In-Situ Fracture Observation and Fracture Toughness Analysis of Ni-Mn-Ga-Fe Ferromagnetic Shape Memory Alloys
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NTRODUCTION
FERROMAGNETIC shape memory alloys can be applied to sensors or magnetic actuators because of their unique magnetic induced deformation properties.[1–5] Therefore, various alloys such as Ni-Mn-(Ga,Al,In), Co-Ni-(Ga,Al), and Fe-(Pd,Pt) were investigated for these applications.[6–10] Ni-Mn-Ga alloys have much higher magnetic displacements than conventional piezoceramics or magnetostrictive materials. They can be applied to dynamic actuators requiring large deformation, because their response time is much shorter than that of conventional actuator materials.[5,11] In order to be applied to practical components, it is required that the martensitic transformation should occur between room temperature and Curie temperature,[12–14] with the sufficient magnetic displacement and the proper ductility.[15] However, their applications are quite limited because they hardly exhibit ductility.[12–16] Much research was undertaken on the control of martensite transformation temperatures and the improvement of magnetic displacements in order to develop ferromagnetic shape memory alloys.[13–15] However, the improvement of ductility remains unsolved for KWANGJUN EUH, Senior Researcher, and JUNG-MOO LEE, Principal Researcher, are with the Structural Materials Division, Korea Institute of Materials Science, Changwon 641-010, Korea. DUK-HYUN NAM, formerly with the Center for Advanced Aerospace Materials, Pohang University of Science and Technology, Pohang 790-784, Korea, is a Senior Research Engineer, now with the Material Research Team, Hyundai Motors Technical Research Laboratory, Hwasung 445-706, Korea. SUNGHAK LEE, Professor, Center for Advanced Aerospace Materials, Pohang University of Science and Technology, is jointly appointed with the Department of Materials Science and Engineering, Pohang University of Science and Technology. Contact e-mail: [email protected] Manuscript submitted December 28, 2009. Article published online July 27, 2011 METALLURGICAL AND MATERIALS TRANSACTIONS A
practical application and development of components. Recently, different studies focused on the improvement of ductility and fracture toughness by distributing ductile particles.[12,16–27] Wang et al.[16] reported that the fracture toughness of Ni-Mn-Ga-Fe alloys increased with increasing the Fe content by the fracture mode change from intergranular to transgranular. However, improvement of fracture properties of ferromagnetic shape memory alloys by particle distribution in relation to the microstructural modification was only rarely reported.[28] For the accurate evaluation of ductility and fracture toughness of these alloys, systematic understanding of the correlation between microstructure, ductility, and fracture toughness is required, and microfracture mechanisms in relation with microstructure need to be elucidated. One of the simplest ways to investigate microfracture mechanisms is a phenomenal observation of fractured surfaces, followed by mathematical quantification of the results in order to describe fracture properties effectively.[29,
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