Structure, Thermal Stability And Mechanical Behavior Of Nanostructured Al-1%Si Alloy

Nanostructured pure metals show enhanced strength but suffer from twofundamental problems: reduced thermal stability due to the high stored energyassociated with the high density of grain boundaries and reduced ductility due to thelack of capability of work hardening. In this thesis we have designed and produced ananostructured Al-1%Si alloy with the addition of1%Si present in the form of dispersedparticles in the Al matrix. The principal objective of the present study is to explore theeffects of dispersed particles on the thermal stability and mechanical behavior of thenanostructured Al-1%Si alloy, with an aim to establish a new strategy to optimize thestructure and mechanical properties of nanostructured metals.High purity Al (99.9996%) was used as the matrix and the addition of1%Si was toform dispersed Si particles as the solubility of Si in Al is almost zero at roomtemperature. The cast Al-1%Si ingot was hot deformed and then cold-rolled to athickness reduction of98%to obtain a nanostructured state of the alloy, which was usedas the base material for investigating the effects of annealing on thermal stability andmechanical behavior. The specimens of as-cold-rolled and after different annealingtreatments were characterized by tensile tests and by microstructural analysis usingtransmission electron microscopy (TEM), scanning electron microscopy/electronbackscatter diffraction (SEM/EBSD). The main findings are summarized as follows:After cold rolling to98%thickness reduction a nanostructured Al-1%Si alloy wasproduced with a nanoscale lamellar structure containing nanoscale (~20nm) Si particles.The average boundary spacings are230nm and950nm along the normal direction (ND)and the rolling direction (RD), respectively. Large amount of second phase particlesdisperse mostly along lamellar boundaries which stabilize the microstructure. Thenanostructured Al-1%Si alloy has a yield strength of210MPa, a UTS of230MPa and atensile elongation of15%. Comparing with nanostructured high purity Al (99.99%),commercial purity Al1050(99.5%) and Al1100(99.2%), the as-processednanostructured Al-1%Si exhibits the best combination of strength and tensile ductility.The thermal stability of nanostructured Al-1%Si alloy was investigated over a widerange of temperature from100–600℃. It is found that the nanostructured Al-1%Sialloy is relatively stable when annealing for1hour at temperatures below200℃where only recovery takes place. Nucleation and growth occur when annealing for1 hour at temperatures higher than200℃. In more detail, partial recrystallization takeplace when the annealing temperature is between200and250℃. After annealing for1hour at temperatures higher than250℃, the material is fully recrystallized. Secondphase particles are coarsening, resulting in an increase in average size with an increasein annealing temperature. Highly strained regions are formed around second phaseparticles with diameters of several micrometers. Nevertheless these large particles donot stimulate preferred nucleation and growth around them during annealing due to thepinning effect of fine particles on the boundary migration. Therefore they have littleeffect on the thermal stability of the nanostructure. Nearly random texture is developedafter low temperature annealing, and rotated cube texture is developed after annealing athigh temperatures. The higher the annealing temperature is, the stronger the textureforms.The effect of annealing on mechanical behavior was investigated over the sametemperature range as for the thermal stability study. The strength drops rapidly withincreasing annealing temperature in the recovery region but the plasticity (totalelongation) shows nearly no change. This observation shows a distinct contrast to thatobserved in nanostructured commerical purity Al alloys, in which the elongation isreduced greatly after recovery annealing. Furthermore, no yield point phenomenonassociated with a yield drop and Lüders deformation appears after annealing at mediumtemperatures, as observed in nanostructured commerical purity Al after a similarannealing treatment. The Hall-Petch slope of the nanostructured Al-1%Si is twice ofthat obtained in the coarse grain range (>10m). This manifests that grain refinementinduced strengthening is much more effective when the grain size is in thesubmicrometer and nanometer range. The above results show a significant improvementin the tensile stability for both the as-processed structure and after recovery annealing.This improvement is related to the fine dispersion of Si particles in the microstructure,which retraded the recovery of dislocations during recovery and enhanced theinteraction with dislocations during tensile deformation.Comparing with nanostructured commercial purity Al alloys with similar nominalcompositions, the present nanostructured Al-1%Si shows a greatly improvedcombination of strength and ductility at strength levels of higher than100MPa.In conclusion, the results obtained in this thesis show that introducing dispersedparticles in nanostructured pure metals can generate significant beneficial effects instabilizing the mechanical behavior (including removal of recovery enhanced flow instability and of the occurrence of yield point phenomena) and the thermal response,and in achieving improved combination of strength and ductility at the higher level ofstrength of pure Al.