Electrochemical Noise Response of Cr₂Nb Powders Applying Mechanical Alloying

Abstract

Cr₂Nb alloys are potential candidates for high-temperature structural materials. The influence of different mechanical alloying parameters (milling time) and sintering processes were studied. After mechanical alloying and observation by scanning electron microscope (SEM), nano powders were characterized and then sintered by spark plasma sintering (SPS). Electrochemical noise (EN) tests were also conducted in order to study the electrochemical behavior. From the current experimental results, it was revealed that ball milling times up to 20 h may explain the influence of Nb–Cr alloys and its association to the Laves phase and corrosion behavior. These insights aimed at improving the samples’ predicted behavior before spending time and resources at high-temperature industrial processes.

1. Introduction

The engineering applications of Cr₂Nb are critically inhibited by its low fracture toughness at room temperature, typically ~1.4 MPa, m1/2. To overcome the brittleness of Cr₂Nb, plenty of work was carried out that can be divided into two main categories, namely, the alloying toughening method and the second-phase toughening strategy [1]. Some time ago, the design of multiple alloys was an option to solve this problem. Alloys of Cr and Nb are one of the potential candidates for high-temperature structural materials due to their high melting temperature, good oxidation resistance, and high strength at elevated temperatures. The properties and the morphologies of nano powders fabricated by mechanical alloying (MA) are affected by different parameters [2]. In this research, the MA advantages were analyzed under characterization results, electrochemical noise tests, and processing images aimed at the improvement of corrosion resistance. Previous studies have referred to Cr alloys precipitated on secondary Laves phase Cr₂Nb alloys at temperatures up to 900 °C [3]. Ball milling time has also been considered as an important factor to be analyzed, indicating that the synthesizing reaction of Cr₂Nb can be sufficiently accomplished when ball milling times are longer than 40 h [4]. Niobium–silicon (Nb–Si)-based alloys can be oxidized to form unprotected porous oxides such as Nb₂O₅, TiNb₂O₇, and TiO₂, which cannot stop the inward diffusion of oxygen at elevated temperatures in oxidizing environments [5]. Furthermore, the influence of Nb was observed to study the corrosion resistance for different alloys. For Nb coating and cold spraying, Nb performed very well in milder corrosion conditions [6]. The presence of alloying elements such as Al, Cr, and Nb led to the formation of protective oxide layers which enhanced the mechanical properties, as well as the corrosion resistance, of these alloys [7]. Nb is an important condition to the behavior of the samples made by ball milling. This condition was observed in this research through characterization by SEM (scanning electron microscopy). The results obtained from other investigations have shown that the handling of the milling time took about 3 h, including 11 h of ball milling, or longer periods of time [8]. In some cases, there was a large mismatch in the working temperatures, demanding resourceful solutions for their joint operation [9]. Similar conditions in the SEM images were observed at different milling times [10] without applying image processing. In a few studies, however, some differences in the SEM images have been observed by using the Fiji software [11].

There is a significant demand for high-temperature structures, while exhibiting better high-temperature strength than in Ni-based alloys, and Nb–Ti–Si-based alloys typically have a lower fracture toughness as well as insufficient oxidation resistance. Alloying has great influence on the constituent phases and precipitates [12]. Therefore, the current research seeks to provide new insights on the influence of an Nb addition to specific alloying samples at different milling times by applying the spark plasma sintering technique (SPS) and electrochemical noise (EN) tests. Some studies have been referred to which use image analysis software (Fiji) in order to find out the flattening ratio (FR) of Al in Ti–6Al–4V alloys in different composite coatings [13].

2. Materials and Methods

Chromium (99.5% purity) and niobium (99.5% purity) powders were obtained from Alfa Aesar (Haverhill, MA, USA). The high-energy SPEX 8000M (SPEX Sample P-rep, Metuchen, NJ, USA) was used with a hardened steel container with 13 mm (Ø) balls as milling media. Inert Ar atmosphere was applied as a control environment, and 3 drops of methanol were used as a process control agent to avoid excessive agglomeration. Milling intervals were 0, 8, and 20 h using alternate cycles of 30 min milling and 30 min resting to avoid overheating. Powder weight ratio was kept 3:2 throughout the experimental runs. Green powders were obtained by pressing milling to a circular die at 950 MPa under uniaxial load, and the specimens were made using a hydraulic press.

