Austenite

The CrCoNi MEA was produced from high-purity elements (>99.9% pure), which were arc-melted under argon atmosphere and drop-cast into rectangular cross-section copper moulds measuring 25.4 × 19.1 × 127 mm. The ingots were homogenized at 1,200 °C for 24 h in vacuum, cut in half length-wise and then cold-forged and cross-rolled at room temperature along the side that is 25.4 mm to a final thickness of ∼10 mm, as shown in Fig. 1a (total reduction in thickness of ∼60%). Each piece was subsequently annealed at 800 °C for 1 h in air leading to sheets with a fully recrystallized microstructure consisting of equiaxed grains ∼5–50 μm in size.
To analyse the microstructure of the material after processing, two pieces were cut from the recrystallized sheets perpendicular to the rolling direction, embedded in conductive resin and metallographically polished in stages to a final surface finish of 0.04 μm using colloidal silica. For optical microscopy analysis, one polished surface was chemically etched using a standard solution for austenitic steels (10 ml H₂O, 1 ml HNO₃, 5 ml HCl and 1 g FeCl₃); the other was analysed as is in an LEO (Zeiss) 1525 FE-SEM (Carl Zeiss, Oberkochen, Germany) scanning electron microscope (SEM) operated at 20 kV in the back-scattered electron mode.

The experimental materials used in this contribution were two special tool steels manufactured by Böhler, namely M390 and M398. Steels are produced using powder metallurgy with the HIP (Hot Isostatic Pressing) method [11 (link)]. The description of the production process is described in the publication by Ciger et al. [12 (link)]. Due to the production method and chemical composition, the steel provides extremely high resistance to mechanical wear as well as corrosion resistance. The prerequisite for the use of steel is the production of screws for injection molding machines. The main concept for increasing the macro-hardness is the high content of MC and M₇C₃ carbides, which can be observed in the microstructure itself in Figure 1. The basic microstructure of test tool steels was investigated by scanning electron microscopy (SEM, Tescan Vega 3, Tescan Orsay Holding, Brno, Czech Republic). The samples in this case were heat-treated (Q + T) to the highest hardness.
These steels have their uses in the plastics industry, specifically to produce injection molding screws. The helix performs a rotational movement which moves the solid granulates from the hopper into the space in front of the screw. It is gradually heated and melted to the injection temperature depending on the type of plastic polymer in the temperature range of 160–250 °C. The injection of plastic takes place by means of a straight movement of the screw to the front position. The back flow cylinder prevents the backflow of the molten plastic granulate. The investigated material M398 is a newly developed material, the task of which will be to replace material M390. Other properties that predetermine M398 steel to produce screws include, high dimensional stability during heat treatment, good resistance to chemical corrosion, and the possibility of polishing to a mirror finish.
Table 1 shows and compares the values of the prescribed chemical composition from Böhler and the chemical composition measured by the authors using a Spectrolab Jr. chemical analyzer. The basic mechanical properties are listed and compared in Table 2. The manufacturer did not provide the exact chemical range of the individual elements.
In the processes of all realized experiments, dry sliding friction on powdered tool steels M390 vs. M398 and comparison of wear in contact with ceramic bearing ball Al₂O₃ at a constant measuring temperature of 200 °C were investigated. The whole measurement process took place on an instrument, the UMT TriboLab (Bruker Austria GmbH, Wien, Austria), where the main changing parameter was the different tempered temperatures of the samples (200 °C, 400 °C, 600 °C). Half of each material was also cryogenically turbid to reduce the residual austenite. The marking of the samples consisted of the marking of the experimental material and the subsequent tempering temperature. The samples are marked with the type of steel, and the subsequent number determines the tempering temperature of the samples. The letters DF (deep freezing) indicate samples that have undergone a cryogenic hardening process. Samples that did not have DF at the end were quenched to 50 °C and then tempered to the prescribed temperatures written on their labels. Wear results and roughness measurements were measured by light optical microscopy (LOM) and atomic force microscopy (AFM, Oxford Instruments, Abingdon, UK), respectively. Using the AFM microscope, 3D topographies of the ceramic balls, as well as the formed grooves, were also obtained, and their final texture surface was determined after individual experiments. All the obtained results for the coefficient of friction and wear are discussed in the next part of the article.

