Thermodynamic calculation and enthalpy determination based on CEMS experimental data of martensitic transformation in Fe-Mn-Si shape memory alloys

J. MARTÍNEZ; S. M. COTES; J. MARTÍNEZ

La Plata, Buenos Aires, ARgentina

Conferencia; Conferencia Latinoamericana Sobre las Aplicaciones del Efecto Mössbauer; 2008

An important study of the effect of chemical composition on alloys with shape memory effect, based on the Fe‑Mn and Fe‑Mn‑Si system was developed on reference [1], where a wide range of compositions, where the fcc/hcp transformation takes place. The previously known information in these system, coupled to the results of those experiments [2][3][1] allowed to assess the parameters that describe the energy of Gibbs of the fcc and hcp phases of these alloys based on the model of phenomenological Redlich-Kist [4], from which it was possible to evaluate the relative stability of the hcp and fcc phases in systems Fe‑Mn and Fe‑Mn‑Si [1]. From this result the so-called transformation driving forces were calculated. These results provide a solid basis for studying the variation of those driving forces in the presence of crystalline defects and magnetic effects, and also to assess the relative stability of the three phases involved (fcc, hcp and bcc) in alloys with low contents of Mn. However, although the thermodynamic model has been widely used in several previous studies [2][3][1], there is a major difficulty for which there is not enough data in the literature on enthalpy transformation, which can be used to an estimation of other thermodynamic properties. This difficulty comes from the fact that the amount of martensite formed is not simple to determine in polycrystalline alloys with a certain texture using normal methods. In this work was carried out a systematic study of the enthalpy transformation, using the transformation heat for a wide range of compositions where the transformation takes place. The martensite phase fractions were obtain by Mössbauer spectroscopy, and the calculation of enthalpy for the ternary system was done a using the thermodynamics description based on the phenomenological model of Redlich-Kist[4]. The importance of using the Mössbauer spectroscopy in the study of the amount of phase that transforms is of paramount importance given the low cost of this technique in comparison to neutron diffraction, and the best approximation to the real value of the amount measured in comparison to macroscopic techniques that do not take into account the grain accommodation as dilatometry.     [1] Cotes, S., Fernández Guillermet, A y Sade, M. Journal of Alloys and Compounds, Vol. 280, (1998), págs. 168-177. [2] Cotes, S., Sade, M. y Fernández Guillermet, A. Metallurgical and Materials Transactions A, Vol. 26A, (1995), págs. 1957-1969. [3] Cotes, S., Fernández Guillermet, A y Sade, M. Journal of Alloys and Compounds, Vol. 278, (1998), págs. 231-238. [4] Redlich, O. y Kister, A. T. Industrial and Engineering Chemistry, Vol. 40, 2, (1948), págs. 345-348.