Thermodynamics of the iron martensitic transformation and the m s temperature of iron
- PDF / 476,589 Bytes
- 5 Pages / 613 x 788.28 pts Page_size
- 59 Downloads / 212 Views
I.
INTRODUCTION
S T U D I E S on the martensitic transformation of ironcarbon (Fe-C) alloys have covered nearly a century, with thermodynamic discussions over five decades. Martensitic transformations, strictly speaking, are classified as nonequilibrium phase transitions which always occur far away from equilibrium and which result in nonequilibrium products. Therefore, the previously published thermodynamic treatments on martensitic transformations of iron-carbon alloys which have been based on equilibrium thermodynamics cannot offer a complete description of real martensitic transformation processes. That is to say, for the sake of scientific certainty and precision, irreversible thermodynamics other than equilibrium thermodynamics should be utilized to discuss martensitic transformations of Fe-C alloys. However, it is now impossible to do so, because the destabilization of austenite of Fe-C alloys under the condition of martensitic transformations is abrupt and the abruptness of austenite destabilization makes detailed kinetic investigation of the transformation so diffcult that, at the present time, relatively sufficient information cannot be obtained. Thus, there is no way to thermodynamically describe martensitic transformations using irreversible thermodynamics. While destabilization conditions for a system of constant pressure and constant temperature given by equilibrium thermodynamics are of universality, though the system of interest is considered as a black box, the results obtained can be used to approximately describe the thermodynamic properties of martensitic transformations of Fe-C alloys. Thermodynamic treatments on martensitic transformations of Fe-C alloys have been carried out by Fisher, [u Aaronson et al. ,i2[ Shiflet et al. ,[31 and Bhadeshia, [4] and Hsu and co-workers, t5-91Hsu wrote the free energy change
WEI QIUMING, formerly Ph.D. Candidate with the Department of Materials Science and Engineering, Northwestern Polytechnical University, is Postdoctoral Researcher, Beijing University of Science and Technology, Beijing, 100083 People's Republic of China. KANG MOKUANG, Professor, is with the Department of Materials Science and Engineering, Northwestern Polytechnical University, Xian Shaanxi 710072, People's Republic of China. Manuscript submitted December 21, 1989. METALLURGICAL TRANSACTIONS A
accompanying martensitic transformation of Fe-C alloys as
[1]
A G ~ M = A G v'-~ + A G 'r''M
where AG T M represents an imagined free energy change corresponding to the transformation of austenite to a bodycentered-cubic (bcc) ferrite embryo of the same composition and AG ~--~M the free energy change of transformation of the above bcc embryo to martensite. Hsu then set [2]
AG~Mlr=u, = 0
as the critical condition of martensitic transformation, namely, the destabilization condition of austenite under the condition of martensitic transformation. From fundamental thermodynamics, one can write ~t
AG ~'~ = (1 -xc)AGFrT M + (1 - x c ) R T l n aF---2 awe aa
+ xcRT In ~
a:
[3]
As for AG ~--'M, it
Data Loading...
