Numéro |
J. Phys. IV France
Volume 112, October 2003
|
|
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Page(s) | 673 - 676 | |
DOI | https://doi.org/10.1051/jp4:2003972 |
J. Phys. IV France 112 (2003) 673
DOI: 10.1051/jp4:2003972
On the role of chemical and microstructural heterogeneities in multistage martensitic transformations
G. Eggeler1, J. Khalil Allafi1, A. Dlouhy2 and X. Ren21 Institut für Werkstoffe, Ruhr-Universitaet Bochum, 44780 Bochum, Germany
2 Institute of Physics of Materials, Zizkova 22, 61662 Brno, Czech Republic
Abstract
The present paper considers chemical and microstructural reasons for unusual multiple step transformations as observed using
differential scanning calorimetry (DSC) for Ni-rich NiTi-alloys on cooling from the B2 regime. In a simplified two-particles/matrix
system a multiple step transformation can be explained based on two elements: (1) The composition inhomogeneity which evolves
during aging as Ni
4Ti
3, precipitates grow. (2) The difference between nucleation barriers for R-phase (small) and B19
' (large). These two elements explain the features of the evolution of DSC charts during aging, which change from two transformation
peaks on cooling after short aging times, to three after intermediate aging times and finally again back to two peaks after
long aging times (2-3-2 transformation behavior). Another reason for multiple step transformations in Ni-rich NiTi alloys
may be that Ni
4Ti
3, particles precipitate heterogeneously near grain boundaries. Then the first two transformation peaks can be attributed to
the formation of R-phase and B19
' in the grain boundary regions while the third peak corresponds to a transformation of B2 to B19
' in the precipitate free interior of the grains.
© EDP Sciences 2003