Kinetics of the phase separation in Cu–Al–Mn alloys and the influence on martensitic transformations

Abstract

We have determined the shape of the miscibility gap of the Cu–Al–Mn alloy system along the Cu₃Al → Mn composition line ((Cu₃Al)_{1 − x} Mn_x with 0 ⩽ x ⩽ 1) using electrical resistance measurements. Electrical resistance measurements have also been used to study the kinetics of the phase separation that takes place within the miscibility gap. The effect of such a phase separation on the magnetic properties of the system and on the martensitic transition displayed by these alloys has also been analysed. The results obtained show that the phase separation produces an enhancement of the ferromagnetic character of the system as a result of the development of ferromagnetic L2₁ domains. In addition, the phase separation gives rise to an increase in the transition temperature and to a reduction in the associated entropy change. Both effects are a consequence of the growth of L2₁ Mn-rich domains that do not transform martensitically.

Acknowledgements

J.M., A.P. and Ll.M. have received financial support from the Comisión Inteministerial de Ciencia y Tecnologia (Spain), project MAT2001-3251, and from the Comissió Interdepartamental de Recerca i Innovació Tecnològica (Catalonia), project 2001SGR00066; M.L.C. and R.R. received financial support from the Consejo Nacional de Investigación Científica y Técnica, Comisión de Investigaciones Científicas de la Provincia de Buenos Aires and Secretaría de Ciencia y Tecnología, Universidad del Centro de la Provincia de Buenos Aires (UnCentro). The collaboration between the two groups has been supported by Fundación Antorchas (Argentina). J.M. also acknowledges financial support from Direcció General de Recerca (Catalonia) under a predoctoral fellowship. R.R. acknowledges the Universitat de Barcelona for its support during his visit to the Departament d’Estructura i Constituents de la Matèria.

Notes

† In order to evaluate e/a we have assumed, as is customary, that the elements Cu and Al respectively contribute with one and three conduction electrons per atom, coming from their incomplete s and p shields. The contribution of Mn atoms is given by (Caroli and Blandin 1966), where m is the magnetic moment of Mn atoms and μ_B is the Bohr magneton. Since in Cu–Al–Mn alloys (Webster and Ziebeck 1988), a contribution of one conduction electron per Mn atom results.

† The α phase corresponds to the fcc phase of cu; the γ phase, also identified as γ₁ or γ₂ in the literature, is an ordered cubic phase (space group, ) with a composition close to Cu₉Al₄.

‡ The sensitivity of the calorimeter depends on temperature. In particular, in the analysed temperature range it increases monotonically from 0.2 V W⁻¹ at T = 100 K to 0.77 V W⁻¹ at T = 300 K. These values are more than ten times the sensitivity of a conventional differential scanning calorimeter.

† We have estimated T _GAP as the temperature for which the relative deviation of the resistance from linear behaviour exceeds 1%.