Grain Boundary Character Distributions Of Rolled And Annealed Pb-Ca-Sn-Al Alloy

The effects of rolling temperature,rolling strain and after-rolling heat treatment on the grain boundary character distribution(GBCD) of Pb-Ca-Sn-Al alloy were investigated by electron back scatter diffraction(EBSD) technique.The results showed that,for a recrystallized sample,rolling at liquid nitrogen(-196℃) was easier than rolling at room temperature and 100℃in promoting the production of special grain boundaries(mainly composed of the low∑-CSL boundaries such as∑3,∑9 and∑27) in the subsequent annealing.In moderate strain(10%～60% thickness reduction) rolling at liquid nitrogen,rolling by 30%strongly promoted the formation of special grain boundaries in the followed annealing.For the sample rolled by 30%at liquid nitrogen,its peak value of special grain boundary fraction out of the total boundaries appeared at 10 minutes during annealing at 270℃.The investigation also demonstrated that the fraction of∑3,∑9 and total special boundaries and the value of(∑9+∑27)/∑3 in the sample rolled at liquid nitrogen by 30%followed by annealing at 270℃for 10 minutes was 66.7%,13.7%, 84.3%and 0.28,respectively,while that in the sample rolled at room temperature by the same strain followed by the same annealing was 47.4%,13.2%,63.7%and 0.21.In these two samples,the connectivity of general high angle grain boundary (HAB) network was interrupted by the special boundaries substantially,implying the GBCDs were optimized significantly.However,the fraction of total special grain boundaries in the sample rolled at 100℃followed by the same annealing was only 38.2%and the connectivity of HAB network was not interrupted by the special boundaries reflecting the negative result of GBCD optimization.Further discussion pointed out that owing to its low stacking fault energy of FCC Pb-Ca-Sn-Al alloy,cross slip and climb of the dislocations was depressed in the sample during rolling at liquid nitrogen and the dislocations were hard to form the conventional cell structures but tended to exist as the form of so called Taylor lattice,it might be in favor of the formation of great deal of annealing twins and resulted in the production of very high fraction of special grain boundaries in the subsequent annealing.The(∑9+∑27)/∑3 value in the sample rolled at room temperature was much higher than that rolled at liquid nitrogen indicated incoherent∑3 boundaries were easier to be produced in the former and it might be the key point that though the fraction of special grain boundaries was not so high, its GBCD was optimized significantly in the former.Dynamic recovery was probably operative in the sample rolled at 100℃because of the low melting point (around 320℃) of the alloy and it could be negative to the formation of special grain boundaries in the followed annealing.Of course,the effects of rolling strain and the holding time of annealing on the GBCDs of the samples as processed might be stemming from their influence on the behavior of recrystallization and boundary migration.These issues need to be studied in detail.