High-resolution equipment for martensitic transformation in shape memory alloys: local studies in stress-strain-temperature

Abstract

A computer-controlled system to study the stress-strain-temperature surface for small sized samples is described. Resolution and reproducibilities are: load 1 mN (full load 20 MPa for samples with section 1 mm2), displacements 0.1 mu m (up to 2 mm) and 0.01 K in temperature (from 280 to 360 K). The equipment can observe the phenomena of friction, nucleation, intrinsic pseudo-elasticity or thermo-elasticity, precipitate effects and minor evolutive phenomena in shape memory alloys. The results achieved with the equipment have been useful to formulate a physical model and to interpret the thermomechanical behaviour of the material.

Low temperature aging behaviour of transformation temperatures in some Cu-based and NiTi SMA

“…When the material undergoes a step like change of temperature the electrical resistance varies quickly via the classical phonon contribution (nearly linear dependence of resistance with temperature), as expected, but it only reaches the exponential way, with relatively large time constants near room temperature, from near 1 h at 373 K to about a year at 293 K in CuZnAl with electronic concentration 1.48 e/a [9]. A similar effect has been reported at higher temperatures in some alloys affected by diffusive atomic ordering processes [12].…”

“…The computerized device, controlling the sample temperature as described in [9], permits cooling and heating to determine the Ms value by electrical resistance measurements. The cycles were done at relatively low speeds, in order to determine properly the temperature (near four hours per cycle).…”

Guaranteed behaviour of shape memory alloys : After quench and long time effects in CuZnAl SMA

“…Careful control and programmation of temperature is needed to obtain reliable results: temperature control and stability to at least O.OTC is necessary to check for small changes in the resistance value, or to look for stability at constant temperature. The temperature control was achieved in a similar way as described in [8]. The electrical resistance was measured with the fourwire technique, and positive and negative readings of current and voltage were acquired to avoid the thermal emf effects.…”

Predictable behavior of smart materials (Cu-Zn-Al SMA)

“…The standard temperature control includes a 'classical' predictorcorrector scheme for the expected reproducibility (usually near 0.003 K). Using two controlled temperature plates (120• mm 2) a stress-strain-temperature (and, eventually, time) equipment has been built [6]. The sample, usually with 25 mm in length and near 1 mm 2 in section (see 1 in Fig.…”

“…Using an adapted instrumentation with improved resolution [5][6][7] (differential and nondifferential conduction calorimeter, thermomechanical equipment's and resistance measurements) permits an evaluation of the irreversible evolution (like dislocation creation), the time evolution of the samples with or without cycling, and the relevant parameters characterizing the properties of the alloys. Accurate and reproducible transformation hysteresis cycles with internal loops were obtained in single crystals [8][9].…”