Influence of interstitial admixtures on phase transitions in invar

Abstract

669.15; 536.413 We investigate the influence of hydrogen and carbon on the kinetics of magnetic and atomic ordering in N36 iron-nickel Invar alloy. It is shown that alloying by carbon (up to 0.3 wt. %) results in an increase of the Curie T c and Kurnakov T k temperatures. In the case of hydrogen Tr decreases but T k remains constant, We determine the optimum carbon concentration (0.17%) improving the operational properties of Invar and propose a physical mechanism of the influence of interstitial admixtures on the temperature coefficient of linear expansion and on the Invar strengthening of N36 alloy.Iron-nickel alloys in the Invar range of concentrations (30-45% Ni) are characterized by anomalous physical properties. The most important among them are the so-called "Invar strengthening" (i.e., a sharp growth of strength parameters due to the phase transition "paramagnetic-ferromagnetic" [1]) and low values of the temperature coefficient of linear expansion in the range 73-373 K [2]. These anomalies are usually explained by the ferromagnetic nature of Invars. According to Komura and Taheda [3], in the range of the presence of magnetism, thermal compression of a crystal lattice during cooling is practically completely compensated for by its expansion due to the appearance of spontaneous magnetization and depends on the level of the effective exchange interaction in alloys. For this reason, a study of the processes of magnetic ordering which occur near the Curie temperature is of great importance for prediction and development of new materials with a prescribed value of the temperature coefficient of linear expansion. In addition, physicomechanical properties of Invar substantially depend on its heat treatment and the content of admixtures. In the first case, it is necessary to take into account the possibility of formation of ordered atomic structures affecting the temperature range of the existence of ferromagnetism [4]. In the second case, interstitial admixtures, in particular carbon, cause a change in magnetization and magnetic permeability [5]. Therefore, by combining the appropriate heat treatment and variation in the content of admixtures, we can improve the operational characteristics of Invar alloys.We investigated the influence of carbon (up to 0.3 wt. %) and heat treatment in hydrogen on the kinetics of magnetic and atomic ordering in N36 Invar alloy. Experimental ProcedureWe studied the processes of atomic and magnetic ordering in N36 alloy by the method of a.c. electric conduction by using an automatic compensator [6] which made it possible to obtain continuous dependences of electric resistance on temperature for a wide range of frequencies of the measuring current (up to 200 kHz). The choice of this method was conditioned by the fact that an anomaly (maximum) of electric resistance appeared in the temperature dependences R(T) at high frequencies. The parameters of this anomaly correspond to the ferromagnetic transition [7]. Moreover, the a.c. electric resistance of specimens (when the frequency i...