Thermodynamics of the Mn-P system
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The free energy is shown to be more negative than in the Fe system, reflecting a stronger interaction between Mn and P atoms than between Fe and P atoms. Presenting the activity coefficient of P with the expression used by Lupis and Elliott, the first and second interaction coefficients are obtained as follows: e~(Mn) = 10.538 + 9728.14/T p~(Mn) = -28.148 -~ 9101.83/T The Gibbs free energy of formation for Mn3P was estimated in the temperature range of 1233 to 1378 K to be 3Mn(l~ + P~t~--- Mn3P(s~ AG~
I.
INTRODUCTION
M A N G A N E S E is one of the important elements in alloy steels. The increasing demand for high quality alloy steels imposes strict specifications for the content of impurities such as P. Refining processes have been understood and developed by means of thermodynamic descriptions of the alloy systems. A sizable quantity of thermodynamic data 1"2 has been organized for these purposes; however, most describe the behavior of solute elements in solvent iron. When the amount of the alloying elements, like Mn in alloy steels, increases substantially, the thermodynamic data need correction. Therefore, adjustments in the data are needed to describe properly the Mn-P system and thereby develop the processes for producing low P containing, high quality alloy steels. The available thermodynamic information for the Mn-P system is limited and often shows discrepancies among the various investigators. This study is intended to obtain internally consistent thermodynamic properties for the Mn-P system. This is accomplished by coupling the available thermodynamic data with phase boundary information.
II.
= -241,461.65 + 65.031T
PHASE DIAGRAM OF THE Mn-P SYSTEM
The phase diagram is available from the compilation by Hansen 3 and is shown in Figure 1. The Mn-P system was determined up to 0.5 mole fraction P and is shown to have four compounds: Mn3P, Mn2P, Mn3P2, and MnP. The Mn3P2 phase is shown to be stable between 1275 and 1363 K, and, as will be shown later, the thermodynamic properties were not previously reported. The compounds appear to be stoichiometric. No solubility of P in solid Mn is reported. This Y.E. LEE is Senior Engineer, Elkem Metals Company, Technology Center, 4625 Royal Avenue, Niagara Falls, NY 14302. Manuscript submitted October 16, 1985. METALLURGICALTRANSACTIONSB
1800 HANSEN
1700
1600
CALCULATION
liquid
j - " - ,,,
1500 z,.
1400
/~'r 1378
el
1363
~I
E
.=
1300 1275 1233
1200
I100
I0000 Mn
01.1
~. (~,3 1~.4 mole fraction, Xp
05
Fig. 1--Phase diagram of the Mn-P system.
is in contrast to the Fe-P system which has a maximum solubility in a-Fe 4 (Xp = 0.0452). The system is shown to have two eutectics (1233 K, Xp -- 0.131 and 1358 K, Xp = 0.426) and two peritectics (1378 and 1363 K). This study accepts the phase diagram as Hansen 3 has presented. VOLUME 17B, DECEMBER 1986--777
IIl.
et al. 10 obtained the entropy of Mn2P from the experi-
T H E R M O D Y N A M I C PROPERTIES OF M A N G A N E S E PHOSPHIDES
Thermodynamic properties of manganese
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