Cr-Si Alloys with Very Low Impurity Levels Prepared by Optical Floating Zone Technique

Abstract

The optical floating zone technique was utilized to purify chromium and a single-phase chromium–silicon alloy in this work. The impurity content (carbon, nitrogen, and oxygen) can be reduced by decreasing the withdrawal speed of samples during the zone refining process, and the coarsening of grains was also observed. The effect of the impurities on mechanical properties was determined by hardness measurements at room temperature, and the hardness of both chromium and the chromium–silicon alloy decreased with lower concentrations of nitrogen and oxygen. In contrast, brittle material behavior is observed in samples prepared by arc melting process with higher concentrations of impurities. To use chromium–silicon alloys for future high-temperature applications, their brittle behavior must be improved, which can be achieved by reducing their carbon, nitrogen, and oxygen concentrations.

1. Introduction

Chromium (Cr)-based alloys show good high-temperature properties and are therefore currently being investigated intensively [1,2,3]. However, brittleness at low-to-moderate temperatures [4,5,6,7,8,9] often prevents their application. Studies show that carbon (C), nitrogen (N), and oxygen (O) promote brittleness in Cr-based alloys [10,11,12,13]. In previous investigations, Cr-based alloys were fabricated using arc melting processes [14], additive manufacturing [15], or investment casting [16]. These manufacturing processes are not reported to lower impurity levels. Here, chemical and physical interactions between the melt and the mold material or a ceramic crucible, respectively, can occur, as Pelchen et al. show in their work [17]. The optical floating zone (OFZ) technique is suitable for producing high-purity materials because it enables remelting without a crucible [18] and promotes outward diffusion [19] of impurities. In addition to this purification effect, the OFZ technique is used to create directionally solidified or single-crystal metal or metal oxide specimens [20]. To the authors knowledge, the OFZ technique has not yet been applied to Cr-Si alloys with the aim of producing high-purity specimens. The instrumentation for the OFZ technique is illustrated in Figure 1.

In this work, Cr-based alloys are produced by OFZ to exploit its purification potential. Concentrations of C, N, and O are determined by glow discharge optical emission spectroscopy (GDOES). Martens hardness, also known as universal hardness, is measured on all specimens. To estimate the brittle behavior of the specimens, crack formation on Vickers indentations is evaluated, based on the model presented by Evans et al. [21] for brittle materials.

2. Materials and Methods

According to Okamoto [22], the solubility of Si in Cr is around 2 at.% below 1000 °C in equilibrium, so the alloy compositions Cr₁₀₀ and Cr₉₈Si₂ (in at.%) are investigated in this work. Pure Cr (Plansee SE, Reutte, Austria, purity 99.95%) and pure Si (HMW Hauner, Röttenbach, Germany, purity 99.999%) are arc melted and solidified in an argon (Ar) atmosphere into cuboid-shaped ingots. For OFZ, cylindrical bars with a diameter of 9 mm and a length of 110 mm are prepared from the arc-melted ingots by electric discharge machining (EDM). The bars are further divided with a length ratio of 4:7, the shorter bars thereafter used as so-called seed rods and the longer bars as so-called feed rods in the OFZ. An OFZ furnace, partially shown in Figure 1b (FZ-T-12,000-X-VII-VPO-PC from Crystal Systems Corporation, Hokuto, Japan) with four xenon lamps (total power 12 kW) is used to produce nine specimens (

Table 1. Specimens are processed by OFZ and associated process parameters. All specimens are processed in high-pressure argon atmosphere of 0.7 MPa, except for Cr98Si2_20LP, which is processed in argon under ambient pressure of 0.1 MPa.

Specimen	Nominal Composition (in at.%)		Withdrawal Speed (in mm·h−1)	Ar Pressure (in MPa)	Ar Gas Flow (in L/min)
	Cr	Si
Cr100_10	100.0	0.0	10	0.7	2.0
Cr100_20			20
Cr100_30			30
Cr100_40			40
Cr98Si2_20LP	98.0	2.0	20	0.1
Cr98Si2_10				0.7
Cr98Si2_20
Cr98Si2_30			30
Cr98Si2_40			40

) with two different alloy compositions. A high-pressure quartz glass tube surrounding the seed and feed rods enables the use of Ar gas in the quality class 6.0 (purity 99.9999%).

