Molecular Dynamics Simulation of Shape-Memory Behavior

Abstract

Mechanical properties of shape-memory alloys (SMAs) are typically represented by the characteristic stress–strain curve, which forms a hysteresis loop in a loading, unloading and shaperecovering process. To represent the deformation behavior of SMAs, various constitutive equations have been developed, and prediction of the macroscopic behavior has been possible using finite-element simulations. The atomistic behavior leading to the deformation and shape-recovery is explained on the basis of the phase transformation between austenite and martensite phases and the characteristics of the crystal structure. One well-known atomistic mechanism is illustrated in Fig. 1. The stable phase depends on the temperature, and phases at high and low temperature are body-centered cubic (bcc or B2) and martensite, respectively. The martensite phase consists of many variants, and each variant has a directional unit cell. In Fig. 1(b), for example, a unit cell of the martensite is illustrated as a box leaning in the positive or negative direction along the x-axis. Cells leaning in the same direction constitute a layer, and the direction of the lean alternates between layers. In this paper, the layer is called a variant, although a realistic variant is defined as a rather larger domain. The martensite phase is generated by cooling the B2 structure shown in Fig. 1(a). Randomly orientated variants are then generated, as shown in Fig. 1(b). When a shear load is imposed on this state, some of the layers change their orientation, as shown in Fig. 1(c). This structural change induces macroscopic deformation. When the external shear load is released, the strain does not return to the original state except for slight elastic recovery. When the specimen is heated to the transformation temperature, the martensite transforms into the B2 structure, and martensite appears again with cooling of the specimen. Since the B2 structure is cubic, the shape of the unit cell is independent of the orientation of the martensite layers. Therefore, the specimen macroscopically regains its original shape. This mechanism is well known but has not been fully verified since direct observation of dynamic behavior in a wide range of temperatures is difficult. Therefore, computer simulation is expected to provide evidence for and further extend the mechanism. The molecular dynamics method has become a powerful and effective tool to investigate material properties and dynamic behavior on an atomistic scale, and it has also been applied in the case of SMAs. The stable structure of Ni3Al, for instance, was investigated by Foiles and Daw (Foiles & Daw, 1987), Chen et al. (Chen et al., 1989) using an interatomic potential based on the embeddedatom method (EAM) with suitable parameters (Daw & Baskes, 1984; Foiles et al., 1986). The phase stability and transformation between B2 and martensite structures in NiAl was also 1