Modeling of the Thermomechanical Response of Shape Memory Alloys

Shape memory alloys (SMAs) are metallic materials that have the ability to ”re member” their previous form when subjected to appropriate thermomechanical stimuli. This ”smart” property, which is attributed to a reversible, diffusionless, solid-to-solid phase transformation from austenite to martensite, renders SMAs desirable for applications in the medical, aerospace, and automotive industries. Recent research also demonstrated that SMAs exhibit tailorable bulk thermal expansion (TE), projected to achieve coefficient of TE (CTE) over a wide range of positive and negative values, via martensite variant texturing by taking advantage of the significant intrinsic TE anisotropy of the martensite lattice. There is, therefore, a remarkable potential of SMAs in applications in which TE is a critical design factor, such as high precision instruments in optical applications and satellite antennas. To ultimately realize the true potential of SMAs in engineer ing applications as ”smart” or low-CTE materials, constitutive models capable of effectively describing their response to thermomechanical stimuli are needed to enable efficient design of SMA-based devices. In this thesis, a constitutive model is proposed that i) can efficiently describe reversible phase transformation from austenite to self-accommodated and/or oriented martensite, (re)orientation of martensite variants, minor loops, latent heat effect and tension-compression asymmetry, and ii) tailor the bulk TE tensor based on an effective description of martensite variants texture. The strengths of the proposed model lie in i) its ability to account for all the aforementioned aspects aspects of the deformation response of SMAs and highlight their collective importance in complex non-proportional thermomechanical loading, and ii) its innovation to accurately tailor the TE evolu tion during deformation processing. The model is validated against experimental results under tension/compression/torsion box loading and on the CTE evolution due to orientation of self-accommodated martensite, and verified by numerical simulations of 3D SMA-based structures.