.2 E12.-zr Aluminum Alloy Heat Treatment, Microstructure And Mechanical Properties

Effects of heat treatment and microstructures on properties of 2E12+Zr alloy were studied in this paper,including fatigue properties testing and tensile properties testing,Optical microscopy(OM), Transmission electron microscopy(TEM)and Scanning electron microscopy(SEM)observations.The main conclusions are as following:(1)DTA test were carried out,the overburnt temperature is 503℃.With the SEM obvervations,the insoluble paritles were AlCu, AlCuMn and AlCuMg paticle.The results showed that 2E12+Zr alloy had better tensile properties after 500℃×40min solution treatment.(2)GPB zone was the comstituent phase after 96h natural aging of 2E12+Zr alloy.The alloy at T3 condition had better strength and lower elongation than T4 condition,because the pre-deformation induced higher dislocation density,promoted the precipitation of GPB zone.(3)S' phase was the comstituent phase after 190℃artificial aging of 2E12+Zr alloy.At T6 condition 10h the strength arrived at the peak value, at T8 condition was 2h earlier.The Tensile strength and Yield strength at T8 condition increased 40MPa,the elongation decreased 6 percent.The results was that the pre-deformation induced higher dislocation density, promoted the precipitation of S' phase,increased strength of the alloy.(4)After 96h natural aging,the Tensile strength of 2E12+Zr alloy decreased 20～30MPa,Yield strength was invariant,the elongation increased 4 percent;After 190℃artificial aging,the Tensile strength of 2E12+Zr alloy decreased 40～50MPa,Yield strength decreased 10～30 MPa,the elongation increased 2～5 percent.The results was that the addition of micro Zr reduced the formation of GPB zone,decreased the precipitation of the S' phase,reduced the effect of age harding.(5)When stress ratio R=0.5,with the same level cycles,the fatigue strength at T3 condition is higher than that at T4 condition,such as the cyclic fatigue strength(σ_max)at 10⁶ cycles is 10MPa higher than that at T4 condition.These notches resulted in reduction of fatigue limit of this alloy.When Kt=3,the fatigue limit is short half of the Kt=1. (6)The fatigue crack growth of 2E12+Zr alloy contained three stages in L-T direction at T3 and T4 condition.On the condition of R=0.5, AK＜5.5Mpa×m^1/2,it was the first stage;when△K was between 5.5Mpa×m^1/2and 18Mpa×m^1/2,it was at the second stage;when△K＞18Mpa×m^1/2,it was at the last stage.With the same level of stress intensity factor△K,the fatigue crack propagation rate(da/dN)at T3 condition is slower than that at T4 condition.(7)At the T3 condition,with the same level of stress intensity factor△K,the fatigue crack propagation rate(da/dN)on the condition of R=0.1 is slower than that on the condition of R=0.5.When R =0.1, AK＜7.3Mpa×m^1/2,it was the first stage;when△K was between 7.3Mpa×m^1/2and 30Mpa×m^1/2,it was at the second stage;when△K=30Mpa×m^1/2,da/dN=3.2×10^-3mm/cycle;when△K＞30Mpa×m^1/2,it was at the last stage.(8)At the T3 condition,when R=0.5 and R=0.1,with the same level of stress intensity factor△K,the fatigue crack propagation rate(da/dN) on the condition of L-T direction is slower than that on the condition of L-T direction,and the diffenerce was small;at the last stage,the diffenerce turned smaller.(9)R=0.landR=0.5,with the same level of stress intensity factor△K, the fatigue crack propagation rate(da/dN)of the 2E12 alloy is slower than that of 2E12+Zr alloy until△K=19Mpa×m^1/2in the initial stage.As the stress intensity factor△K increased,the diffenerce turned small.(10)The fracture appearance of 2E12+Zr alloy consisted of fatigue crack intiation zone,fatigue crack zone and final fracture zone.The locations of initial fatigue cracks generally were on the surface stress concentrated or on the different defects in alloy,the appearance of facets had fishbone-like.It was obvious to see fatigue stripe in fatigue crack zone and some secondary cracks.It was just like tensile fracture appearance in the final fracture zone with circular dimples and elongated dimples.