Phase Transformation And Superelasticity Of FeMnAlNiC Alloy

The traditional iron-based shape memory alloys(Fe-SMAs)represented by FeMnSi alloys often have non-thermoelastic martensitic transformation,no obvious superelasticity at room temperature,and shape memory effects are degraded during cyclic loading,therefore,the development of Fe-SMAs is greatly restricted.Fe-SMAs with thermoelastic martensitic transformations,such as FeNiCoAlTaB and FeMnAlNi alloys,can be obtained by introducing ordered precipitation phases to change the martensitic transformation type by aging treatment,which breaks through the limitation of Fe-SMAs without superelasticity and it is expected to become the most promising iron-based shape memory alloy.In this paper,based on FeMnAlNi polycrystalline alloy,a small amount of element C was doped to prepare a superelastic FeMnAlNiC shape memory alloy,and then the phase transition process and its superelasticity were studied.The FeMnAlNiC alloy obtained by C doping has an ?+? dual phase structure under its as-cast condition.And the microstructure of a single phase can be obtained after solution heat treatment at 1275? for 2 h.After the aging treatment,the ?-NiAl phase coherent with the mother phase is precipitated.The effect of the aging treatment on its superelasticity was studied.The optimum aging condition of the as-cast sample of FeMnAlNiC alloy was 200?-6 h.Under these conditions,in the tensile test of incremental strain,when the applied strain reaches 6%,the superelastic recovery strain is 1.5%,and the recoverable strain is 3.09%.In the incremental strain compression test,When the applied strain reaches 7%,the superelastic recovery strain is 2.3%,and the recoverable strain is 6.07%.Comparing the tensile/compression tests under the same heat treatment conditions,the difference in the number of stress-induced martensite variants is the main reason for the different superelasticity under different stress states.The thermal deformation behavior was studied by thermal compression simulation and the thermal processing equation was established.The results show that the hot workability is better at 950?1100? and the strain rate is 0.1?1 s-1.The thermomechanically processed samples were heat treated at different temperatures.The results showed that the ?-Mn phase was precipitated between 550? and 800?;bct-martensite(?')was present between 800? and 1000?;When the temperature is above1250?,the ? single phase structure can be obtained.After high-temperature solution treatment or cyclic heat treatment at 1250?/900?,the needle-like 18R(51)3-martensite appears.By studying the microstructure evolution and superelasticity of the hot-rolled samples after solution/cyclic heat treatment,it is found that as the grain size increases,the stress-induced martensite critical stress decreases and the superelastic strain increases.After 5 cycles of heat treatment,its grain size reached more than 6.81 mm,and its superelastic recovery strain increased to about 5.14%.Comparing the superelasticity of as-cast and hot-rolled samples,it was found that under the same aging conditions,The superelasticity(?max=2.30%)of the as-cast sample after the solution treatment at 1275? for 2 h is similar to the superelasticity(?max=2.39%)of the hot-rolled sample after 2 cycles heat treatment.