Finite Element Analysis of a Copper Single Crystal Shape Memory Alloy-Based Endodontic Instruments

Stress induced martensitic transformations and phases stability in Cu–Al–Be shape-memory single crystals

Recent experimental results on the deformation field of single crystal CuNiAl shape memory alloys (SMA) by using Moire´ interference technique are reported and two kinds of austenite-martensite (A-M) interfaces with different deformation features are identified. The experimental discovery questioned the invariant plane hypothesis used in crystallographic theory of martensitic transformation. Some fundamental issues on the property of A-M interface and the related micro- and macro-deformation features are discussed.

Rate dependent damping of single crystal CuAlNi shape memory alloy

NiTi shape memory alloy is a wonder material that is a solution looking for problems. The material finds wide biomedical applications like endodontic files for root canal treatment and cardiovascular stents. This material has rendered the surgical procedure simple compared to that with the existing Stainless Steel (SS) or titanium ones. NiTi as an endodontic file would cause less discomfort to the patients in comparison to that with far stiffer SS or titanium ones. Here nearly equi-atomic 50:50 commercial NiTi rods were subjected to low temperature aging at 300 to 450°C. The wear resistance of the as-received and the heat-treated samples was studied using adhesive wear tests on hardened steel counter face. Abrasive wear tests were run against Alumina disc to simulate the working of endodontic drills and files against dental hard and soft tissues. The abrasive wear resistance is expected to be proportional to the Vickers Hardness of the material and is high for the 450°C heat-treated sample. A correlation between the mechanical properties and microstructures of this material is attempted

Determination of the interaction energy in the martensitic state

The electro-discharge machining (EDM) characteristics of TiNi shape memory alloys (SMAs) have been investigated in this study. Experimental results show that the material removal rate of TiNi SMAs in the EDM process significantly relates to the electro-discharge energy mode, involving the pulse current IP and pulse duration τP. It also has a reverse relationship to the product of the melting temperature and thermal conductivity of TiNi SMAs. In addition, a longer pulse duration τP and a lower pulse current IP should be selected to have a precise EDM machining of TiNi SMAs. Many electro-discharge craters and re-cast materials are observed on the EDM surface of TiNi SMAs. The re-cast layer consists of the oxides TiO2, TiNiO3 and the deposition particles of the consumed Cu electrode and dissolved dielectric medium. The thickness of the re-cast layer initially increases, reaches a critical value, and then decreases with increasing pulse duration τP. The specimen's hardness near the outer surface can reach 750 Hv for EDM TiNi SMAs. This feature originates from the hardening effect of the re-cast layer. The EDM TiNi SMAs still exhibit a nearly perfect shape recovery at a normal bending strain, but a slightly reduced shape recovery at a higher bending strain due to the depression of the re-cast layer. All the Ti49Ni51, Ti50Ni50 and Ti50Ni40Cu10 SMAs exhibit similar EDM characteristics although they have different crystal structures and mechanical properties at room temperature.

Ultra-high mechanical damping during superelastic bending of micro pillars and micro beams in Cu-Al-Ni shape memory alloys

Shape memory alloy cables have emerged as an alternative to conventional steel cable restrainers for preventing the bridge spans from unseating during an extreme earthquake. Feasibility of high-cost NiTi shape memory alloy restrainers in retrofitting the bridges has been numerically investigated, and promising results have been published; however, considering the economic impacts, the effect of different types of shape memory alloy such as Cu-based and Fe-based shape memory alloy restrainers has not been discussed yet. The objective of this study is to address this problem in detail in order to propose the most cost-effective shape memory alloy restrainer suitable for bridge engineering applications. Seismic fragility and life-cycle loss (both direct and indirect) assessments are analytically performed on an isolated simply-supported highway bridge retrofitted by four types of shape memory alloy restrainers (i.e. NiTi, FeNiCoAlTaB, CuAlMn, and FeMnAlNi). Results showed that for all retrofitted bridges performed in the range of design displacement, the effect of type of shape memory alloy is significant on the damage probability and long-term seismic loss of the bridges. All the bridges retrofitted with shape memory alloy restrainers have a very low probability of collapse (less than 7%). It is also found that the bridge retrofitted with Fe-based shape memory alloy restrainers (SMA-II and SMA-IV) performed better as compared to the other cases. Compared to the bridge without restrainers and with NiTi shape memory alloy restrainers, Fe-based shape memory alloy restrainers can reduce the long-term loss by about 87% and 11%, respectively, at the design earthquake event specified in CHBDC-2014. The probabilistic risk analysis of highway bridges retrofitted with shape memory alloy restrainers can aid in paving the way toward widespread application of such smart materials in structural applications.

