The alpha-gamma phase boundaries and the T 0 line for Fe-Mn alloys
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I.
INTRODUCTION
U N D E R S T A N D I N G of the gamma to alpha phase transformation in HSLA type steels demands quantitative knowledge of the dilute Mn range of the Fe-Mn phase diagram in the temperature range 580 ~ to 911 ~ Our recent kinetic studies of such steels ~ led us to a reassessment and redetermination of the thermodynamic system in the range up to 5 mole pct. In this investigation we are led to an optimum diagram which takes account of all the relevant thermodynamic data and direct experiments which have been carried out to date. Troiano and McGuire 2 determined part of the Fe-Mn phase diagram by crystal structure and lattice parameter determinations and examination of the microstructures of equilibrated alloys. In particular, they determined the a r i a + 3/) phase boundary between 400 ~ and 800 ~ They were unable to locate the (a + 3/)/3/boundary with the same level of accuracy, so they extrapolated it from the data at 600 ~ and below on the basis of qualitative observations of the character of the microstructures and the diffraction patterns. Hillert et al 3 experimentally determined one point on the a / ( a + 3/) boundary at 820 ~ using equilibration and diffusion couple methods which is in agreement with the experimental trend of Troiano and McGuire 2 (Figure 1). They fitted theoretical a r i a + 3/) and (a + 3/)/3/phase boundaries between 911 ~ and 500 ~ using existing thermodynamic data and a regular solution model. The calculated a r i a + 3/) boundary passing through their single experimental point lies close to that of the Troiano and McGuire diagram, while the calculated (a + 3/)/3' boundary lies substantially lower than the estimate of Troiano and McGuire (911 ~ > T > 600 ~ Gilmour et al 4 reevaluated the Fe-Mn phase diagram between 911 ~ and 700 ~ using the a / ( a + y ) boundary of Troiano and McGuire and the Wagner thermodynamic formalism, s Their (a + 3/)//7 boundary lies a little higher than that of Hillert et al. Kirchner et al 6 used regular solution thermodynamics to evaluate the Fe-Mn phase diagram between 911 ~ and 400 ~ Their calculated diagram is essentially in agreement with that of Hillert et al. K.K. SRIVASTAVA is Research Engineer with Westinghouse Electric Corporation, Bloomfield, NJ 07003. J.S. KIRKALDY is Professor of Metallurgy and Materials Science and Director, McMaster Institute for Energy Studies, McMaster University, Hamilton, Ontario, Canada, L8S 4KI. Manuscript submitted April 27, 1982.
METALLURGICALTRANSACTIONS A
In view of the indications that the (a + 3/)/3/boundary is lower than that estimated by Troiano and McGuire and the absence of experimental tie-line information in the high temperature region (>600 ~ further experiments were judged to be necessary.
II.
T H E R M O D Y N A M I C ANALYSIS
The partial molar free energies, Gi, or the chemical potentials,/zi, for each component are equal for the two phases ferrite (a) and austenite (3/) in equilibrium. Thus G~ = G~o
[1]
GM, = G~,
[2]
where by definition GF~ = ~ GM, = ~
+ R T In 3/F~Fo
[3]
+ R T I
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