On the multiplication of dislocations during martensitic transformations in NiTi shape memory alloys

• Published in: Acta materialia Vol. 58; no. 5; pp. 1850 - 1860
• Main Authors: Simon, T., Kröger, A., Somsen, C., Dlouhy, A., Eggeler, G.
• Format: Journal Article
• Language: English
• Published: Kidlington Elsevier Ltd 01.03.2010; Elsevier
• Subjects: Applied sciences; Dislocation multiplication mechanism; Dislocations; Exact sciences and technology; Martensite variants; Martensitic transformations; Metals. Metallurgy; NiTi; NiTi; Martensitic transformations; Dislocation multiplication mechanism; Martensite variants; Dislocations; Titanium alloy; Mechanism; Dislocation; Phase transformation; Shape memory alloy; Martensitic transformation; Martensite; Nickel alloy
• ISSN: 1359-6454; 1873-2453
• DOI: 10.1016/j.actamat.2009.11.028

In situ and post-mortem diffraction contrast transmission electron microscopy (TEM) was used to study the multiplication of dislocations during a thermal martensitic forward and reverse transformation in a NiTi shape memory alloy single crystal. An analysis of the elongated dislocation loops which formed during the transformation was performed. It is proposed that the stress field of an approaching martensite needle activates an in-grown dislocation segment and generates characteristic narrow and elongated dislocation loops which expand on {1 1 0} B2 planes parallel to {0 0 1} B19′ compound twin planes. The findings are compared with TEM results reported in the literature for NiTi and other shape memory alloys. It is suggested that the type of dislocation multiplication mechanism documented in the present study is generic and that it can account for the increase in dislocation densities during thermal and stress-induced martensitic transformations in other shape memory alloys.