The Effect of Cr Content on the Glass Forming Ability of Fe68-xCrxNb8B24 (x =8,10,12) Alloys

Based on the Fe60Cr8Nb8B24 alloy, reported in the literature as good Glass Forming Ability (GFA), in this study the GFA of two new compositions was proposed, Fe68-xCrxNb8B24 (x=10,12). They were evaluated as good candidates to be new compositions of Bulk Metallic Glasses (BMG) with better corrosion resistance due to the high Cr content. Rapidly solidified glassy ribbons were processed, and based on their thermal characteristics, the critical thickness of the glassy structure formation (Zc) was estimated. The critical thickness (Zc) obtained experimentally using the wedge-shaped casting method was evaluated and it presented a much lower value than that estimated theoretically. However, the GFA of the compositions analyzed was ranked and this ranking (i.e. whichever has the most or least GFA) is in agreement with the result predicted theoretically. The GFA of Fe58Cr10Nb8B24, which presented a maximum thickness of the amorphous region of wedge-shaped samples of about 0.6 mm (but estimated it would be 3.56 mm), offered good prospects to be a new Fe-based glass former alloy which has better resistance to corrosion than the Fe60Cr8Nb8B24 alloy reported.


Introduction
Over the last few years, extensive research has been carried out to improve the properties of metallic glasses, such as the physical, chemical, magnetic and mechanical properties.The corrosion behavior of Bulk Metallic Glasses (BMG) is important when these materials are used in aggressive and hostile environments (high temperatures, oxidizing atmospheres and corrosive media) 1 .In Fe-based alloys, adding some elements such as Cr is known to improve the corrosion resistance of theses alloys, however it also reduces the Glass Forming Ability (GFA) 2 .The percentage of Cr in the glass former alloy could be lower than that of stainless steel as only 2% is sufficient to be resistant to pitting and crevice corrosion.In 1 N HCl solutions, the Fe 67.6 C 7.1 Si 3.3 B 5.5 P 8.7 Cr 2.3 Mo 2.5 Al 2.0 Co 1.0 alloy containing 2.3 at% Cr showed a corrosion resistance similar to the 304 stainless steel that contains high Cr (19 at% Cr) and Ni (9 at% Ni) contents 3 .This behavior indicates a promising replacement of stainless steel alloys by glassy alloys in various applications.
Despite the fact that Cr reduces the GFA of the alloy, it has been reported in the literature that a Fe-based BMG can be obtained containing up to 15 at.%Cr and 16 at.%Cr 4,5,6,7,8 and the corrosion resistance can be significantly improved due to this higher amount of Cr.
Besides Cr, molybdenum and niobium additions have also been extensively studied because both are also effective in promoting anodic passivation (similar to what happens to Cr), even in acidic chloride solution 2 .Kiminami et al. reported that a small quantity of Nb and/or Mo, lower than the minimum concentration generally used in commercial stainless steel (0.6 wt.%), is sufficient to increase the general corrosion resistance, as well as the pitting corrosion resistance of glassy Fe-Cr-based alloys 9 .It has been shown that Mo is more effective in enhancing corrosion resistance than the Nb in 4.0 M HCl solution, and the alloy containing both Nb and Mo presented more overall corrosion resistance than the alloy containing only one of these elements 10 .Furthermore, the presence of these two elements favors the increase in the GFA of the alloy.
Regarding the design and selection of compositions in Fe-based BMG with additions of Cr, Nb and B elements, Cheney and Vecchio 11 reported a modeling criterion that simultaneously analyzes the thermodynamics and kinetics of the vitrification behavior in a potential glass-forming alloy.The authors used a liquidus model (parameter α) to determine and rank the presence of deep eutectics.Moreover, this model was cross-checked with an elastic strain model (parameter ε) in a pseudo-ternary phase diagram.The authors analyzed 19 samples of this Fe-Cr-Nb-B system, and both theoretical To analyze the liquidus temperature calculations, a dimensionless parameter, α, is used to quantify the depth of the eutectic, where a high value of alpha means a potentially good glass-forming alloy 11,12 .The parameter α is described by Eq. ( 1), where x i is the atomic percent of the ith element and T l is the liquid temperature of the alloy.evaluated using differential scanning calorimetry (DSC) and differential thermal analysis (DTA) at a heating rate of 0.67 K/s under a flowing Ar atmosphere.

