Effect of Sn Addition in the Microstructure Refinement and Corrosion Resistance of Cu-Zr-AlAg Alloy

In this paper, a composition of known high glass-forming ability, Cu42Zr42Al8Ag8 and a novel Cu42Zr42Al8Ag4Sn4, were produced by suction-casting. After rapid solidification of both alloys, X-ray diffraction patterns along with crystallite size estimation by Scherrer equation and scanning electron microscopy images revealed a refined microstructure, composed mainly by AlCu2Zr, CuZr2 and Cu10Zr7 phases. Transmission electron microscopy indicated the presence of small crystalline precipitates in the Cu42Zr42Al8Ag4Sn4 suction-casted sample. Polarization curves showed a higher passivation current in the Sn containing sample, suggesting a decrease in corrosion resistance.


Introduction
Bulk metallic glasses, BMG, and nanocrystalline alloys represent new possibilities for microstructural engineering of materials.One way to measure the glass forming ability (GFA) is the critical cooling rate to avoid the ''nose'' of the time-temperature-transition (TTT) curve for a specific alloy.The nucleation and growth mechanism determines the shape and position of the TTT curve in the temperaturetime space.In most of cases, by avoiding the nose, external infuencens prevent inherent homogeneous crystallization [1][2][3][4] .If a critical cooling rate is achieved for a stated dimension 5 , compositions that possess high glass-forming ability can be produced as amorphous solids by rapid solidification processes, such as suction-casting 6 .
Systems that possess high GFA usually follow the conventional proposed criteria involving relations between crystallization, melting and glass transition temperatures, along with a negative heat of mixing, topological factors, critical cooling rate and critical dimension 7 .Eutectic Copper-Zirconium based alloys exhibit high GFA, the rapid solidification of these alloys can lead to the formation of BMG's [8][9][10] .Near eutectic Cu-Zr based systems have been extensively studied due to their interesting properties, such as high corrosion resistance 11,12 and good mechanical strength 13,14 .These systems are thus useful as engineering materials.Zr-(Cu-Ag)-Al bulk metallic glasses can be produced by suction-casting in copper molds with large diameters, the Zr 48 Cu 38.5 Ag 5.5 Al 8 composition achieving 20 mm 15 and the Cu 43 Zr 43 Al 7 Ag 7 , 8mm 16 .
However, contrary to crystals, BMGs suffer from a lack of crystalline defects such as dislocations and commonly do not present ductility in uniaxial mechanical tests 17 .In such case, BMGs lead to shear bands formation, localizing the plastic deformation 18 .This restricts the overall plasticity of BMGs in most cases to less than 1% and hampers the use of BMGs as a structural material in several possible utilization.For the sake of overcoming the problem of the poor plasticity of BMG at room temperature and allow the development of structural materials, attempts have been concentrated on the production of BMG composites.A diversity of composite materials have been produced either by adding a crystalline phase into the liquefied BMG 19 or by in situ precipitation of crystalline phases throughout partial crystallization 20,21 .Such a methodology allows enhanced plasticity in BMG composites by generating multiple shear bands, which will interact with each other, thus limiting a single shear band from crossing within the material.
A simple way of producing such a composite is by not reaching the critical cooling rate.The product of solidification may then present a second phase embedded in the amorphous alloy, improving the yield 22 and flexural strength 23 , and ductility 24 .
Sn was chosen as substitutional element due to the improved corrosion resistance in Zirconium based alloys when small amount of this element is added 25

Experimental
An arc-melter Edmund Bühler GmbH furnace, with 5mm diameter copper mold, was used to produce suction-casted samples of Cu 42 Zr 42 Al 8 Ag 8 and of the proposed composition Cu 42 Zr 42 Al 8 Ag 4 Sn 4 .To crystallize the alloys during suction casting the water flow in the refrigeration system was reduced, also reducing the exchanged heat.Melting processes were performed under Argon atmosphere.To prepare these alloys Cu powder (99.99 % purity), Zr nuclear quality 99.9% purity, Al 99.99% purity, Ag 99.99% purity and Sn 99.9% purity were used.To solve the problem of Sn evaporation, Zr was positioned above Sn and these elements were previously molten together, in order to form the α-Zr and Zr 4 Sn phases in equilibrium, with the melting temperature of approximately 1000ºC 26 .After this step, this material was molten along with other elements.
XRD measurements were performed in G8-Discover Bruker equipment with Cu-Kα radiation, λ =1.5418 Å. Present phases were identified by comparison with the XRD patterns simulated by Powder Cell software, based on crystallographic data obtained from literature 27,28,29 .The Scherrer equation was used for crystallite size estimation from the XRD patterns, along with Gaussian fittings.The chosen shape factor was k = 0.9., and these phases along with of AlCu 2 Zr are reported in the literature after copper mold casting of Cu 40 Zr 44 Al 8 Ag 8 28 .The red arrows in Figures 2  and 3 point to the analyzed peaks, used to generate the full width at half maximum (FWHM) and crystallite size data in Table 1.The suction-casting process led to remarkably higher FWHM and smaller crystallite sizes for all analyzed peaks in both compositions.The rapid solidification process promoted great microstructure refinement instead of leading to the formation of amorphous alloys.It can also be noted for AlCu 2 Zr, CuZr 2 and Cu 10 Zr 7 analyzed peaks, that the rapid cooling promoted similar crystallite sizes for both compositions considering the uncertainties.The addition of Sn in Cu-Zr-Al-Ag system did not lead to a significant difference in microstructure refinement obtained by the performed suction-casting.

