Stress-induced twinning / detwinning processes in tetragonal martensite of Ni-Mn-Ga alloy studied by in-situ transmission electron microscopy
Ni-Mn-Ga is a ferromagnetic shape memory alloy which transforms from the cubic high temperature phase to thermoelastic martensite and undergoes a magnetic transition during cooling. The alloy shows remarkable properties, e.g. magnetic-field-induced-strain, superelasticity or thermally induced shape memory effect. In this context the mobility of twin boundaries is a critical factor. We study stress-induced motion of twin boundaries by tensile experiments performed in-situ in a Jeol transmission electron microscope (equipped by a double tilt straining stage constructed in the IP ASCR, (see
Z.Dlabáček, A.Gemperle, J.Gemperlová, 13th EMC, Antwerp 2004, p.417) and by ex-situ high resolution TEM (TU Helsinki). It was found that the twinning / detwinning processes of the martensite are implemented through the plate by plate transformation of one martensite variant into the other. This process occurs via movement of partial twinning dislocations emitted from the interface between two twinned bands [
Y.Ge, N.Zárubová, Z.Dlabáček, I.Aaltio, O.Söderberg, S.-P.Hannula, ESOMAT 2009, DOI: 10.1051/esomat/200904007].
Fig. 1. Contrast on twin dislocations in disappearing plates of one martensite variant.
Fig. 2. Twin dislocations emmited from the boundary between two twinned bends.
back
Nonconventional final thermomechanical treatment of shape memory NiTi filaments by DC electric current
The last step of the technological route from Ni and Ti metals to final superelastic or shape memory NiTi wires consists of a final thermomechanical treatment commonly done in air furnaces at temperatures around 400 °C. The shape, functional properties including their stability and fatigue properties are in large extent controlled by the final thermomechanical treatment. That is why this treatment is a very important step in the NiTi production technology and has always been given a special attention in the field. The FMC group developed, in the framework of a European research program
AVALON focused on the technologies for production of NiTi hybrid fabrics and their industrial applications, a method of nonconventional final thermomechanical treatment of NiTi by DC electric current /FTMT-EC method/ (see the scheme in Fig. 1). It allows for the heat treatment of continuous thin NiTi filaments on a textile compatible equipment and NiTi filaments already integrated in fabrics together with natural or polymeric yarns capable to resist only temperatures up to ~300°C. Moreover, this heat treatment proceeds under precisely controlled conditions that determine the final thermomechanical behaviour of the NiTi wire (see Fig. 1).
A patent PV 2009-279 has been filed in the Czech Industrial Property Office on this heat treatment method.
Fig. 1. Nonconventional final thermomechanical treatment of NiTi by DC electric current /FTMT-EC method/ developed by FMC group - scheme of the method, first prototype NiTiTec and tensile stress-strain curves of NiTi wires heat treated under different conditions.
back
Thermomechanical characterisation of shape memory NiTi filaments
Applied research within the Functional Materials and Composites group is partially focused on the development of smart NiTi textiles incorporating the shape memory NiTi filaments. To allow a design of high value added NiTi textiles it is crucial to understand the thermomechanical behaviour of NiTi filaments under different loading modes that a single NiTi filament integrated into a textile fabric is subjected to when the fabric is being loaded. Therefore a systematic method towards thermomechanical characterization of thin NiTi filaments under different loading modes has been developed. Within this research unique miniature testing equipment has been designed allowing for termomechanical testing in tension and combined tension torsion (see Fig. 1) of thin NiTi filaments down to few microns of diameter. Moreover, a characterization of thermomechanical behaviour of NiTi filaments using a set of parameters and a non-equilibrium stress-strain diagram (Fig. 3) has been developed that enables one to asses in an objective way the properties of a given NiTi filament [
Heller L. et al., Quasistatic and dynamic functional properties of thin superelastic wires, The European Physical Journal Special Topics, (2008) 128: 7–15].
Fig. 1. Dedicated laboratory equipment (ATTUT) for testing shape memory alloy NiTi micro-wires under combined tension-torsion loading mode designed and developed by FMC group
Fig. 2. An example of results delivered within an automated testing procedure by ATTUT testing equipment (see above). The results are related to a 100 microns thin NiTi wire with austenite finish temperature at ~0°C.
Fig. 3. An example of the stress-temperature non-equilibrium used for assessment of thermomechanical behaviour in tension of shape memory aloy NiTi wires. It is constructed using data gathered during a set of predefined thermomechanical tests.
back
SMA velcro fastener - a novel shape memory application
A novel design of the velcro fastener has been developed, employing hooks made of NiTi shape memory wires. It overcomes disadvantages of conventional velcro design such as sharp sound when pulling apart the two tied parts and wearing out after several thousands cycles of connecting/disconnecting. Moreover, it features new functionalities such as outstanding strength, customized fastening and temperature driven properties.
[
Tearing apart Velcro design for silent use ( eStrategies, 2008),
Shape Memory Hooks Employed in Fasteners (JMEPEG (2009) 18:706–710) ].
The patent "Silent Velcro Fastener" has been filed in the Czech Industrial Property Office (PV 2008-568)
Fig. 1. Scheme and prototype of the SMA velcro fastener
back