Results and Discussion
Figure 1 presents the thermograms of the as-quenched ribbon.The arrows in Figure 1a indicate the glass transition temperature (T g ) and the crystallization temperature (T x ), and the arrows in Figure 1b indicate the liquidus temperature (T l ).These measured thermal parameters (T g , T x and T l ), and the calculated ΔT x (supercooled liquid region, ΔT x = T x -T g ), γ parameter (T x /(T g +T l )) 13 and Z C (critical thickness of the composition calculated using the relationship Z c =2.80e(−7) exp (41.70γ)) 13 are summarized in Table 1.According to Lu and Liu 13 , γ values for BMGs vary from 0.350 to 0.500, therefore it would be possible to obtain a BMG with these alloy compositions, and it is theoretically possible to obtain an amorphous structure up to 3.5 mm for the Fe 58 Cr 10 Nb 8 B 24 alloy and up to 1.6 mm for the Fe 56 Cr 12 Nb 8 B 24 alloy.The Fe 60 Cr 8 Nb 8 B 24 alloy obtained a higher ΔTx (=60 K) and Z C (=4.35 mm) value than that reported by Cheney and Vecchio (ΔT x =55 K and Zc=3.44 mm) 11 .
Figure 2 shows the X-ray diffraction patterns of the as-quenched ribbons.It can be observed that all the alloys have a halo without any peaks, which is a characteristic of an amorphous structure, i.e. indicating that there was a formation of an amorphous structure.Despite these XRD results for the Fe 56 Cr 12 Nb 8 B 24 alloy, Figure 3b shows the SEM micrograph in which the crystalline phases can be observed in the cross-section of the ribbon at a quantity that the XRD certainly did not detect.The SEM micrograph of the melt-spun Fe 58 Cr 10 Nb 8 B 24 ribbon showed no evidence of crystalline phases, as shown in Figure 3a.This result, which showed higher GFA of Fe 58 Cr 10 Nb 8 B 24 compared with Fe 56 Cr 12 Nb 8 B 24, was predicted by the γ parameter (0.39 and 0.37 respectively) and by the critical thickness Z C (3.56 and 1.63 mm respectively), as presented in Table 1.
The low tendency of the GFA of the Fe 56 Cr 12 Nb 8 B 24 alloy is attributed to the high Cr content.Nevertheless, in the literature, cases can be found reporting BMG for alloys with a high Cr content, such as in the Fe 43 Cr 16 Mo 16 C 10 B 5 P 10 (at%) 5 and Fe 43 Cr 16 Mo 16 C 15 B 10 (at%) 7 alloys, and this behaviour has been attributed to the high Mo content in these alloys, which is favorable in terms of increasing the GFA 14 .Moreover, in these alloys, the partial substitution of B by C and/or P also helps to increase the GFA 5 .
The Fe 58 Cr 10 Nb 8 B 24 alloy bulk sample, which has a thickness as high as 45 mm processed by the suction casting method, was analyzed by SEM. Figure 4a shows the SEM micrographs of a region (thickness around 1.0 mm) where some crystallized phases in the wedge-section of the Fe 58 Cr 10 Nb 8 B 24 alloy can be observed.In this region, the microstructure consists of a few crystalline phases embedded The parameter ε, which calculates the elastic strain energy that must be stored in an emerging crystalline nucleus, is described in ref. 5.This elastic strain criterion states that the mean local strain must exceed 0.054 in order to destabilize the crystal lattice formation and encourage amorphization 11,12 .
Based on these modeling criteria and on the Fe 60 Cr 8 Nb 8 B 24 alloy reported by Cheney and Vecchio 11 as a good GFA, for this study the Fe 68-x Cr x Nb 8 B 24 (x=10,12) compositions were considered as candidates to be new compositions of BMG with higher corrosion resistance due to the higher Cr content.It can be observed that these compositions have the desirable conditions established by Cheney and Vecchio 11 , i.e., high value of alpha, ε > 0.054 and a chemical short range order parameter (CSRO)< (-0.04).A Nb concentration above 8 at.% was not used because it reduces the GFA 11 .Concerning the B element, a concentration above 24 at.%slightly increases the GFA 11 , thus it was maintained at this concentration as the Fe 60 Cr 8 Nb 8 B 24 alloy.Thus, in order to increase the Cr, it was decided to decrease the Fe content and not the B one because it favors the GFA 4 .
This paper analyzes the effect of the increase in the Cr content on the structure and thermal stability of Fe 68- x Cr x Nb 8 B 24 (x=10,12) alloys.For the sake of comparison, the Fe 60 Cr 8 Nb 8 B 24 alloy reported by Cheney and Vecchio was also evaluated in this work.