During TEVAR, a 28–28-150 mm Medtronic Valiant SG (Medtronic Vascular, Santa Rosa, California) with proximal bare metal stent was implanted. The SG geometry was created in Solidworks (Dassault Systèmes, France) following the dimension and specification of the Valliant product which consists of a Nitinol stent scaffold and polyethylene terephthalate (PET) fabric graft (Fig. 2a). The Nitinol stent was meshed into linear hexahedral elements with reduced integration (C3D8R) in Abaqus®. A superelastic material property was used, to reproduce the mechanical behaviour of Nitinol with parameters shown in Table 1 (Kleinstreuer et al. 2008 (link)). PET fabric graft was modelled as a tube with 0.1 mm thickness and meshed into membrane elements with reduced integration (M3D4R). The material property of PET fabric was simplified by assuming it as an isotropic elastic material with parameters taken from the same study (Kleinstreuer et al. 2008 (link)).Fig. 2

Summary of the steps in the simulation of stent-graft (SG) deployment and model variations. a The 28–28-150 mm Medtronic Valiant SG was used in TEVAR procedure and was covered by the virtual sheath. b The SG was compressed by the virtual sheath to its crimped state. c A curved tube opened up the local narrowing in the compressed true lumen. d The SG was delivered and deployed at the targeted position. e overall workflow and model variations

Table 1

Superelastic material parameters for Nitinol (Kleinstreuer et al. 2008 (link))

Austenite elastic modulus \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${E}_{\mathrm{A}}$$\end{document}EA, MPa	51,700
Austenite Poisson’s ratio \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\nu }_{\mathrm{A}}$$\end{document}νA	0.3
Martensite elastic modulus \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${E}_{\mathrm{M}}$$\end{document}EM, MPa	47,800
Martensite Poisson’s ratio \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\nu }_{\mathrm{M}}$$\end{document}νM	0.3
Transformation strain	0.063
Start of transformation (loading) \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\sigma }_{\mathrm{L}}^{\mathrm{s}}$$\end{document}σLs, MPa	600
End of transformation (loading) \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\sigma }_{\mathrm{L}}^{E}$$\end{document}σLE, MPa	670
Start of transformation (unloading) \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\sigma }_{\mathrm{U}}^{\mathrm{s}}$$\end{document}σUs, MPa	288
End of transformation (unloading) \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\sigma }_{\mathrm{U}}^{\mathrm{E}}$$\end{document}σUE, MPa	254
Start of transformation stress in compress \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\sigma }_{\mathrm{CL}}^{\mathrm{S}}$$\end{document}σCLS, MPa	900
Reference temperature \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$T$$\end{document}T, ℃	37
Density \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\rho$$\end{document}ρ, g/cm3	6.5

The Nitinol stent and PET fabric graft were then assembled together by using the tie constraint which prevents sliding or separation of the two components. A tubular surface with a diameter of 29 mm was created outside of the SG and meshed into surface elements (SMF3D4R); this represented the virtual delivery sheath and was employed to crimp and deliver the SG into the aortic dissection.

