Effect of Partial Thermal Cycles on Non-Chemical Free Energy Contributions in Polycrystalline Cu-Al-Be Shape Memory Alloy

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 ( ) 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 Af and the minimum temperature was gradually increased from a value below Mf. 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 T temperature interval with T0.5 and <0.5, respectively. On the other hand the d() functions show a maximum at about the central point of the sub-cycles, and deviate considerably from the d() function obtained from the full cycles. This is also reflected in the  dependence of the integral value of the dissipative energy, D(): 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.

Investigation of the bainitic reaction in a CuAlNiMnFe shape memory alloy

Despite their favorable properties, brittle nature of the CuAlNi shape memory alloys limits their suitability. To increase their ductility, Mn and Fe were added to the base CuAlNi alloy. To reveal the applicability of the developed CuAlNiMn and CuAlNiMnFe alloys as functional materials, the effect of ageing on the thermoelastic martensitic transformation was investigated. During the first heating of the aged samples the thermoelastic ?? ? ? transformation occurred, which was followed by a bainitic transformation. This transformation inhibited the further thermoelastic martensitic transformations. The present paper covers heat flux DSC, SEM, and TEM investigations of the bainitic transformation. A feasible mechanism of the bainitic transformation in these alloys is suggested based on the results of the examinations.

Martensitic Transformation Cycling and the Phenomenon of Two-Way Shape Memory Training

ABSTRACTThe character and mechanism of two-way shape memory (TWSM) “training” has been investigated in a Cu-Zn-Al alloy by means of detailed thermomechanical evaluation and TEM observations. The progressive effects of training on the TWSM behaviour, as well as an accompanying substructural evolution, have been established. The results indicate a relationship between the substructural effects of cyclic thermoelastic martensitic transformation and the ability to exhibit TWSM. Microscopic studies reveal that as the number of cycles of thermoelastic transformation under stress increases, specific physical features develop in the parent phase submicrostructure. These take the form of dislocation structures which evolve into “vestigial” martensite markinqs. These in turn assist in the nucleation and growth of a preferred martensite plate arrangement on cooling which is similar to that induced under stress during the training cycles.