Metallographic techniques were also applied on the samples to analyze the microstructural behavior. Observations were made by using thescanning electron microscope (SEM, Napoles, Mexico) JEOL-5800-LV and X-ray diffraction (XRD, Madrid, Spain)with a Panalytical X’Pert Pro diffractometer (40 kV, 35 mA) with Cu Kα radiation (λ = 0.15406 nm).

Samples were made with 5 g of powder, having a composition of Cr₂Nb. Such a composition was coated with resin and rinsed in an H₂SO₄ solution for half an hour for stabilization, and the open circuit potential was measured with a multimeter; the OCP samples were −180 and −194 mV vs. SCE for 0 h, 8 h and 20 h milling time, and then sintered by SPS. Work electrodes were usedat room temperature. All samples were analyzed according to the ASTM G1 and ASTM G199.The reading points were at 1024 and Gill AC-ACM Instruments equipment was used.

SPS operations were conducted with a Dr. Sinter1020 apparatus (Sumitomo Coal Mining, Tokyo, Japan) at temperatures of 900 °C, 1000 °C, 1100 °C and 1200 °C. The specimens were made by 10.0 g of powders of Cr₂Nb. The powder to be sintered was stuck on a rectangular graphite dies set. Dr. Sinter1020 conditions were established as 1400 A, 3 V, and 15 MPa.

Image processing converted the image acquired from microscopes into mathematical functions to enhance important zones to be analyzed. Surface morphology of the samples obtained by the SEM was studied by the free software Fiji. The ImageJ 1.53c version of Fijiwas used in this research. The Fiji Image software tool based on Java handled several algorithms, and various tools were applied. In this case, ROI (region of interest) using the segmentation algorithm was also used. This algorithm takes two vectors and computes over the image selected.

3. Results and Discussion

The oxidation resistance in this kind of intermetallic alloy, and the oxidation temperatures with different kinds of processes, have been studied for many years. In this study, the ball milling process was found to be of great benefit due to the oxidation handling process before the sample preparation. EDS (energy dispersive spectroscopy) analysis and microanalysis showed less degree of oxidation (Figure 1). It is noteworthy that a minimum quantity of aluminum was observed during the X-ray study made by ball milling and Figure 2 show alloy microanalysis. Figure 2. For the Cr sample, the higher average concentration was found at 20 h.

The deformation behavior of the Cr₂Nb intermetallic compounds consists of a single phase. This structure has not been studied at all. In this research, the microstructure of the Cr₂Nb intermetallic compounds prepared by ball milling was initially characterized. In Figure 3, is possible to see XDR with mayor concentration of Cr at 20 h of ball milling.

Cr₂Nb-based intermetallic compounds have been reported to undergo the phase transition from Cl4 (MgZn, hexagonal crystal structure) to Cl5 (MgCu, cubic crystal structure) at approximately 1597 °C, thus having oxidation resistance as an important property for high-temperature structural materials [14]. Some other investigations were focused on the oxidation temperature of the Laves phase NbCr₂ below 1150 °C and the oxidation kinetics of the NbCr₂ alloy and Cr₂Nb (JCPDS card No. 47-1638).Therefore, under the current experimental high-temperature conditions, it is also expected to present such an oxidation process and similar phase transitions.

Powders of a Cr₂Nb alloy are shown in Figure 4. Under these experimental conditions, the powders were observed to present a better milling without the presence of oxygen. A previous study related to Cr has been an important influence in super oxidation resistance [15]. From the current electrochemical tests, this resistance was clearly observed. In some cases, an interesting Cr₂Nb reaction has also been observed at 1100 °C [16]. The weight gain per unit area vs. time can be seen in Figure 5 and Figure 6. The best parabolic behavior was observed at 1000 °C and 0 h of ball milling (Figure 5) in comparison to those obtained at 1100 °C (Figure 6). In this case, the increase in temperature reduced the parabolic behavior, while a better ball milling time was observed at 20 h (“C”). In this respect, studies have shown that mass change was negligible at 900 °C and 1000 °C for Zr and Al alloys, respectively [17]. Moreover, it was found that the mass gain curves followed a parabolic law with little change in temperature [18]. Similar results were found in the current investigation using only a mixing of Cr and Nb powders.

