| Magnesium(Mg)alloys have wide application prospects in the fields of transportation,aerospace,and national defense,due to their low density and high specific strength.Because of low production cost and good processability,dilute Mg alloys,i.e.low-alloyed Mg alloys,attract widespread attention among researchers at home and abroad.However,it is difficult to form a high volume fraction of precipitates to restrict grain growth in conventional dilute Mg alloys during thermal mechanical processing,which is not conducive to achieving high strength-ductility synergy at room temperature and superplasticity at elevated temperatures.Through the new low-alloy and high-performance composition design principles,grain boundary(GB)segregation and nano-sized particles can be introduced in the dilute Mg alloys to modify the microstructure thermal stability,facilitating the improvement of strength-ductility synergy at room temperature and superplasticity at elevated temperatures.However,the studies related to improved thermal stability by GB segregation mainly focused on the static annealing process,and the relationships between GB segregation and superplasticity of low-alloy Mg alloys remain unclear.Besides,the sub-rapid solidification(SRS)technology can alleviate the micro-segregation and improve the solubility of alloying elements in the matrix,which is beneficial to the development of high-performance dilute Mg alloy sheets.In recent years,the strength of dilute Mg alloys can be greatly improved by the formation of clusters,solute segregation,and G.P.zones via strain-aging treatment.However,high-temperature solid-solution treatment prior to strain-aging treatment inevitably causes grain coarsening and great strength loss,which is hard to be compensated by the formation of nano-sized precipitations.Therefore,it is of great importance to explore new strengthening mechanisms or realize the combination of strain-aging strengthening and fine-grain strengthing in the development of dilute Mg alloys with high strength-ductility synergy.Based on the above research status and existing problems,a low-alloyed Mg-1.0Zn-0.5Ca-0.3Sn-0.2Mn(wt.%)alloy was designed in this paper,and the dilute Mg alloy sheet fabricated by SRS and multi-pass rolling exhibited a fine-grained microstructure,accompanied by a high strength-ductility synergy.At the same time,the relationship between GB segregation and superplastic deformation of Mg-Zn-Ca-SnMn alloy was studied in this paper.Based on dislocation-induced solute aging repartitioning behavior,an impressive strain-aging strengthening effect was realized in the fine-grained Mg-Zn-Ca-Sn-Mn alloy.The rapid natural-aging(NA)and roomtemperature dynamic strain-aging precipitation behaviors of Mg-Zn-Ca-Sn-Mn alloys were studied,and a remarkable cluster strengthening was achieved in the alloy.The main conclusions are as follows:(1)The effects of solidification rate on the microstructure and mechanical properties of Mg-Zn-Ca-Sn-Mn alloy were studied.Compared to the conventional solidification(CS)counterpart,the SRS alloy had a significantly finer primary α-Mg phase and eutectic phase,and the average grain size was refined from ~100 μm to ~240 μm.After hot-rolling and annealing,the SRS alloy exhibited a finer grain size(~3 μm)in comparison with that(~5 μm)of the CS counterpart.Moreover,the sizes of eutectic phases in the R&A SRS alloy were also smaller,and the distribution was more uniform than that in the R&A CS alloy,reducing the stress concentration and thus the possibility of crack initiation.Therefore,the R&A SRS alloy exhibited a high yield strength(YS)of ~270 MPa and high elongation of ~21%.(2)The roles of GB segregation on the superplastic deformation at elevated temperatures in the low-alloyed Mg-Zn-Ca-Sn-Mn alloy were revealed.A uniform fine-grained(~4 μm)Mg-Zn-Ca-Sn-Mn alloy was fabricated via extrusion and multi-pass rolling,and the alloy exhibited an impressive superplasticity with elongation ~410% at 573 K and 10-3 s-1.The superplastic deformation was co-dominated by grain boundary sliding(GBS)and solute drag creep.The solute segregations of Zn and Ca atoms at GBs and nano-sized precipitates exerted a threshold stress(~4 MPa)for the operation of GBS,however,the resulting superior thermostability(<6 μm)guaranteed the superplastic deformation ability.(3)The strain-aging hardening mechanisms in the low-temperature annealed Mg-ZnCa-Sn-Mn alloy with fine-grained microstructure were illuminated.The uniform fine-grained(~2.5 μm)Mg-Zn-Ca-Sn-Mn alloy exhibited an impressive YS increment of ~30 MPa after ~2% pre-tension and 175 oC/30 min treatment.The YS of pre-strained or directly-aged alloy had no obvious change.After lowtemperature annealing,a large number of Zn and Ca atoms were enriched at GBs.During aging process,the Zn and Ca atoms segregated at the GBs and dissolved in the matrix diffused toward the dislocation cores,which was facilitated by the fast "channel" effect provided by pre-deformation-induced basal <a> dislocations.Subsequently,the formation of dislocation segregation pinned the dislocation movement,contributing to the achievement of strain-aging hardening in the finegrained dilute Mg alloy.(4)The fast natural aging(NA)strengthening mechanisms of dilute Mg-Zn-Ca-SnMn alloy were elucidated.After ~2% pre-tension and 48 h NA treatment,the YS of Mg-Zn-Ca-Sn-Mn alloy witnessed an improvement of ~25 MPa,while the YS of pre-strained or directly naturally-aged alloy had no obvious change.Basal <a> dislocations and non-equilibrium vacancies were introduced by pre-strain treatment,and the Zn and Ca atoms diffused toward the dislocation cores during NA process,forming dislocation segregation and thus improving the YS.(5)The rapid dynamic strain-aging hardening mechanisms of dilute Mg-Zn-Ca-SnMn alloy were elucidated.A considerable YS improvement of ~81 MPa was observed in the Mg-Zn-Ca-Sn-Mn alloy after undergoing cyclic deformation(CD)at room temperature.Moreover,the elongation was only reduced from ～18% to ～16%.A large amount of non-equilibrium vacancies were injected into the alloy by cyclic deformation,facilitating the formation of high-density(~5.7 × 1024 m-3) nanosized solute clusters(Zn-Ca-Mn,Zn-Ca,Zn-Zn,Zn-Mn,Ca-Ca,and CaMn).Therefore,the YS of dilute Mg alloy can be greatly improved with little sacrifice of elongation. |
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