Order Processes in Cu-Zn-Al Shape Memory Alloys Quantitative Approach to Ms Values by Resistance measurements

Analysis of the experimental results on Cu-Zn-Al shape memory alloys indicates that the transformation temperatures (for instance, M_s) fluctuate in long-term records. Continuous measurements of electrical resistance R (five significant figures) are made with controlled and programmed temperature (resolution 0.005 K). The experimental analysis suggests a partitioning of resistance into two additive contributions: phonon and order (or R^*). The time behaviour of R^* represents the dependence of M_s time. The results indicate that the effects of time and temperature in the austenite phase can be represented by two independent differential equations: the long-time fluctuations in the transformation temperature are predictable and the uncertainty reduces to ±0.15 K. This revised version was published online in July 2006 with corrections to the Cover Date.     

Non-Differential Conduction Calorimeter,a Tool to Study the After Quench Behaviour in Shape Memory Alloys

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 μW and a long time fluctuation of the base line lower than 1 μW 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. This revised version was published online in July 2006 with corrections to the Cover Date.     

Metastable effects on martensitic transformation in SMA

The Clausius-Clapeyron coefficient (CCC or α_CC) or relation of the stress to transform a thermoelastic SMA sample and temperature, is revisited for 1-dimensional stressed SMA when polycrystalline materials are used. The experimental method is discussed in the frame of equilibrium thermodynamics for polycrystalline materials. Suggestions for evaluation of the experimental value in polycrystalline NiTi and CuAlBe specimens are proposed. The analysis for NiTi wire gives a CCC of α_CC=6.3±0.3 MPa K⁻¹. On the other hand CuAlBe provides a value of α_CC=2.2±0.4 MPa K⁻¹ for tensile stress.     

Metastable effects on martensitic transformation in SMA part VII. Aging problems in NiTi

Shape memory alloys (SMA) are interesting for applications in damping of civil engineering structures. To achieve the SMA as a guaranteed material for damping of structures requires a reliability study of the static and dynamic properties adapted to each type of application. Here we present static (temperature and time effects) and dynamic actions in pseudo-elastic NiTi SMA. We concern with long time effects of temperature and time mainly in beta phase. NiTi results are presented, including measurements of electrical resistance as a function of temperature and time, DSC to −80°C (TA Instruments), non-conventional conduction calorimetry (to 80 K), and several X-ray diffraction spectra at different temperatures. Diffusion effects are present, and all the measurements show that transformation temperatures change with time of aging even at moderate temperatures (i.e., near 100°C), depending on time and temperature. It is possible to visualize the diffusion change in the R phase transformation via classical X-ray characterization. We include some results of pre-stressing effects applying the stress at different zones of the hysteresis cycle. The experimental analysis shows that coupling between stress and temperature enhances the changes. For long times (10, 20 years) and direct sunny actions, more deep analysis is required. Finally, some dynamic effects in cycling affecting the SMA creep are outlined.     

From adapted and computerized thermomechanical equipments to modelling and the time-evolution behaviour in Cu−Zn−Al shape memory alloys

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.