Study On Micro-deformation Mechanisms Of Ti-Ni-Hf Based High Temperature Shape Memory Alloys

In the paper, the Ti-Ni-Hf based alloy ribbons were fabricated by the meltspinning. The microstructures, transformation behaviors, mechanical properties andshape memory behaviors of Ti-Ni-Hf based alloy ribbons were systematicallystudied by the X-ray diffraction （XRD）, differential scanning calorimeter （DSC）,dynamic thermomechanical analysis （DMA）, micro-tensile tester and transmissionelectron microscopy （TEM）, respectively. The microstructure evolutions of theTi-Ni-Hf based alloy ribbons were introduced during tensile deformation and themicro-deformation mechanisms of the Ti-Ni-Hf based alloy ribbons were revealed.The results show the B19′martensites and （Ti,Hf）2Ni phase were obseved inthe annealed Ti-Ni-Hf based alloy ribbons at room temperature. The B19′martensitevariants show spear-like morphologies and its substructure mainly is （001）compound twins. While, many nano-scale grains and coarsen grains coexist in theribbons. With the annealing temperature increaes or annealing time extends, the（Ti,Hf）2Ni particles begin to precipitate at the grain boundaries of nanoscale grainsand in the coarsen grain interior, respectively. At the same time, the size of thenano-scale grains and （Ti,Hf）2Ni particles increase.The results of DSC test show that only single-stage stransformation B2B19′was observed in the annealed Ti-Ni-Hf based ribbons. The transformationtemperatures increase with the annealing temperature increases or the annealingtime extends. When the annealing temperature lowers than1073K or the annealingtime shorts than5h, there were more than one transformation peaks in the DSCcurves of the annealed Ti-Ni-Hf based ribbonsThe results of the shape memory behavior show that the recoveral strain wouldup to6.1%in the Ti₃₆Ni₄₁Hf₁₅Cu₈alloy ribbons annealed at773K for1h. When theannealing temperature is up to873K, the5.5%tensile strain would not fully recoverafter heating. The shape memory behavior of the Ti₃₆Ni₄₁Hf₁₅Cu₈alloy ribbonsreduces with the annealing temperature increases.The TEM results illustrate that when the Ti₃₆Ni₄₁Hf₁₅Cu₈alloy ribbonsannealed at500℃for1h are deformed to2%, the （001） compound twins begin toreorient. At this stage, the deformation is conducted by the movement of （001）twinning domain boundaries. When the ribbons are deformed to4%, the furtherreorientation would occur for the （001） compound twins. All the （001） compoundtwins are in an orientation and the detwinning of （001） compound twins is observedin some regions. When the strain reaches6%, the further detwinning of （001） compound twins is observed in many grains, even there are no twins can be found insome grains.The tensile deformation of the Ti₃₆Ni₄₁Hf₁₅Cu₈alloy ribbons annealed at500℃for1h mainly involves the elastic deformation of the B19′martensites, thereorientation and the following detwinning of （001） compound twins.