The Influence Of Pre-fatigue Deformation On Size Effect Of Tensile Behavior Of Al-4.0wt.%Cu Alloy

In recent years,with the rapid development of the miniaturized advanced manufacturing technology represented by MEMS(Micro Electro Mechanical Systems)and microsystem technology,the studies on mechanical properties of the small scale materials which are suitable for such miniaturization systems have been extensively concerned.The phenomenon of "the smaller the weaker" appeared in the sub-millimeter-scale materials stems from the rapid annihilation of dislocations on the free surface.Whereas it is found that the strength of matierals is increased as the point defects with high density are introduced into or the oxide layers are fomred on surface in the nanometer and some submicron-scale materials.However,if the second phases introduced into the materials can postpone the occurrence of size effect or improve the mechnaical properties of small scale materials?So far,the reports on the research have not been found yet.In addition,many research findings have indicated that the strength of the materials can be improved by a suitable pre-fatigue deformation.Therefore,in the present work,the precipitation hardening Al-4.0wt.%Cu alloy is chosed as an eperimental material,and the sheet thickness-dependent tensile behavior and the influence of pre-fatigue deformation on the dependency as well as the effect of pre-fatigue deformation on tensile behavior of underaged alloy are systematically studied.The aim is to provide reference data for enriching the micro-mechanisms of material size effect and the influence of pre-fatigue deformation on the mechanical behavior of materials.For Al-4.0wt.%Cu alloy treated by solution at 530?/5h and aging at 160?/16h,with the decrease of t,?YS does not exhibit an obvious thickness effect;?UTS decreases gradually and the maximum decreasing amplitude appears at t=0.45 mm,with continuously decreasing t to?0.30 mm,there is no obvious change in ?UTS;? first increases and then decreases as t<0.60 mm,with continuously decreasing t to?0.30 mm,there is no notable change in ?.The fracture surfaces consist of the fibrous and radiation regions for thinner specimens,and the fibrous regions appear at the intersections of frature surface and side surface.Whereas only the fibrous regions are observed for thicker specimens,and the dimple size obviously increases.After the specimens with various t are loaded to tensile rupture,the dislocation density is higher nearby the ?^// precipitates,especially in thicker specimens.Whereas in the regions without ?^// precipitates,with the decrease of t,fine dislocation cells are evloved into larger cells,finally to dislocation networks and tangles.The decrease in dislocation density leads to weaker interactions among dislocations and between dislcoations and precipitates,further causing the decrease in ?UTS with t,but as t? 0.30 mm,there are no obvious change in ?UTS and ? as a result of the impediment of ?^// precipitates to the annihilation of dislocations on free surface.After Al-4.0wt.%Cu alloy treated by solution at 530 ?/5h and aging at 160?/16h is pre-fatigued to D=20%(D=Ni/Nf,where Ni and Nf are the applied cycles and fatigue life,respectively),?YS and ?UTS have no obvious change as t? 0.60 mm,with the continuous reduction of t,they decrease,and ?YS remains basically constant as t?0.30 mm;? first increases and then decreases after t=0.70 mm.Only the fibrous regions are observed on fracture surfaces for various t specimens,and with the increase of t,the orientaion and density of slip bands(SBs)increase.In comparison with the un-fatigued specimens with the same t,the pre-fatigue deformation leads to the increased density of SBs,and the bulge in grain interior as well as the intrusion and extrusion of grain boundaries(GBs)become weaker.The pre-fatigue deformation promotes not only the precipitation and the growth of ?^//phase but also the increase in dislocation density,further resulting in the increased strength of pre-fatigued alloy,but the thickness value below which the decreases in ?UTS ocurs is decreased from 1.00 mm at the un-fatigued state to 0.60 mm at the pre-fatigued state.After Al-4.0wt.%Cu alloy treated by solution at 530?/5h and aging at 160?/10h is pre-fatigued to D=5%,20%and 50%,the strength notably increases and ? obviously decreases compared with the un-fatigued state,but the dependency of tensile properties on D is not behaved.After underaged alloy is pre-fatigued,the obvious slip steps and the cracks along the GBs are observed on the surfaces of tensile specimen,and slight necking occurs,and the dimples are mainly distributed at the intersections of fracture surfaces and side surfaces or at two ends of fractures along width direction,meanwhile,the cracks are observed on fractures.The pre-fatigue deformation pormotes the precipitation of a number of needle-like fine ?^// precipitates with smaller spacing,and their distribution is very non-uniform,especially at lower D,meanwhile,the dislocation density is increased,further leading to the formation of dislocation networks and tangels and even loose cells.After tension rupture,with the increasing of D,loose dislocation cells are developed into the cell block structures.The formation of cell blocks is closely related to the precipitation of needle-like fine ?^// precipitates with smaller spacing.