Effects of Nb microalloying on properties of Zr-Al-Fe-Cu glassy alloy

The effects of Nb-microalloying on glass forming ability, thermal properties and mechanical properties of (Zr0.6032Cu0.2256Fe0.0995Al0.0717)100-xNbx (x = 0, 1, 2, 3, 4) alloys were investigated. The best glass former was obtained for (Zr0.6032Cu0.2256Fe0.0995Al0.0717)97Nb3, which could be fabricated into full glass with diameter up to 6 mm at least. In addition, the origin of enhancing GFA of ZrAlFeCu amorphous alloy by means of the minor addition of Nb, Gd and Hf, was also discussed from the aspects of clusters and mixing entropy, which might provide a method of understanding the mechanism of enhancing glass-forming ability via microalloying, and choosing minor alloying element with an aim of enhancing glass-forming ability. It was found that the thermal stability reduced as the content of Nb increased along with the supercooled liquid region decreased. Nb-microalloying decease the fracture strength. However, moderate Nb microalloying could enhance the room temperature plastic strain.


Introduction
Owing to the high glass-forming ability (GFA), good biocompatibility, excellent mechanical properties and high corrosion resistance, Zr-based bulk metallic glasses (BMGs) have been chosen as a candidate for biomedical applications [1][2][3]. In recent years, a series Be-free Zr-based BMGs have been developed, such as in Zr-Al-Fe, Zr-Al-Fe-Cu, Zr-Al-Ni, Zr-Al-Cu and Zr-Al-Co-Ag etc systems [4][5][6][7][8]. Among the designed Zr-based glass formers, those containing Ni element are blamed for being allergic and possibly carcinogenic [7]. Besides, considering the cost of manufacturing, glass-formers containing noble metal, might hinder the application. Based on this, taking into account various factors such as manufacturability, the cost and the mechanical properties, Zr-Al-Fe-Cu BMGs are attractive, and the experimental results have shown the feasibility of biomedical devices applications reported by Jin and Han et al researchers [2,9].
In our previous work, a method combining clusters and mixing entropy was applied to understand glass formation and design good glass formers [19]. Under the guidance of this method, a novel Ni-free Zr-based glass former Zr 60.32 Cu 22.56 Fe 9.95 Al 7.17 was designed with a critical diameter up to 5 mm, superior to the well-known composition Zr 60 Cu 25 Fe 5 Al 10 BMG under the same laboratory condition, which shows a good prospect for future application as biomedical materials [20].
As mentioned above, Nb-microalloying always plays an important role in enhancing GFA and changing properties. However, the discussion on the mechanism of Nb-microalloying understood via microstructure is quite few. Based on this, the effects of minor addition of Nb on GFA, thermal properties and mechanical properties of Zr 60.32 Cu 22.56 Fe 9.95 Al 7.17 have been investigated. The reasons for influencing GFA, thermal properties and mechanical properties were discussed from the aspect of clusters and mixing entropy with an aim of offering a method of understanding the mechanism of microalloying, which might lay a solid foundation of helping us quickly and exactly choose the beneficial microalloying elements.

Materials and methods
Zr-based alloys with compositions of (Zr 0.6032 Cu 0.2256 Fe 0.0995 Al 0.0717 ) 100-x Nb x (x=0, 1, 2, 3, 4) and (Zr 0.6032 Cu 0.2256 Fe 0.0995 Al 0.0717 ) 97 Gd 3 were prepared in a Ti-gettered argon atmosphere. To ensure the homogeneity, the master alloys were melted four times. The purities of elements are 99.99 wt. % for Al, Fe, Cu and Nb ,99.95 wt. % for Zr and Gd. Bulk samples with diameters of 2 mm and 6 mm were produced by copper mold suction casting. The ribbons (0.02 mm×1.2 mm) were fabricated by melt spinning. The structure of the sample was identified by x-ray diffraction (XRD, Philips PW 1050, Cu Kα). The thermodynamic parameters of glassy rods, such as the glass transition temperatures and the liquids temperatures etc, were evaluated using differential scanning calorimetry (DSC, Netzsch DSC 404C) at a heating rate of 0.67 K s −1 . The compression properties were tested, using samples having 4 mm long and 2 mm diameter, by Instron testing machine at a strain rate of 5.0×10 −4 s −1 . The fracture features of the specimens were observed by scanning electron microscope (SEM, Supra 35). Figure 1 shows the XRD patterns of the casted (Zr 0.6032 Cu 0.2256 Fe 0.0995 Al 0.0717 ) 100-x Nb x (x=0, 1, 2, 3, 4) alloys with diameters of 6 mm. Under the same laboratory environment, the XRD pattern of Zr 60.32 Cu 22.56 Fe 9.95 Al 7.17 (x=0) with a diameter of 5 mm was examined and shown in our previous paper [20]. The results showed that Zr 60.32 Cu 22.56 Fe 9.95 Al 7.17 s could form full amorphous alloy with diameter up to 5 mm. However, as shown in figure 1, the XRD pattern of Zr 60.32 Cu 22.56   As can be concluded above, the critical diameter of Zr 60.32 Cu 22.56 Fe 9.95 Al 7.17 increased from 5 mm to 6 mm with the addition of 3at% Nb. As one of effective ways of enhancing glass forming ability, the internal mechanism of minor alloying is still unclear from the aspect of clusters. It is essential to figure out the origin of high GFA of Zr 60.32 Cu 22.56 Fe 9.95 Al 7.17 before understanding the mechanism of enhancing GFA via microalloying.

