Influence of Cold Rolling and Subsequent Annealing Processes on the Functional Fatigue Properties of High Temperature NiTiHf Shape Memory Alloys

Abstract

Shape memory alloys are very special alloys that can be used as actuators in various applications due to their capability of returning to a prearranged shape via heating under applied load. Their high power per weight ratio makes them an important alternative to conventional pneumatic, hydraulic and motor kind of solid-state heavy actuators, primarily in the aerospace industry. The majority of applications in the aerospace industry demand continuous actuation with transformation temperatures exceeding 100 °C. To judge the feasibility of adopting shape memory alloys in industrial applications, the stability of shape memory properties in terms of actuation strain and transformation temperatures at high temperatures during operational life time is essential. Therefore, shape memory alloys, which have transformation temperatures higher than approximately 100°C, are being widely studied for their utilization in high-temperature applications. Adding hafnium to NiTi binary alloys is one of the most effective ways to create a High Temperature Shape Memory Alloy (HTSMA). HTSMAs undergo thermal, mechanical or thermomechanical treatments to enhance their actuation abilities and stability of shape memory properties and make them a valid option for actuation applications. In this study, the effects of cold rolling with different reduction thicknesses and subsequent annealing conducted at different temperatures on the functional fatigue properties Ni50.1Ti19.9Hf30 (at%) were investigated. The material utilized in this work was produced by vacuum induction melting using high purity Ni, Ti, and Hf elements and the cast billet was hot extruded at 900°C for achieving chemical homogeneity. To analyze the influence of the cold rolling and subsequent annealing on the functional parameters, all of the samples were homogenized to get rid of any texture formation in the material. The first set of samples was cold-rolled by 2% and the second set was cold-rolled by 5%. Then, three samples in each set were annealed at the following different temperatures: 500, 550, and 600 °C. In each set, one sample was kept in cold rolled condition. The transformation temperatures of all samples were first examined by Differential Scanning Calorimetry (DSC). Afterward, the samples with the selected conditions were thermally cycled under 300MPa with a 15°C/sec heating cooling rates to determine the functional fatigue properties such as the evolution of fatigue life, actuation strains, accumulated irrecoverable strains, and transformation temperatures. DSC measurements showed that an increase in cold work percentage reduced the transformation temperatures since cold work led to an increase in dislocation density and thus, suppressed the phase transformation. On the other hand, transformation temperatures increased as the annealing temperature was increased since annealing caused partial annihilation of dislocations, thus reduced dislocation density in the sample. Functional fatigue test results revealed that cold rolling with subsequent annealing stabilized the shape memory parameters and increased the actuation strains since annealing removed the rolling induced dislocation with the internal stress fields in the matrix and increased the amount of transforming volume. However, 30at% of hafnium content made the material extremely hard to deform, and micro-cracks were induced during cold rolling and these micro-cracks were propagated fast during functional fatigue tests and shortened the fatigue lives. Additionally, Ni50.1Ti19.9Hf30 (at%) alloy showed very high plastic deformation with 300MPa of loading since full austenite transformations of the alloy can be achieved at approximately 600°C under this stress magnitude.