Three NiTi rotary systems with different cross-sectional geometries but the same sizes were selected including convex triangle (CT), S-type (S), and triple-helix (TH) with the size and taper of 25/04. Detailed 3-dimentional geometries of these three files were designed using the SolidWorks software (Dassault Systèmes, Vélizy-Villacoublay, France).
Cross-sectional and longitudinal geometries of instruments are shown in Figure 1.
The zaxis was chosen along the length of the instruments, i.e., normal to the cross section. Root canal models with curvature angles of 45ᵒ and 60ᵒ and radii of 2 and 5mm with the same size as files (25/04) and a total length of 15mm were regenerated according to clinical information.
By combining the two cited parameters, four types of canal geometries were evaluated (Figure 2).
All models were transferred to ABAQUS software V2018 (SIMULIA, Providence, RI, USA) and multiple numerical simulations were performed to evaluate the stress distribution in different endodontic files. Modeled files were advanced continuously, without repetitive up and down pecking or brushing movements to reach the apex of the modeled root canals. During the insertion, the instruments rotated at the speed of 300 rpm (5 revolutions per second) and the von Mises stress distribution was evaluated based on the finite element method.
The mechanical characteristics of NiTi alloy and dentin component of the root canal were setting as: Young’s modulus 36 GPa, the Passion’s ratio 0.30, the stress range for austenite to martensite phase transformation 504-600 MPa for the NiTi alloy [ 12 (link)
] and the Young’s modulus 18.60 and the Poisson’s ratio 0.30 for dentin [ 2 (link) ].
The accumulation of plastic deformation due to cyclic loading in the pseudo-elastic range and the shear strains due to friction of the instrument blade into the canal wall were neglected as model simplifications.

The Fe-Ni alloys investigated in the present work were produced using the arc-melting procedure which has previously been used to produce high-purity Ni-Ti alloys [58 (link),59 (link)]. Melting was performed using Fe and Ni feedstock in a Buehler AM arc melter (Edmund Buehler GmbH, Bodelshausen, Germany). Fe was obtained from HMW Hauner GmbH&Co. KG (Roettenbach, Germany) in the form of granulate (pieces < 20 mm) with a purity of 99.99 wt.%. HMW also provided the Ni in form of granulate (pieces < 20 mm) with a purity of 99.98 wt.%. The Fe:Ni ratios were controlled using an analytical balance (Mettler Toledo LLC, Columbus, OH, USA) with an accuracy of ±0.001 g. Prior to the first melting step, the reaction chamber was evacuated (vacuum: 3 × 10⁻³ bar) and filled with Ar (purity: 99.9999 vol.%, pressure: 0.6 bar). This cycle was repeated three times. The chamber of the arc melter contained a Ti getter, which was melted for 60 s, to minimize the content of residual oxygen. Subsequently, twelve re-melting cycles and a final drop-casting step were applied. The final ingots had masses of ~50 g, rectangular cross-sections of 8 × 20 mm², and lengths of 60 mm. Casting was followed by hot and cold rolling in a rolling mill of type DWU-30 (Friedrich Krollmann GmbH, Altena, Germany). During six hot rolling steps (with intermediate short-time anneals at 1073 K) the cross section was reduced from 8 to 4.5 mm thickness. Subsequently, during eight cold-rolling steps, the cross-sectional thickness was further reduced from 4.5 to 2.5 mm. The total degree of rolling deformation prior to the subsequent homogenization/austenitization heat treatment was 58.8%. The thermomechanical treated materials were subsequently sealed into evacuated quartz capsules under a vacuum of around 5 × 10⁻⁵ mbar along with a Ti getter. High-temperature annealings (for homogenization and austenitization) were conducted for 72 h at 1473 K, followed by water quenching, which resulted in an austenitic material state. Small (40 to 50 mg) samples were prepared by cutting, grinding, and polishing (down to a grit size of 1000) and cleaned for 10 min in an ethanol ultrasonic bath. The processing route applied in the present work is documented in Figure 2.
Using this procedure, a total number of eleven ingots was produced. Their target compositions, together with the actual compositions measured after processing using energy dispersive X-ray analysis (EDX) in the SEM and inductively coupled plasma-optical emission spectrometry (ICP-OES), are given in Table 1 for six of the twelve casted ingots. Please note that the experimentally determined concentrations (columns 3 and 4 of Table 1) are very close to the target compositions (column 2 of Table 1). From these eleven ingots, 29 specimens were cut for further investigations. After processing, all ingots were in the austenitic state, with an austenite grain size in the mm range.