A Novel Ni-Free Zr-Based Bulk Metallic Glass with High Glass Forming Ability, Corrosion Resistance and Thermal Stability

Abstract

Zr-based Bulk metallic glasses exhibit incredible corrosion resistance and glass forming ability, however, these properties need further enhancement to meet the practical use. In this study, Zr₆₃Fe_2.5Cu₂₃Al_11.5, a new type of Zr-based bulk metallic glass was fabricated. Potentiodynamic polarization techniques were used to measure the corrosion resistance of this alloy. Furthermore, crystallization behavior and kinetics of Zr₆₃Fe_2.5Cu₂₃Al_11.5 bulk metallic glass were investigated by using differential scanning calorimetry of non-isothermal model. Kissinger and Ozawa methods were used for calculating activation energies of crystallization and the mechanism of crystallization was analyzed by Johnson-Mehl-Avrami-Kolmogorow methods. The results suggest that this specified metallic glass system possesses a relatively high thermal stability and glass forming ability. Moreover, the crystallization procedure is mainly dominated by nucleation with an increasing rate. The study demonstrates that the slight composition adjustment of Zr–Fe–Cu–Al system bulk metallic glass can make a considerable contribution to higher glass forming and thermal stability as well as corrosion resistance.

1 Introduction

Zr-based BMGs have drawn an increasing attention in recent years for their unique properties, such as high hardness, superior strength, excellent fracture toughness, enhanced elastic limit as well as improved corrosion resistance [1, 2]. As strongly potential materials in biomedical industries, most of the Zr-based BMGs contain Ni or Be elements, which may cause allergies in human body [3]. Then Zr₆₀Fe₁₀Cu₂₀Al₁₀, a developed Zr-based Ni-free alloy is proposed for substitution, which possesses rather good glass forming ability (GFA) [4]. However, limited GFA is a major restriction in the application of BMGs as structural materials. Composition adjustment is therefore used to improve the GFA of BMGs [5, 6].

BMGs are essentially metastable, which inherently has the possibility to crystallize into a stable structure during thermal variation [7, 8]. To build practical BMG parts, hot forming during super-cooled liquid region is often employed [9], the occurrence of partial crystallization may take place because of the thermal effect. Generally, the corrosion resistance of Zr-based bulk metallic glasses (BMGs) with amorphous structure is 1–2 orders of magnitude higher than their crystalline counter parts, for the lacking of structural defects such as dislocations or grain boundaries in BMGs [10,11,12,13]. Thus, the building up of high performance BMG parts calls for a good thermal stability of BMGs to avoid the formation of crystalline phases. Therefore it is of great significance to recognize both the thermal characteristics and crystallization processes. The understanding of crystallization from kinetic aspect is important since the competition between nucleation and growth during the generating of crystalline phase can be quantified through the kinetic characters [14]. Non-isothermal method is widely applied with a fixed heating rate to reveal the crystallization behavior and thermal stability of bulk metallic glasses, particularly on Zr based BMGs, for its rapidity and efficiency [15]. An et al reported that the crystallization procedure of Zr_62.5Al_12.1Cu_7.95Ni_17.45 bulk metallic glass can be manifested into two stages. Rashidi et al. [16, 17] figured out the thermal stability and crystallization behavior of Cu₄₇Zr₄₇Al₆ and (Cu₄₇Zr₄₇Al₆)₉₉Sn₁ bulk metallic glasses.

In this work, Zr₆₃Fe_2.5Cu₂₃Al_11.5 BMG, a new type of Zr–Fe–Cu–Al system BMG with better glass forming ability and thermal stability, was prepared by suction-casting method. The corrosion resistance of the Zr₆₃Fe_2.5Cu₂₃Al_11.5 BMG was tested by potentiodynamic polarization techniques and then compared to Zr₆₀Fe₁₀Cu₂₀Al₁₀ BMG, 316L stainless steel (traditional structure material) as well as Ti-6Al-4V alloy (pitting resistant structural material). The thermal stability and crystallization kinetics of Zr₆₃Fe_2.5Cu₂₃Al_11.5 alloy were investigated by non-isothermal DSC methods. In particular, the crystallization kinetics were illuminated by studying the crystallization activation energy and crystallization mechanism.

