Study of relaxation and crystallization kinetics of NiTi made amorphous by repeated cold rolling

• Published in: Acta materialia Vol. 58; no. 20; pp. 6637 - 6648
• Main Authors: Peterlechner, M., Bokeloh, J., Wilde, G., Waitz, T.
• Format: Journal Article
• Language: English
• Published: Kidlington Elsevier Ltd 01.12.2010; Elsevier
• Subjects: Applied sciences; Bulk amorphous materials; Calorimetry; Crystallization; Exact sciences and technology; Forming; Intermetallics; Mathematical models; Metals. Metallurgy; Nickel base alloys; Nickel compounds; Nickel titanides; Production techniques; Relaxation; Repeated cold rolling; Rolling; Shape memory alloys; Titanium compounds; Relaxation; Calorimetry; Repeated cold rolling; Bulk amorphous materials; Crystallization; Cold rolling; Forming; Kinetics
• ISSN: 1359-6454; 1873-2453
• DOI: 10.1016/j.actamat.2010.08.026

An amorphous structure of NiTi was obtained by repeated cold rolling. Relaxation and crystallization were studied applying a combination of sensitive calorimetric methods, X-ray diffraction and transmission electron microscopy. Upon deformation, a nanostructured mixture of crystalline and amorphous phase occurs. With increasing degree of RCR, the fraction of the crystalline phase decreases; at an equivalent strain of 16.8, an almost completely amorphous phase arises. Isothermal calorimetry yields a monotonously decreasing signal followed by a peak occurring by relaxation of the amorphous phase and nanocrystallization, respectively. The relaxation signal is well described by a Kohlrausch–Williams–Watts function. Crystallization kinetics agree with those expected from the Johnson–Mehl–Avrami model. The kinetic exponent agrees with three-dimensional growth and mixed nucleation or nucleation with a decreasing rate. The crystallization kinetics of NiTi made amorphous by severe plastic deformation differ from those of amorphous NiTi obtained by melt spinning or sputter deposition.