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A simple, low-cost apparatus has been designed and constructed for measurement of the thermal conductivities of samples with low cross-sections (∼10−7 m2). This apparatus has been used to determine variations in the thermal conductivity of the metallic glass Fe80B20 (Metglas 2605) in the crystallization process induced by thermal treatment.

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Abstract  

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.

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Abstract  

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 quench effects.

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Abstract  

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.

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Metastable effects on martensitic transformation in SMA

Part VI. The Clausius-Clapeyron relationship

Journal of Thermal Analysis and Calorimetry
Authors: A. Isalgue, V. Torra, A. Yawny, and F. Lovey

Abstract  

The Clausius-Clapeyron coefficient (CCC or αCC) 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 αCC=6.3±0.3 MPa K−1. On the other hand CuAlBe provides a value of αCC=2.2±0.4 MPa K−1 for tensile stress.

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Abstract  

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.

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Metastable effects onmartensitic transformation in SMA

Part 4. Thermomechanical properties of CuAlBe and NiTi observations for dampers in family houses

Journal of Thermal Analysis and Calorimetry
Authors: C. Auguet, A. Isalgué, F. Lovey, F. Martorell, and V. Torra

Abstract  

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.

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Abstract  

The fatigue-life (traction-traction only) is experimentally studied mainly for pseudoelastic NiTi wire of 2.46 mm of diameter for eventual application in damping of structures under wind or rain. Thermal effects are highly relevant in determining the fatigue-life. The results shows that the fracture level overcomes 130000 cycles when the moving air is used for cooling. When the number of working cycles overcomes 30000, the frictional energy decreases 40% vs. N roughly with an exponential behavior.

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