2 Experiments

2.1 Materials Preparation

To avoid adding superfluous elements, the master ingots with nominal composition of Zr₆₀Fe₁₀Cu₂₀Al₁₀ (in at%) and Zr₆₃Fe_2.5Cu₂₃Al_11.5 (in at%) were fabricated by using arc-melting high-purity ingots of Zr (>99.9 wt%), Fe (>99.99 wt%), Cu (>99.99 wt%) and Al(99.99 wt%) for four times at least to ensure the chemical homogeneity. And cylindrical rods in diameters of 1‒14 mm were fabricated under Ar atmosphere by suction-casting in water-cooled copper molds.

2.2 X-ray Diffraction Measurement

The structural information of the as-cast rods were acquired by X-ray diffraction (XRD, Philips X’Pert Pro) using Cu–Kα radiation with the determine range from 20° to 90°. Before the measurement, the cylindrical rods were cut into pieces with the height of 1 mm and the cross section of the rods were grind by abrasive paper then polished on a polishing cloth with diamond polishing agent. The diffraction profiles were used to evaluate whether the as-cast rods were in an amorphous status.

2.3 Thermal Behavior and Crystallization Kinetics Measurement

The differential scanning calorimetry (DSC) tests were carried out within the temperature ranging from 300 K to 838 K under heating rates of 10, 20, 40 and 80 K/min, respectively. The Zr₆₃Fe_2.5Cu₂₃Al_11.5 specimens tested in DSC were thinned before loading in the DSC cells to ensure the accuracy of the test.

2.4 Corrosion Test

The as-cast rods of Zr₆₃Fe_2.5Cu₂₃Al_11.5 alloy with diameters of 5 and 6mm and the Zr₆₀Fe₁₀Cu₂₀Al₁₀ alloy with diameters of 1 and 3mm were cut into pieces, and then fixed in epoxy resin and the untested side of specimens were linked with copper wires. These specimens were mechanically polished by an automated lapping machine, then sonicated in anhydrous alcohol for 600 s and sequentially dried by Ar flow. The corrosion behavior of Zr₆₃Fe_2.5Cu₂₃Al_11.5 samples were characterized by potentiodynamic polarization techniques which were conducted on an electrochemical workstation (AutoLab 302N, Metrohm). A three-electrode system was consist of a Zr₆₃Fe_2.5Cu₂₃Al_11.5 working electrode, a 4 cm² Pt net as the counter electrode and a saturated calomel electrode as the reference electrode. The electrolyte was NaCl solution (3.5 wt%), and the electrolytic cell was commercial flat electrolytic cell. For comparison, the corrosion resistance of Zr₆₀Fe₁₀Cu₂₀Al₁₀, 316L stainless steel and Ti-6Al-4V alloy were tested as well.

3 Results and Discussion

3.1 Glass Forming Abilities of Zr–Fe–Cu–Al BMGs

Figure 1 exhibits the XRD results of cylindrical rods in different diameters of Zr₆₀Cu₂₀Fe₁₀Al₁₀ and Zr₆₃Fe_2.5Cu₂₃Al_11.5 alloys. Due to the instrument we used in the present work, the critical diameter of Zr₆₀Cu₂₀Fe₁₀Al₁₀ BMG is 1 mm (Figure 1a), while the critical diameter of Zr₆₃Fe_2.5Cu₂₃Al_11.5 BMG is 5 mm (Figure 1b), which shows that the GFA of Zr₆₃Fe_2.5Cu₂₃Al_11.5 BMG is much better than that of Zr₆₀Cu₂₀Fe₁₀Al₁₀. The results indicate that Zr₆₃Fe_2.5Cu₂₃Al_11.5 BMG may have extensive application prospect compared with Zr₆₀Cu₂₀Fe₁₀Al₁₀ BMG in building practical parts.

