| Ultrafine-grained(UFG) and nanostructured(NS) materials exhibit many novel physical and mechanical properties as compared to those of their coarse grain(CG) counterparts due to the nanometer scaled structure. High volume fraction of grain boundaries accounts for these superior properties. Severe plastic deformation(SPD) is an epidemic technique to produce UFG and NS materials. By imposing a very high strain on a bulk solid under an extensive hydrostatic pressure and without the introduction of any significant change in the overall dimensions of the sample, SPD can produce considerable grain refinement during which dislocation multiplication occurs. Moreover, SPD-processed materials are free of contamination and porosity. High pressure torsion(HPT) refers to processing in which the sample, generally in the form of a thin disk, is subjected to torsional straining under a high pressure. HPT has the strongest grain refining ability among SPD methods and it can produce NS materials with high angle grain boundaries. With good corrosion resistance, weldability, formability and moderate strength, Al–Mg aluminum alloys are widely used in pressure vessels, automobile body sheets and ship structure. SPD can bring an obvious rise on the strength of Al–Mg alloys through grain refinement. Therefore, it will be of great importance in investigating the HPT technique of Al–Mg alloys.In this paper, a commercial pure Al and three binary Al–Mg alloys with high Mg content(6, 8, 10 wt. % Mg) were processed by HPT. Microstructures including grain boundaries, dislocation, and stacking faults(SFs) have been investigated by X-ray diffraction(XRD), transmission electron microscopy(TEM), high-resolution TEM(HRTEM) and scanning TEM(STEM). Thermal stability of the HPT-processed pure Al has been studied by differential scaning calorimetry(DSC), TEM and hardness test. In addition, mechanical properties including microhardness and strength have been analyzed by means of hardness and tensile tests. Furthermore, strengthening and toughening mechanisms have been discussed. The main conclusions are as follows:(1) XRD results show that the average grain sizes of the HPT Al–8Mg andAl–10Mg alloys are 62 nm and 60 nm respectively, reaching NS scale. It is 65 nm of the HPT Al–8Mg alloy measured by TEM, which is close to the XRD result. TEM dark-field technology indicates the HPT pure Al is refined to UFG scale with the average grain size of 363 nm.(2) The dislocation density is up to 3.7×1014 m-2 in the HPT Al–8Mg and 4.6×1014 m-2 in the HPT Al–10Mg alloy, repectively. It is noteworthy that the local dislocation density approaches 1017 m-2 in some large grains. In the SPD processing, dislocation lines, dislocation walls and dislocation cells are formed in original grains at first. Then, dislocation walls transform into sub-boundaries and dislocation cells convert into sub-grains. These sub-boundaries will further turn into high angle grain boundaries with more plastic deformation. In this way, grain refinement is eventually produced due to the evolution of these dislocation structures as well as the interaction with solute Mg atoms.(3) Two STEM observation models including plan-view and end-on view using the [111] and [110] zone axes repectively have been applied to investigate the microstruce of the HPT pure Al. Results show that a high density of SFs and nanotwins with the width of 5~20 nm exist in the HPT pure Al. Notably, the local SF density reaches 1016 m-2. These SFs are observed to be formed by the 60° mixed dislocation dissociation into 90° and 30° Shockley partials, and the 0° screw dislocation dissociation into two 30° Shockley partials. These results demonstrate the formation mechanism of SFs and nanotwins in fcc nanocrystalline metals is also applicable to fcc UFG materials.(4) DSC analysis suggests that recovery, recrystallization, grain growth and secondary recrystallization occur successively within the range from 100 °C to 500 °C in the HPT pure Al. The temperature of the recovery and recrystallization is lower than that of the as-homogenized pure Al, which indicates the thermal stability of the HPT pure Al decreases due to the high strain introduced by plastic deformation.(5) After annealing at 195 °C for 30 minutes, deformation texture recovers and residual stress releases in the HPT pure Al, which results in a slight decline of the hardness from 649 MPa to 588 MPa. Furthermore, after annealing at a higher temperature of 380 °C for the same duration, the grains suffer a sharp growth with size about micrometer scale. The pure Al transforms from work hardening state to equilibrium condition. Accordingly, the hardness decreases considerably to 270 MPa, approximately equal to that of the ashomogenized pure Al.(6) The hardness of the pure Al and Al–Mg alloys increases significantly after HPT processing. For instance, the hardness of the HPT Al–8Mg and Al–10Mg alloys rises from 854 MPa and 990 MPa to 2141 MPa and 2396 MPa, 2.5 and 2.4 times higher than that of their undeformed counterparts respectively.(7) HPT processing also drasticly increases the strength of the Al–Mg alloys. In detail, the yield strength and ultimate tensile strength of the HPT Al–8Mg alloy are as high as 830 MPa and 855 MPa respectively, much higher than that of 160 MPa and 290 MPa before HPT.(8) Three strengthening mechanisms have been quantitatively analyzed. Solution strengthening, grain size strengthening and dislocation strengthening contribute 148 MPa, 160 MPa and 134 MPa to the yield strength of the HPT Al–8Mg alloy respectively. The three strengthening mechanisms account for 56% of the total strength, which indicates non-equilibrium grain boundaries and grain boundary segregation also play an important role in the strength of the alloy.(9) NS Al–Mg alloys with high Mg content usually have relatively low ductility and ultrahigh yield-strength ratio due to a diminishing work hardening capacity, which results from the depressed dislocation accumulation in nanometer scaled grains. |
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