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|Martensitic transformation in shape memory alloys
|The shape memory effect gives name to a set of alloys which exhibit an unusual capability to recover large deformations. Among these alloys one finds NiTi and several copper based alloys, like Cu-Zn-Al and Cu-Al-Ni. The effect is linked to a martensitic transformation, a difussionless structural phase change. The transformation can be induced by cooling the alloy, by increasing the hydrostatic pressure, or by increasing a tensile or compressive uniaxial load. Depending on the composition of the alloy and the thermomechanical treatment of the samples, the transformation takes place at different temperatures or loads and presents hysteresis between the forward and the reverse transformation paths. Due to the symmetry of the high temperature phase (bcc structure) there can appear several variants of martensite during cooling (for example, 24 in the case of Cu-Zn-Al). On heating these variant disappear transforming back to the high temperature structure. If the sample is deformed in the martensitic phase there is a transformation between variants. Those variants which decrease the stress will grow more than the others and, macroscopically, the maximum deformation, before plastic deformation, can be quite large. If the sample is now heated, the variants retransform to the parent phase. Thus, the macroscopic shape of the sample is recovered at the same time than it retransforms to the high temperature structure. Technologically, the capability of the samples to produce work during the reverse transformation is of large interest. In addition, these alloys present some other interesting effects like pseudoelasticity and the two-way shape-memory effect. Recently, research have been initiated on the effects of aging on the martensitic transformation of Ni-Ti ribbons.
|The martensitic transformation has been studied with several home-made devices which provides some features absent in standar commercial systems. These include several kinds of differential scanning calorimeters (with higher resolution than some standar commercial systems), dilatometric devices (which allow loads up to about 50 N, much higher than standar commercial dilatometers), and temperature controlled stages (resolution near 1 mK) for optical microscopy observation and video recording, with simultaneous measurement of calorimetry, resistance or acoustic emission.