Cyclic pseudoelastic behavior of friction stir processed NiTi shape memory alloy: Microstructure and W-alloying

Abstract

A Ni_49.2Ti_50.8 (nitinol) shape-memory alloy was prepared using a vacuum re-melting furnace. The surface of the alloy was modified, and tungsten surface alloying was applied using the friction stir processing (FSP) technique, with varying tool rotational speed as the crucial processing parameter. The aim was to engineer microstructural evolutions and phase transformations in surface-modified and alloyed NiTi materials and assess the developments in mechanical properties. The research focused on the pseudoelastic behavior of processed shape memory materials upon cyclic loading and unloading tensile experiments, depending on microstructural details and alloy design. The electron backscatter diffraction (EBSD) analysis of stirred zones of NiTi and NiTi/W alloys revealed significant grain structural refinement undergoing the operative dynamic recrystallization (DRX) mechanism and the formation of an equiaxed microstructure with an average size of less than 9.6 μm, with a preferred substantial shear orientation majority. The hardness of NiTi alloy increased up to ∼344 Vickers by friction stir modification. Implementing FSP under the rotational speed of 800 rpm and traverse velocity of 50 mm/min yielded the best pseudoelastic properties for the modified alloy up to around 8 % strain, concerning the 10 % strain response of the primary cast alloy before modification. Meanwhile, applying thermal aging at an optimized temperature of 500 °C for 2 h improved the pseudoelastic behavior of the FSP-treated alloy up to ∼20 % strain, compared to the upgraded value of ∼14 % strain for the base alloy after the same post-heat treatment. The micro-mechanisms beyond such evaluation trends in pseudoelastic properties depending on the thermo-mechanical action of FSP were discussed and clarified.