Microstructural change and mechanical properties of Fe-Ni-Al-C alloys have been studied using X-ray diffractometer, optical microscope, transmission electron microscope, hardness test and tensile test. All alloys reveal austenitic single phase after solution treatment. From the aging experiments at 1023K and 973K, it is thought that $Ll_2$ phase formation boundary exists between these two temperatures. The amount of $Ll_2$ ordered phase increases as the increase of Al content.
If aging temperature decreases, grain boundary reaction with lamellar type structure evolves in addition to the $Ll_2$ ordering in the matrix. More grain boundary reaction is found with increase of Al content and decrease of aging temperature.
Two step hardening can be seen in the hardness test of 823K aging-treated alloys. One may be related to the increase of short range order parameter and the other to the actual formation of $Ll_2$ ordered phase. This phenomenon is enhanced with decrease of both aging temperature and Al content. Prolonged aging brings about the rapid decrease of hardness, which is caused by the coherency loss of second phase.
From the tensile tests of the alloys aged at 873K and 823K, aging treatments generally increase YS and UTS while elongation decreases with furthur aging. YS of 921.3MPa, UTS of 1163 MPa and 22.5% elongation at room temperature are obtained from the Fe-38.8 at.%Ni-8.25 at %Al-2.01 at.%C alloy aged at 873K for 100 hours. These values are comparable to those of commercially used Ni and Fe-base superalloys. Of course, these tensile properties and closely related to the microstructure of the alloys.