Results and Discussions
The back-scattered electrons SEM image of the Cu 42 Zr 42 Al 8 Ag 8 arc-melted sample (Figure 4.a) presents two main types of precipitated morphologies: "drop-like" and "cross", dispersed in the matrix.It is noteworthy that the "cross" morphology resembles dendritic structures.After the suction-casting process, the scanning electron microscopy image (Figure 4.b) shows more refined "cross" structures, with the "drop-like" structure not exhibiting a substantial     5.a and the atomic percent results are in Table 2. Point 1 is marked inside the "drop-like" precipitate and point 2 in the matrix.The results of the EDS analysis allow for inferring that all elements are present in both regions, and point 1 contains less Cu and Ag.These regions could not be assigned to specific phases.The presence of Oxygen in point 1 is probably due to an oxidation process.White oriented columns, cross structures, and other structures without defined shape are dispersed inside the matrix, in the microstructure of the Cu 42 Zr 42 Al 8 Ag 4 Sn 4 arcmelted sample (Figure 4.c).After rapid solidification, the formation of white oriented columnar structures is suppressed and the white phase is quenched into smaller structures (Figure 4.d).To investigate the composition of the different regions of the Cu 42 Zr 42 Al 8 Ag 4 Sn 4 suction-casted sample, EDS analysis was performed.The selected points for EDS analyses and results are shown in figure 5.b and Table 2. Point 3 is marked on the white phase and presents Sn , Zr, Cu, and Al in its composition.Point 4 is marked inside the matrix and presents Ag, Zr, Cu and Al in its composition.Therefore, the white phase contains Sn and not Ag, while in the matrix region this relation is the opposite.The relevant atomic percent of Sn exhibited on point 3 suggests that the white columnar phases contain this element and these structures may contain the AlSn 3 Zr 5 phase.Figures 4.e and 4.f show the TEM image of Cu 42 Zr 42 Al 8 Ag 4 Sn 4 suction-casted sample and the SAEDP.The presence of small crystalline precipitates in grain boundaries, pointed by red arrows, is remarkable.Furthermore, in SAEDP, small dispersive points in diffusive halo can be noted.This behavior can be related to small crystalline structures 31,32 .
Figure 6 presents the polarization curves and the Table 3 displays the passivation current (I) for all analyzed samples.For Cu 42 Zr 42 Al 8 Ag 8 composition, the peak at the beginning of anodic polarization and passivation current became lower after the suction-casting.This behavior suggests better corrosion resistance after microstructure refinement 11 , as expected.Considering the Cu 42 Zr 42 Al 8 Ag 4 Sn 4 composition, although the suction-casted sample presented a lower peak at beginning of anodic polarization, the passivation current was higher.This suggests that the microstructure refinement promoted by rapid solidification process in the proposed composition did not lead to an increase in corrosion resistance.The passivation currents for arc-melted and suction-casted

Conclusions
The Cu 42 Zr 42 Al 8 Ag 8 and the novel Cu 42 Zr 42 Al 8 Ag 4 Sn 4 suction-casted samples presented great microstructural refinement, as shown by XRD, SEM and crystallite size estimation.After rapid solidification, the crystallite size estimation of the common peaks resulted in similar values.The rapid solidification of the proposed composition led to the formation of small crystalline precipitates at grain boundaries, as noted in TEM images.The addition of Sn led to high passivation currents.These results allow for inferring that Sn addition did not clearly affect microstructural refinement, however, it decreased the corrosion resistance.

