| Metals have a wide range of applications in engineering.Light metals such as aluminum alloys and magnesium alloys play a great role in engineering.The mechanical properties are important indicators for selecting materials.Some engineering structural components require long-term service in high temperature environments,and the deformation mechanism of materials and related research have important guiding significance for engineering design.In this paper,the fatigue and time-dependent properties of commonly used aluminum alloys(5083)and magnesium alloys(AZ31B)are explored experimentally.The mechanism of material deformation is analyzed by using related microscopic experimental equipments.In fact,there are all kinds of external loads,researchers and engineers focus on fatigue due to its vital role in industrial.To explain the mechanism of fatigue damage of materials quantitatively is a major part of this paper.Besides,time-dependent properties also greatly affect the stability and fatigue life of materials.Thus,linking the deformation behavior of the material reasonably and accurately with the time-dependent properties experimentally and giving accurate quantification analysis are the purposes of this article.The cyclic behavior and fatigue performance of coarse and ultrafine-grained(UFG)5083 Al alloy produced by Equal Channel Angular Pressing were compared,from low to very high cycle fatigue.At high plastic strain amplitude(and only in this regime),crack initiation-mostly from intermetallic particles-was much easier in the UFG alloy,which showed a better resistance to micro-cracks growth.The micro-cracks grew across the largest grains,and along the boundaries of the smallest grains.Due to the complex interplay of mutual shielding and cracks coalescence,the UFG alloy showed a lower resistance to plastic-strain controlled LCF.Its resistance to stress-controlled HCF was hardly better than that of its CG counterpart until 2×10~6 cycles,but became 43%higher in VHCF,until 5×10~8 cycles.Beyond that point,a transition towards internal crack initiation,with the formation of“fish-eye”patterns on the fracture surface occurred,and the fatigue resistance of the UFG material decreased,which was explained by fracture Mechanics.Besides,A creep strain rate equation with one damage variable is adopted.Two kinematic hardening models related to time-independent plasticity and viscoplasticity,respectively,are incorporated.Combining the creep equation and kinematic hardening theory,two models are proposed:a creep-plasticity model and a nonunified viscoplasticity model.Experiment data for Al5083 was used to verify the utility of the proposed models.Three typical time-dependent tests include of the rate jump,creep,and stress relaxation at room temperature and 393K were conducted to address the time-dependent properties of rolled AZ31B Mg alloy.The tensile curves were rate dependent while the compression ones were totally rate-insensitive at both temperatures.In creep and stress relaxation tests,weaker creep strain and decay stress in the compression were noticed than that in tensile curve.The viscosity increased at 393K since the dislocation motion was thermally activated.The thermal activation theory was used to analysis the results of the repeated stress relaxation tests.According to the results,cross-slip and dislocation nucleation are the operating mechanisms for creep and stress relaxation.Stress-controlled cyclic tension-compressin expereiments with loading rates changed were conducted.to study the low cycle fatigue behavior of rolled AZ31B magnesium(Mg)alloy at 393K and room temperature.The relationship between dislocation slipping and twinning/detwinningof retchetting deformation was analysed.According to the results,the ratchetting behaviors of AZ31B magnesium alloy was temperature-dependent.Ratchetting was more apparent at 393K than that at room temperature,especially the cases under higher loading amplitude with larger tension peak stresses.The‘ratchetting recovery’was found and discussed.The rates influence on ratchetting was dependent on the testing temperature,that is,at 393K ratchetting was rate-sensitive,while at room temperature ratchetting was rate-insensitive.The mechanisms of different hysteresis loops and the influence of rates on peak and valley strains were discussed.To study the interaction between creep and fatigue in rolled AZ31B magnesium alloy at room temperature,a series of stress-controlled tests was designed.The influence of the amplitudes of mean stress and loading sequences on the creep-fatigue tests were explored with higher mean stress,strain accumulation during the test was due to creep instead of ratchetting.However,for tests with lower mean stresses,twinning/detwinning played the dominantly role on the shifting of peaks/valley strain. |
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