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Nickel-titanium shape memory alloys: fracture and fatigue properties

Abstract:

The extraordinary high shape recovery properties of the near equiatomic Nickel-Titanium (NiTi) system were firstly discovered by accident at the National Ordance Laboratory in early 1970s, whose acronym provided the original trade name NiTiNOL. These shape recovery properties are due to a solid-solid reversible phase transition between two distinct crystal structures, the parent body centered austenite and the product orthorhombic or monoclinic martensite, known as thermoelastic martensite transformation (TMT). TMT can be trigged either by stress (SIM, stress-induced martensite) or temperature variations (TIM, thermally-induced martensite) between the so-called transformation temperatures of the alloy (TTs).

Thank to these phase transition mechanisms, NiTi alloys exhibit two interesting functional features at the macro scale, known as superelastic effect (SE) and shape memory effect (SME). In particular, SE is linked to SIM and represents a very large pseudo-elastic response of the material whereas SME defines the thermal recovery properties attributed to TIM.

Starting from the early successful applications in the medical field, NiTi alloys have been becoming exceptional candidates in several industrial application due to the unique combination of functional and structural properties as well as to the good corrosion resistance and chemical stability. In such applications both the SE and SME are increasingly exploited to develop smart components with active and/or tunable functional responses, incorporating load bearing capabilities typical of high strength metals. Unfortunately, standard design procedures/methods based on solid mechanic theories cannot be directly applied to predict the complex thermo-mechanical response of SMAs, especially when dealing with crack formation and propagation mechanisms occurring during cyclic thermo-mechanical loading. This actually represent one of the major issues for widespread application of SMAs in several industrial fields.

Within this context, the seminar will focus on fracture and fatigue properties of NiTi alloys, highlighting the main microstructural features causing material damage, that is by considering the local stress and/or thermally-induced phase transition mechanisms that are mainly responsible for the crack formation and propagation. The unusual fracture and fatigue properties of NiTi alloys will be analyzed by ad-hoc theoretical models and experimental observations, taking into account their complex thermo-mechanical response.

Bio: 

Carmine Maletta is a professor of mechanical engineering at University of Calabria (Unical) - Italy. He got the degree in mechanical Engineering in 1999 and in 2006 he obtained the PhD in Structural Engineering at Unical. From 2000 to 2002 he was employed as a business consultant in the international consulting company Accenture. Since 2016 he has been a cooperation associate at the European Organization for Nuclear Research (CERN), within the context of a CERN-Unical research project on smart pipe coupling technologies based on shape memory alloy (SMA), where he serves as technical coordinator for the University of Calabria.

He is co-founder and CEO at 2SMArtEST S.r.l, an innovative start-up on SMA-based smart solutions and technologies established in 2019. He is a scientific coordinator of the Material testing and mechanical design laboratory at Unical and he serves as a scientific coordinator of several national and EU R&D projects.

The main research interests cover both numerical methods and experimental techniques in the fields of mechanical engineering and materials science. In particular, special focuses were devoted to fatigue and fracture of engineering materials and to the thermo-mechanical analysis of shape memory alloys. So far, Dr. Carmine Maletta has authored more than 100 research papers.