Copper Based Bulk Metallic Glasses for Medical Devices

Bulk metallic glasses (BMGs) represent a viable alternative for replacing classic materials used in medical devices. This paper presents the research conducted in order to obtain copper based BMGs using two different chemical compositions: Cu48Zr47Al5 and Cu45Zr45Al5Ag5. The samples were obtained by copper mold casting and their structure and properties were investigated using X-Ray diffraction (XRD), differential scanning calorimetry (DSC) and optical microscopy.


Experimental part
Pure elements Cu, Zr, Al and Ag were mixed together, than the mixture was melted using an Arc Melter AM 200 furnace, obtaining thus 10 grams batches of Cu48Zr47Al5 and Cu45Zr45Al5Ag5 master alloys. Afterwards, the master alloys were re-melted and casted using a copper mold, obtaining rods with 3 mm diameter and 100 mm length. No special conditions were required during casting, as the process was conducted at room atmosphere.
The structure of the samples was investigated using SEM/EDAX and X-ray diffraction XRD. X-ray diffraction was conducted using an X'Pert³ Powder diffraction system, with the radiation of a Cu anode with a wavelength λ = 1.54 Å. Differential scanning calorimetry (DSC) was used to investigate the thermal stability, on a Netzsch STA 441 Jupiter apparatus under a flow of purified nitrogen. The glass transition temperature Tg, the crystallization temperature Tx and the melting temperature Tm were determined as the onset temperatures of the glass transition the crystallization and melting peak, respectively, and the liquidus temperature Tl as the offset of the melting peak during heating with a constant rate of 0.33 K/s. Vickers hardness HV was measured using a Wolpert Micro-Vickers Hardness Tester machine at room temperature using a 100 grams load. At least 10 measurements were conducted, in order to ensure the accuracy of the results. Olympus BX51M microscope with Analysis 5.0 software was used to microscopically analyze the alloys microstructure.

Results and discussions
In order to confirm the structural homogeneity of the master alloys, SEM analysis was conducted on both batches. The images are presented in figure 1. The Cu48Zr47Al5 alloy shows a casting structure with dendrites, specific to crystalline materials, while the Cu45Zr45Al5Ag5 alloy exhibits only slight traces of dendrites embedded in an eutectic structure, indicating thus a higher amorphization susceptibility. X-ray diffraction shows the presence of high peaks for the Cu48Zr47Al5 master alloy (figure 2a), characteristic to crystalline materials, which together with the low eutectic quantity indicate a lower glass forming ability (GFA). The second X-ray diffraction (figure 2b) confirms the results given by the SEM image, that the alloy Cu45Zr45Al5Ag5 is suitable for amorphization. The rods casted in a copper mold (figure 3) were macroscopically analyzed. The examination emphasized the metallic luster and the absence of defects such as pores or cracks. The presence of broad peaks in diffraction image (figure 4) confirms that the structure is amorphous only for the Cu45Zr45Al5Ag5 alloy. The structure of the Cu48Zr47Al5 alloy is microcrystalline, with peaks corresponding to compounds such as CuZr3, ZrCu, Zr2Cu, Zr2Al and Zr.  In case of the Cu48Zr47Al5 alloy, the DSC curve shows an exothermic inflexion which marks the recrystallization of the microcrystalline metastable phase, obtaining as a result of the rapid cooling. Heating over Tx = 472 o C leads to obtaining a crystalline structure, similar to the master alloy.
In case of the Cu45Zr45Al5Ag5 alloy the DSC curve exhibits an exothermic peak which marks the crystallization event and an endothermic peak for the melting. The glass transition temperatures Tg, the crystallization temperatures Tx and the ΔTx = Tx-Tg parameter which estimates the glass forming ability was calculated for this alloy. The values are listed in table 1. For a good GFA the supercooled liquid region ΔTx, should be as large as possible [19], while a high Tx means a higher stability of the glass. It can be seen from table 1 that the addition of silver ensures the glass transition temperature Tg of 430 o C and a good the thermal stability, with crystallization temperature Tx of 487 o C. Meanwhile, the melting temperature slightly moves to lower values. ΔTx, the most common parameter used to determine the GFA, is 57 o C for the alloy that contains the Ag addition and it is well known that a high value for ΔTx ensures a good GFA.
Regarding the mechanical properties, it was found that the alloys containing silver have a slightly lower microhardness than the Cu48Zr47Al5 alloy.
Since silver tends to form eutectics with Cu, Zr, Al, according to the confusion principle, addition of silver to the chemical composition leads to the formation of a quaternary eutectic. As a result, the melting temperature decreases and it favors the transformation of the liquid into an amorphous solid. Meanwhile, it is well known that silver has low hardness and its tendency of forming solid solutions with Cu, Al, Zr can explain the decrease of hardness exhibit by the Cu45Zr45Al5Ag5 alloy [20].
The microscopic image of the arc melted Cu45Zr45Al5Ag5 alloy ( Figure 6) presents two compounds with distinct shapes precipitated in the matrix. Different studies [5,16] reported the presence of structures similar to those presented in this research. After casting, the structure is refining, but does not substantially change (Figure 7).

Conclusions
Cu48Zr47Al5 and Cu45Zr45Al5Ag5 bulk metallic glasses were obtained in the shape of 3 mm diameter and 100 mm length rods using the copper mod casting method. The novelty consists of the fact that no special conditions were required during casting, as the process was conducted at room atmosphere.
It was found that the addition of silver in the chemical composition leads to a value for ΔTx (the most common parameter used to determine the GFA) of 57 o C. However, the hardness decreases slightly when adding silver in the chemical composition of the alloy. Considering the results, further investigations are required for these alloy class and future tests will include compression and corrosion tests.