Effect of high magnetic fields on the martensite transformation
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T is well established that the application of a magnetic field can influence the formation of ferromagnetic martensite from a nonferromagnetic austenite. The magnetic field favors the formation of the ferromagnetic phase, and hence raises the martensite start (Ms) temperature. The clearest and most extensive experiments on martensite formed during cooling appear to be those of Satyanarayan, Eliasz, and Miodownik;' who give references to earlier (mostly Russian) work. We do not consider the effect of magnetic fields on the formation of isothermal martensite, which has also been fairly extensively investigated. Satyanarayan et al conducted experiments on two alloy steels, whose compositions are given in Table I. They followed the formation of martensite in fields of 0 and 16 kOe, using the metallographic Gremnger-T'roiano" technique, and found a clear increase in the Ms temperature in the presence of the magnetic field. The Satyana ray an et al analysis of the results is based on that of Estrin." It is shown that the only significant effect of the magnetic field is to lower the free energy of the ferromagnetic phase relative to the nonmagnetic phase by an amount IH, where I is the saturation magnetization of the ferromagnetic phase and H is the effective magnetic field. If I is in cgs emu/cm" (Gauss/4rr) and H is in Oersteds, the quantity IH is in erg/ern"; this can be converted to cal/rnol for comparison with thermodynamic quantities as conventionally expressed. If t.T is the increase in martensite start temperature due to a field H, !H/t.T = &(t.G)!aT = ssr>', where t.G is the free energy difference between austensite and martensite at T = Ms, and e:,sY-Ci' is the entropy of the transformation. The quantity !H/t.T can also be regarded as an approximate measure of t.G,jTo - Ms, where t.G E is the strain energy associated with the formation of the first martensite and To is the temperature of thermodynamic equilibrium between austenite and martensite. Somewhat less accurately, IH/ t. T is approximately equal to t.Go/T o, where t.G o is the free energy difference beRICHARD FIELDS is Research Student, Department of Engineering, Cambridge University, England. C. D. GRAHAM, Jr., is Professor of Metallurgy and Materials Science, University of Pennsylvania, Philadelphia, PA 19174, where Richard Fields was formerly a student. Manuscript submitted September 12,1975. METALLURGICAL TRANSACTIONS A
Table I. Alloy Compositions Alloy
C
Mn
Si
Ni
Cr
Mo
*Comp.2 *Camp.1 52100 410
0.3 1.0 1.02 0.1\
0.6 0.5 0.36 0.44
0.2 0.2
2.8 0.007 0.16 0.2
0.6 1.5 1.41 12.18
0.6
0.37
*Ref. 1
tween austenite and martensite at absolute zero, and To is as previously defined. Satyanarayan et al give a neat graphical derivation of these expressions. Values of t.G o, t.G E, and To can only be obtained indirectly for the austenite-martensite transformation, and experiments with magnetic fields are useful to provide confirmation of the estimates of these quantities. The entropy of transformation, however, is a direct experimental value r
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