EFFECT OF PARTIAL THERMAL CYCLES ON NON-CHEMICAL FREE ENERGY CONTRIBUTIONS IN POLYCRISTALLINE CU-AL-BE SHAPE MEMORY ALLOY

摘要

Thermoelasic martensitic transformations are controlled by the local equilibrium of chemical and non-chemical free energy contributions (D and E being the dissipative and elastic energies, respectively). The derivatives of non-chemical free energies ((∂D)/(∂ξ) = d(ξ),(∂E)/(∂ξ) = e(ξ)) as a function of the transformed martensite fraction (ξ) can be expressed from the experimental data obtained from the temperature-elongation, temperature-resistance, etc hysteresis loops. This method, developed in our laboratory, was used for the investigation of non complete, partial thermoelastic transformation cycles. In the first set of experiments the subsequent cycles were started below the Mf temperature and the maximum temperature was decreased gradually from a value above Af (series U). In the second (L) set the cycles were started above the A_f and the minimum temperature was gradually increased from a value below M_f. In the third (UL) set the minor loops were positioned into the centre of the two phase region (i.e. the cycling was made with an increasing AT temperature interval with AT0.5 and ξ<0.5, respectively. On the other hand the d(ξ) functions show a maximum at about the central point of the sub-cycles, and deviate considerably from the d(ξ) function obtained from the full cycles. This is also reflected in the ξ dependence of the integral value of the dissipative energy, D(ξ): its value for the partial loops is lower than the dissipative energy calculated from the full cycle for the same transformed fraction interval. An opposite tendency (i.e. higher values for the partial loops) was obtained for the integral value of the elastic energy, E. The relative values of the dissipated energies, D, (calculated from the areas of the minor loops and normalized to the area of the major loop) are not very sensitive to the details of the cycling process, i.e. they are very similar for all sets.