Table 1 provides an overview of the specimens created in the OFZ and the associated process parameters. The withdrawal speed given in Table 1 is often referred to as growth rate in the literature, i.e., the speed at which the seed and feed rod are moved vertically through the focal point in the OFZ furnace. The withdrawal speed is chosen within the range of 10 to 40 mm·h⁻¹, since a stable melting zone is achieved within these rates. To determine the influence of the Ar gas pressure in the quartz glass tube on impurity input, the pressure is varied between 0.1 MPa and 0.7 MPa (see Table 1).

After OFZ, specimens are cut lengthwise using EDM so that plane-parallel surfaces are created along the longitudinal axis (Figure 2c,d). Both surfaces are ground stepwise from a grain size of 320 to 1000 on a wet grinding machine and diamond sandpaper and then polished with an aqueous SiO₂ suspension. For electron backscatter diffraction (EBSD) analysis, specimens are vibration polished for 24 h with a 0.06 µm SiO₂ suspension and then ultrasonically cleaned in a bath with water and ethanol.

Martens hardness measurements are carried out at room temperature using a Fischerscope^® (Sindelfingen, Germany) HM 2000. A total of 200 measurement points of Martens hardness with a test force of 300 mN are recorded along the longitudinal axis within the build rod (see Figure 2d) of every specimen. Vickers hardness measurements (Matsuzawa VMT-X S3, Akita, Japan) with a testing force of 98.1 N (=HV10) are carried out on all OFZ-processed specimens. The Vickers indentations are investigated for cracks with scanning electron microscopy (SEM).

Fracture toughness K_IC, as a measure of brittleness, is estimated using the model of Evans et al. [21] and calculated according to Equation (1). To estimate K_IC, the parameters Vickers hardness HV, mean crack length c, and the length of the half-diagonal a of the Vickers indentation are used. Table 2 lists the boundary conditions within which Equation (1) is applicable.

$$K_{IC}=0.16·{\left({\displaystyle \frac{c}{a}}\right)}^{-1.5}·HV·\sqrt{a}$$

(1)

$K_{IC}:$ fracture toughness in $\mathrm{MPa}\sqrt{\mathrm{m}}$

$c:$ mean crack length in $\mu \mathrm{m}$

$a:$ half-diagonal of Vickers indentation in $\mu \mathrm{m}$

$HV:$ Vickers hardness in $\mathrm{G}\mathrm{P}\mathrm{a}$

Table 2. Boundary conditions within which Equation (1) is applicable. Adapted from Ref. [21].

Properties	Limits for Which Equation (1) Is Applicable
Vickers hardness HV in $\mathrm{G}\mathrm{P}\mathrm{a}$	1…70
fracture toughness KIC in $\mathrm{MPa}\sqrt{\mathrm{m}}$	0.9…16
Poisson’s ratio $\upsilon$	0.2…0.3

Table 2. Boundary conditions within which Equation (1) is applicable. Adapted from Ref. [21].

Properties	Limits for Which Equation (1) Is Applicable
Vickers hardness HV in $\mathrm{G}\mathrm{P}\mathrm{a}$	1…70
fracture toughness KIC in $\mathrm{MPa}\sqrt{\mathrm{m}}$	0.9…16
Poisson’s ratio $\upsilon$	0.2…0.3

Microstructures are examined using optical microscopy (OM, Zeiss^®, Oberkochen, Germany, Axioplan 2) and SEM (Zeiss^® Gemini 300, Zeiss^® Sigma 300 VP and Zeiss^® 1540 EsB Cross Beam). Backscattered electron images (BSE) are taken of all specimens. Element distribution maps are recorded by energy dispersive spectroscopy (EDS). Grain sizes are determined from electron backscatter diffraction (EBSD) images. If grains with diameters larger than 500 µm occur, the grain size is additionally determined by macro etching and OM examination. The chemical composition along the longitudinal specimen axis is measured by µXRF (EDAX^®, Mahwah, NJ, USA, Orbis PC) with a measuring spot of 30 µm in diameter. Phase determination is carried out by X-ray diffraction (XRD) using a Bruker^® (Billerica, MA, USA) D8 Discovery.

