Atomic-Scale Mechanism Of Invar Effect For Low-Expansion Fe-Ni Alloy And Thermal Expansion Of Fe-Ni Clusters

Fe-Ni Invar alloy has a near-zero linear expansion coefficient at room temperature.Since its discovery,it has been widely used in a wide range of aspects of production and life.At the same time,the mechanism of various abnormal physical properties of Invar alloy has also attracted the attention of scientists and has been under extensive investigation,but there is still no widely accepted explanation so far,which should be ascribed that the nature of the Invar effect involves the magnetism and electronic structure of materials,the related theories of which are not developed enough,and that the calculations are very complicated.Exploring the nature of the Invar effect can deepen people’s understanding of the magnetism of materials and expand the application range of Invar alloy,which is of extreme importance in theory and application.In this paper,we combined the first-principles calculations and classical molecular dynamics simulation to investigate the atomic-scale mechanism of anomalously low thermal expansion of Fe-Ni Invar alloy.We also explore the thermal expansion properties of Fe-Ni clusters for the first time and explain the Invar effect from the calculated and the previous experimental results.The main content of this article is as follows:(1)The atomic-scale mechanism of the magneto volume effect of Fe-Ni Invar alloy was studied using first-principles calculations.We mainly analyze the relationship between magnetic moment and atomic structure as well as the relationship between magnetic moment and volume in ferromagnetic and antiferromagnetic states.The results imply that the magnetic moment of Fe atoms would decrease with increasing number of nearest neighbor Fe atoms,and it would show an antiferromagnetic state in the local Fe-rich region,but there is no obvious rule betwwen the magnetic moment of Ni atoms and the number of nearest neighbor Fe atoms.The decrease of magnetic moment of Fe atoms would lead to the decrease of lattice constant,so does the transition of Fe atom from ferromagnetic state to antiferromagnetic state.However,the change of the magnetic moment of Ni has no effect on the volume.The magnetovolume effect of Fe-Ni invar alloy mainly comes from FCC Fe.(2)Atomic structure and atomic-scale thermal expansion of Fe-Ni Invar alloy were studied using classical molecular dynamics.Atomic structure analysis reveals that there is a strong mutual attraction between Fe and Ni atoms.We also found the Fe atoms that are located in local Fe-rich regions accounts for 14%of total atoms,and this proportion would increase as temperature rises.The analysis of thermal expansion of chemical bonds shows that the Fe-Ni bond has the shortest bond length and the smallest thermal expansion among the three types of bonds in solid Invar alloy.According to statistics,it is found that Fe-Ni bonds account for about half of all bonds in numbers,hence Fe-Ni bonds must make an important contribution to the abnormally small thermal expansion coefficient.(3)The structural,magnetic and thermal expansion properties of Fe-Ni clusters were studied using first-principles study.We found that all 13-atom Fe-Ni clusters have icosahedral structures,and the bond length decreases with the increase number of Ni atoms in the clusters.The bond length between the surface atoms is longer than that between the center and the surface atoms.The analysis shows that the magnetic moment of the surface atom is larger than that of the central atom,and only the Fe13 cluster has a significant magnetovolume effect.We find that the thermal expansion of the Fe13 cluster is abnormally small,which should be ascribed to its magnetovolume effect.This phenomenon is a reflection of the Invar effect on the scale of clusters.