Martensite

The experimental material M398, developed by BÖHLER, aims to meet high demands in plastics processing. It is a martensitic chrome steel with a high carbon content produced by the powder metallurgy method. Thanks to the production method and chemical composition, the steel provides extremely high resistance to mechanical wear and corrosion. The prerequisite for using steel is the production of injection molding screws. The main concept for increasing the macro-hardness is the high content of MC and M₇C₃ carbides, which we can observe in the microstructure in Figure 1. All samples were etched with a special etchant designed for steels that are highly alloyed with Cr and Ni. The composition of the etchant consisted of 10 mL HNO₃, 10 mL H₂O₂, 20 mL HCl, and 20 mL glycerin [47 (link)]. Thanks to the increased occurrence of carbide particles in the M398 steel, we can state that it would be possible to create screws enabling the processing of plastics with increased content of glass fibers [48 (link)].
The chemical composition of the experimental sample was determined and compared with the data from the steel manufacturer using spectral analysis on a SPECTROMAXx LMX10 device. The results of the spectral analysis, together with the chemical composition values provided by BÖHLER, can be seen in Table 1.
The material in its unheated state is suitable for machining; it has favorable cutting conditions thanks to the content of elements such as Mo and Mn.
With the chemical composition and method of steel production using powder metallurgy, the primary goal of which is to ensure a fine-grained structure, we expect that the steel will already have high strength and low susceptibility to plastic deformation in its basic state [49 (link),50 (link)]. In order to determine the ultimate tensile strength Rm, the measurement was carried out on the INSTRON 5500R tensile test machine. For the measurement, samples were made according to Figure 2b. After the measurement, tensile curves were generated, which, as expected, do not have a significant yield strength. The highest average measured value of the tensile strength reached 1078.5 MPa (Figure 2a); this value was used as input data for later simulated calculations in the JJMatPro^® API v7.0 simulation software.

An Fe₂₁Mn₅₈Al₁₇Ni₄ alloy of 3 g was prepared from the pure metals by arc melting. The resulting ingot (dimensions: cylinder with 1.5 cm diameter and 5 mm height) was afterwards heat-treated at 1473 K in an argon atmosphere for 24 h and quenched in ice water. The chemical compositions were checked by energy-dispersive X-ray spectroscopy (EDS). Further details regarding the sample preparation and annealing procedures can be found in the work of Walnsch et al. (2019a ▸ ,b ▸ ). Thermally induced martensite was generated by water quenching from the heat-treatment temperature.
Scanning electron microscopy (SEM) on metallographically prepared cross sections (special effort has been made to prevent re-heating of the samples during the preparation process) after grinding and polishing (final stage: vibrational polishing) has been used to acquire the shown figures and BKD patterns. The austenite grains have a size of several hundred µm, making powder X-ray diffraction an unsuitable method for investigating the crystal structure and microstructure. Therefore, electron backscatter diffraction (EBSD) was chosen as the investigation method.
EBSD was carried out using a JEOL JSM 7800F scanning electron microscope with an acceleration voltage of 30 keV and beam current of approximately 10 nA. The BKD camera (EDAX Hikari Super Elite) has a resolution of 480 × 480 pixels and the output patterns are in a 16-bit format, making a precise analysis of the diffracted intensities possible. The BKD patterns were acquired using the EDAX TEAM software.
Special efforts were made to acquire BKD patterns with the best possible quality:
(i) The individual BKD patterns were acquired with an acquisition time of more than 100 ms. Ten patterns of the same spot were summed to reduce noise.
(ii) Background subtraction was done on the basis of BKD patterns acquired from embedded and polished Fe₄₀Ni₄₀B₂₀ metallic glass having a similar electron density to the presently investigated alloy [see Fig. 2 ▸(a)]. This method is superior in the present case of a very coarse-grained alloy to averaging many differently oriented grains of the actual alloy, as the latter procedure leads to residual Kikuchi bands in the averaged background pattern dedicated to serve as background for the BKD indexing procedure.
(iii) The acquired BKD patterns were used in their 16-bit format and treated by low-pass filtering and adjusting of the gray value histogram [compare Fig. 2 ▸(b) and Fig. 2 ▸(c)] to reduce noise in the pattern, originating from an imperfect background subtraction.
The BKD patterns were analyzed by means of Hough-space-based indexing using the TSL OIM DC 7 software (EDAX, 2017 ▸ ). In order to relate the experimentally measured patterns to possible structure models, dynamic simulation of the BKD patterns, using the structure models, was carried out using the Bruker DynamicS software (Winkelmann et al., 2007 ▸ ).

