|dc.description.abstract||Recently, new shape memory alloy (SMA) MAX phase composites (e.g. NiTi- Ti2AlC and NiTi − Ti3SiC2) have been developed to take advantage of their unique combination of thermomechanical responses. Of particular interest are the reversible martensitic transformation of the SMA phase and the recoverable and irrecoverable deformations associated with kinking in the MAX phases. By using such behaviors in conjunction with each other, a novel composite material is produced for use in extreme environments. Specifically, the nonlinear behavior of the two constituents leads to strong mechanical damping and the possibility of developing controllable residual stress states. To take advantage of these possibilities, detailed understanding of the interactions of not just the mechansisms but also the composite microstructure is needed. Therefore, a series of models are constructed to explore the various composite behaviors.
To address this need, in this work a series of models are developed to explore the local and global responses of this system with an emphasis on the response of these composites to thermal loadings. First, the effective transformation characteristics of SMA composites with a stiff, elastic matrix are modeled using an efficient micromechanical scheme and the inﬂuence of a stress redistribution associated with transformation on the effective phase diagram is determined. The interaction of the reversible SMA phase with an elastic-plastic MAX phase through an constant stress, thermal cycle is then studied through image based ﬁnite element approaches. Specifically, two microstructure models are generated. The ﬁrst being a reference two-phase system and the latter being an actual NiTi-Ti2AlC composite. By analyzing response through such loadings it is demonstrated that martensitic transformation may be used to induce permanent deformations in the MAX phase and thereby develop residual stress states. Additional analysis of the actual composite system reveals that these residual stress states may induce an effective two-way shape memory behavior. Comparison to experimental results also shows reasonable agreement. Further simulations are used to explore the effect of the microstructure and identify the impact of different features on the eﬀective composite response.
The previous results emphasized only permanent deformations in the MAX phase even though additional irrecoverable deformations have been experimentally observed. To incorporate these eﬀects, an enhanced constitutive model considering both responses is needed. Currently, no such model exists in the literature. There-fore, to address this need, a constitutive model of the MAX phase behaviors is established and implemented in this work. Specifically, a three dimensional multi-mechanism model considering recoverable, irrecoverable, and damage is constructed. Both the theoretical development and numerical implementation are presented while a three dimensional material subroutine is created. The resultant model shows good agreement with experimental results and demonstrates the necessity of all three mechanisms. This enhanced constitutive routine is then used with composite models to investigate the impact of the different phases.||en