Mechanism for the Pseudoelastic Behavior of FCC Shape Memory Nanowires

Abstract

Pseudoelasticity and shape memory have been recently discovered in single-crystalline FCC nanowires of Cu, Ni, Au and Ag. The deformation mechanism responsible for this novel behavior is surface-stress-driven reorientations of the FCC lattice structure. A mechanismbased continuum model has been developed for the lattice reorientation process during loading through the propagation of a single twin boundary. Here, this model is extended to the nucleation, propagation and annihilation of multiple twin boundaries associated with the reverse reorientation process during unloading. The extended model captures the major characteristics of the loading and unloading behavior and highlights the dominating effect of the evolution of twin boundary structure on the pseudoelasticity.

Surface-induced structural transformation in nanowires

“…After reorientation, the square cross-section of the nanowires changes into a rhombic shape. This surfaceinduced structural reorientation has been observed from Ni [68,70], Au [20,23,68,70], Pd [67], Ag [66,70], and Cu [14,[68][69][70] nanowires. The difference in the reorientation mechanisms stems from the difference in the unstable stacking fault energy (USFE) and orientation dependence of the electron density when different atom potentials are used [20].…”

“…Hence, surface effects are dominant factors affecting the structure of nanowires and can even induce thorough structural transition. On the heels of these discoveries, other interesting materials properties such as surfaceinduced lattice reorientation, martensitic phase transformation (MT), pseudo-elastic behavior, and shape memory effects (SMEs) have been studied and observed from fcc [14,[19][20][21][22][23]28,[63][64][65][66][67][68][69][70][71][72][73][74][75], bcc [76][77][78][79][80][81][82], and hcp [83,84] single-element, layered composite [85][86][87][88][89], intermetallic alloy [90][91][92][93][94][95], and even metal oxide [96][97][98][99][100] or nitride [101] compound nanowires. The reversible strain can be as high as 40-70% and is much lar...…”

“…Liang et al [70,72,159] have proposed a continuum model to describe the lattice reorientation process during loading and unloading. In this model, the lattice reorientation is decomposed into two parts, a reversible, smooth transition between a series of phase-equilibrium states and a superimposed irreversible, dissipative process.…”

“…18 stress-strain curves almost coincide with those derived from MD simulation, suggesting that this model can derive the major characteristics of structural reorientation, particularly the size and temperature effect. The continuum model has been extended to the study of nucleation, propagation, and annihilation of multiple twin boundaries in reverse reorientation under unloading [70]. Although the model can disclose the main features of pseudoelasticity and SMEs of metal nanowires, it is difficult to determine the dynamic processes and propagation velocity of the twinning boundaries because of the lack of an accurate kinetics relationship [72].…”

Deformation mechanisms and pseudoelastic behaviors in trilayer composite metal nanowires

“…[11][12][13][14][15] During tensile loading, at a critical-resolved shear stress a leading 1 / 6͗112͘ partial dislocation nucleates at the surface and propagates in the nanowires. For sizes larger than a critical value, a trailing partial emits on the same plane resulting in the formation of a full dislocation and leading to permanent deformation ͑slip͒.…”