The use of shape memory alloys for micro-actuators constitutes a field of application in which copper–aluminum-based alloys find their usefulness because they can reach higher activation temperatures and are easier to produce than titanium-based alloys, particularly by the method proposed in this work. SMA tapes are a two-dimensional structure that offers many design options such as stamping, punching, and deep drawing, but they are also suitable for laser cutting, engraving, stamping, and EDM machining. This work has been made to study the manufacture of copper-based shape memory alloys (SMAs) using the cold co-rolling process also called the cold-roll bonding (CRB) process. In this process, a thin metal sandwich can be produced with a rolling machine. This sandwich consists of layers of CuNiBe master alloy and Al. During the rolling phase, the sandwich has no shape memory effect (SME) or superelastic effect (SE), so thin strips can be easily produced. After the rolling phase, the sandwich is subjected to a complex heat treatment to gain the SME. To validate this process to produce Cu-based SMAs, several alloys with different CuAlNiBe compositions have been tested. The SMAs obtained were characterized by optical microscopy, scanning electron microscopy (SEM) and X-ray diffraction (XRD) techniques. The martensitic transformation was studied by Differential Scanning Calorimetry (DSC) and SME and SE were studied by three-point bending tests. This work shows that the CRB is a good process for making a wide variety of Cu-based SMA ribbons.

Full-field stress and strain measurements revealing energy dissipation characteristics in martensitic band of Cu-Al-Mn shape memory alloy

The examination of solid state processes leading to the degradation of the shape memory behaviour is essential with respect to the suitability of shape memory alloys. Besides degradation processes occurring during relatively long periods of time called ageing, bainitic reactions that suddenly degrade the shape memory behaviour were also observed in many Cu-based shape memory alloys. The mechanisms and effects of the bainitic reactions on the shape memory characteristics were investigated in many Cu-based systems, but the kinetic of the reaction was not examined so far. In the present paper, an examination was carried out on a CuAlNiMn and a CuAlNiMnFe shape memory alloy to reveal what kinetic model describes the bainitic reaction occurring and thus completely destroying the shape memory effect during one stage of heating.

A finite element analysis of evolution of stress–strain and martensite transformation in front of a notch in shape memory alloy NiTi

In this paper, a methodology based on three-dimensional (3D) finite element procedures is developed and used to simulate the superelastic behavior and the shape memory effect (SME) of shape memory alloys (SMAs). A 3D constitutive model for SMAs is implemented as a user defined subroutine for the finite element code ABAQUS. The kinetic law based on an experimentally defined stress-temperature phase diagram provides the phase fraction history for a given loading path and specified initial value of the martensite fraction. With a simple 3D model, we demonstrate the capability of the proposed methodology to simulate the shape memory effect, pseudoelasticity and hysteresis behavior. This methodology provides a computational tool for the design of 3D actuators using SMAs.

A non-invariant plane model for the interface in CuAlNi single crystal shape memory alloys

Ni-Ti, a shape memory alloy (SMA), can recover from deformation to its initial shape when heated. With using this effect, Ni-Ti SMA is applied for several industries such as a medical industry, an aerospace, electrical application on a part of micro structure. The Ni-Ti alloy used for SMA is composed of approximately 56% nickel and 44% titanium. With this composition, Ni-Ti alloy cannot be machined efficiently using traditional machining tools and methods such as the lathe, milling, and drilling because it shares the poor heat dispersion characteristics of titanium. Thus, Ti-Ni SMA should be machined using non-traditional machining methods. Electrochemical micro machining (EMM) is one form of non-traditional machining that can be applied to Ni-Ti SMA. As an anodic dissolution process, EMM allows the machining of complex shapes in Ti-Ni SMA without the generation of heat and without causing tool wear during the machining process. In this study, through the experiment that making the micro-groove, the characteristic of short pulse electrochemical machining (SPECM) process is accomplished to Ni-Ti SMA. And an evaluation of various machining factors for Ni-Ti SMA is also performed, through the substitution of different types of power source and machining time, and through simulation of the material removal rate (MRR). The experimental and simulation results are analyzed and compared.