Prakt. Met. Sonderband 30 (1999) 359 
Eig Structure investigation of TiNi shape memory alloy with R-phase transformation and two- 
um ar way shape memory effect. 
Mang. 
Wig Peter Filip, Karel Mazanec and Albert C. Kneissl’ 
ding 7 TU Ostrava, Institute for Materials Engineering, Ostrava, Czech Republic, “Montanuniversity 
a. Leoben, Institute of Physical Metallurgy and Material Testing, Leoben Austria. 
üSchaften 
Abstract 
NZ durch The so-called two-way shape memory effect (TWSME) represents a behavior in which a reversible 
Kleine temperature change of a “memory alloy” is accompanied with spontaneous reversible macroscopic 
en elasti deformations (as a rule of order of 1%). It was generally accepted that internal structural defects, 
re Rück: which are introduced during the so-called training procedure, are “responsible” for this 
phenomenon. We have recently shown that formation of so-called ghost martensite may contribute 
to the enhanced reversibility in binary TiNi systems without R-phase formation. The aim of this 
study was to investigate TiNi materials in which R-phase formation occurs. For this purpose, we 
prepared Ti-50.4 at. %Ni alloy, which was thermally cycled under several conditions (common 
feature was: €=4%, and heating cycles performed between 20 and 135°C). It was found that trained 
ch der specimens contain dislocation arrays, stabilized martensite and ghost martensite. The structure of 
a ghost martensite is related to dislocations and also to distribution of interstitial atoms. It is expected, 
owing to the specific crystallographic relation, that ghost martensite formation can facilitate both 
the R-phase formation as well as B19’ martensite formation and in this way support the reversible 
memory of TiNi materials. 
a Introduction 
The two-way shape memory effect (TWSME) is a special kind of shape memory behavior. It is 
characterized by a macroscopic shape change, which depends only on temperature. No external 
i tik stress has to be applied on the material. ‚The physical basis of TWSME isa reversible martensitic 
edd transformation. Spontaneous macroscopic shape change is linked with the dominant formation of 
=o the most favorable (preferential) martensite variant (“cold shape”) and retransformation of 
martensite to high temperature phase (“hot shape”) (1). The TWSME is not an inherent property of 
) shape memory alloys (SMA). The dominant and spontaneous formation of a preferential martensite 
s zurück: variant during the cooling process accompanied with transformation plasticity (“cold shape”) can 
ert only be induced after a particular thermomechanical training procedure (1 to 6). The training 
form generates some kinds of microstructural asymmetry (4). Heating to the high temperature phase 
(accompanied with a retransformation of martensite and a recovery of “hot shape”) should not 
destroy this asymmetry. The two main structural changes, responsible for the TWSME discussed in 
the literature are either based on i) generation of dislocation arrays or ii) presence of stabilized 
martensite (1, 2, 4) 
The most widely accepted mechanism is based on the observation of dislocation arrays generated 
during training routes. These dislocation arrangements are produced by thermomechanical 
treatment (1, 4, 7) or by thermal cycling (1, 2, 8, 9). Based on these observations, the TWSME has 
been attributed to the oriented residual stresses accompanying the dislocation arrangements (1, 2). 
The oriented residual stresses only favor the nucleation and growth of the preferential variant of 
martensite. Since any martensite formation is accompanied by shear like deformation, the residual 
stresses are relaxed by a shape change (1). The preferentially formed variant of martensite grows 
without any external stress assistance during cooling. Heating and reverse transformation of 
martensite to high temperature phase (austenite) leads again to the generation of oriented residual 
stresses in matrix. This process can be repeated during following thermal cycles. Based on the