Effect of grain size on the thermal hysteresis in martensitic transformations

P. LA ROCA, P. VERMAUT, P. OCHIN, J. MALARRÍA

Paris

Simposio; Colloque Plasticité 2013; 2013

Société Française de Métallurgie et de Matériaux

In a thermally induced martensitic transformation the thermal hysteresis is originated in dissipative processes [1]. Polycrystalline Cu-based shape memory materials that undergoes a martensitic transformation to a monoclinic 18R structure,  displays an hysteresis of about 10 K if their grain size (g) is large, i.e. with a characteristic length above 100 μm. A similar value is observed in single crystals, where the dissipative mechanism is related to frictional work in interphases displacement released as irreversible heat [2]. However, when the grain size decreases the hysteresis width increases as can be seen in Figure 1, where data from a series of ribbons with reduced grain size obtained by rapid solidification techniques are collected. To rationalize the observed behavior, we propose a model that considers that strain incompatibilities between martensite variants and granular domains are responsible for the occurrence of microplasticity. Strain incompatibilities occurring at grain boundaries lead to larger irreversible heat, and plastic deformation can explain this behavior. Microplasticity derived from grain constraint introduces an additional energy barrier that must be overcome. This increases the frictional term and hence the thermal hysteresis. The assumption of an energy barrier proportional to the grain surfaces lead to a suitable fitting of experimental data. [1] J. Ortin, A. Planes , Acta Metall 36 (1988) 1873-1889 [2] V. Recarte, J.I. Perez Aldazabal, P.P. Rodriguez, E.H. Bocanegra, M. L. No, J. San Juan, Acta Mat 52 (2004) 3941-3948