Carbon (C), nitrogen (N), and oxygen (O) impurity contents are determined using glow discharge optical emission spectroscopy (GDOES, Spectruma^®, Hofheim am Taunus, Germany, GDA-Alpha). Integral impurity contents of pure Cr from Plansee, pure Cr from EVOCHEM, and arc-melted Cr₉₈Si₂ (in at.%) are measured by inductively coupled plasma optical emission spectroscopy (ICP-OES, LECO ONH 836, St. Joseph, MI, USA) to evaluate proprietary GDOES standards.

3. Results

All specimens listed in Table 1 are produced twice to ensure reproducibility of the measured values. All specimens are subjected to the same tests with identical testing parameters. No significant deviation is observed regarding the test results from both batches. Figure 3 shows a sliced Cr₉₈Si₂ (in at.%) specimen after OFZ, analyzed by SEM, EBSD, and EDS. Within the build rod volume, elongated grains are observed (Figure 3a). The maximum grain length achieved is 41 mm. The grain size is additionally determined by macro etching and OM investigations. Figure 3b shows the BSE image with corresponding EDS mappings for Cr and Si on the Cr98Si2_20 specimen as an example for all EDS investigations. The EDS mappings verify the single-phase microstructure of all Cr₉₈Si₂ specimens.

Table 3. Length of the largest grain and number of grains within the build rod of the OFZ specimens.

Specimen	Nominal Composition (in at.%)		Length of Largest Grain (in mm)	Number of Grains Within the Build Rod
	Cr	Si
Cr100_10	100.0	0.0	>30	12
Cr100_20			>20	16
Cr100_30				17
Cr100_40			9.8	>30
Cr98Si2_20LP	98.0	2.0	0.6	>50
Cr98Si2_10			>40	11
Cr98Si2_20				12
Cr98Si2_30			>20	22
Cr98Si2_40				26

shows the measured length and number of grains in the build rod of the OFZ specimens. Large grains elongated in the withdrawal direction are observed for all specimens, as shown in Figure 3. For the pure Cr specimens, the measured grain length is slightly shorter compared to the Cr₉₈Si₂ specimens.

Table 4. Results of the µXRF measurements and pore fraction analysis by OM on the specimens with Si content.

Specimen	Nominal Composition (in at.%)		µXRF Results (in at.%)		Porosity (Measured at the Surface) (in %)
	Cr	Si	Cr	Si
Cr100_10	100.0	0.0	100.0 ± 0.0	below resolution limit (<0.1 at.%)	0.15
Cr100_20					0.20
Cr100_30
Cr100_40					0.25
Cr, pure from Plansee®					0.85
Cr, pure from EVOCHEM®					0.80
Cr98Si2, arc melted	98.0	2.0			0.70
Cr98Si2_20LP			97.6 ± 0.1	2.4 ± 0.1	0.30
Cr98Si2_10			97.8 ± 0.1	2.2 ± 0.1	0.20
Cr98Si2_20			97.9 ± 0.2	2.1 ± 0.2	0.35
Cr98Si2_30			97.7 ± 0.2	2.3 ± 0.2	0.40
Cr98Si2_40			98.1 ± 0.2	1.9 ± 0.2

shows the results of the µXRF measurements of all specimens containing Si. For every specimen, the mean element concentration is calculated from 20 measurements along the longitudinal axis. The alloy compositions do not deviate more than 0.3 at.% from the nominal compositions. OM investigations reveal porosities below 0.5% for all Cr-Si specimens. The porosity of pure Cr specimens is below 0.25%.

XRD patterns reveal a lattice constant of $a_{Cr100}^{100} = 288.3$ pm for the pure Cr specimens and a lattice constant of $a_{Cr98Si2}^{100} = 288.1$ pm for the Cr₉₈Si₂ (in at.%) specimens prepared by OFZ, respectively. An influence of the withdrawal speed on the lattice constant is not observed.

Table 5. Impurities measured by GDOES; proprietary standard specimens marked with an asterisk *.

