Crystallization And Phase Transformation Behavior Of ZrCu-based Shape Memory Alloy Melt-spun Ribbons

ZrCu-based shape memory alloys as a potential high-temperature shape memory alloy,the phase transition temperature is higher,and can be adjusted within a wide range.In addition,it has a strong ability to form an amorphous alloy.ZrCu-based shape memory–reinforced metallic glass composites greatly improve the plasticity of the amorphous matrix.In this paper,Zr₅₀Cu₂₅Ni_xCo?25-x??x=5,7.5,10,12.5?shape memory alloy ribbons were prepared by the single-roll stripping method.The crystallization behavior,microstructure and martensitic transformation of the alloy ribbons were studied.The optimum technological parameter of Zr₅₀Cu₂₅Ni_xCo?25-x? alloy ribbons were as follows: spray casting pressure 0.04 MPa,copper roll speed 3000r/min,the distance between the nozzle and the copper roller is 2mm,the diameter of the nozzle 1mm,injection temperature 1160?.The addition of Ni and Co reduces the amorphous forming ability of ZrCu alloy.Through the study of the microstructure of Zr₅₀Cu₂₅Ni_xCo?25-x? alloy ribbons,it is found that the microstructure of the as-cast alloy ribbon is heterogeneous.Along the thickness of the alloy ribbon,The microstructure of the alloy ribbon from the wheel side to the free side is composed of amorphous,an amorphous matrix containing nanocrystals and crystallized.Homogeneous microstructure can be obtained after complete crystallization.The microstructure of the alloy ribbon is relatively small after different crystallization processes and most grain sizes are between 50-100 nm.Morphology of martensitic is parallel strip and the structures of martensitic are micro twins.The width of partial lath martensite is mostly 2-5nm.Most twin plane boundary are straight and have good coherence.Occasionally small steps appear at twin plane boundary because of thermal stress or deformation.The content of Ni and Co and annealing temperature have significant effects on the microstructure of Zr₅₀Cu₂₅Ni_xCo?25-x? alloy ribbons.After annealing at 460°C,510°C,800°C,martensite in Zr₅₀Cu₂₅Ni₅Co₂₀ alloy ribbons gradually increased and the parent phase gradually decreased until disappeared with the increase of annealing temperature.After 510? annealing,Microstructure of Zr50Cu25Ni10Co15 and Zr50Cu25Ni12.5Co12.5 alloy ribbons are twin martensites,?021?type I twins.Zr50Cu25Ni12.5Co12.5 alloy ribbon has the best effect on inhibiting the second phase.The grain sizes of Zr₅₀Cu₂₅Ni_xCo?25-x? alloy ribbons are much smaller than that of the bulk material of the same composition and it results that the phase transition temperature is lower than that of the bulk.With the increase of Ni content,the phase transition temperature of Zr₅₀Cu₂₅Ni_xCo?25-x? alloy ribbons gradually increases.The peak temperature of martensite phase transition of Zr50Cu25Ni12.5Co12.5 alloy ribbon was 388.9?.Through the study of the microstructure of parent phase of Zr₅₀Cu₂₅Ni₅Co₂₀ alloy ribbons 3% tensile strain at room temperature after 460? annealing,it is found that the mother phase occurs stress-induced martensitic transformation.Obvious steps appear at twin plane boundary.Through the study of the microstructure of martensite phase of Zr₅₀Cu₂₅Ni_7.5Co_17.5 alloy ribbons with 3% tensile strain,it is found that small deformation twin crystals formed between martensite variants.The stacking faults are observed near the twin plane boundary.The deformation twin crystals are formed to relieve stress relaxation.