Magnetic Effect on the Phase Equilibria of FCC(Fe, Ni) Alloys at Low Temperatures
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MAGNETIC EFFECT ON THE PHASE EQUILIBRIA OF FCC(Fe,Ni) ALLOYS AT LOWTEMPERATURES JEN-CHWEN LIN. YING-YU CHUANG AND Y. AUSTIN CHANG Engineering, and Mineral of Metallurgical Department Wisconsin-Madison, Madison, WI 537Ub
University
of
ABSTRACT Phase stabilities of iron alloys at low temperatures are strongly The appearance of certain type of equilibria infuenced by magnetic effect. is often due entirely to magnetic contribution to the Gibbs energy of the The appearance of the stable and metastable equilibria in pertinent phase. fcc(Fe,Ni) alloys are discussed in terms of the magnetic interaction. INTRODUCTION We have recently analyzed the thermodynamic and phase equilibrium data The of Fe-Ni and Fe-Cr from the liquid phase down to about 500 K [1.21. thermodynamic properties of the liquid and various solid phases were fixed At high primarily by the thermochemical data measured at high temperatures. temperatures, the phases are paramagnetic and the measured values represent With the chemical contribution to the total Gibbs energies of these phases. The contributions to decreasing temperatures, these phases become magnetic. the thermodynamic properties of these phases due to magnetic ordering were The described using the equations proposed by Chuang, Schmid and Chang [3]. proposed equations contain two parameters, the Curie temperature Tc and the mean magnetic moment per atom ;3. Values of Tc and !3for the pertinent phases From a knowledge of T in Fe-Ni and Fe-Cr are available in the literature. and 3, the magnetic portions of the Gibbs energies were obtained and addeh Knowing the Gibbs energies of all pertinent phases, to the chemical terms. phase equilibria, both stable and netastable ones, were calculated for Fe-Ni and Fe-Cr. In the present communication, we wish to limit our discussion to the magnetically induced phase equilibria in Fe-Ni at low temperatures and how they can be used to rationalize some of the results obtained under electron irradiation. THE Fe-Ni BINARY Fig. 1 shows the calculated Fe-Ni phase diagram from 1200 to 500 K as With the exception compared to the data reported in the literature [4-9]. of a few data points by Owen and Sully [81, there is good agreement between Phase the calculated and experimentally measured phase boundaries. separation of the fcc phase to 'yl(paramagnetic) and Y2 (ferromagnetic) is due to the magnetic Gibbs energy of the phase. The nonmagnetic Gibbs energy of the fcc phase is concave everywhere (holds water) within the composition range of interest. But the compositional variations of the magnetic Gibbs energy are such when it is added to the nonmagnetic term, a hump is produced in the total Gibbs energy curve. This hump causes the phase separation. Fig. 2 shows the same phase diagram without any of the experimental data. Moreover, the T (a+ý) line is shown as well as metastable extensions of the gap are various phase Boundaries and of T'. The spinodals for the -y Since some of Gibbs energy equations used are valid down to 500 also shown. and the spinodals to K, extens
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