Authors:Richard DV. Espiritu and Alberto V. Amorsolo Jr.
Shapememoryalloys (SMA) are metallic materials that undergo thermoelastic martensitic phase transformation which demonstrates their ability to return to some previously defined shape when subjected to an
Authors:J. Rouchaud, M. Fedoroff, A. Dubertret, and V. Agafonov
The transition temperatures and ageing of shape memory alloys (SMA) depend on the chemical composition and purity. Methods for the deterrnination of trace impurities, doping and major elements in NiTi, CuZnAl and CuNiAl were developed using neutron activation analysis and -spectroscopy. For traces in Zn containing alloys, a chemical separation based on anion exchange resins was developed. Multielement analysis leads to detection limits ranging from 0.0001 to 1 g·g–1. For major elements, optimization of the irradiation and spectrometry parameters enables standard deviation better than 0.5%.
Authors:G. Airoldi, G. Riva, B. Rivolta, and M. Vanelli
The unusual mechanical properties (i.e. shape memory effect and superelasticity) of shape memory alloys (SMA) rely on the thermoelastic martensitic transformation (TMT) which is a first-order solid-solid, non-diffusive phase transition, athermal in character.
Differential scanning calorimetry (DSC) is often used as a convenient method of investigating the thermal properties ofSMAs. The common practice of standard temperature calibration, required for a correct instrument performance, is here critically
discussed in relation to the study of both the direct exothermic transformation on cooling, and the reverse endothermic transformation
on heating in a NiTiSMA. The DSC results show that, with the standard temperature calibration, the instrument is calibrated on heating but un-calibrated
on cooling. A general method is advanced to overcome this problem, intrinsically related to the dynamic character of DSC.
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.
Shapememoryalloys (SMAs) have two crystalline structures, martensite and austenite, stable at two different temperatures. Consequently, the material is sensitive to a specific thermal cycle transforming from
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
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.
–Mn–Si-based shapememoryalloys (SMAs), which possess a low stacking fault energy (SFE), show γ to α ′, γ to ∊, and ∊ to α ′ martensite transformations.
As early as 1984, Sato et al. [ 16 ] studied the SME in single crystal Fe–(20–32)Mn–(1–6.5)Si SMAs
Authors:Sonia Degeratu, P. Rotaru, Gh. Manolea, H. Manolea, and A. Rotaru
For two typical actuators of intelligent systems (Ni–Ti SMA cantilever and SMA helical spring), the evaluation of their thermal
characteristics is presented. In order to determine the transformation temperatures and other thermal parameters of the two
studied elements, the attention was concentrated on thermal analysis experiments. For each actuator configuration, comprehensive
graphical interfaces have been developed, to run in Visual Basic, with respect to the results of performed thermal analyses.