Low Nickel Content FCC Alloys: Recent Evolution and Applications

Invar-type austenitic Fe-Ni alloys have been studied for a long time due to their noticeable physical anomalies and their varied martensite behavior below 28%Ni content. Yet these two aspects are not directly connected and offer the place for a field of low Curie point alloys, between 28 and 36%Ni. Their magnetic behavior has been first interpreted as a mixture of two magnetic phases ferromagnetic or not, easily represented on a Bethe-Slater curve, and later as the occurrence of 2 simultaneous and distinct electronic states high and low spin, respectively ferro- (FM) and antiferro- (AFM) magnetic whose energies are very near in the case of Invar type Fe-Ni alloys. The copper addition to such alloys, especially studied in this paper, allows Curie point Tc and polarization to saturation Js to be significantly increased as coercive field also does. These facts strengthen the assumption of a preferred copper solid solution in the FM phase, increasing in there the exchange interaction stiffness J by the way of internal stresses and magnetoelasticity, but also moderately damaging the soft magnetic properties. Starting from the studied and quantified ?-gene effects of Cr and Cu, and from the Cu induced T_c and J_s improvements, soft magnetic Invar - Fe-Ni-Cr-Cu alloys (H_c=2.5 to 12 A/m), exhibiting a stable and single phase fcc structure at ambient temperature, low T_c (100-250?C), medium J_s (0,4 to 1,2T) have been designed between 27 and 32%wt-Ni. Among already known FeNi usual applications, such material progress opens the door to new industrial alloys, with a special attention paid for precise chemical content, impurities reduction and segregation homogenization. These new alloys are nowadays developed towards technical applications in mass production such as watch stepper motor or advanced induction heating which will be presented at the end to emphasize the performances of the new FeNiCrCu alloys.