Anisotropic tensile and actuation properties of NiTi fabricated with selective laser melting

Moghaddam, Narges Shayesteh; Saghaian, Sayed Ehsan; Amerinatanzi, Amirhesam; Ibrahim, Hamdy; Li, Peizhen; Toker, Guher P.; Karaca, H.E.; Elahinia, Mohammad

doi:10.1016/j.msea.2018.03.072

“…While this level of superelasticity under compressive loading is of great interest, a next critical step is to evaluate the superelasticity of the as-fabricated Ni-rich NiTi alloy under tension. In a recently published paper 63 , however, it has been shown that the SLM NiTi alloy presents premature failure due to the presence of numerous un-melted powders concentrated in the edges of the specimens, which could act as crack initiation sites. Should this problem be successfully addressed, a higher level of recoverable strain under tension is expected, probably 1.5 times larger due to the differences in deformation mechanisms with loading direction and the unidirectional nature of twin deformation 64–68 .…”

Section: Discussionmentioning

confidence: 99%

Achieving superelasticity in additively manufactured NiTi in compression without post-process heat treatment

Moghaddam

¹

,

Saedi

²

,

Amerinatanzi

³

et al. 2019

Sci Rep

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Shape memory alloys (SMAs), such as Nitinol (i.e., NiTi), are of great importance in biomedical and engineering applications due to their unique superelasticity and shape memory properties. In recent years, additive manufacturing (AM) processes have been used to produce complex NiTi components, which provide the ability to tailor microstructure and thus the critical properties of the alloys, such as the superelastic behavior and transformation temperatures (TTs), by selection of processing parameters. In biomedical applications, superelasticity in implants play a critical role since it gives the implants bone-like behavior. In this study, a methodology of improving superelasticity in Ni-rich NiTi components without the need for any kind of post-process heat treatments will be revealed. It will be shown that superelasticity with 5.62% strain recovery and 98% recovery ratio can be observed in Ni-rich NiTi after the sample is processed with 250 W laser power, 1250 mm/s scanning speed, and 80 µm hatch spacing without, any post-process heat treatments. This superelasticity in as-fabricated Ni-rich SLM NiTi was not previously possible in the absence of post-process heat treatments. The findings of this study promise the fast, reliable and inexpensive fabrication of complex shaped superelastic NiTi components for many envisioned applications such as patient-specific biomedical implants.

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“…This was associated with the changes in the chemical composition of the materials with the energy input as a result of Ni evaporation in melt pool, reported also by other authors (Ref 13 7). The superelasticity of NiTi alloys in the as-deposited state is moderate-they exhibited a recoverable strain of about 2-3% (Ref [16][17][18]. This results mainly from the wide temperature range of the martensitic transformation observed for alloys in as-deposited state, associated with the formation of Ni-rich precipitations during the deposition and slight fluctuation of chemical composition of the matrix (Ref 16).…”

Section: Introductionmentioning

confidence: 99%

Microstructure, Mechanical Properties, and Martensitic Transformation in NiTi Shape Memory Alloy Fabricated Using Electron Beam Additive Manufacturing Technique

Rogal

¹

,

Kalita

²

,

Kawałko

³

et al. 2021

J. of Materi Eng and Perform

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The electron beam additive manufacturing (EBAM) method was applied in order to fabricate rectangular-shaped NiTi component. The process was performed using an electron beam welding system using wire feeder inside the vacuum chamber. NiTi wire containing 50.97 at.% Ni and showing martensitic transformation near room temperature was used. It allowed to obtain a good quality material consisting of columnar grains elongated into the built direction growing directly from the NiTi substrate, which is related to the epitaxial grain growth mechanism. As manufactured material showed martensitic and reverse transformations diffused over the temperature range from −10 to 44 °C, the applied aging at 500° C moved the transformation to higher temperatures and transformation peaks became sharper. The highest recoverable strain of about 3.5% was obtained in the as-deposited sample deformed along the deposition direction. In the case of deformation of the alloy aged at 500 °C for 2h, the formation of martensite occurs at significantly lower stress; however, at about 2.5% the stress begins to increase gradually and only a small shape recovery was observed due to a higher martensitic transformation temperature. In situ SEM tensile deformation in the direction perpendicular to deposition direction showed that the martensite began to appear at the surface of the sample and at the grain boundaries due to heterogeneous nucleation. In situ studies allowed to determine the following crystallographic relationships between B2 and B19’ martensite: (100)B2||(100)B19’ and (100) B2 || (011)B19’; (011)B2|| (001)B19’ and $${(011)}_{\mathrm{B}2}||{\left(11\bar{1 }\right)}_{\mathrm{B}1{9}^{\mathrm{^{\prime}}}}$$ ( 011 ) B 2 | | 11 1 ¯ B 1 9 ′ . Samples aged at 500 °C exhibited fully austenitic microstructure; however, with increasing degree of deformation, the formation of martensite was observed. The majority of needles were tilted about 45° with respect to the tensile direction, and the presence of type I (11 $$\bar{1 }$$ 1 ¯ ) invariant twin boundaries was observed at higher degrees of deformation.

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“…ADDITIVE MANUFACTURING FROM THE POINT OF VIEW OF MATERIALS… Mechanically produced powder Ni-rich Ni-Ti alloys required aging treatment [91] Solutionized and aged Ni-Ti had a better shape memory response [92] Ni-rich Ni-Ti showed superelastic behavior [93] Shape memory effect recovery and unstable oriented/de-twinned martensite [94] NiTi50 showed excellent mechanical properties as compared to NiTi45 and NiTi55 [95] Gas atomized powder Using low scanning speed reduced the obtained shape memory effect [96] Superelastic behavior [96] Loss of nickel in the process [97] L-PBF High relative density (>97%) and hardness reported [98,99] Excellent compression fatigue resistance [100] with an irreversible stain behavior [99] Wider hatch distance decreased relative density [101] Superelastic response (95%) [102][103][104][105][106][107][108] Shape memory effect [94,105,109,110] Recovery above 5.5% [106][107][108]111] Desired stiffness was achieved by regulating the level of porosity and stiffness reduced from 69 GPa to 20.5 GPa for 58% porosity [112] Loss of nickel in the process [113,114] Wider hatch spacing led to a highly irrecoverable strain [114] Microstructure influenced the shape memory response and mechanical behavior [109,115] Martensite twins were formed easily after annealing process [116] Heat treatment above 400 °C decreased the shape recovery and transformation strain [117] Tens...…”

Section: Additive Manufacturing Of Stimuli-responsive Materialsmentioning

confidence: 99%