Calorimetric and thermal analysis set-up applied to study the martensitic transformations in shape-memory alloys is described. The information obtained are as follows: transformation temperature, enthalpy/entropy change and the dynamics of the phenomena. Hysteresis loop and the description of the macroscopic features of the transformation are given.
After the development of differential conduction calorimeters realized by E. Calvet around 1946, the standard equipment always
used a differential configuration. In home made systems for special purposes, the instrumentation available nowadays suggests
that it is possible to use non-differential conduction calorimeters. In order to prove this, a simple and cheap design was
constructed and tested. A sensitivity of 700 mV/W near 298 K (in agreement with the detecting semiconductors), a noise around
0.3 W and a long time fluctuation of the base line lower than 1 W were obtained. The reliability of the system was evaluated
by analyzing the changes of single crystals of Cu-Zn-Al Shape Memory Alloys after different thermal treatments. The calorimeter
allowed the determination of a reproducible set of time constants related to the heat treatments and to the mass (or shape)
of the sample. It is concluded that the experimental configuration used is suitable for this isothermal analysis.
Using two similar high resolution computer controlled stress-strain-temperature set-up of equivalent resolution (1 mN, 0.1
μm, 5 mK) the detailed study of the martensitic transformation in single crystals of the Cu−Zn−Al shape memory alloys is realized.
The devices can obtain 20 or 150 N in applied force, 2 or 4 mm in length and can be operated near room temperature (between
280 and 360 K). The analysis of the hysteresis domain in single crystals clearly visualises the intrinsic characteristics
of the material (pseudoelasticity, nucleation, interface friction) and enables the obtenton of parameters for physical models
of the hysteretic behaviour in force—lengthening—temperature and, eventually, time-dependent processes. The observation of
time evolution shows the ‘recoverable martensite creep’ associated to a microstabilization process.
A phenomenological approach, in the parent phase of Cu-Zn-Al shape memory alloy, establishes a predictable model (or mathematical
equations) relating the dependence of Ms with the temperature over a long period of time (i.e. seasonal or yearly room temperature). High-resolution resistance and
temperature measurements vs. time are used. The long time Ms tracks the external room temperature via two temperature dependent time constants. In steady state, the changes in Ms approach17 per cent of the ‘room’ temperature change. The detailed analysis shows the puzzling disappearance of the after
Authors:A. Isalgue, H. Tachoire, A. Torralba, V. R. Torra, and V. Torra
Reliability is a critical word in industrial applications of Shape Memory Alloys. Accurate and reproducible transformation hysteresis cycles and internal loops were obtained in single crystals using a high resolution automatized equipment. From a mechanical model formulated for a single martensite plate, the shape of the hysteresis cycle is obtained by generalizing the representation toN plates. The observed time effects on the hysteresis loops related to diffusion processes were also taken into account. It allows to explain the martensite recoverable creep and the micromemory effects. Also, the room temperature effects on the parent phase (for instance, summer to winter) acting over the transformation temperature are quantified.
Authors:A. Amengual, A. Isalgue, F. Marco, H. Tachoire, V. Torra, and V. R. Torra
We describe an experimental set-up of high resolution thermal analysis (HRTA) activated by Peltier effect and computer-controlled by the signal processing that allows a high resolution programming of the temperature and a reproducibility better than ±0.01 deg.
Authors:C. Auguet, A. Isalgué, F. Lovey, F. Martorell, and V. Torra
The behavior of shape memory alloys (SMA) allows their use as a passive
smart material. In particular, the existence of a hysteretic cycle in the
domain of the elementary coordinates strain-stress-temperature (σ, ε, T) suggests its application for damping
in mechanical and/or in civil engineering. We are working in the application
of SMA as dampers for earthquakes in small houses as family homes. For dampers
installed in the inner porticos of the house, the suggested SMA is the CuAlBe
and, eventually, the NiTi. At room temperature the used SMA wires induces
forces situated between 2–3kN/wire. The properties related with the
damping applications for CuAlBe and NiTi, i.e., the SMA creep and the self-heating
will be presented, together with some other minor stress and temperature effects
on NiTi modifying the hysteretic behavior.