TY - JOUR

T1 - Analysis of multistep transformations in single-crystal NiTi

AU - Johnson, A. J.Wagoner

AU - Hamilton, R. F.

AU - Sehitoglu, H.

AU - Biallas, G.

AU - Maier, H. J.

AU - Chumlyakov, Y. I.

AU - Woo, H. S.

N1 - Funding Information: One of the authors (HS) acknowledges the partial support of AFSOR, Aerospace and Materials Sciences, and the National Science Foundation (Grant No. CMS-0428428). HJM was supported by Deutsche Forschungsgemeinschaft.

PY - 2005/4

Y1 - 2005/4

N2 - 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.

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.

UR - http://www.scopus.com/inward/record.url?scp=17744381399&partnerID=8YFLogxK

U2 - 10.1007/s11661-005-0286-x

DO - 10.1007/s11661-005-0286-x

M3 - Article

AN - SCOPUS:17744381399

VL - 36

SP - 919

EP - 928

JO - Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science

JF - Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science

SN - 1073-5623

IS - 4

ER -