a
Centro de Tecnologia -Ilha do Fundão, Programa de Engenharia da Nanotecnologia, Universidade Federal do Rio de Janeiro, Caixa Postal 68505, Rio de Janeiro, RJ, Brasil b Programa de Engenharia Metalúrgica e de Materiais, Universidade Federal do Rio de Janeiro, Caixa Postal 68505, Rio de Janeiro, RJ, Brasil effects promoted by Sn addition.The rapidly solidified alloys were produced by the suction-casting method and characterized by X-ray diffraction (XRD) along with crystallite size estimation by Scherrer equation, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) techniques.Polarization curves were also obtained for corrosion resistance inference.
SEM images in back-scattered electrons mode were acquired with a Jeol JSM 6460LV.TEM images of the Cu 42 Zr 42 Al 8 Ag 4 Sn 4 suction-casted sample were acquired with FEI TECNAI G2 F20 microscope.The potentiodynamic polarization curves were performed in H 2 SO 4 1M solution under open air at room temperature and conducted in a three-electrode cell using a platinum counter electrode and a Hg/HgCl 2 reference electrode.These curves were measured at a rate of 600 mV/h after open-circuit immersion for about 30 min when the open-circuit potential became almost steady.The minimum and maximum applied potential were -2V and 2V.

Figure 1
Figure 1 shows the Cu 42 Zr 42 Al 8 Ag 4 Sn 4 suction-casted sample.Figures 2 and 3 show the diffraction patterns for the Cu 42 Zr 42 Al 8 Ag 8 and the Cu 42 Zr 42 Al 8 Ag 4 Sn 4 samples, under the two studied conditions.Cu 10 Zr 7 was markedly the most present phase.AlCu 2 Zr and CuZr 2 peaks were also identified in all four samples.CuZr 2 and Cu 10 Zr 7 are present in the Cu-Zr binary diagram30 , and these phases along with of AlCu 2 Zr are reported in the literature after copper mold casting of Cu 40 Zr 44 Al 8 Ag 828 .The red arrows in Figures2 and 3point to the analyzed peaks, used to generate the full width at half maximum (FWHM) and crystallite size data in Table1.The suction-casting process led to remarkably higher FWHM and smaller crystallite sizes for all analyzed peaks in both compositions.The rapid solidification process promoted great microstructure refinement instead of leading to the formation of amorphous alloys.It can also be noted for AlCu 2 Zr, CuZr 2 and Cu 10 Zr 7 analyzed peaks, that the rapid cooling promoted similar crystallite sizes for both compositions considering the uncertainties.The addition of Sn in Cu-Zr-Al-Ag system did not lead to a significant difference in microstructure refinement obtained by the performed suction-casting.The back-scattered electrons SEM image of the Cu 42 Zr 42 Al 8 Ag 8 arc-melted sample (Figure4.a)presents two main types of precipitated morphologies: "drop-like" and "cross", dispersed in the matrix.It is noteworthy that the "cross" morphology resembles dendritic structures.After the suction-casting process, the scanning electron microscopy image (Figure4.b)shows more refined "cross" structures, with the "drop-like" structure not exhibiting a substantial

Figure 4 .
Figure 4. From a-b images of Cu 42 Zr 42 Al 8 Ag 8 samples: SEM of arc-melted and SEM of suction-casted, respectively.From c-d images of Cu 42 Zr 42 Al 8 Ag 4 Sn 4 samples: SEM of arc-melted and SEM of suction-casted, respectively.Images (e) and (f): TEM of Cu 42 Zr 42 A l8 Ag 4 Sn 4 suction-casted sample with nanocrystalline precipitates pointed by red arrows and its SAEDP, respectively.

Figure 6 .
Figure 6.Potentiodynamic polarization curves for arc-melted (AM) and suction-casted (SC) samples.The selected points for passivation current density (I) acquisition are indicated by arrows.
and its low cost in comparison to Ag.Furthermore, the atomic radius and electronegativity of Ag and Sn are very similar, thus allowing keeping the good GFA of the alloy.This work aims to synthesize by a rapid solidification method Cu 42 Zr 42 Al 8 Ag 8 and Cu 42 Zr 42 Al 8 Ag 4 Sn 4 alloys, infer about the microstructure refinement and corrosion resistance

Table 1 .
28sults of full width at half maximum (FWHM) and crystallite sizes for arc-melted (AM) and suction-casted (SC) systems, with uncertainties (σ).difference.A study of the microstructure of Cu 40 Zr 44 Al 8 Ag 8 rapidly solidified in copper mold28reported the presence of a "skeletal phase" with an approximate stoichiometry of CuZr 2 , which resembles the observed "cross" structure.The existence of a "blocky phase" is also reported with a stoichiometry of approximately Cu 10 Zr 7 , resembling the "droplike" structures observed in this study.The points selected for EDS analysis of Cu 42 Zr 42 Al 8 Ag 8 suction-casted sample are shown in Figure