that the matrix has higher grain boundary energy and its thermal stability is worse when Mg content is higher.?6?Annealing temperature and holding time have important effects on recovery process of HPT pure Al and Al-5.0Mg alloy and annealing temperature has a significant effect on recrystallization process.Grain size will obviously coarse-grained and grain size will increase from 486 nm to 1130 nm when annealing temperature of pure Al increases from 125°C to 280°C.TEM characteristics of HPT Al-5.0Mg alloy are similar to those of HPT pure Al.Trigeminal grain boundary structure with adjacent angles of 120°can be observed at only 280°C,which is an obvious feature of local recrystallization.