Investigations in Achieving and Tailoring Negative Thermal Expansion in Cuznal and TINB Shape Memory Alloys

Abstract

Thermal Expansion is a material property that must be accounted for when designing objects where the operating temperature is larger or smaller than the ambient temperature, i.e. cryogenic tanks, and in precision instruments such as lasers, optical instruments. Currently, metals used in such applications have positive coefficients of thermal expansion. When the instruments become operational, the metals used will contract if cooled or expand when heated, causing critical distances, such as focal lengths in optics and lasers, to change, and fasteners to loosen. Having a metal that with a negative coefficient of thermal expansion can give designers a way to improve these instruments. Research in investigating inexpensive methods of producing negative thermal expansion in metals was conducted. Two different compositions of Copper Zinc Aluminum Shape Memory Alloy were investigated to see if applying known methods of producing negative thermal expansion in shape memory alloys would produce the same effect. Titanium Niobium is known to have negative thermal expansion properties. Different cold processing methods were investigated to see the effects in the coefficient of thermal expansion. Two different methods of tailoring the thermal expansion coefficient were investigated as well: welding TiNb to Ti , and post heat treating TiNb after having a negative coefficient of thermal expansion. CuZnAl showed that the coefficient of thermal expansion can be lowered, but it would remain positive. The different methods of processing TiNb all showed to produce negative coefficients of thermal expansion in TiNb to different degrees. Joining TiNb to Ti by welding produces a coefficient of thermal expansion that is between that of TiNb and Ti. Post heat treating TiNb showed that the coefficient of thermal expansion can be altered by controlling the temperature and time of the heat treatment. The two compositions of CuZnAl cannot be used in applications needing negative thermal expansion. TiNb has multiple method of having negative thermal expansion produced and multiple ways of tailoring the thermal expansion properties, making it an optimal material for application.