Study On Microstructure And Thermal Expansion Properties Of Nanocrystalline FeNiCo Alloy Prepared By Surface Mechanical Grinding Treatment

Invar alloy is widely used in many fields such as microwave communication and precision instruments due to its excellent low expansion performance,but its micro-mechanism for producing Invar effect is still controversial.In recent years,studies have found that as the temperature decreases,the Fe Ni Invar alloy exhibits a phenomenon of martensite pre-transformation.After proper deformation treatment,the Invar alloy can simultaneously exhibit the Elinvar effect,which is pre-transformed with martensite.closely related.Based on this,we speculate that the mechanism of the Invar effect is also related to the martensite pre-transformation behavior.The nano-crystals suppress the martensitic phase transformation,thereby broadening the pre-transition temperature range,thereby obtaining low expansion characteristics.we designed a three-component FeNiCo alloy and treated the FeNiCo alloy by surface mechanical grinding treatment（SMGT）to prepare a nanocrystalline FeNiCo alloy.Using experimental methods such as XRD,TEM,electrical resistance and thermal expansion,the influence of SMGT treatment on the microstructure and thermal expansion properties of FeNiCo alloy was studied;the effect of heat treatment on the microstructure and thermal expansion properties of nanocrystalline FeNiCo alloy was studied.The results are as follows:Using SMGT methods,nanocrystalline Fe₆₅Ni₃₁Co₄alloy,Fe₆₈Ni₂₉Co₃alloy and Fe₇₀Ni₂₉Co₁alloy were obtained,and their average grain sizes were 32nm,74nm and132nm,respectively.The Fe₆₅Ni₃₁Co₄alloy and Fe₇₀Ni₂₉Co₁alloy consist of a singleγphase after grinding,while the Fe₆₈Ni₂₉Co₃alloy partially undergoes martensitic transformation during the grinding process,and the nanocrystalline alloy consists ofγandαphases.The nanocrystalline Fe₆₅Ni₃₁Co₄alloy and the nanocrystalline Fe₇₀Ni₂₉Co₁alloy exhibit an invar effect as the temperature decreases,and the thermal expansion coefficients are respectivelyα_{（-150℃～25℃）}=1.7×10～（-6）/℃andα_{（-150℃～25℃）}=3.7×10～（-6）/℃,and the thermal expansion coefficient of the mixed phase nanocrystalline Fe₆₈Ni₂₉Co₃alloy is larger,and its value isα_{（-150℃～25℃）}=7.9×10～（-6）/℃.When the nanocrystalline Fe₆₅Ni₃₁Co₄alloy is annealed at 350°C for 5 min,the grain size is 60 nm,the grain size is 95 nm at 10 min,and the grain size is 101 nm at 30min.After annealing treatment,the thermal expansion coefficient of the nanocrystalline FeNiCo alloy changes little.After the aging treatment of the nanocrystalline Fe₇₀Ni₂₉Co₁alloy at 200℃10h,the thermal expansion coefficientα_{（-150℃～25℃）}=2.1×10～（-6）/℃.