Specimen	Withdrawal Speed (in mm·h−1)	C (in ppm)	N (in ppm)	O (in ppm)
Cr100_10	10	10 ± 2	32 ± 4	144 ± 5
Cr100_20	20	11 ± 2	40 ± 3	151 ± 5
Cr100_30	30	13 ± 2	44 ± 5	150 ± 5
Cr100_40	40	12 ± 2	118 ± 7	259 ± 5
Cr98Si2_10	10	12 ± 3	66 ± 9	210 ± 8
Cr98Si2_20LP	20	12 ± 2	82 ± 7	211 ± 7
Cr98Si2_20		12 ± 2	76 ± 6	207 ± 6
Cr98Si2_30	30	13 ± 2	83 ± 6	213 ± 6
Cr98Si2_40	40	12 ± 2	114 ± 9	244 ± 6
Cr, pure from Plansee® *	--	12 ± 2	144 ± 5	267 ± 5
Cr, pure from EVOchem® *		20 ± 10	1163 ± 50	140 ± 50
Cr98Si2, arc melted *		12 ± 2	209 ± 9	315 ± 7

lists C, N, and O levels measured by GDOES. Table 5 shows an almost constant concentration of C for all specimens produced by OFZ. The concentration of N varies considerably between the specimens. The specimens without Si content have the lowest N content, while the specimens that are not processed with OFZ show significantly higher N concentrations. This also applies to the O content. With increasing withdrawal speed, an increase in the impurities N and O can be observed for both alloys.

Figure 4 shows the sum of the impurities C, N, and O for all OFZ specimens plotted over the withdrawal rate. Additionally, the initial materials for the OFZ process are given in Figure 4 to highlight the purification potential of the OFZ. The scatter bars shown in Figure 4 are derived from the five repeated GDOES measurements on each specimen. For the Cr₁₀₀ specimens (Figure 4a), the concentrations of C, N, and O increase with increasing withdrawal speed. The same applies to the Cr₉₈Si₂ specimens (Figure 4b). For the specimen produced with 0.1 MPa Ar gas pressure (Cr98Si2_20LP), a slightly increased concentration of impurities is observed.

Figure 5 shows the Vickers hardness values of all specimens. Each specimen is tested with 20 evenly distributed Vickers indentations. For the OFZ specimens, the indentations are evenly distributed over all grains, and indentations which fall onto grain boundaries are not taken in account. Figure 5a shows the Vickers hardness values for the Cr₁₀₀ specimens. A slightly higher hardness is observed for the initial materials (Cr Plansee^®, Cr EVOchem^®). With increasing withdrawal speed, the hardness of the Cr₁₀₀ OFZ specimens increases, but the scatter of the measurement points is larger than the hardness differences. Figure 5b shows the Vickers hardness values for the Cr₉₈Si₂ specimens. The initial material for the OFZ process (arc-melted Cr₉₈Si₂) shows a slightly higher hardness than the OFZ samples.

Figure 6 shows the Martens hardness results. For the measured values in Figure 6a,b, the hardness values averaged across all grains are given for each specimen, with scatter bars indicating the deviation. Figure 6a shows the pure Cr specimens. The hardness increases with increasing impurity concentration. The starting material (Cr Plansee^®) shows the highest hardness at the highest impurity content. Figure 6b shows an increase in Martens hardness with increasing impurities. The arc-melted master alloy shows the highest hardness and the highest C, N, and O content. For both alloys in Figure 6a,b, a grain size effect is ruled out for OFZ specimens due to similar grain sizes. Since the OFZ specimens have very large grains, grain-orientation-dependent hardness measurement is possible. Figure 6c,d show increased Martens hardness in grains where the indentation is perpendicular to the (001) plane of the grain, whereas the hardness perpendicular to the (111) plane is reduced (the scatter bars for each specimen are smaller than the symbol size in Figure 6c,d). This observation applies to both alloys.

All Vickers indentations on all OFZ specimens are examined for crack formation using SEM (20 indentations for each specimen). At a resolution of 0.1 µm, no crack formation is observed in any of the OFZ specimens. Figure 7a,b show two indentations on the OFZ specimens with the highest impurity content (withdrawal speed = 40 mm·h⁻¹). Vickers indentations are also performed on the initial materials. Pure Cr from Plansee^® shows no crack formation on any of the 20 indentations (Figure 7c), whereas the arc-melted Cr₉₈Si₂ specimen shows crack formation in the corners of 18 indentations out of 20 (Figure 7d). Palmquist crack formation (cracks forming at the edges of the Vickers indentations) is ensured by gradually grinding the specimens until all cracks have disappeared. The average crack length c, determined from all cracks in all 18 cracked indentations of the arc-melted Cr₉₈Si₂ specimens, is 105 µm. The half-diagonal a (averaged over 18 indentations) of the indentations is 357 µm. According to Equation (1), a fracture toughness of K_IC ≈ 6.7 ± 0.4 $\mathrm{MPa}\sqrt{\mathrm{m}}$ is calculated for arc-melted Cr₉₈Si₂. The fracture toughness K_IC according to the model of Evans et al. [21] of the other specimens cannot be determined due to the lack of crack formation.

