Σ3 Grain Boundary Charater Distributions In A Pb-Ca-Sn-Al Alloy

Pb-Ca-based alloys,which are currently used to make anode grid of lead acid batteries,exhibit poor resistance to intergranular corrosion attack under the service conditions.It usually shortens the service life of batteries because of the intergranular corrosion-resultant collapse of anode grid and hereto the failure of electrical charge and discharge.Although the novel materials for anode grid may be developed in the future,grain boundary engineering(GBE)or grain boundary character distribution(GBCD)optimization,which means enhancing the fraction of so-called corrosion resistant low Σ(1<Σ<29)coincidence site lattice(CSL)boundaries(special boundaries or SBs)in the bulk materials so as to higher the intergranular corrosion resistance,is a promised approach to prolong significantly the service life of the lead-acid batteries.Researches suggest that the lead alloy is a low stacking fault energy face-centered cubic metals,its GBCD can be optimized based on a mechanism of twin-induced GBE and Σ3 boundaries constitute the main part of the so-called special grain boundaries.In present work,Pb-0.07Ca-1.8Sn-0.026 Al alloy(mass fraction,%),which is used to make anode grid of maintenance-free lead-acid batteries,is used as experimental material.Electron backscatter diffraction(EBSD),single section trace analysis and five parameter analysis(FPA)are employed to investigate the effects of solute treatments,initial microstructure,rolling temperature,thickness reduction and holding time of annealing on the Σ3 grain boundary character distributions in the lead alloy.The main results are summarized in following:(1)Solution treatments have significant influence on the Σ3 GBCD in the lead alloy which is subsequently cold rolled and annealed.It is found that when the naturally aged lead alloy is subjected to cold rolling and annealing without solution treatment in advance,it only produces a small amount of Σ3 boundaries and the Σ3 boundary fraction out of the entire boundaries is less than 30%(length fraction).However,when the alloy is solution-treated at 310℃ for 1h at first and then subjected to cold rolling and annealing,it usually yields a lot of Σ3 boundaries and the Σ3 boundary fraction out of the entire boundaries is greater than 57%.Lengthening the holding time of solution treatment from 1h to 5h at the same temperature leads to a slight decrement of Σ3 boundaries when the alloy is given a subsequent cold rolling and annealing.(2)The texture combination of {0 1 1}<1 0 0>(Goss),{1 1 0}<1 1 2>(B or Brass),{0 0 1}<1 0 0>(Cube)and{112}﹤111﹥(Copper),which were introduced by the pre-treatment,is the crucial requirement for the development of Σ3 boundaries in Pb alloy during the subsequent processing of 30% rolling plus annealing at 270℃.During the subsequent rolling and annealing process,the incoherent Σ3 boundaries in Pb alloy are major part of Σ3 boundaries,and with annealing proceeding the incoherent Σ3 boundaries tend to tuned into coherent Σ3 boundaries.(3)Rolling temperature has a minor effect on the Σ3 GBCD in the lead alloy.The Σ3 GBCD obtained by a rolling at room temperature and a annealing at 270℃ is very similar to that achieved by a rolling at liquid nitrogen temperature(-196℃)and a annealing at 270℃,the Σ3 boundary fractions in both processing are increasing with thickness reduction(10%-30%)and the holding time of annealing(3min-300min).Non-coherent Σ3 boundaries formed during the early stage of annealing are transforming into low-energy stable coherent Σ3 boundaries during further annealing.The greater the amount of deformation,the faster the conversion of Non-coherent Σ3 boundaries into coherent Σ3 ones is.The maximum of coherent Σ3 boundary fraction out of the entire Σ3 boundaries is 66%.(4)The thickness reduction of rolling and the holding time of annealing have remarkable effects on the Σ3 GBCDs.For the thickness reduction of 50-70%,a great deal of Σ3 boundaries as well as big-sized(around 200 micron)Σ3n(n=1,2,3)are developed when the alloy is given a subsequent annealing at 270℃for short-time.During further annealing,the extensive migration of high angle grain boundary(or random boundaries)takes place.Due to the “sweeping” effect of the high angle grain boundary,the Σ3 boundary fraction decreases rapidly.At the same time,non-coherent Σ3 boundaries formed during the early stage of annealing are transforming into low-energy stable coherent Σ3 boundaries during further annealing.The maximum of coherent Σ3 boundary fraction out of the entire Σ3 boundaries is 70%.