"Process-Microstructure-Properties Relationships of NiTi Shape Memory Wires for Actuators"
One of the critical aspects of this materials class is the long sequence of thermo-mechanical processing steps which are required to manufacture SMA components. It is well known that cold-working, annealing and training can strongly affect the shape and texture of individual NiTi austenite grains, dislocation densities, precipitation of particles, grain boundary properties and can induce specific martensite variants and twinning. The functional properties of SMA components are directly related to the material’s microstructure. Information on microstructural and crystallographic changes associated with different processing routes may help to improve actuator performances.
In the present work the effects of thermo-mechanical treatments on the microstructure of fine NiTi wires have been investigated by transmission electron microscopy (TEM). The functional and fatigue behavior of such wires has been analyzed by differential scanning calorimetry (DSC), tensile testing and thermo-mechanical cycling. Due to the high number of process parameters that can affect the microstructure and the functional properties, experiments were planned by appropriately combining select material states and input parameters. The results were analyzed in terms of clear trends and relationships between microstructural and functional input and output parameters.
Results on grain sizes, dislocation densities, precipitates and/or inclusions will be presented. Evidence for and the significance of relationships between process parameters, microstructure and shape memory properties will be presented and discussed. Finally, it will be shown how the results have helped to optimize the shape memory properties of fine wires used in automotive applications. A brief outline of the actuator, the product behind our research will also be given.