Martensitic phase transformation induced large elastic lattice strains in Nb3Sn lamellae in NiTi-Nb3Sn eutectic composite

It is known that large elastic lattice strain, which is a measure of the distortion of the interatomic bonds in solids, can influence the many physical and chemical properties of materials. However, it is not as easy to achieve ultralarge elastic strains close to their theoretical limits in bulk engineering materials as it is for nanomaterials. A breakthrough was made in a bulk NiTi-Nb composite in which Nb nanowires were induced to sustain ultra-large elastic lattice strains up to 6.5% by the martensitic transformation of the NiTi matrix via the principles of "lattice strain matching" and "collective atomic load transfer". This study was conducted to trial out these novel concepts in an in situ NiTi-Nb3Sn eutectic composite system. The intermetallic A15-Nb3Sn is a very brittle compound and is widely used as a superconductor. Therefore, this study offers three new aspects: (i) it tests out the applicability of the principle of lattice strain matching for inducing ultra-large elastic strains in brittle materials, (ii) it investigates a material of high potential use, and (iii) it explores a novel fabrication approach for in-situ composites via eutectic solidification. The Nb3Sn lamellae in the composites were found to achieve large elastic lattice strains up to -2.4%, which are an order of magnitude higher than what may be expected of free-standing Nb3Sn or 3-4 times what have been achieved in Nb3Sn microfilaments embedded in other metal matrices, demonstrating the extraordinary effect of the NiTi martensitic transformation matrix. The micromechanical behaviours of lattice strain evolutions of the two phases were investigated by in-situ synchrotron x-ray diffraction analysis.