Experimental Procedures
The Fe 68-x Cr x Nb 8 B 24 (x=8,10,12) alloy ingots were produced by arc-melting the pure elements under an argon atmosphere.Rapidly solidified ribbons, approximately 1.4 mm wide and 45 μm thick were produced from the ingots, using a single-roller melt spinner at a tangential wheel speed of 40 m/s in an argon atmosphere.Bulk wedge-shaped samples with 0.5 µm up to 5 mm thick and 40 mm long were produced by the suction casting method in a copper mold.The samples were structurally characterized by X-ray diffraction (XRD) with Cu-Kα radiation and scanning electron microscopy (SEM).The liquidus temperature (T l ) of the alloys and the thermal stability of the glassy samples were    in an amorphous matrix (featureless region).Figure 5 shows the X-ray diffraction patterns of three different regions of this sample (with 0.6 mm, 1.0 mm and 3 mm thicknesses).
The diffraction pattern shows a fully glassy structure of 0.6 mm thickness and this is in agreement with Figure 4, where it can be observed that the early crystallized phases appear in the range of 0.6 to 1.0 mm.Combining the SEM and XRD analysis, the experimental critical thickness, Z C , of around 0.6 mm for Fe 58 Cr 10 Nb 8 B 24 alloy is lower than that theoretically predicted (3.56 mm as shown in Table 1).
For the Fe 60 Cr 8 Nb 8 B 24 alloy, the same procedure of characterization of the bulk sample was performed by XRD and SEM.Combining the SEM and XRD analysis, the experimental critical thickness (Z C ) achieved was approximately 1 mm for this alloy, which is lower than that (*) composition reported by Cheney and Vecchio 11 .theoretically predicted in this paper (4.35 mm as shown in Table 1) and also lower than that theoretically predicted by Cheney and Vecchio (3.4 mm) 11 .
Studies to determine the experimental critical thickness by using the bulk wedge-shaped casting method, as seen in this paper, is widely used in this field of research.Nevertheless, the reproducibility of this method is a challenge, as reported by Sanders et al. 15 , where the experimental critical thickness for Al 86 La 5 Ni 9 alloy reported was 685±95 µm after processing three samples.In view of this, the critical thicknesses for the Fe-Cr-Nb-B presented here might be higher, by improving the experimental procedures such as optimizing the alloy preparation and processing parameters.Therefore, the three compositions considered in this paper were ranked and this is in agreement with that predicted theoretically: the GFA decreases with increasing Cr content.

Conclusions
Rapidly solidified glassy ribbons were processed, and based on their thermal characteristics, the theoretical critical thickness (Z C ) was obtained.The estimated Z C of the two new compositions with a higher Cr content were lower than those estimated by Cheney and Vecchio 11 .Furthermore, the critical thickness (Zc) was obtained experimentally using the wedge-shaped casting method, however this value of Zc was much lower than the one estimated theoretically.Furthermore, the GFA ranking among the three compositions studied here was established experimentally and is in agreement with that predicted theoretically, which foresaw that GFA decreases when the Cr percentage is increased.The glass forming ability of Fe 58 Cr 10 Nb 8 B 24 , with a fully amorphous bulk sample which was around 0.6 mm thick according to the DRX analysis (estimated theoretically as high as 3.56 mm), offers a good prospect for a new Fe-based glass former alloy with improved corrosion resistance compared with the already reported Fe 60 Cr 8 Nb 8 B 24 alloy.

Figure 1 :
Figure 1: (a) DSC thermograms for as-quenched ribbon.Arrows indicate glass transition temperature (T g ) and crystallization temperature (T x ); (b) DTA scans for the glass-forming alloys.Arrows indicate liquidus temperature (T l ).

Figure 4 :
Figure 4: (a) Scanning electron micrograph (BSE) showing the critical thickness for the wedge specimen of Fe 58 Cr 10 Nb 8 B 24 alloy; (b) morphology of the crystallized phases.

Figure 5 :
Figure 5: X-ray diffraction patterns of samples extracted from regions of different thicknesses of a copper mold wedge cast alloy.
Fe 60 Cr 8 Nb 8 B 24 .However, the authors did not consider the compositions with a high percentage of both Cr and Nb, which can offer better resistance to corrosion.

Table 1 :
Thermal characteristics of as-quenched ribbon and calculated critical thickness for glassy structure formation (Z C ).