Institute of Fundamental Technological Research

In the paper a change of material (mechanical) parameters of NiTi shape memory alloy subjected to ion implantation treatment is investigated. The spherical indentation tests in micro- and nano-scale and tension test have been performed to study an evolution of local superelastic effect in different volumes of nonimplanted and nitrogen ion implanted NiTi alloy. The differential scanning calorimetry has been applied to measure the change of characteristic temperatures due to ion implantation treatment. The structure of implanted material has been investigated using electron microscopy technique. It has been found that the ion implantation process changes the properties not only in a thin surface layer but also in bulk material. In the layer the pseudoelastic effect is destroyed, and in the substrate is preserved, however its parameters are changed. The characteristic phase transformation temperatures in substrate are also modified.

Ion implantation, NiTi shape memory alloy, Micro- nano-indentation

In response to engineering needs of measuring moderate or large strains in metallic structures’ progress in resistance strain gages, technology resulted in widening the measurement range of this type of sensors to a range of 10–15%. For precision strain measurement applications, especially when a considerable number of measurement points are involved, single strain gage sensors are commonly used in quarter Wheatstone bridge configurations. Extension of measurement range to moderate strains requires taking into account inherent nonlinearities in signal transfer functions of the elements present in measurement chain. This work discusses in detail the most important ones, that is resistive gage sensor and quarter bridge measurement configuration nonlinearities to indicate that they are of the same order for moderate strains. Quite straightforward, shunt calibration procedure, taking into account these nonlinearities, is delivered or proposed enabling elimination of systematic errors. Thus, optimum accuracy of strain measurements matching the characteristics of components of strain measurement system can be conveniently achieved upon following simple steps. Inverse hyperbolic tangent—ϵLn =tanh−1(2 ∙e0/Eex)—linking scalar logarithmic strain and bridge reduced output voltage is shown to be a transfer function of strain measurement system with gage sensor in quarter bridge configuration for moderate or large strains.

Experimental Data Acquisition, Data Processing, Nonlinear Transfer Functions, Nonlinear Signal Conditioning, Experimental Strain/Stress Analysis, Accurate Finite Strain Measurements, Resistance Strain Gages, Nonlinear Sensors, Small/Technical Strain, Logarithmic/Natural Strain, Moderate Strains, Quarter Bridge Configuration, Wheatstone Bridge Circuits, Shunt Calibration, Scaling Amplifier Gain

TiNi shape memory alloy (SMA) specimens have been subjected to tension carried out at various strain rates. The goal was to investigate a nucleation and development of the stressinduced martensitic transformation by infrared (IR) and acoustic emission (AE) techniques. Therefore, both the infrared radiation and acoustic emission data were recorded using a fast infrared camera and acoustic emission set-up, respectively. It has been shown that the initial, macroscopically homogeneous transformation initiates in the elastic stage of the deformation even before the stress-strain curve knee and formation of the localized transformation bands. It has also been found that the homogeneous transformation occurs at similar stress level for all strain rates applied, while the localized martensitic transformation depends on the strain rate. Nucleation and development of the localized transformation bands, detected by the infrared camera, were confirmed by acoustic emission technique. The differences between the IR and AE activities were recorded during the TiNi SMA loading and unloading process, manifesting different dynamics of the stress-induced martensitic forward and reverse transformation.

shape memory alloy, TiNi, superelasticity, martensitic transformation, tension test, acoustic emission

Powder metallurgy technology was elaborated for consolidation of shape memory NiTi powders. The shape memory alloy was compacted from the prealloyed powder delivered by Memry SA. The powder shows Ms = 10°C and As = -34°C as results from DSC measurements. The samples were hot pressed in the as delivered spherical particle's state. The hot compaction was performed in a specially constructed vacuum press, at temperature of 680°C and pressure of 400 MPa. The alloy powder was encapsulated in copper capsules prior to hot pressing to avoid oxidation or carbides formation. The alloy after hot vacuum compaction at 680°C (i.e. within the B2 NiTi stability range) has shown similar transformation range as the powder. The porosity of samples compacted in the as delivered state was only 1%. The samples tested in compression up to ε = 0.06 have shown partial superelastic effect due to martensitic reversible transformation which started at the stress above 300 MPa and returned back to ε = 0.015 after unloading. They have shown also a high ultimate compression strength of 1600 MPa. Measurements of the samples temperature changes during the process allowed to detect the temperature increase above 12°C for the strain rate 10-2 s-1 accompanied the exothermic martensite transformation during loading and the temperature decrease related to the reverse endothermic transformation during unloading.

Powder metallurgy technology, shape memory alloys, DSC measurements, compression test, exothermic martensite transformation, endothermic reverse transformation

This work compares the thermomechanical behavior of TiNi shape memory alloy subjected to the compression test performed under various strain rates. The mechanical characteristics of the material were elaborated. The specimen temperature changes related to the exothermic martensitic forward and endothermic reverse transformation were measured in a contact-less manner by a very fast and sensitive infrared camera. It has been found that in this experimental conditions the stress-induced transformation and reorientation processes occurred homogeneously, since the temperature distribution observed on the specimen surface was uniform. The obtained temperature variation depends on the strain rate applied. At the higher strain rate the higher temperature changes were obtained, because the heat flow to the surroundings was lower and the process was closer to adiabatic conditions. The temperature changes influence significantly the SMA stress-strain curves and their mechanical properties.

shape memory alloy, various strain rates, compression tests, exothermic martensitic forward, endothermic reverse transformation, fast and sensitive infrared camera