Figure 1

XRD results of cylindrical rods in different diameters: a Zr₆₀Cu₂₀Fe₁₀Al₁₀, b Zr₆₃Fe_2.5Cu₂₃Al_11.5

3.2 Corrosion Resistance of Zr₆₃Fe_2.5Cu₂₃Al_11.5 BMG

Zr₆₃Fe_2.5Cu₂₃Al_11.5, Zr₆₀Fe₁₀Cu₂₀Al₁₀ BMGs, 316L stainless steel and Ti–6Al–4V alloy in 3.5% NaCl solution are illustrated in Figure 2, and corresponding corrosion data are listed in Table 1. It can be seen that all the Zr-based BMGs exhibit better corrosion performance than traditional structural materials (316L). With respect to Zr-based BMGs with the same composition, the presence of a small amount of crystals leads to an order of magnitude increase in corrosion rate than that of amorphous phase. In addition, slightly composition adjustment can change the corrosion resistance of Zr-based BMGs to some extent. With the decrease of the composition of Fe from 10 at.% to 2.5 at.% in Zr–Fe–Cu–Al BMG, the corrosion potential (E_corr) increases and the corrosion current density (I_corr) reduces 80%, indicating that the corrosion resistance of the Zr–Fe–Cu–Al BMGs is improved significantly. It is worth noting that the corrosion resistance of Zr₆₃Fe_2.5Cu₂₃Al_11.5 reaches the level of structural materials (Ti-6Al-4V alloy). It is mainly due to the reduction of amorphous material defects and the absence of grain boundaries.

Figure 2

Potentiodynamic polarization curves of as-cast specimens of Zr₆₀Cu₂₀Fe₁₀Al_10, Zr₆₃Fe_2.5Cu₂₃Al_11.5, Ti-6Al-4V and 316L stainless steel

Table 1 Polarization data for Zr₆₃Fe_2.5Cu₂₃Al_11.5, Zr₆₀Fe₁₀Cu₂₀Al₁₀, 316L stainless steel and Ti-6Al-4V alloy

3.3 Thermal Stability of Zr₆₃Fe_2.5Cu₂₃Al_11.5 BMG

Figure 3 shows typical non-isothermal DSC curves measured from the Zr₆₃Fe_2.5Cu₂₃Al_11.5 BMG under different heating rates. All the curves exhibit a single exothermic peak. The corresponding transition temperatures of inner structural, including glass transition temperature (T_g), crystallization temperature (T_x), and peak temperature (T_p), are listed in Table 2. It can be seen from Table 2 that with the increasing of heating rate, all the temperatures move to a higher value, which is characteristic of a dominant kinetic process. The curves show extended super-cooled liquid regions ΔT_x of width 76-86K, which are close to the reported ΔT_x of similar component BMGs [18] and higher than that of Cu–Zr–Al system BMGs [19], suggesting that the specified metallic glass system has a relatively high thermal stability.

Figure 3

DSC curves of Zr₆₃Fe_2.5Cu₂₃Al_11.5 BMG with different heating rates

Table 2 Characteristic temperatures of Zr₆₃Fe_2.5Cu₂₃Al_11.5 BMG with different heating rates

The activation energies can be caculated by Kissinger equation as shown below [20]:

$$\ln \left( {\frac{{T^{2} }}{\theta }} \right) = \frac{E}{RT} + {\text{constant}},$$

(1)

where T is the transition temperature (T_g, T_x and T_p) under a given heating rate $\theta$, E is the activation energy and R is the gas constant. The activation energy is determined by the slope and standard error of the linear fitted lines from the plot of $\ln \left( {\frac{{T^{2} }}{\theta }} \right)$ vs. 1/T. The values are calculated for the activation energies at glass transition temperature (E_g), beginning of crystallization (E_x) and crystallization peak (E_p) as 301.4±16.5, 256.91±8.6 and 216.15±2.7 kJ/mol, respectively (Figure 4a).

Figure 4

Determined activation energies (E_g, E_x, E_p) by Kissinger and Ozawa methods of Zr₆₃Fe_2.5Cu₂₃Al_11.5 BMG

E can also be calculated by Ozawa equation, which is shown below [21]:

$$\ln \theta = - 1.0561\frac{E}{{{R}T}} + {\text{constant}},$$

(2)

where $\theta$ is the heating rate, T is the specific temperature (T_g, T_x and T_p), E is the activation energy and R is the gas constant. From the slopes and standard errors of the linear fitted lines (a plot of $\ln \theta$ against 1/T), the activation energies were estimated as 285.90 ± 13.5, 256.07 ± 3.9, 217.74 ± 2.0 kJ/mol for T_g, T_x, T_p, respectively (Figure 4b).