Results and discussion
In our previous work, a method combining clusters and mixing entropy was applied to understand glass formation and design good glass formers [19]. Essentially, this method of understanding glass forming ability balances both microstructure and thermodynamics. Under the guidance of this method, clusters were treated as the basic units of glass formers. The coefficients of clusters are calculated based on the premise that composition owns the corresponding largest mixing entropy.
Under the guidance of this method, glass formation in Zr-Al-Fe-Cu system was studied in our previous work [20]. The best glass former Zr 60.32 Cu 22.56 Fe 9.95 Al 7.17 could be expressed as Fe(Fe 3 +Zr 9 )+0.961Al(Zr 8 +Al 2 )+1.511Cu(Cu 5 +Zr 5 ). These Zr-Fe, Zr-Al and Zr-Cu topological clusters Fe(Fe 3 +Zr 9 ), Al(Zr 8 +Al 2 ) and Cu(Cu 5 +Zr 5 ) are the basic units of this glass former. The high topological packing in clusters and high entropy are the origin of high GFA of Zr 60.32 Cu 22.56 Fe 9.95 Al 7.17 .
The enthalpies of mixing of the atomic pairs Nb-Zr, Nb-Al, Nb-Fe and Nb-Cu at equi-atomic compositions are respectively ΔH Nb-Zr =4KJ mol −1 , ΔH Nb-Al =−18KJ mol −1 , ΔH Nb-Fe =−16KJ mol −1 and ΔH Nb-Cu =−3KJ mol −1 [21]. The negative enthalpies of mixing mean the tendentiousness of gathering together to form clusters. However, it has been pointed that, to enhance GFA of basic composition, the microalloying element enjoying negative enthalpies of mixing between component elements is a prerequisite but not sufficient condition. The key to enhancing GFA is introducing topological clusters.
As analyzed above, Nb-Fe and Nb-Al pairs are negative. They are likely to gather together to form clusters. Miracle pointed that, the cluster was similar to the microstructure of competing phases [22]. As for Nb-Fe pairs, a novel Fe-centered Fe-Nb 7.5 Fe 4.5 could be obtained as shown in figure 1. By calculating the radius ratio, the degree of close packing in clusters could be evaluated.
The Goldschmidt radiuses of Fe and Nb are 0.128 nm and 0.147 nm. As for cluster Fe-Nb 7.5 Fe 4.5 , the radius of center atom Fe is 0.128 nm. The average radius of atoms in the cluster's shell is 0.1399. The ratio of the radius of center atom to the average radius of atoms in the cluster's shell is 0.9151. The ideal radius ratio has been calculated according to the coordination number (CN) [23]. The CN of cluster Fe-Nb 7.5 Fe 4.5 is 12. The ideal radius ratio for CN12 cluster is 0.902 [23]. The deviation of calculation result and theoretical value is only 1.45%. The cluster Fe-Nb 7.5 Fe 4.5 meets the requirement of topological packing. As for Nb-Al binary pair, it has been reported that Nb and Al could form topologically packed Al-centered Al-Al 8 Nb 4 cluster. These topologically packed clusters could increase the degree of atomic packing state, which is beneficial for glass-formation. Moreover, Nb is adjacent to Zr in the periodic table of the elements. Nb element might take the place of the position of Zr in Zr-based clusters, namely changing from Zr-Al, Zr-Fe binary clusters to Zr/Nb-Al and Zr/Nb-Fe clusters, which might enhance the degree of clusters' topological packing.
