Mechanisms of nanocrystallization and amorphization of NiTiNb shape memory alloy subjected to severe plastic deformation

Abstract

Abstract NiTiNb shape memory alloy (SMA) has been widely used for pipe coupling because it possesses the wider phase transformation hysteresis. In the present study, Ni 47 Ti 44 Nb 9 (at.%) SMA, which is composed of B2 austenite phase and β-Nb phase, was subjected to local canning compression at the deformation degrees of 25%, 50% and 75%, respectively. Local canning compression causes Ni 47 Ti 44 Nb 9 SMA to be in a three-dimensional compressive stress state, which contributes to severe plastic deformation of the involved alloy. Severe plastic deformation leads to nanocrystallization and amorphization of Ni 47 Ti 44 Nb 9 SMA. Dislocation slip plays a dominant role in plastic deformation of Ni 47 Ti 44 Nb 9 SMA. Physical mechanisms for nanocrystallization and amorphization of Ni 47 Ti 44 Nb 9 SMA were revealed on the basis of statistically stored dislocation (SSD) and geometrically necessary dislocation (GND). As a soft phase, β-Nb phase is subjected to plastic yield more easily than B2 austenite phase. However, nanocrystalline phase and amorphous phase are more easily induced in B2 austenite phase than in β-Nb phase. The dislocations emitted from β-Nb phase move toward the interface between β-Nb phase and B2 austenite phase and consequently pile up at the corresponding interface, which plays a significant role in nanocrystallization and amorphization of Ni 47 Ti 44 Nb 9 SMA subjected to severe plastic deformation. The formation of nanocrystalline phase deals with the transformation from low angle grain boundary to high angle grain boundary. Nanocrystalline phase can be viewed as a transition from coarse grain to amorphous phase. The occurrence of amorphous phase is attributed to the fact that a high density of dislocations are induced during severe plastic deformation of Ni 47 Ti 44 Nb 9 SMA.