| The Fe-Mn-Al-Ni-based shape memory alloy has the advantages of excellent processing performance,higher strength,good superelasticity and lower cost.It has attracted widespread attention in recent years,and it is a new type of Fe-based memory alloy with great potential for development.In order to further improve its strain characteristics,in this investigation,a new type of Fe-Mn-Al-Ni shape memory alloy was prepared by adding a small amount of Nb and C elements to Fe-Mn-Al-Ni alloy.The structure evolution of the as-cast alloy during the heat treatment process was systematically investigated,the difference of the shape memory effect,superelastic and corrosion resistance between the as-cast and hot-rolled alloys under one-step aging and two-step aging conditions was compared and analyzed.The as-cast alloy has a dual phase structure of ?+?.Thermal expansion experiments shows the ?-Mn phase is precipitated in ? phase when heated to600°C?650°C;the bct structured martensitic phase is produced within the ? phase at800°C?950°C,and the ? single phase could be obtained after solution treated at1200°C for 2 h.In addition,it was observed by TEM that Ni Al phase was precipitated in the matrix of the specimens which aged at 200°C.With the further extension of the aging time,the precipitations gradually grew up and the distribution became more uniform.After aged at 200°C for 6 h,the size of the second phase particles grew from9 nm which aged for 0.5 h to about 32 nm.Meanwhile,Nb C particles precipitated in the matrix and were accompanied by the Ni Al phase after the two-step aging at800°C+200°C.After the aging treatment,the shape memory effect and superelasticity of the alloy were significantly improved,the recovery rate of the as-cast alloy is increased from 20% to 45%;the hot-rolled alloy is increased from 21% to 50%.And after the two-step aging,the recovery rate is increased to 51% and 60%,respectively.In the tensile test,it was found that when the applied strain is 4%,the maximum superelastic of the one-step aging as-cast and hot-rolled alloys is 1.12% and 1.15%,respectively.After two-step aging,it is increased to 1.22% and 1.25%,respectively.In the compression test,when the applied strain is 4%,after two-step aging,the superelastic of as-cast alloys is increased from 1.25% to 1.39%,the hot-rolled alloy increased from 1.36% to 1.47%.The difference in superelastic between the tensile and compression tests is related to the difference in the number of martensite variants produced during the deformation process.AFM and in-situ SEM revealed that the amount of stress-induced ?' martensite reduced constantly during heating temperature between 100°C and 400°C,it was indicated that the ?'?? transformation has occurred.The results of polarization curve and electrochemical impedance spectroscopy shows that one-step aging could improve the corrosion resistance of as-cast and hot-rolled alloys;due to the precipitation of Nb C particles,the content of C in the matrix decreases,and the matrix composition around the particles is not uniform.In addition,there is a potential difference between the Nb C particles and the matrix.And the potential of such particles is usually higher than that of the matrix,therefore,in the corrosion process,the matrix near the particles is preferentially dissolved.Macroscopically,it shows a certain degree of decrease in the corrosion resistance of the material. |
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