Studies Of Microstructures And Invar Effect Of The Fe-Ni-Co Alloys

The Fe-Ni-Co alloys have been prepared by a non-consumable tungsten electrode arc melting furnace with different chemical composition. Doppler broadening spectrum of positron annihilation radiation has been used in this work to investigate the behaviors of3d electron in Fe-Ni-Co alloys. The microdefects and the electron density of the Fe-Ni-Co alloys have been investigated by the measurements of positron lifetime spectra. The resistivities of the Fe-Ni-Co alloys have been measured by a Hall Effect instrument. The Vickers hardness of Fe-Ni-Co alloys has been measured by a Vickers Hardness tester. The thermal expansion coefficients of the Fe-Ni-Co alloys with different chemical compositions under the different heat treatment regime have been measured by a Mchelson interferometer. The influence of the chemical composition and the heat treatment regime on the thermal expansion coefficients of the Fe-Ni-Co alloys has been discussed. The main experimental results of present work are listed in the following:(1) There are3d electrons in Fe, Ni and Co atoms, some of them without coupling, and the d-d electron interaction will be enhanced when these atoms combined to form Fe-Ni-Co alloy. The results obtained from the measurements of the positron coincidence Doppler broadening spectra of the Fe-Ni-Co alloys with different chemical compositions indicate that the3d electron signal of Fe64Ni32Co4alloy is the highest in the alloys tested. Deviation from the composition of the Fe64Ni32Co4alloy, the signal of3d electron decreases.(2) The results obtained from the measurements of the positron lifetime spectra of the Fe-Ni-Co alloys with different chemical compositions show that the bulk electron density of the Fe64Ni32Co4alloy reaches the highest values in the alloys tested, while the open volume of defects in this alloy is the smallest in the alloys tested. Deviation from the composition of the Fe64Ni32Co4alloy, the open volume of defects will increase and bulk electron density will decrease.(3) The results obtained from the measurements of the thermal expansion coefficients of the Fe-Ni-Co alloys with different chemical compositions by using a Michelson interferometer show that the thermal expansion coefficient of Fe64Ni31Co5reaches the lowest value in the alloys tested. Deviation from the composition of the Fe64Ni31Co5alloy, the thermal expansion coefficients of the alloys increase.(4) With the increase of the Ni content in Fe-Ni-Co alloys, the vicker hardnesses of the alloys increase. When Ni atoms are replaced by Co atoms, with the increase of the Co content in the alloys, the Hardnesses of the alloys decreased first and then increased. The resistivity of the super invar alloy Fe-Ni-Co is relatively low, and resistivity of the Fe-Ni-Co alloy increases after annealing. (5) The heat treatment regime plays an important role in the microstructure and property of the Fe-Ni-Co alloy. After the annealing at800℃for20h, the stress of the alloy can be eliminated, the concentration of defects will decrease and the thermal stability of the alloy can be improved; the resistivity and the thermal expansion coefficient of the Fe-Ni-Co alloy will increase, while the hardness of the alloy will decrease. In comparison of the parameters of positron lifetime spectra of the Fe-Ni-Co alloys annealed at800℃for20h, it has been found that the mean positron lifetime of the Fe64Ni32Co4alloy is the shortest, that is, the mean electron density is the highest in the alloys tested.