International Journal of Lightweight Materials and Manufacture (Dec 2020)
Microstructure evolution of Ni47Ti44Nb9 shape memory alloy in high-temperature deformation
Abstract
An important and efficient way to improve the memory performance potential of shape memory alloys (SMAs) is by tailoring the microstructure such as grain refining in thermal deformation working. To explore the microstructure evolution and its mechanism under different thermal-mechanical conditions, the hot deformation behavior and constitutive relation of Ni47Ti44Nb9 (at.%) shape memory alloy are investigated by conducting hot compression tests under the temperature range of 700–1000 °C, the strain rate range of 0.001–1 s−1 and different strains of 0.22, 0.5 and 0.9, respectively. The microstructural evolution is characterized by grain orientation spread (GOS) map and misorientation angle (MA) which can reflect the microstructure information such as grain size, grain boundary, sub-grain boundary and quantified orientation statistics. The major findings include: (1) The self-heating effect of Ni47Ti44Nb9 alloy occurs due to the excessive strain rate. The highest temperature rise (ΔT) is 39.7 °C under the compression temperature of 700 °C which can promote dynamic softening. The ΔT decreases with the increase of deformation temperature and its effect on the dynamic softening decreases; (2) The established strain-dependent Arrhenius-type conservative model well predict the hot deformation behavior of Ni47Ti44Nb9 alloy, and the maximum relative error between the experimental value and the calculated value is about 5.4%. Due to dynamic softening, the thermal activation energy (Q) decreases with the increase of strain; (3) The discontinuous dynamic recrystallization (DDRX) mainly occurs in the initial stage of strain. With the increase of deformation, the deformation of eutectic structure weakens the strain energy between adjacent grains and inhibits the further occurrence of DDRX. The continuous dynamic recrystallization (CDRX) becomes the main nucleation mechanism and grain refinement mechanism, the grain size of Ni47Ti44Nb9 alloy reduced from initial 100 μm to 10–40 μm after strain 0.9 under 900 °C and 0.01 s−1 hot deformation.
