Thermal expansion in Fe-Ni alloys: A DFT study

Iron-nickel alloys are known for a very special behavior of the coefficient of thermal expansion with respect to temperature, known as the Invar effect. Being exposed to a temperature gradient within a certain temperature interval, these alloys tend to maintain their crystal lattice constant unchanged in contrast to most of the metallic alloys that expand their lattice upon heating. This effect has found its use in various metrological and high precision applications. An exact theory behind this effect is still debated. This thesis investigates the origin of the Invar effect in iron-nickel alloys using density functional theory (DFT) and a Debye-Grüneisen model. Density functional theory is used for the electronic structure calculation. The Debye-Grüneisen model is used for the description of atomic vibrations that lead to thermal lattice expansion. Both DFT and the Debye-Grüneisen model results are found to be in reasonable agreement with existing experimental and theoretical data on the Fe-Ni alloy system and illustrate that a major contribution to the Invar effect comes from the magnetic ordering and associated magnetic entropy effects. The calculated results on the coefficient of thermal lattice expansion have been used to resolve a long-time standing discrepancy on the Invar behavior of Fe30Ni70 Invar alloys and to rationalize the experimental data. In addition the Young¿s modulus for Fe 97atm.% Ni 3atm.% alloy was studied. By using a relation between the grüneisen constant and Possion¿s ratio the Young¿s modulus was calculated.