Shape Memory Alloys (SMAs) can recover up to 6% strain, undergo an increase in elastic modulus of over 100%, and generate large stresses through a temperature and stress-dependent transformation between their martensite and austenite phases. In this research, SMAs are embedded within an aluminum matrix through Ultrasonic Additive Manufacturing (UAM), a rapid prototyping process. Being a low temperature process, UAM enables the construction of metal-matrix composites with embedded thermally sensitive materials such as SMAs. Six composites were constructed by embedding NiTi ribbons within an Al 3003-H18 matrix with fiber volume fractions of up to 20% and various levels of NiTi prestrain. The resulting composites are the subject of modeling and experimental characterization.
The developed model describes the response to thermomechanical loading of an SMA composite with arbitrary fiber volume fractions and prestrain levels. The model is based on a new bivariant SMA formulation which has a continuous kinetic law through the mixed stress and temperature-induced transformation region of the phase diagram. This model is used to analyze experimental data and aid in the design of NiTi-Al composites.
The experimental characterization of the composites focuses on controlling thermally-induced strain, tuning the response of the composites to static and dynamic mechanical loads via temperature changes, and characterization of the NiTi-Al interface. Through analysis of the thermally-induced strain responses of the composites, two key parameters for controlling the Coefficient of Thermal Expansion (CTE) were identified: NiTi fiber volume fraction and prestrain of the embedded NiTi. Using a 15.2% fiber volume fraction of fully detwinned NiTi ribbons, a minimum CTE of 7.6 ppm/°C is obtained compared to 23.2 ppm/°C for Al~3003.
The change in static stiffness, natural frequency, and damping ratio of composites 1-4 were observed as a function of temperature. Composites exhibit a maximum increase in static stiffness of 10.0% at 100°C due to the increased modulus of the austenitic phase. Dynamic behavior was observed for clamped-free and clamped-clamped boundary conditions. The inclusion of NiTi mitigates the decrease in natural frequency associated with the decreasing Al modulus at elevated temperatures under the clamped-free boundary condition. Under the clamped-clamped boundary condition, the maximum change in natural frequency is +25.0% due to the blocking stress generated by the NiTi ribbons. In both boundary conditions, the composites exhibit a damping ratio similar to that of solid Al.
The NiTi-Al interface was characterized through scanning electron microscopy and differential scanning calorimetry. These methods were used to investigate the nature of bonding between the ribbon and matrix and determine interface shear strength. The interface is found to be dominated by friction and has a shear strength of 7.36 MPa.
This research provides three primary contributions. The first is a new model that can guide the design of SMA composites using only material and composite properties. The second contribution is the development and construction of SMA metal-matrix composites at low temperatures with variable thermally-induced strain and tunable static and dynamic properties. The third contribution is the experimental characterization of the unique thermally-dependent behaviors and analysis of the SMA-matrix interface.