4. Discussion

The brittle-to-ductile transition temperatures of metals and alloys with bcc structure (e.g., Cr-Si alloys) are generally above room temperature [7,23,24], meaning the specimens tested in this work are considered to be in a brittle state. In 1935, Kroll [9] showed that pure Cr has ductile properties at room temperature, hence a brittle-to-ductile transition temperature below room temperature, when the oxygen content is reduced to a technically feasible minimum by multi-stage reduction. The period from 1950s to the 1970s represents a peak in research into the ductility of Cr and Cr alloys. During this period, studies proved an increase in ductility at low temperatures through measures such as purification or grain refinement: Edwards et al. [25] investigated the insertion of the impurities hydrogen (H), N, O, sulfur (S), phosphorus (P), and other elements during the electrolytic extraction of pure Cr from chromite and determined that Cr with a purity of 99.98 at.% exhibits ductile, hence plastic, deformation at room temperature and above. They also showed that impurities accumulate preferentially at grain boundaries and concluded that pure Cr should have a coarse-grained microstructure to reduce the risk of impurities entering during exposure to higher temperatures [25]. A more recent study showed improvements in the ductility and fracture toughness of Cr and Cr alloys by sintering and purifying Cr [26]. A low N content is crucial for Cr-Si alloys, since previous studies have shown that N leads to embrittlement in Cr-Si alloys [27].

The Cr and Cr-Si specimens produced in this work using OFZ exhibited an impurity concentration (sum of C, N, and O) of 186 to 389 ppm (Figure 4 and Table 5). The impurity concentration depends on the alloy composition and the withdrawal speed. By reducing the withdrawal speed from 40 to 10 mm·h⁻¹, the impurity concentration is reduced by approximately 25% for Cr₁₀₀ and around 30% for Cr₉₈Si₂. The initial materials (Cr Plansee^®, Cr EVOchem^®, and arc-melted Cr₉₈Si₂) show significantly higher impurity concentrations (Figure 4). For arc-melted Cr₉₈Si₂, the Vickers indentation method leads to the formation of Palmquist cracks, which allows an estimation of the fracture toughness K_IC according to the model of Evans et al. [21], subject to the boundary conditions (Table 2). In this study the impurities N and O are again identified to be crucial for the brittle-to-ductile transition of pure Cr and dilute Cr-Si alloys. Additionally, the impurities show a significant influence on hardness: By reducing the impurities of the OFZ specimens by about 30%, the Martens hardness is also reduced by about 30% (Figure 6a,b).The melting metallurgical process OFZ turned out to purify Cr and dilute Cr-Si alloys to an extent that the brittle to ductile transitions take place below room temperature. The lower the OFZ withdrawal speed, the lower the impurity levels in the processed alloys. In addition, OFZ-processed alloys have low porosities and homogeneous microstructures. OFZ generates coarse, elongated grains in the range of centimeters, which is not possible with other melting metallurgical processes such as arc melting or investment casting [16,27]. The metallurgical melting process OFZ achieves a reduction in the impurities C, N, and O and is therefore an alternative to other purification processes such as multi-stage reduction [9] and electrolytic extraction [25].

5. Conclusions

Cr and Cr-Si alloys can be purified by OFZ. In particular, the OFZ process results in very low N and O levels, as low as 176 ppm. The slower the OFZ withdrawal speed, the lower the impurity levels. Low N and O levels together with grain coarsening through OFZ processing affected the deformation behavior of Cr and Cr-Si alloys. OFZ-purified and coarsened Cr and Cr-Si alloys have decreased hardnesses, but do not show crack formation at room temperature when tested for their brittle behavior with the indentation method developed by Evans et al. [21]. The effect of solid solution hardening by adding Si is surpassed by the effect of hardening due to increasing impurity concentrations. By applying the OFZ technique, specimens with centimeter-long grains are produced, which, to the authors’ knowledge, has not been reported for Cr-Si alloys before. The large-grained structure of the OFZ specimens allows the grain-orientation-dependent investigation of the hardness of Cr-Si alloys. The conclusions drawn in this work apply to single-phase Cr-Si alloys with low Si content.