For the Zr-based amorphous alloys, except Nb element, rare-earth element was also one of the important microalloying elements [24]. To further study the mechanism of rare-earth element microalloying, similarly, 3 at% Gd was also microalloying to Zr 60.32 Cu 22.56 Fe 9.95 Al 7.17 . As shown in figure 1, the GFA of Zr 60.32 Cu 22.56 Fe 9.95 Al 7.17 increased from 5 mm to at least 6 mm. The enthalpies of mixing of the atomic pairs Gd-Zr, Gd-Al, Gd-Fe and Gd-Cu at equi-atomic compositions are respectively ΔH Gd-Zr =9KJ mol −1 , ΔH Gd-Al =−39KJ mol −1 , ΔH Gd-Fe =−1KJ mol −1 and ΔH Gd-Cu =−22KJ mol −1 [21]. In our previous study, a CN12 cluster Al-Al 6 Gd 6 was obtained from Al 2 Gd [19]. Similarly, as shown in figure 2, a CN12 cluster Fe-Fe 6 Gd 6 could be obtained from phase Fe 2 Gd. The Goldschmidt radiuses of Gd, Fe and Al are 0.180 nm, 0.128 nm and 0.143 nm, separately. Similar to the calculation of Nb-Fe cluster, the actual radius ratios of Fe-Fe 6 Gd 6 and Al-Al 6 Gd 6 are 0.831 and 0.885. The deviation of calculation results and theoretical value are −7.87% and −1.89%. The above two Fe-Fe 6 Gd 6 and Al-Al 6 Gd 6 can be treated as topologically packed clusters. The new topologically packed clusters introduced by Gd-microalloying, could increase the degree of atomic packing state, thus enhancing the GFA of basic composition.
Similarly, it has been proven effective in enhancing GFA of ZrAlFeCu amorphous alloys via Hfmicroalloying [2]. It could also be explained via clusters. The enthalpies of mixing of the atomic pairs Hf-Zr, Hf-Al, Hf-Fe and Hf-Cu at equi-atomic compositions are respectively ΔH Hf-Zr =0KJ mol −1 , ΔH Hf-Al =−39KJ mol −1 , ΔH Hf-Fe =−21KJ mol −1 and ΔH Hf-Cu =−17KJ mol −1 [21]. The enthalpies of mixing of Hf-Al, Hf-Fe and Hf-Cu binary pairs are negative, which means the tendency of gather together to form clusters. As shown in figure 3, an Archimedean octahedral anti-prism CN10 cluster Al-Al 2 Hf 8 is obtained from phase AlHf 2 . Two CN12 clusters Fe-Fe 6 Hf 6 and Fe-Fe 3 Hf 9 are obtained from phase Fe 2 Hf and FeHf 2 . The Goldschmidt radiuses of Hf, Fe and Al are 0.159 nm, 0.128 nm and 0.143 nm, separately. The ideal radius ratios of CN10 and CN12 cluster are 0.799 and 0.902 [23]. As for Al-Al 2 Hf 8 , Fe-Fe 6 Hf 6 and Fe-Fe 3 Hf 9 clusters, the actual radius ratios are 0.918, 0.892 and 0.846. The deviation of calculation results and theoretical value are 14.89%, −1.11% and −6.21%. It can be seen that, Fe-Fe 6 Hf 6 and Fe-Fe 3 Hf 9 clusters are topologically packed clusters. As for Hf-Cu binary system, a series of topologically packed clusters Cu-Cu 8 Hf 4 , Cu-Cu 7 Hf 5 and Cu-Cu 5 Hf 5 have been found and used to design good glass-formers in Cu-Hf-Al system [25]. Moreover, Hf and Zr are in the same group on the periodic table. They share similar atomic radius, physical properties and chemical properties. Hf might take the place of the position of Zr in Zr-based clusters, namely changing from Zr-Al, Zr-Fe and Zr-Cu binary clusters to Zr/Hf-Al, Zr/Hf-Fe and Zr/Hf-Cu clusters, which might enhance the degree of topological packing.  It can be seen that, Nb-, Gd-and Hf-microalloying could enhance the GFA of ZrAlFeCu amorphous alloys. On one hand, from the point of clusters, with the addition of Nb/Gd/Hf element, the newly introduced topologically clusters might enhance the degree of atomic packing state, which is beneficial for glass formation. On the other hand, from the point of entropy, with the addition of Nb/Gd/Hf, the degree of chaos of the system rising, it would be more difficult for the precipitation of crystalline phases which is also beneficial for glass formation.