A technique with several steps was applied to produce the reinforced specimens. This technique is schematically presented in Figure 1 and Figure 2. Commercial powders of WC powders (99.0 wt.% purity) and Fe (99.0 wt.% purity), from Alfa Aesar, ThermoFisher (Kandel, Germany) GmbH were used to produce green compacts to be inserted in the mold cavity. First, the morphology and granulometric distribution of the powders were characterized by scanning electron microscopy (SEM), using a FEI QUANTA 400 FEG (FEI Company, Hillsboro, OR, USA) with an energy-dispersive detector (EDS) and dynamic light scattering (DLS, Laser Coulter LS230 granulometer, Beckman Coulter, Inc., Brea, CA, USA). In the second step, the WC and Fe powders were mixed in a volume fraction of 40:60, which, according to the literature, is within the range that it is expected to guarantee the best wear performance. The mixture was homogenized in Turbula shaker-mixer (Willy A. Bachofen AG, Muttenz, Switzerland) for 7 h, and bound with sodium silicate (1.5 mL). Then, the mixture was uniaxially cold-pressed at approximately 70 MPa in a metallic mold to produce green compacts with dimensions of 31 mm × 12 mm × 7 mm. SEM analysis was performed to study the characteristics of both the powder mixtures and the green compacts. The composition of the mixture was selected from data the literature.
In the final step, the green compacts were inserted in the mold cavity, and the high-chromium white cast iron, melted in a medium frequency induction furnace with a capacity of up to 1000 kg, was poured at a temperature of 1460 °C. The nominal chemical composition of the base metal (see Table 1) was analyzed by optical emission spectrometry (MAXx LMM05, Spectro, Germany).
A cross-section of the specimen was cut by wire electrical discharge for visual control of the inner zones of the composite and the bonding between the composite and the base metal. Metallographic samples were prepared and etched with Beraha-Martensite reagent. The microstructure was characterized by optical microscopy (OM) using a Leica DM 4000M with a DFC 420 camera (Leica Microsystems, Wetzlar, Germany) and SEM secondary electron (SE) and backscattered electron (BSE) image. Electron backscatter diffraction (EBSD) analysis has been used to assist with phase identification. The data obtained from EBSD were submitted to a dilation clean-up procedure, using a grain tolerance angle of 15° and the minimum grain size of 10 points, to avoid inaccurate predictions.
A detailed characterization of the phases formed was performed in transmission electron microscopy (TEM) using a JEOL 2100 (JEOL Ltd., Akishima, Tokyo) operated at 200 keV. For that, thin foils were prepared in a dual-beam focused ion beam (FIB) FEI Helios NanoLab 450S (FEI Company, Hillsboro, OR, USA). On TEM, the phases were fully identified through selected area electron diffraction (SAED). X-ray diffraction (XRD, Cu Kα radiation, Bruker D8 Discover), with a scanning range (2θ) of 20° to 100°, was used to complement the characterization of the phases.

Tensile test specimens were fabricated from a Ni₅₀Ti₅₀ (at.%) alloy powder utilizing the AconityMINI® PBF-LB system. The processing conditions were set to 180 W laser power, 1200 mm/s scan speed, 70 µm hatch spacing, spot size was 50 µm, layer thickness was 40 µm, and the scan pattern was bi-directional as per simulation pattern No3 (0°, 90°), shown in Fig. 2c. Testing was carried out according to ASTM E8/E8M standard, using four (4) small size round specimens of length, 29 mm; diameter 4 mm and gauge length 10 mm. Tensile testing was performed to failure using a 50 kN Zwick/Roell testing system with a crosshead speed of 1 mm/min and a preload of 5 N in the specimen build direction. A Class B-1 axial extensometer (Epsilon Technology Corp) was utilized to record specimen elongation during testing. The resultant material properties used for the simulation are presented in Table 2.Table 2

Average tensile test results for Ni₅₀Ti₅₀ martensitic printed via PBF-LB, n = 5

Property	Value
Yield strength	188.87 MPa
Elastic modulus	34.03 GPa
Poisson ratio	0.33

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.

EDX experiments were performed using a LaB₆ JEOL JSM 840A type SEM (Tokyo, Japan) operating at 30 kV. EDX maps were recorded from areas of 0.25 mm². To obtain high-quality images of martensitic microstructures and to study the distributions of the ξ-angles to differentiate between different types of martensite, an FEI Quanta FEG 650 SEM (Hillsboro, OR, USA) was used. During the EBSD measurements, the specimens were tilted by 70° and an acceleration voltage of 30 kV was used at a working distance of ~17 mm. Investigations were performed on vibro-polished metallographic cross-sections. Areas of 100 × 100 µm² in the center of large prior austenite grains (~mm size) were scanned using a step size of 100 nm resulting in 500,000 crystallographic data points per material state. EBSD measurements were conducted using an EDAX Inc. type Hikari XP camera (Mahwah, NJ, USA). The EBSD data were evaluated following the approach which was recently documented in Ref. [8 (link)] and implemented into the MATLAB [60 ] toolbox MTEX [61 (link),62 (link)]. In the present work, two procedures are used which were introduced in previous studies, see Ref. [8 (link)]. First, the distribution of the three ξ-angles, which represents the deviation between the actual orientation relationship between the austenite parent lattice and the ideal Bain orientation relation is considered. The three angles are denoted as ξ₁, ξ_2, and ξ₃, where indices ₁, _2, and ₃ correspond to the x, y, and z-axes of the Bain cell respectively., see Figure 3a. The symmetry of the austenite-martensite transformation yields 24 possible martensite variants which can form in one austenite grain. Secondly, a martensite variant color coding which was introduced in [8 (link)], Figure 3b, is applied to visualize information about the emerging variants forming during the martensitic transformation.