High Strength And Plasticity Of Zirconium Alloy Induced By L1₂ Type Intermetallics

Due to excellent diversity properties,Zr and its alloys can be used as structural materials in harsh service environments.In general,crystalline Zr alloys have been strengthened by solution strengthening,but at the cost of ductility loss.The intrinsic characteristic of HCPα-Zr is the insufficient slip system,which results in poor uniform plastic deformation ability.To obtain high strength and plasticity of Zr alloy,innovative alloy design concept is needed.In this paper,a novel design strategy for HCPα-Zr and FCC L1₂ intermetallic compound binding Zr alloys has been systematically developed by combining Sn,which is favorable for activating and promoting non-basal plane slip of HCP tissue,with light Al,which is favorable for introducing L1₂ intermetallic compound.By regulating Al content,for as-cast and rolling state Zr Sn Al alloy with HCP structureα-Zr,the lattice distortion caused by the solid solution strengthening effect of Al increased dislocation motion resistance,8at.%Al increases the compressive yield strength of as-cast alloy by 31.54%,6at.%Al increases the tensile yield strength and ultimate strength by 85.3%and 54.8%,respectively,but both of them are accompanied by the decrease of compressive plasticity and tensile elongation.When Al content reaches12at.%,large numbers of ultrafine B8₂intermetallics are precipitated in as-cast alloy,which are dispersed at grain boundaries and embedded inα-Zr matrix,and compressive yield stress is improved obviously.The L1₂ intermetallics with FCC structure were observed in hot-rolled Zr Sn_1.5A₈ to Zr Sn_1.5A₁₆ alloys with multi-phase structure ofα/α’+β+L1₂.The phase content of L1₂ in the Zr Sn_1.5A₁₆ alloy（60.5%）is much higher than that ofα-Zr（36.3%）.In the composition range of Zr Sn_1.5A₆ to Zr Sn_1.5A₁₄,the ultimate strength increases from 846MPa to 1080.84MPa,and total elongation increases from15.35%to 28.59%,simultaneously.Zr_84.5Sn_1.5Al₁₄（at.%）alloy with excellent comprehensive mechanical properties was selected,The B8₂-type HCP hard brittle phase in the as-cast structure was transformed into L1₂-type FCC phase with more slip systems and stronger capacity to accommodate dislocation through peritectoid reaction by further pretreatment.The holding temperature was lower than decomposition temperature of L1₂（960℃）and higher thanα→βtransition termination point（863℃）.B8₂ was completely transformed into L1₂ after holding at 900℃for 60min.After rolling,air-cooling sample has excellent properties of 21.98%uniform elongation and 26.53%total elongation.Different hot rolling temperatures show that the average grain size is smallest at 750℃（α-Zr:0.72±1.36μm;L1₂:0.52±0.66μm）,corresponding to highest yield strength（1275.7MPa）;At 800℃,it has high degree of equiaxed microstructure and the best comprehensive mechanical properties（yield and ultimate strength is 1200.9MPa and 1305.1MPa,uniform and total elongation is 19.4%and 19.49%）.The presence of acicularα′-Zr martensite with high dislocation density at900℃results in highest ultimate strength（1407.27MPa）and work hardening capacity.By analyzing the strengthening,toughening and deformation mechanism of dual-phaseα+L1₂ zirconium alloy with ultrafine size,in which yield and ultimate strength is 1203.84MPa and 1309.04MPa,uniform and total elongation is 24.49%and 35.27%.For the mechanism analysis,the high yield strength mainly derives from dislocation strengthening effect,grain size effect and hetero-deformation induced strengthening effect.Addition to activation of non-basal dislocations inα-Zr,incrementally hierarchical stacking-fault networks and high-density Lomer-Cottrell locks inside L1₂ during tensile deformation synergically induce multi-stage work-hardening response and uniform plastic deformation ability,resulting in unusually high tensile ductility.Yield strength is the critical stress of material resistant to plastic deformation.Plasticity is the ability of material to resist crack propagation,which requires initiation of large numbers of dislocation.In current paper,FCC L1₂ intermetallics were introduced into HCP Zr alloys with few slip systems.The synergistic strengthening of dislocation and L1₂ ordered phase was realized,and the high strength and plasticity of Zr Sn Al alloy was obtained,which enriched the theoretical system of strengthening and toughening Zr alloy.