Prakt. Met. Sonderband 41 (2009) 121 
Thermomechanical training and microstructure of NiTi melt-spun 
ribbons 
K. Mehrabi', M. Bruncko'?, M. Hirtel', A. C. Kneissl' 
' Department of Physical Metallurgy and Materials Testing, University of Leoben, Austria 
? Faculty of Mechanical Engineering, University of Maribor, Slovenia 
Abstract 
The present work aims to study the influence of thermomechanical training and two-way shape 
memory effect (TWSME) on the transformation behaviour of Ni-50.3 at. % Ti melt-spun ribbons by 
differential scanning calorimetry (DSC). The results showed that the transformation temperatures 
decrease with an increase in the number of thermal cycles. The austenitic transformation remains a 
one-stage transformation, while the martensitic transformation changes to a two-stage 
transformation. These changes are due to the appearance of an intermediate phase which was 
stabilized probably by the accumulation of defects introduced by thermomechanical training. The 
phase that is present after training was studied by transmission electron microscopy. 
1 Introduction 
NiTi alloys are the most technological important shape memory alloys (SMAs) for a wide range of 
applications, often as actuators and sensors, since with a shape memory element a pre-determined 
response can be obtained very easily by thermal or electric stimulus. The possibility to realize 
complicated movements with an element of simple design and compact size makes shape memory 
actuators very attractive [1,2]. There is also a trend to very small-dimensioned shape memory 
elements which could be used as microactuators. In these applications the NiTi components often 
experience repeated transformation cycles, either under external load or free of constraint. One 
critical concern in the development of such devices is the stability of the shape memory properties. 
Obviously, unstable transformation cycles with associated difficulties in accurate predictions of 
materials behaviour and device design in the presence of these instabilities are not desirable for 
actuator applications. 
From the several modes of shape memory effect, the two-way shape memory effect (TWSME) is 
the most suitable to apply in actuators since no resetting force has to be considered in design. 
However, it is not inherent to SMAs, but can be exhibited after specific repetitive 
thermomechanical treatments known as training procedures through which SMAs memorize the 
low temperature shape. To date, more than 20 different training procedures have been investigated 
and published in efforts to develop a TWSME of both high magnitude and stability which have the 
common feature that external applied stress should be used, such as tension, compression, torsion or 
bending. There are some training methods without applied load, which induce internal stress by 
quenching, ion irradiation or external magnetic fields [4]. It is generally perceived that the 
development of TWSME originates from the preferential nucleation and growth of martensite