It can be inferred that, due to the topologically packed Al-Al 8 Nb 4 , Fe-Nb 7.5 Fe 4.5 , Fe-Fe 6 Gd 6 , Al-Al 6 Gd 6 , Fe-Fe 3 Hf 9 , Fe-Fe 6 Hf 6 clusters, Nb-, Gd-and Hf-microalloying might be effective in enhancing the GFA of the amorphous alloys containing with element Al or Fe. Furthermore, these Hf-based and Gd-based binary topologically packed clusters could help researchers further develop Hf-based and Gd-based glass-formers. Figure 4 shows the DSC curves of as-cast (Zr 0.6032 Cu 0.2256 Fe 0.0995 Al 0.0717 ) 100-x Nb x (x=0, 1, 2, 3, 4) metallic glass alloy series, showing their crystallization and melting behaviors. As shown in figure 4, the glass transition temperatures (T g ), crystallization temperatures (T x ), solidus temperatures (T m ) and liquidus temperatures (T l ) of these alloys are marked with arrows. Thermal parameters are given in table 1. A series of typical thermal parameters T rg (= T g /T l ) [26], γ (= T x / (T g +T l )) [27] and γ m (= (2T x -T g ) / T l ) [28] has been proposed to predict glass forming ability. These parameters are summarized in table 2.
A series of crystallization behaviors of as-cast (Zr 0.6032 Cu 0.2256 Fe 0.0995 Al 0.0717 ) 100-x Nb x (x=0, 1, 2, 3, 4) metallic glass alloys are shown in figure 4(a). The heating rate is 40K min −1 . It can be seen that, with the temperature increased, all of the curves appear one distinct endothermic platform and two distinct exothermic peaks, which correspond to the glass transition and crystallization process respectively, showing typical amorphous crystallization behaviors. Combining figure 4(a) and table 1, it can be found, with the addition of  Nb, the glass transition temperature (T g ) of this system increases along with the content of Nb increases. When the content of Nb does not exceed 3 at%, the crystallization temperature (T x ) of this system increases along with the content of Nb increases, while the content of Nb increases to 4 at. %, the crystallization temperature slightly decreased. Figure 4(b) shows the melting behaviors of (Zr 60.32 Cu 22.56 Fe 9.95 Al 7.17 ) 100-x Nb x (x=0, 1, 2, 3, 4) metallic glass alloys. It can be seen that minor addition of Nb doesn't change the melting process of Zr 60.32 Cu 22.56 Fe 9.95 Al 7.17 . All of the curves show a distinct endothermic peak and a weak endothermic peak, indicating these alloys have multiple melting phases.
It can be seen that, with the addition of Nb, the supercooled liquid region ΔT decreases, along with the thermal stability getting worse. However, moderate minor Nb addition could enhance the glass forming ability. It can be inferred that, in the system of Zr-Al-Fe-Cu-Nb, the width of the supercooled liquid region ΔT could not properly describe the glass forming ability. As stated above, a series of thermal parameters T rg , γ and γ m have been proposed to predict the glass forming ability. However, combining table 2 and figure 1, all of the parameters couldn't represent the glass-formation ability well.
As can be seen in figure 5 and table 2, with the addition of Nb, the supercooled liquid region ΔT decreases, along with the thermal stability get worse. As analyzed above, the enthalpies of mixing of the atomic pairs Nb-Zr and Nb-Cu are positive quite close to zero, Nb has an aggregation phenomenon. With excessive Nb element doping, the phenomenon of aggregation is enhanced, which would promote the formation of Nb-rich clusters, increase the possibility of nucleation and precipitation, decrease the thermal stability of alloys in supercooled liquid region, thus causes narrowing of the width of supercooled liquid region in DSC curves.
