PSI - Issue 69

Muhammad Asim et al. / Procedia Structural Integrity 69 (2025) 41–46

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1. Introduction The unique properties of shape memory alloys (SMA) make them useful in numerous fields, including aviation, medicine, robotics, and construction. Typically, SMA can exhibit either the shape memory effect (SME) or superelasticity (SE), depending upon its specific composition, deformation temperature, and thermal treatment. SE in SMA possesses the ability to induce substantial levels of reversible deformation without the need for external stimulus [1]. NiTi-based compositions, such as NiTi and NiTiHf, [2] have been extensively researched and widely used in various applications, while other materials systems like Co and Cu-based alloys with different elemental compositions have also demonstrated shape memory properties [3]. Despite the extensive utilization of NiTi, particularly within the medical domain [4], concerns regarding biocompatibility remain because ofthe potential release of the harmful Ni element into the human body [5]. TiNb-based SMAs have been developed to address this limitation and offer mechanical properties that are equivalent to existing materials while also exhibiting improved biocompatibility [6], [7]. Zirconium is added to enhance the superelastic stress-strain curve of Ti-Nb Alloy, and Sn is added for enhanced biocompatibility [8], [9], [10]. However, these alloys have a limitation related to their functional fatigue properties[11]. When subjected to cyclic loading, the superelastic nature degrades which leads to loss of functionality. Various efforts have been dedicated to enhance the functional fatigue properties by optimizing the elemental composition, thermomechanical treatment, and grain size to limit SE strain degradation [12] [13], [14]. Laser Shock Peening (LSP) is a widely accepted technique to enhance the fatigue life of metallic materials. The structural fatigue in metallic materials is mainly attributed to the initiation of cracks at the surface that propagate to the bulk leading to fracture. LSP introduces compressive residual stresses in the material that counteracts with the tensile stresses generated in practical applications, thus enhancing the fatigue life of metallic components [15]. This research aims to study and evaluate the deterioration of SE strains in the material under cyclic loading. Moreover, this study aims to explore potential methods for improving functional fatigue properties of SMAs by utilizing Femtosecond laser shock peening (F-LSP). 2. Material and Experimental Setup Ingots of composition Ti 67 Zr 19 Nb 11.5 Sn 2.5 (at. %) were casted and the purity level of each element was > 99.5 wt.%. A homogenization treatment was carried out on all the casted ingots at 1100 °C for 24 hrs in an inert Ar atmosphere. Rectangular strips were machined out of the homogenized ingots using EDM and subjected to severe cold rolling leading to 95% reduction in thickness. Subsequently, dog-bone tensile specimens (8 mm gauge length and a cross section of 3 x 0.3 mm 2 ) were machined from the rolled strips. All samples were prepared with the tensile axis aligned to the rolling direction. However, it should be noted that based on EBSD measurements, no significant crystallographic texture was observed in the microstructure following the homogenization heat treatment. To prevent oxidation during heat treatment, the samples were encapsulated in quartz tubes, evacuated and filled with Argon. For laser processing, the femtosecond laser system was utilized. The Femtosecond laser system can produce laser pulses with an average power of up to 150 Watts, with a pulse duration between 7 fs to 300 fs (1030 nm wavelength), and a repetition rate that can be tuned from 50 kHz to 150 kHz. A detailed representation of the system's fundamental configuration is shown in Figure 1(b) . To achieve the highest possible level of energy concentration at the surface of the SMA specimen, the focused laser beam was directed onto the surface of the specimen. The translation stage makes it easier to achieve precise alignment of the beam and the specimen, which in turn makes it possible to exercise effective control over the laser-treated area and the parameters of laser processing. The range of the laser parameters that were explored is shown in Figure 1 (a). The samples were immersed in de-ionized water for the entrapment of shockwaves to be directed inside the sample and also to minimize the surface detriment of the material caused by the laser processing. Multiple experiments were conducted by varying laser power, scanning speed, and line spacing to see the effect of the laser parameters combination on the functional response of the alloy.