Characterization of negative thermal expansion of shape memory alloys

• Main Author: Wong, Yu Lian.
• Other Authors: Liu Yong
• Format: Final Year Project
• Language: English
• Published: 2009
• Subjects: DRNTU::Engineering::Materials::Metallic materials::Alloys
• Online Access: http://hdl.handle.net/10356/16984
• Institution: Nanyang Technological University

Characterization of materials is important so as to understand the different properties of the material. This aims to ensure that they can be fully utilized in all of their present and future applications. The present research aims to understand the occurence of the negative thermal expansion phenomenon associated with shape memory alloys through experimental means. Two parameters were varied to understand this phenomenon, namely stress applied and initial pre-straining on the specimen. From the research, it was shown that this phenomenon was only a characteristic of the martensitic phase. It was not observed in the austenitic phase. An increase in the stress applied onto the specimen during thermal cycling has a decreasing effect on the negative thermal expansion. Above a critical stress level, negative thermal expansion effect was not observed. Instead, the material exhibits the positive thermal expansion. Also, it was shown that the effects of initial pre-straining could be seen on this characteristic. With an increase in initial pre-straining, negative thermal expansion becomes more pronounced until it reaches a critical level. After which, the effects were shown to be slightly reduced. Lastly, the effects of constraint stress and initial pre-straining was also seen on the transformation temperatures of the specimen. In conclusion, the effect of constraint stress and initial pre-straining were studied and their effects were shown. This will allow for more specific studies to be targeted in future.