Incommensurate modulated structure and its influence on the martensitic transformation temperature span of single phase multielement Ni-Cu-Co-Mn-Ga two-way shape memory single crystals

Abstract

Significant two-way shape memory effect (TWSME) was achieved in single crystals of single-phase multielement Ni42–xCu8CoxMn37Ga13 (8 ≤ x ≤ 12) alloys by performing thermomechanical training. However, anomalous dependence of the martensitic transformation temperature span on Co content was observed. Before training, quite a narrow temperature span of the martensitic transformation, nearly independent of the Co content, was observed in all single crystals. After training the temperature span was still narrow for 8 ≤ x ≤ 10.9 but was obviously expanded for 10.9 < x ≤ 12. High-resolution transmission electron microscopy revealed that at the atomic scale, there exists incommensurate modulated structure in the single phase single crystals, as evidenced by nonperiodic satellite spots in the selected area electronic diffraction patterns. Moreover, the modulated wave vector of the satellite spots was increased by higher Co contents. Combining first principal calculations it was considered that the incommensurate modulated structure originates from the formation of Co–Co pairs. After training arrays of ordered dislocations with the same Burgers vector were introduced for 8 ≤ x ≤ 10.9 but the network of dislocations was formed for 10.9 < x ≤ 12. Based on analysis of transmission electron microscopy, geometric phase, thermodynamics, and Landau theory, it was considered that the austenite/martensite phase interface was pinned by the network of dislocations, expanding the temperature span of the martensitic transformation. This work supplies new insights for understanding the microstructure and martensitic transformation of Ni-Mn-Ga-based alloys.