TY - JOUR
AU1 - Wagoner Johnson, A.
AU2 - Sehitoglu, H.
AU3 - Hamilton, R.
AU4 - Biallas, G.
AU5 - Maier, H.
AU6 - Chumlyakov, Y.
AU7 - Woo, H.
AB - The effects of composition and heat treatment on the thermally induced phase-transformation behavior of single-crystal NiTi with compositions of 50.1, 50.4, 50.8, and 51.5 at. pct Ni are presented in this article. Differential scanning calorimetry (DSC) experiments reveal that a heat-treated 50.1 at. pct Ni alloy exhibits an unprecedented multiple-step transformation (MST) on both heating and cooling, with up to four peaks. This behavior is absent in the higher-Ni-content alloys. In polycrystalline NiTi alloys, MSTs have been attributed to microstructure heterogeneities such as grain boundaries and dislocations, which influence precipitation. In-situ scanning electron microscopy (SEM) results show that the MST in the 50.1 at. pct Ni alloy is associated with single-crystal defects such as dendrites and low-angle boundaries. A heterogeneous precipitate distribution is observed in transmission electron microscopy (TEM) images of the same low-Ni alloy, also associated with the defects, creating conditions that have been shown in other studies to promote the MST in polycrystals. These MSTs are not observed for high-Ni single-crystal alloys containing the same defects. In this article, we describe the origin of the extraordinary forward and reverse MSTs in the low-Ni alloy and the absence of the MST in high-Ni alloys. Transformation sequences are proposed based on the contrasting precipitate microstructures.
TI - Analysis of multistep transformations in single-crystal NiTi
JF - Metallurgical and Materials Transactions A
DO - 10.1007/s11661-005-0286-x
DA - 2005-04-01
UR - https://www.deepdyve.com/lp/springer-journals/analysis-of-multistep-transformations-in-single-crystal-niti-DXQLcHaNPC
SP - 919
EP - 928
VL - 36
IS - 4
DP - DeepDyve
ER -