Kissinger and Ozawa methods are the most commonly used ways of calculating non-isothermal kinetic like crystallization activation energy E or Avrami exponent n. As expected, the activation energies obtained by the two methods are close thereby indicating a good coherence.

Generally, E_g represents the energy barrier of the structural transformation from the glassy structure to super-cooled liquid region, while E_x gives information about the difficulty for the transition from the super-cooled liquid to the crystalline phase [22]. From Table 3, the E_g value is higher than the E_x, which means that the energy barrier for glass transition is higher than that at starting point of crystallization. The thermal stability of BMGs can be evaluated by comparing the activation energies, namely the higher the activation energy, the harder the crystallization proceeds. Compared with our previous work on Zr₆₀Cu₂₀Fe₁₀Al₁₀ BMG, the E_g and E_x of Zr₆₃Fe_2.5Cu₂₃Al_11.5 BMG are higher than those of Zr₆₀Cu₂₀Fe₁₀Al₁₀, which indicates the Zr₆₃Fe_2.5Cu₂₃Al_11.5 BMG has a better thermal stability than Zr₆₀Cu₂₀Fe₁₀Al₁₀.

Table 3 The activation energy values (E_g, E_x, E_p) of Zr₆₃Fe_2.5Cu₂₃Al_11.5 and Zr₆₀Cu₂₀Fe₁₀Al₁₀ BMGs calculated by Kissinger and Ozawa methods

3.4 Non-isothermal Crystallization Kinetics of Zr₆₃Fe_2.5Cu₂₃Al_11.5 BMG

3.4.1 Local Activation Energy (LAE)

Examining the crystallization peaks in the DSC curves obtained under different heating rates (Figure 5a) shows that the range of the crystallization peaks became larger as the heating rate increased. Figure 5a displays a partial DSC curve of Zr₆₃Fe_2.5Cu₂₃Al_11.5 BMG at different heating rates. The crystal volume fraction x at any temperature is x = St/S, where S is the total area of the crystallization peak between the temperature at which crystallization starts and the temperature at which crystallization is ended, and St is the portion of the crystallization peak up to the temperature T area. Thus, Figure 5b shows the volume fraction of crystallization (x) at different temperatures, which is a typical S-shape and consistent with other developed amorphous alloys [23, 24].

Figure 5

Non-isothermal crystallization procedures of Zr₆₃Fe_2.5Cu₂₃Al_11.5 BMG under different heating rates: a plots of the crystallization peaks; b plots of crystallization volume fraction x versus temperature

Since the local activation energy (LAE) changes in different stages of crystallization procedure [25], the change in LAE with the crystallization volume fraction can be determined. Figure 6a plots the linear fitted relationship between ln(θ) and 1000/T with different crystallization volume fractions (x) from non to all, where the evolution of the LAE calculated by the Ozawa method was expressed in Figure 6b.

Figure 6

Non-isothermal crystallization of Zr₆₃Fe_2.5Cu₂₃Al_11.5 BMG under different heating rates: a plots of ln(θ) vs. 1000/T with different volume fractions of crystallization; b plot of local activation energy as a function of x based on the Ozawa–Flynn–Wall method

E_c(x) shows a downward trend with the increase of x, indicating that the energy barrier reduces during the crystallization reaction, makes it more available for the crystallization proceeds. At the starting stage (x < 0.1), the activation energy decreases rapidly as well as the increasing of x, indicating that the process is dominated by nucleation, and the nucleation needs to overcome the high energy barrier. As the crystallization proceeds (0.1 < x < 0.9), the curve shows a gradual decreasing tendency due to structural relaxation of the amorphous structure around the crystalline phase. In the ending stage, the activation energy drops sharply, which reveals the crystallization procedure is dominated by growth.

