Creep of binary Ni-rich NiTi shape memory alloys and the influence of pre-creep on martensitic transformations

The present work represents an exploratory study of creep in a Ni-rich NiTi shape memory alloy (50.7 at.% Ni) in the temperature range 470-530 degreesC at stresses between 90 and 150 MPa. Creep of Ni-rich NiTi alloys is characterized by stress exponents of 2 (for the minimum creep rate data) and of 4.7 (for creep rates measured after 10% of accumulated strain). During creep, Ni4Ti3 precipitates (which nucleate and grow during heating under a small pre-load) coarsen. The interparticle spacing increases and this is why the resistance of the material to creep decreases. It was not possible to rationalize this creep softening on the basis of a traditional Orowan process. Instead a wrapping process, where particles were wrapped in by migrating subgrain boundaries, was observed. The sharp minima in the creep curves and the strain dependence of the stress exponent as observed in the present study can be rationalized on the basis of an initial climb controlled bypassing of particles followed by this wrapping process, which becomes more intense as interparticle spacings increase. When comparing crept materials with materials which were aged at the same temperature for the same time there only is a small difference between the corresponding differential scanning calorimetry (DSC) charts. Therefore we conclude that the precipitation and coarsening of Ni4Ti3 particles during creep is more important in affecting DSC chart features than the evolution of a creep dislocation structure.