During the oxidation process, a positive effect of the Nb was observed [19,20]. To identify whether Cr or Nb would influence the reaction into the oxidation process and improve the toughening, an image processing technique was employed by using the Fiji software. Some studies showed that Nb in microstructural analyses played a major role in increasing the strength in steel [21]. In this investigation, electrochemical experiments showed satisfactory results in corrosion behavior during the EN analysis. Figure 7 related to SEM studies was used in Fiji software to understand behavior of Nb concentration.

For the powder density, the process of ball milling showed little difference during the milling time (

Table 1. Density of milling samples with Cr₂Nb powders ¹.

Volume	Density	Milling Intensity (h) SPS
0.3935 cc	7.2485 g/cc	0 h
0.3935 cc	7.5317 g/cc	8 h
0.3925 cc	7.6733 g/cc	20 h

¹ Comparison of density powders at time milling.

). The average powder density was found to be 7.4845 ± 0.22 g/cc. The electrochemical behavior of the samples was analyzed at 0 h (Figure 8), 8 h (Figure 9), and 20 h (Figure 10), where Rn (noise resistance) was denoted with the alloy name Cr₂Nb. In order to understand the results from the EN analysis, the obtained signals consist in studying the structure of the EN time record in the time domain. In the current investigation, the behavior of the samples showed the good resistance of the corrosive environment, meaning that the passivity breakdown on the metal surfaces under different milling hours were similar, and therefore indicating that the corrosion type was located. The increase in corrosion may be explained by the chromium reaction, since the image processing studies showed that Nb behaves under certain patterns. This view has been supported elsewhere [22,23]. Niobium has also been found to exhibit the refining of grains for steel alloys, improving microstructure uniformity and enhancing the mechanical and welding properties [24,25]. In this context, the presence of oxygen, chromium, and niobium concentrations (Figure 11) show that Cr₂Nb alloys are attractive candidates for high-temperature structural materials. Density powders were tested on the Quantacrome ultrapycnometer model 1000.

In order to understand the influence of the position of Cr or Nb particles on the toughening and oxidation resistance, MA was applied during the alloy processing [26]. The imaging process was focused on the ROI part of the samples. In medical areas, ROI has been given prior attention to find brain tumors and other clinical pathologies [27]. Therefore, the ROI was selected from the images of the SEM. The processing (via the Fiji software) can be seen in Figure 7, while the sample behavior can be observed in Figure 12. The segmentation technique was used in some cases to appreciate the difference between the solid material and the porosity [28,29]. In this case, the main observation accounts for the Nb particles. The experimental results showed that the Nb samples may display a behavior pattern that can be correlated with the time milling. The extraction of the Nb particles by segmentation is shown in Figure 9 (indicated by a red color and the mapping by yellow numbers). Likewise, the way of each position was measured by the ROI manager.

4. Conclusions

The combination of various analysis techniques (EN, SEM, and SPS) were found to be satisfactory in order to study the prediction behavior of Cr₂Nb alloys at high temperatures. The current results showed that the powder density increases when increasing the ball milling times, and the niobium addition enriches the alloy to withstand corrosion damage. Ball milling times up to 20 h may be sufficient to explain the influence of the Nb–Cr alloys and its association to the Laves phase and corrosion behavior.

Despite the encouraging results obtained in this investigation, further research work should be carried out in order to better understand the mechanisms and applications of these kinds of nano powders in the light of new alloys withstanding high temperatures for industrial processes.

Concerning the toughening strategy, designing multiple alloys is considered to be an expensive and exhaustive technique, since time milling has shown a better approach. To improve these observations on the SEM images, Fiji software was used. Processing image studies may indicate a pattern behavior of Nb particles, showing the relevance of using other computational tools such as machine learning.