Study On The Influencing Factors Of Invar Effect Of Fe-Ni Alloy

Fe-Ni Invar was first discovered by the French physicist Guillaume in 1896 and won the Nobel Prize in Physics;because of its low coefficient of expansion(Invar effect)in a certain temperature range,Widely used in the manufacture of precision instruments in aerospace and other fields.However,the low expansion mechanism of Invar has been controversial for centuries.In this paper,the influencing factors of low expansion coefficient of Fe-Ni alloy are studied,and the mechanism of Invar effect is explored.Based on the research of standard Invar 4J36(Fe64Ni36,at%)and super Invar alloy 4J29(Fe54Ni29Co17,at%),17 kinds of FeNi alloys with different Ni contents were designed and passed through vacuum induction melting,forging and drawing.Fe-Ni alloy wire was prepared by annealing and annealing,and the influencing factors of Invar effect were explored by using tensile,three-dimensional atom probe,scanning electron microscope,transmission electron microscope and thermal expansion instrument.We have discovered for the first time in the world that three-dimensional atom probes have a nanoscale(10-15 nm)chemical composition fluctuation in Fe64Ni36(at%)alloy.The surface of the 4J36 alloy sample(room temperature polished)after plastic deformation was observed by scanning electron microscopy.An abnormal bending of the slip belt occurs.It is found by thermal expansion test that the Martensitic transformation occurs in the FeNi alloy during the cooling process with the decrease of Ni content.When the Ni content is reduced to 30%(atomic percent),the Fe70Ni30(at%)starts to decrease in the martensite phase.Before the initial temperature(Ms),the expansion coefficient is abnormal(invar effect);the tensile test results at different temperatures(below room temperature)show that the typical FeNi Invar alloy(4J29,4J36,4J42,4J50)decreases with the stretching temperature.The yield strength of the sample increases,showing an abnormal relationship between yield strength and temperature.The study also found that heat treatment temperature,time and plastic deformation have little effect on the Invar effect.Based on the above results,we suspect that the Invar effect of the FeNi alloy may be due to the presence of nanoscale(10-15 nm)chemical composition fluctuations(uneven distribution of Ni elements).This chemical composition undulation leads to the presence of a lower,high martensitic transformation start temperature Ms(controlled by chemical composition)in the nanometer-scale two regions.During the cooling process,when the temperature is lower than Ms,the nanoscale high phase The variable temperature(low Ni)region attempts to undergo martensitic transformation(volume expansion),but is constrained by the surrounding low phase transition temperature(high Ni)region;although it cannot undergo martensitic transformation during cooling,it will A certain degree of volume expansion occurs,offsetting the volumetric contraction during the original cooling process,resulting in a lower coefficient of expansion(invar effect).