Research On The Microstructures And Properties Of Fe-Ni-Al-Nb Based Alloys

Fe-based shape memory alloy has many merits, such as excellent mechanical properties, ease to process, cheap and so on. So it has been a hot research topic in shape memory alloys. In order to research the effect of Nb content, different heat treatment conditions and thermo-mechanical training on properties and organization of Fe-Ni-Al-Nb alloys, this paper attempted through some experimental methods,such as x-ray diffraction analysis, scanning electron microscope, metallurgical microscope, energy dispersive spectrometer, compression-test machine, microhardness test, thermo-mechanical training and so on. The conclusions are as follow:(1) After solid solution treatment, there are many twin crystals in Fe59.5-XNi30Al10.5NbX(x=0,1,2,3)alloys. The grain size of alloy with Nb element is smaller than that of the alloy without Nb element, and there were only matrix phase ?and primaryphase ? in the alloys. But the phase ?' precipitated in the alloys with Nb element after aging treatment. With the increase of Nb contents, the hardness of as-soluted alloys increased, but the hardness of aged alloys increased at first then decreased. The hardness of aged Fe57.5Ni30Al10.5Nb2 alloy is the maximum, reaching394.4 HV. Adding Nb element to the alloy can reduce the expansion coefficient of the alloys. The pseudo-elasticity of Fe57.5Ni30Al10.5Nb2 alloy is the best in the four alloys,the recovery strain is the maximum, reaching 6.637%, and the residual strain is the minimum, reaching 0.2475%.(2) After diverse aging time treatment at 600 ?, there were many phase in the alloys. After aged 20 h, there were matrix phase ?, primaryphase ? and infinitesimal precipitated phase ?' in the alloys. With the aging time increased, the quantity of phase ? and phase ?' gradually increased. While aged 70 h, the size of the phase ?'was further increased. With the aging time increased, the hardness of aged alloys increased at first then decreased. When aged 60 h, the hardness of the alloy reaches the maximum and the pseudo-elasticity of Fe57.5Ni30Al10.5Nb2 alloy is the best.(3) In thermo-mechanical training, when the heating temperature is 600 ?and the pre-strain is 4%, the pseudo-elasticity of Fe57.5Ni30Al10.5Nb2 alloy can beappropriately improved. In thermo-mechanical training, the stress can induce martensitic transformation. With the increase of thermo-mechanical training times,the amount of martensite and the deformation recovery rate increased. After training twice, the deformation recovery rate of alloy reached the maximum, 8.8%. But, after training three times, the martensite had appeared different orientation and the deformation recovery rate of alloy decreased because of the permanent deformation increased. Thermo-mechanical training can improve the pseudo-elasticity of Fe57.5Ni30Al10.5Nb2 alloy.