NiTi-Nb in-situ Nanowire Composite – Extending the Boundaries of Materials Elastic Strain Limit, Young’s Modulus and Strength

This paper reports the creation of a NiTi-Nb nanowire in-situ composite of remarkable mechanical properties. This composite is based on a new materials design concept of elastic strain coupling between the NiTi phase transforming matrix and metallic nanowires, Nb in this case. Freestanding nanowires generally have ultrahigh elastic strain limits (4–7%) and strengths (typically on the order of gigapascals), but harnessing their extraordinary intrinsic mechanical properties in bulk composite materials has proven to be challenging. This challenge is commonly known as a “valley of death” in nanocomposite design. In this study, the intrinsic mechanical properties of nanowires are exploited in a phase-transforming matrix based on the concept of elastic and transformation strain matching. By engineering the microstructure and residual stress to couple the elasticity of Nb nanowires with the pseudoelasticity of a NiTi shape-memory alloy, we have created an in situ composite that possesses a large quasi-linear elastic strain of over 6%, a low Young's modulus of 28 GPa and a high yield strength of 1.65 GPa. This elastic strain-matching approach allows the exceptional mechanical properties of nanowires to be harnessed in bulk materials. In this regard this work represents a breakthrough in nanowire composite design, by overcoming a long-standing challenge to materials scientists for over three decades. (This work has just been published on Science 339 (2013) 1191-1194.)