3.4.2 Kinetic Parameter-Evolution of Local Avrami Exponent

To figure out the competition between the nucleation and growth, the crystallization of Zr₆₃Fe_2.5Cu₂₃Al_11.5 amorphous alloy was determined based on Johnson-Mehl-Avrami-Kolmogorow (JMAK) model:

$$x = 1 - \exp \left( { - kt^{n} } \right),$$

(3)

where x and n are crystallization volume fraction and Avrami exponent [26], k is the rate constant related to temperature:

$$k = A{ \exp }\left( { - \frac{E}{RT}} \right),$$

(4)

where A and E are the pre-exponential term and activation energy, T represents the temperature, R is the gas constant and. Figure 7a shows the JMAK plots of the data measured under various heating rates. As n is variable along the entire crystallization procedure, the local Avrami exponent is employed to represent the corresponding dynamic characteristics as the equation below:

$$n\left( x \right) = - \frac{R}{Ec}\frac{{\partial \ln \left[ { - \ln \left( {1 - x} \right)} \right]}}{{\partial \left( {\frac{1}{T}} \right)}}.$$

(5)

Figure 7

a ln[−ln(1−x)] vs. 1000/T plots measured under different heating rates; b The plots of local Avrami exponent n(x) as a function of crystallized fraction x under heating rates of 10, 20, 40, 80 K/min

The JMAK equation is often employed to describe the isothermal crystallization behavior of BMGs originally, then it is extended to non-isothermal applications [27]. Therefore, in order to measure the Avrami exponent, first we need to plot $\ln[ - \ln \left( {1 - x} \right)$] versus $1/T$ as shown in Figure 7a, the Avrami exponent can be determined by the slopes of the curves as shown in Figure 7a. To understand the crystallization behavior of Zr₆₃Fe_2.5Cu₂₃Al_11.5 BMG, diffusion-controlled growth theory is often applied. According to this theory, the definition of Avrami exponent is used to determine the variation of nucleation and growth during crystallization procedure [28]. Considering the crystallization in the super-cooled liquid is diffusion-controlled, the Avrami exponent can be divided into the followings [29]: n < 1.5 denotes the growth of crystals with an appreciable initial volume; n = 1.5 indicates the growth dominates the crystallization procedure with no nucleation occurs; 1.5 < n < 2.5 indicates the growth governs the crystallization procedure with a lower growth rate and a decreasing nucleation rate; n = 2.5 reflects growth of crystals with a constant nucleation rate and n>2.5 indicates the growth of small crystals with an increasing nucleation rate.

Figure 7b reveals the evolution of n(x) depending on x under different heating rates of 10, 20, 40, 80 K/min. n(x) tends to decrease gradually, while the crystallization volume fraction increases. The variation of nucleation and growth rates during crystallization can be speculated by the variation of local Avrami exponent. In particular, the LAEX reduced from 12 to 0.8 under the heating rate of 80 K/min. From Figure 7b it can be seen that if x < 0.9 and n(x)>2.5, which refers the generating of nuclei although most of the matrix of Zr₆₃Fe_2.5Cu₂₃Al_11.5 BMG has been crystallized, and the nucleation rate increase rapidly from 0 < x < 0.1 and became smooth from 0.1 < x < 0.9. When 0.9 < x < 0.98 and 1.5 < n(x) < 2.5, indicated that the rate of nucleation is reducing and growth rate increased. When x > 0.98, the nucleation procedure tended to end. It can be inferred that the crystallization of Zr₆₃Fe_2.5Cu₂₃Al_11.5 BMG is mainly dominated by nucleation with a decreasing nucleation rate during most of crystallization procedure within 0 < x < 0.9, after that growth took control and filled the rest of the amorphous matrix.

4 Conclusions

In this work, a high glass forming ability Zr-based BMG with the composition of Zr₆₃Fe_2.5Cu₂₃Al_11.5 was fabricated. The BMG has excellent corrosion resistance, with the corrosion rate of 2.03×10⁻⁸ A/cm² in a 3.5 wt% sodium chloride solution.

The overall crystallization activation energies of Zr₆₃Fe_2.5Cu₂₃Al_11.5 BMG, calculated by Kissinger and Ozawa methods, were 256.91±8.6 and 256.07±3.9 kJ/mol, respectively, which denotes that the specified metallic glass system has a relatively high thermal stability. Then the crystallization mechanism was investigated by JMAK method. The results suggest that the crystallization procedure is mainly dominated by nucleation with decreasing nucleation rate.