The Effect Of Fatigue Pre-deformation On The Mechanical Behavior Of Al-4.0wt.%Cu Alloy

A variety of engineering structural components are usually subjected to alternating loads under service conditions,which will lead to the change of their mechanical properties,in particular,the degradation of mechanical properties induced by alternating loading will seriously affect the production safety.Many studies have thus been focused on the cyclic pre-deformation effect on the mechanical behavior of materials in recent years.However,up to now,less studies are focused on the effect of fatigue pre-deformation on the mechanical behavior of solution-and aging-treated alloys.Therefore,in the present work,an Al-4.0wt.%Cu alloy was selected as the target material,and the influence of pre-cyclic level D(=No/Nf,where Ni and Nf are the applied cycles and fatigue life,respectively)on the tensile behavior of solution-treated,and solution+aging-treated Al-4.0wt.%Cu was systematically investigated,and the corresponding microstructures,damage features on lateral surfaces near fracture surfaces and facture surfaces were observed.The aim is to provide reference date for enriching the micro-mechanisms for the fatigue pre-deformation effect on the mechanical behavior of materials.The results show that with the prolonging of aging time,the hardness first increases and basically remains the range from 45 to 50 HV for Al-4.0wt.%Cu alloy aged at 160℃for 1～16 h,ultimate tensile strength σUTS and yield strength σYS increase,and uniform elongation δ decreases.After the solution-treated alloys are pre-fatigued to different D in the range from 5%to 75%and then loaded to tensile rupture,in comparison to the as-solution-treated state,the σYS obviously increases and the δ decreases,and they do not exhibit obvious change with D,whereas the σUTS at different D is basically comparable to that in the as-solution-treated state.After the alloy aging-treated at 160℃for 16 h is pre-fatigued to different D(=5～75%)and then loaded to tensile rupture,the notable increase in σYS takes place as compared to the as-aging-treated state,and the δobviously increases at D=5%and 20%,here,the σUTS is basically comparable to that in the as-aging-treated state.With continuously increasing D,the σUTS and δ start to decrease.Therefore,the cyclic pre-deformation at D=20%on the solution-and aging-treated Al-4.0t.%Cu alloy leads to a notable improvement in tensile properties.During an aging treatment at 160℃,with the prolonging of time,the type of precipitates is transferred from the flaky θ//into the disc-shaped θ//,and then into the coexistence of disc-shaped θ/and a small amount of granular θ;and the content of precipitates increases.After the solution-treated alloy is loaded to tensile failure,the dislocation networks were formed,and fine granular θ phase is precipitated.Whereas for the aging-treated alloy,with the prolonging of aging time,the dislocation density increases after tension,and the structures are transformed from tangled to celluar structure,meanhwile,the precipitate content increases and the type of precipitates is transferred from the flaky θ//into the disc-shaped θ/.As time is 13 h,fine granularθ phase is observed,with prolonging time to 16 h,the θ phase growth occurs.The cyclic pre-deformation on the solution-treated alloy promotes the precipitation of fine θ particles,and its content increases with increasing D,and the dislocations exist in the form of single state.After tension,with increasing D,fine θ particles increase,and the dislocation structures are changed from the cellular arrangements into the network form.In contrast,as the alloys solution and aging-treated at 160℃ for 16 h were pre-deformed to D=5～75%and subsequently loaded to tensile rupture,with increasing D,the dislocation density raises and the transformation from the tangles and cells to networks and tangles takes place.The precipitates are mainly disc-shaped θ/,and it becomes wider and its content increases with D.After D>20%,a small amount of granular θ phase is precipitated,which leads to the decrease in strength and elongation.The surface deformation of solution treated alloy is more severe than that of solution and aging-treated alloy after fatigue pre-deformation and subsequent tension.The cracks are mainly formed at grain boundaries(GBs)after tensile deformation of the solution-and aging-treated alloy,and some cracks formed at GBs extend to grain interior along slip bands at higher D.nd the obvious GB extrusion are observed in the solution treated and then pre-deformed alloy.The cyclic pre-deformation at D=5%and 20%on solution-and aging-treated alloy results in the extension of fibre zone on fracture surface and the increased and uniformly distrubited dimples.