Research On The Microstructures And Mechanical Properties Of Fe-Ni-Co Based Poly Crystalline Shape Memory Alloys

Compared with Ni-Ti based and Cu based shape memory alloys, Fe based shape memory alloys have the advantages of high strength, good plasticity, flexibility, low price and so on; so in the field of application of shape memory alloy, it has very important research value. In this paper, based on a new type of Fe based polycrystalline shape memory alloys—Fe-Ni-Co based shape memory alloys which have Nb element as research object, the effects of content of Nb element on microstructure and mechanical properties of Fe41.5-xNi30Co18Al10.5Nbx(x=0,1,2,3) shape memory alloy have been studied. The effects of aging duration and aging temperature on microstructure and mechanical properties of the alloys were investigated by X-ray diffraction, SEM, pressure test machine, metallurgical microscope, and energy disperse spectroscopy. The results show that:(1) At the aging temperature of 873 K, Fe41.5-xNi30Co18Al10.5Nbx shape memory alloys with different content of Nb element were aged treatment for 60 h. With aging duration increasing, γ’ phase and β phase precipitated successively, the precipitation of γ’ phase strengthens the austenite matrix, while the precipitation of β phase reduces the ductility of the alloy. Nb element can refine the grain and improve the strength of the alloys. With the increasing of the content of Nb element, the recoverable strain maximum strain and compressive strength of the alloy increase at first and then decrease. When the content of Nb element is 2 %(at. %), the compressive strength, the recoverable strain and the maximum strain of the alloys all reach maximums, the recoverable strain reaches 14.01 %. And at this moment, the pseudo-elasticity of alloy is the best. The Vickers hardness of the aged alloys also gradually increase at first and then decrease. When the content of Nb element is 2 %(at. %), the hardness of the aged alloys reaches maximum, reaching at 535.7 HV. Adding Nb element to the alloys can decrease the mean thermal expansion coefficient of the alloys; When the content of Nb element is 2 %(at. %), the mean thermal expansion coefficient is minimum relatively.(2) The Fe39.5Ni30Co18Al10.5Nb2 shape memory alloys were aged treatment 60 h at 723 K、873 K and 1023 K. With aging duration increasing, γ’ phase and β phase precipitated successively. The pseudo-elasticity curves show that, with aging duration increasing, the compressive strength and recoverable strain of the alloys increase at first and then decrease. When the aging temperature is 873 K, the compressive strength and the recoverable strain of the alloy both reach maximums and are 1818.9 MPa, 14.01 %. The Vickers hardness of the alloys also gradually increase at first and then decrease with the increasing of aging temperature. When the aging temperature is 873 K, the hardness of the alloy reaches maximum, reaching at 535.7 HV. During the aging process, the size and morphology of the precipitated γ’ phase and β phase can change the mean thermal expansion coefficient of the alloys. When the aging temperature is 873 K, the mean thermal expansion coefficient is minimum relatively.(3) The aging duration has a significant influence on microstructure and mechanical properties of Fe39.35Ni30Co18Al10.5Nb2B0.15 shape memory alloy. At the aging temperature of 873 K, the alloys were aged treatment for 20 h, 45 h, 60 h, 72 h. With aging duration increasing, the compressive strength, recoverable strain and Vickers hardness of the alloys increase at first and then decrease. When the aging duration is 60 h, the compressive strength, the recoverable strain and the hardness of the alloy all reach maximums. And at this moment, the pseudo-elasticity of alloys is the best.