Abstract
This work aims at contributing to the development of a revolutionary technology based on shape memory alloy (SMA) coatings deposited on‐site to large‐scale metallic structural elements, which operate in extreme environmental conditions, such as steel bridges and buildings. The proposed technology will contribute to improve the integrity of metallic civil structures, to alter and control their mechanical properties by external stimuli, to contribute to the stiffness and rigidity of an elastic metallic structure, to safely withstand the expected loading conditions, and to provide corrosion protection. To prove the feasibility of the concept, investigations were carried out by depositing commercial NiTinol Ni50.8Ti (at.%) powder, onto stainless steel substrates by using high‐velocity oxygen‐fuel thermal spray technology. While the NiTinol has been known since decades, this intermetallic alloy, as well as no other alloy, was ever used as the SMA‐coating material. Due to the influence of dynamics of spraying and the impact energy of the powder particles on the properties of thermally sprayed coatings, the effects of the main spray parameters, namely, spray distance, fuel‐to‐oxygen feed rate ratio, and coating thickness, on the quality and properties of the coating, in terms of hardness, adhesion, roughness, and microstructure, were investigated.
Highlights
Ni-Ti shape memory alloys are extremely interesting materials both for their ability of showing the shape memory effect (SME) and for their elevated strength and ductility [1].An attractive Ni-Ti-based SMA is NiTinol, a nearly equiatomic intermetallics of nickel and titanium
Results demonstrate that Rockwell Hardness 15 N decreases monotonically by spray distance, decreases up to a plateau value with kerosene to oxygen feed rate ratio up to 22 l/h-860 l/min, whereas with coating thickness, a uctuating behavior consisting of an initial increase up to 0.3 mm thick coating followed by a decrease thereon is noted. e maximum hardness value of 89 is attained for a spray distance of 300 mm and a kerosene to oxygen feed rate ratio of 20 l/h-800 l/min, where the coating thickness is 0.3 mm
It is therein observed that adhesion strength decreases with spray distance while a uctuating behavior consisting of an initial decrease followed by increase is noted with kerosene to oxygen feed rate ratio. e maximum tensile adhesion strength corresponds to sample “E” having been thermally sprayed from a distance of 300 mm with the highest combustion energy stemming from a kerosene to oxygen feed rate ratio equal to 25 l/h-900 l/min
Summary
Ni-Ti shape memory alloys are extremely interesting materials both for their ability of showing the shape memory effect (SME) and for their elevated strength and ductility [1].An attractive Ni-Ti-based SMA is NiTinol, a nearly equiatomic intermetallics of nickel and titanium. Ni-Ti shape memory alloys are extremely interesting materials both for their ability of showing the shape memory effect (SME) and for their elevated strength and ductility [1]. In common with other SMAs, NiTinol shows two important mechanical features: shape memory effect and pseudoelasticity. E former is related to fatigue and fracture resistances of an alloy and consists in its ability to return to its initial shape upon heating to the austenite phase (high-temperature phase having B2 cubic structure) after having been deformed in the martensite phase (low-temperature monoclinic phase) [3,4,5]; the latter is due to the stress-induced martensitic transformation upon loading and the subsequent strain recovery upon unloading at temperatures above the austenite temperature Af [1, 6]. NiTinol’s ability to undergo a thermal- or stressinduced martensitic phase transformation and its recoverable strains that are much greater than those in traditional alloys, between 8% and 10% [3, 7], make it the most popular shape memory alloy [3].
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