The number of short-range order clusters and activation energy have a close relationship [29]. Based on the DSC curves of (Zr 0.6032 Cu 0.2256 Fe 0.0995 Al 0.0717 ) 100-x Nb x (x=0, 1, 2, 3, 4) amorphous ribbons at the heating rates of 0.167 K s −1 , 0.333 K s −1 , 0.5K s −1 , 0.667 K s −1 and 0.833K s −1 , under the guidance of Kissinger equation [30], the corresponding Kissinger plots of (Zr 0.6032 Cu 0.2256 Fe 0.0995 Al 0.0717 ) 100-x Nb x (x=0, 1, 2, 3, 4) are shown in figure 6. The activation energies for crystallization growth E p of (Zr 0.6032 Cu 0.2256 Fe 0.0995 Al 0.0717 ) 100-x Nb x (x=0, 1, 2, 3, 4) have been obtained. For x=0,1,2,3 and 4, the E p are separately 190.5 kJ mol −1 , 218 .9 kJ mol −1 , 218 .0 kJ mol −1 , 222 .2 kJ mol −1 and 282.6 kJ mol −1 . Compared to primary composition Zr 60.32 Cu 22.56 Fe 9.95 Al 7.17 , with Nb-microalloying, the activation energies for crystallization growth E p for (Zr 0.6032 Cu 0.2256 Fe 0.0995 Al 0.0717 ) 100-x Nb x (x=0, 1, 2, 3, 4) increased. Ep stands for the activation energy of the growth of crystal phases. One of the steps of growth of crystal phases is breaking down the local structure (cluster) to form ordered crystalline phases. However, the clusters in this paper obtained according to the Miracle theory, enjoy special asymmetry and high degree of topological packing. Firstly, from the point of thermal dynamics, the high topologically packed clusters would increase viscosity of molten alloy, which would increase the difficulty of the growth of crystalline phases. Secondly, from the point of energy, the high topologically packed clusters would also decrease the thermodynamic free volume, thus decreasing the energy of system, leading a more stable state, which would also increase the difficulty of the growth of crystalline phases [25]. Thirdly, from the shape of clusters, forming ordered crystalline phases need break down more clusters due to the special asymmetry of obtained clusters, which would also enhance the difficulty of crystalline phases   growing. Nb microalloying might bring more topologically packed clusters. As analyzed above, these clusters would greatly enhance the difficulty of the growth of crystal phases. Therefore, the activation energy of the growth of crystal phases Ep would increase along with Nb doping. As shown in figure 7, during the compression process, (Zr 0.6032 Cu 0.2256 Fe 0.0995 Al 0.0717 ) 100-x Nb x (x=0, 1, 2, 3) alloys have elastic strain and plastic strain. However, with 4 at. % Nb addition, the compressive deformation behavior displays a ductile-to-brittle transition. The compressive deformation behavior is sensitive to compositions.
The mechanical parameters of as-cast (Zr 0.6032 Cu 0.2256 Fe 0.0995 Al 0.0717 ) 100-x Nb x (x=0, 1, 2, 3, 4) glassy alloy series are shown in table 3. σ y , σ m and ε f represent yield stress, maximum compressive strength and fracture strain, separately. As can be seen in table 3, moderate Nb-microalloying could enhance the fracture strain. As can be seen in figure 8(c), for the primary alloy composition Zr 60.32 Cu 22.56 Fe 9.95 Al 7.17 , there exists a small number of parallel and crossed shear bands. These shear bands are mostly in one direction. A certain amount of shear bands corresponds to the high plasticity. However, based on this, with 2 at% Nb further doping, the amount of shear bands increases significantly. The shear bands of (Zr 0.6032 Cu 0.2256 Fe 0.0995 Al 0.0717 ) 98 Nb 2 have different directions. As can be in figure 9(b), interaction, restraint and divarication take place among the shear bands in different directions. Compared to the primary alloy composition Zr 60.32 Cu 22.56 Fe 9.95 Al 7.17 , the shear bands of (Zr 0.6032 Cu 0.2256 Fe 0.0995 Al 0.0717 ) 98 Nb 2 are denser. These crossed, biforked and more dense shear bands in different directions correspond to the increase of compassion plasticity. Conversely, with 4 at% Nb further doping, there exist no shear bands of (Zr 0.6032 Cu 0.2256 Fe 0.0995 Al 0.0717 ) 96 Nb 4 . It can be more easily seen that the shear bands are closely relevant to plasticity.