Study On The Regulation Of Mechanical Properties And Invar Effect Mechanism Of Fe-Mn Alloy

Invar alloy has important and wide applications due to its low coefficient of thermal expansion,but the low strength limits its application in high-tech fields.In order to obtain high strength,previous studies generally used strengthening methods such as precipitated phase,nanocrystalline and solid solution.However,these methods not only lost the plastic deformation ability of the alloy,but also significantly increased the coefficient of thermal expansion,resulting in the weakening of the low expansion property of Invar alloy.In recent years,many studies have found that regulating the microstructure of the alloy and constructing heterostructures,such as bimodal grains,nano twins and recrystallization mixed structure,can make the alloy have high strength and high plasticity.These methods may become a new direction of Invar alloy strengthening,and how these microstructures affect the thermal expansion property needs to be further studied.In addition to the mechanical properties of Invar alloy limiting its engineering application,the magnetic property of Invar alloy also limits its use in high magnetic field environment.At present,the most widely studied Fe-Ni based Invar alloys exhibit ferromagnetism,and their Invar effect is easily affected by the surrounding magnetic field environment.Antiferromagnetic Invar alloy can make up for this deficiency.Therefore,we have systematically studied the Invar effect and mechanical properties of antiferromagnetic Fe-Mn Invar alloy.The main research contents and results are as follows:Firstly,the Invar effect mechanism of Fe-27Mn antiferromagnetic Invar alloy is studied by in situ synchrotron radiation high-energy X-ray diffraction by comparing with premartensitic transformation in Fe-24Mn alloy.It is found that the full width at half maxima（FWHM）of the diffraction peaks of the two alloys have similar temperature dependence.The FWHM of diffraction peaks of Fe-27Mn Invar alloy increased abnormally with the decrease of temperature during Invar effect.This phenomenon is reversible and isotropic.The results show that the abnormal variation of FWHM of diffraction peak is closely related to Invar effect.Fe-24Mn alloy also has the abnormal increase of the FWHM of diffraction peaks,which is due to the change of internal stress state caused by the formation and growth of martensite embryos in premartensitic transformation.TEM experiments show that there are additional diffuse scattering spots around the main diffraction spots of austenite in the two alloys.The dark field images corresponding to these diffuse scattering spots show that nano-scale martensite embryos dispersed in austenite.These results indicate that the Invar effect of Fe-Mn alloy may also be related to the formation and growth of nano-scale martensite embryos in premartensitic transformation.On the basis of understanding the Invar effect mechanism of antiferromagnetic Fe-Mn alloy,this paper also makes a study on the mechanical property strengthening of Fe-Mn Invar alloy.Using the maximum cold drawing deformation obtained the highest initial yield strength,and then appropriate heat treatment methods produced the heterostructure with ultrafine grains（hard zone）surrounded by micron-scale coarse grains（soft zone）.The heterostructure Fe-24Mn alloy has high yield strength（1280 MPa）,tensile strength（1344 MPa）and excellent plasticity with the uniform elongation of 11%.The results of cyclic loading and unloading experiments show that the high yield strength of heterostructure Fe-24Mn alloy comes from the hetero-deformation induced（HDI）stress produced by ultrafine grains and coarse grains,and the high plasticity comes from the HDI strain hardening,which is caused by the plastic incompatibility from the heterogeneity of microstructure.In situ synchrotron radiation experiments during tensile deformation gave the results that TRIP effect also contributes greatly to the work hardening ability of the sample.Therefore,the comprehensive mechanical properties of high strength and high plasticity can be obtained.In addition,grain refinement and high dislocation density inside the grains inhibit the martensitic transformation,as a result the sample shows a low coefficient of thermal expansionα_{（-100℃⁰℃）}=（9.15±0.15）*10^-6/℃.There are dispersed martensite embryos in austenite due to the premartensitic transformation.The low thermal expansion performance may be due to the growth of these martensite embryos,which counteracts the thermal expansion and cold contraction behavior caused by anharmonic thermal vibration of lattice.In addition,Fe-30Mn Invar alloy wire containing nano-scale twins was prepared by cold drawing deformation and heat treatment.Nano-twinned Fe-30Mn alloy shows high strength and good plasticity.The yield strength reaches 1065 MPa and the total elongation reaches 24.5%.The high yield strength of the alloy comes from the nano-twinned structure.It is also found that nano-twinned Fe-30Mn alloy has lüders band deformation,and strain induced mechanical twins,namely TWIP effect,make the alloy sample obtain high plasticity.The thermal expansion coefficient of nano-twinned Fe-30Mn alloy isα_{（-100）℃～0℃}=（9.3±0.15）*10^-6/℃,which is close to that of the coarse-grained sample.The grains of Fe-30Mn alloy in solid solution and nano-twinned samples contain a large number of nano-scale martensite embryos,which may be related to the generation of Invar effect.As a potential strengthening method for Invar alloy,the nano twins strengthening method can significantly improve the yield strength of the alloy without increasing the coefficient of thermal expansion,and does not lose its plastic deformation ability.