Ferroic materials exhibit hysteresis loops when driven accross a first order phase transition by sweeping an external control variable. While the equilibrium trajectories in the generalized force-displacement phase space are independent of the external control variable, there is no reason to expect that out-of equilibrium hysteresis loops obtained when controlling the generalized force will be similar to those obtained when controlling the generalized displacement. To address such an issue, we have studied the hysteresis loops obtained for a Cu-Zn-Al single crystal which has been driven across its martensitic transition. Experiments have been performed using two different experimental devices: a commercial tensile machine, which controls the displacement while the load is monitored (strain-driven), and a specifically designed dead-load device which enables a fine control of the applied load while the displacement is continuosly monitored (stress-driven). Results have revealed significant differences in the hysteresis loops depending on which control variable used. The strain-driven curves show re-entrant behaviour which is not observed in the stress-driven case and the dissipated energy in the stress-driven loops is larger than in the strain-driven ones. In situ optical microscopy has also revealed differences in the tranformation mechanisms. The obtained results conform to the recent theoretical predictions proposed within the general framework of athermal systems.

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