The influence of cold and warm rolling on the thermo-mechanical behavior of near-equiatomic NiTiHf20 high-temperature shape memory alloyWednesday (16.05.2018) 09:25 - 09:50 Part of:
The effects of cold and warm rolling on the thermo-mechanical stability of a near-equiatomic NiTiHf20 high-temperature shape memory alloy (HTSMA) were investigated. 15% cold rolling ratio was achieved at room temperature without noticeable cracks and intermediate annealing. Some of the cold rolled samples were subsequently annealed at various temperatures (450°C – 600°C) for 30 minutes. Differential scanning calorimetry (DSC) and Vickers microhardness experiments revealed that 550°C annealing results in the best transformation stability and high hardness values as compared to the initial wrought alloy. Warm rolling experiments with different thickness reductions were carried out between 300°C and 700°C. Scanning electron microscopy (SEM) investigation and Wavelength Dispersive Spectroscopy (WDS) analyses revealed that the alloy contains noticeable amount of Ni(Ti+Hf)2 type particles. DSC analyses showed that both cold rolling and warm rolling caused decrease in all transformation temperatures, except for the 700°C rolling condition, since dislocations caused by the plastic deformation prevent the propagation of phase interfaces. Cold rolled and annealed, and warm rolled specimens were subjected to incremental- and constant-stress thermal cycling tests. The effect of different rolling conditions on the recoverable transformation strains and irrecoverable strains under tensile loadings were evaluated. While rolling led to increase in dimensional stability, recoverable strains dropped as compared to that of the as-received sample. The best actuation response was obtained in the 15% cold rolled and 550°C, 30 min annealed sample which exhibited comparable recoverable transformation and much lower irrecoverable strain as compared to the as-received sample. This particular condition resulted in only 1.40% cumulative irrecoverable strain under 200 MPa tensile stress after 100 thermal cycles.