| Shape memory alloy is a functional material with shape memory effect through martensitic transformation and its reverse transformation.The Fe-Mn-Si-Cr-Ni-based shape memory alloy with the most development value,in addition to the shape memory performance,can be smelted and processed on a large scale under the existing production conditions of iron and steel enterprises,and has the advantages of high strength and good rigidity.It is suitable for use as structural materials and also as special purpose materials,which has broad prospects in application.At present,it has been used in earthquake-resistant damping alloys in building structures,and the development of iron-based Fe-Mn-Si-Cr-Ni shape memory alloys and their key technologies for industrial production have become one of the focuses of research and development of damping materials for earthquake-resistant buildings.In this paper,three kinds of Fe-Mn-Si-Cr-Ni series shape memory alloys with different C content were prepared in a hot forging laboratory,and a systematic comparative study was carried out.A variety of testing methods were used to analyze the influence of the content of C in the alloy on the shape memory performance of the alloy.On this basis,according to the principle of optimal C element addition content,combined with actual production conditions,the hot rolling process is used for industrial trial production.Through the solid solution+aging and direct aging processes,the changes and influence mechanisms of various properties of the experimental alloys under different heat treatment systems are explored.The main research results of this article are as follows:(1)The influence of C element content on the mechanical properties of the alloy matrix structure,the?martensite transformation point,and the shape memory performance are obtained.Under the conditions of solid solution+aging heat treatment,the strength of the alloy matrix phase is affected by solid solution strengthening and precipitation strengthening.The austenite matrix phase comprehensive strengthening increment increases with the addition of C element.The increase in the strength of the matrix phase can inhibit the slip of the complete dislocation,which makes the alloy proceed???with Shockley incomplete dislocations at a higher stress level;when it deforms near the M?s point,it is easy to induce martensitic transformation.The?martensite transformation point M?s decreases with the increase of C element content;its shape memory effect first increases and then decreases with the increase of carbon content.With the increase of carbon content,the strengthening of austenite matrix is the dominant factor to promote???.When the carbon content is higher than the critical value,if the carbon content is continuously increased,the M?s of the alloy will decrease due to the higher carbon content,which will further deviate from the working temperature,resulting in poor shape memory performance.(2)For the precipitation strengthening of the second phase,the precipitation behavior and precipitation control of the Cr23C6 phase were studied.The results show that a large amount of Cr23C6 phase precipitates after aging treatment,which can strengthen the austenite matrix strength of the alloy,thereby improving the shape memory effect.By calculating the precipitation kinetics of Cr23C6 phase,the formulas of solid solubility product in the austenite matrix phase of the test alloy and ratio interface energy formula are deduced as:7)2){[][]}=92.96616-135147/+5.17663×10/and(J?m)=0.6989-0.3149×10.The theoretical calculation of thermodynamics and precipitation kinetics is used to find the best precipitation temperature of the second phase,which provides theoretical guidance for obtaining the most excellent shape memory effect aging treatment process.(3)The effects of direct aging process and traditional solid solution+aging heat treatment process on various properties of industrial trial-produced hot-rolled alloys are obtained.Different from the traditional aging heat treatment process after solution heat treatment,the direct aging process is better than the solid solution+aging process,which can retain the number of stacking faults in the structure as the nucleation position of the?martensitic transformation;avoiding the re-dissolution of the second phase particles,resulting in the disappearance of the effect of pinning the grain boundary and the occurrence of abnormal growth,and avoiding the formation of annealing twins that are not conducive to the shape memory effect in the solution heat treatment.Through the quantitative characterization of the second phase precipitation mass fractions of M23C6 and Nb C under the two heat treatment processes,the direct aging process can obtain more second phase precipitation due to the influence of the hot-rolled alloy deformation storage energy,which further improves the shape memory of the test alloy effect.When the 800?direct aging process is adopted,the best shape recovery rate of80%can be obtained. |
PDF Full Text Request