Effect of Casting Temperature Control on Microstructure and Properties of Continuously Cast Zr-Based Bulk Metallic Glass Slabs

Abstract

In this study, a novel crawler-type continuous casting (CC) technology was designed to efficiently and cost-effectively produce bulk metallic glass (BMG) slabs. As a crucial process parameter, casting temperature has a significant impact on the operation of CC devices and the quality of slabs. CC experiments of the Zr_41.2Ti_13.8Cu_12.5Ni_10.0Be_22.5 (Vit1) BMG slab were carried out at the casting temperatures of 1073 K, 1123 K, and 1173 K, and the microstructure and properties of slab samples were analyzed and studied. The experimental results indicate that the BMG slabs can be prepared by CC at 1173 K and 1123 K. When the temperature is reduced to 1073 K, the Be₁₂Ti crystal phase precipitates inside the CC slab, which has a certain impact on the thermal stability and compressive performance of the slab. The control of casting temperature does not affect the glass-forming ability (GFA) of the slab in the CC process.

1. Introduction

Compared with crystal alloys, bulk metallic glasses (BMGs) exhibit a long-range disordered microstructure without grains and grain boundaries [1,2,3]. Therefore, they have excellent mechanical properties such as high strength, high fracture toughness, and high corrosion resistance [4,5,6]. Currently, BMGs have been applied to precision parts, aerospace, consumer electronics, the automotive industry, medical devices, and other fields [7,8,9,10]. However, the GFA of BMGs is limited [11], and the existing preparation methods of BMGs have high cost and low efficiency. The metallic glass is called bulk metallic glass (BMG) when its diameter is larger than 1 mm [12]. Suction casting [13], die casting [14,15], and gravity casting [16,17] are the preparation methods for BMG, which are widely applied at present. Arc melting technology is generally used in copper die suction casting, whose casting temperature is difficult to control. Both die casting and gravity casting require a higher pouring temperature, and the cost of the mold is higher. It is necessary to prepare BMGs with high efficiency and low cost, and the glass-forming ability (GFA) of BMGs should also be guaranteed.

Continuous casting (CC) has become the most widely used method for casting metals such as steel, copper, aluminum, and other crystalline alloys due to its low energy consumption and high productivity. At present, the CC technology of BMG is in its infancy. Zhang et al. prepared BMG slabs with a cross-sectional size of 50 mm× 6 mm using a self-designed horizontal CC technology [18,19] and obtained BMG bars with a diameter of 10 mm [20,21]. Based on the CC process and dynamic formation mechanism of amorphous alloy, Ren et al. designed a crawler-type CC device for the CC of Zr-based BMG slabs, and they produced Zr_41.2Ti_13.8Cu_12.5Ni_10.0Be_22.5 (Vit1) BMG slabs with dimensions of 200 mm× 160 mm× 5 mm [22].

As an important parameter in the metallurgical process, casting temperature has a great influence on the continuous casting process and slab structure. In the CC process of steel, the casting temperature affects the center macro-segregation of the slab [23]. In the copper mold casting process of BMG, the casting temperature affects the dynamics of the solidification process, and a high pouring temperature may lead to the precipitation of the nanocrystal phase [11]. A previous study [22] has shown that under the process parameters of a casting temperature of 1123 K and CC speed of 21 mm/s, a Vit1 BMG slab was successfully prepared by the crawler-type CC technology. It not only increased the size of the BMG but also ensured that the compressive performance of the samples was essentially consistent with the BMG prepared by copper mold casting [24]. Based on the successful preparation of a BMG slab by the crawler-type CC technology, the influence of different casting temperatures on the crawler-type CC technology and the quality of the slabs can be further studied and analyzed.

In this study, based on previous research, slabs with the Vit1 composition were prepared by the crawler-type CC technology at casting temperatures of 1073 K and 1173 K, with the continuous casting speed unchanged. The glass properties of slab samples were characterized, and the effects of casting temperature on the CC process, slab structure, and mechanical properties of the slabs are discussed.

2. Materials and Methods

As the most distinctive Zr-based metallic glass [25,26,27], Vit1 has a very strong glass formation ability and its critical cooling rate is as low as 1 K/s [28,29]. Therefore, this study utilized Vit1 as the experimental material for CC. The CC process is shown in Figure 1. The CC device was placed in a vacuum tank. The alloy ingots with the Vit1 composition were placed in the crucible of an induction melting furnace, and the vacuum tank was evacuated and filled with argon gas. The master alloy ingots were heated and melted, and the temperature of molten metal was monitored by a thermocouple. After reaching the casting temperature, the molten metal was poured into the tundish with a heat preservation function by the crucible, and then the molten metal was poured from the tundish into the steady flow trough, which led the molten metal into the casting cavity (the cross-sectional size of the casting cavity was 160 mm × 5 mm). The molten metal was cooled and solidified in the casting cavity to form a slab. At the casting temperatures of 1073 K and 1173 K, the CC process was adopted for slab CC.

XRD (D/MAX-2600/PC, Japan) was employed to identify the amorphous phase of the slab. TEM (FEI Talos F200X, USA) was used to analyze the microstructure of the slab. The thermal physical parameters of the plate samples were determined by using DSC (Netzsch 404 F3, Germany) with a heating rate of 20 K/min under argon flow. The compression mechanical properties of slab samples were tested by a universal testing machine (Instron 5982, USA), and the strain rate was 5 × 10⁻⁴ s⁻¹.

3. Results and Discussion

The slabs prepared by this CC technology at pouring temperatures of 1073 K and 1173 K are shown in Figure 1. When the casting temperature is 1073 K, the size of the prepared slab is 50 mm × 160 mm × 5 mm. When the casting temperature is 1173 K, the size of the prepared slab is 290 mm × 160 mm × 5 mm. Combined with the previous research [22], it can be seen that the slab length increases with the increase in casting temperature. When the casting temperature was reduced, the molten metal was cooled and solidified rapidly. The gate was blocked by the slab and the molten metal could not flow out, resulting in the inability of CC to continue.

Figure 2a–c show the XRD and TEM structure characterization of slab samples. As shown in Figure 2a, when the casting temperature is 1073 K, the XRD pattern shows a crystallization peak at 2θ = 34.562°, lattice fringes appear in the high-resolution transmission electron microscopy (HRTEM), and crystal diffraction spots can be seen by selected-area electron diffraction (SAED). As can be seen in Figure 2b,c, when the casting temperature is 1123 K and 1173 K, there is no crystal structure diffraction peak in the XRD patterns, indicating that the crystal structure of the sample is not detectable within the XRD resolution range. The HRTEM images display a disordered structure, and the SAED patterns only exhibit diffraction rings of the amorphous phase, which further indicates that the slab samples were amorphous. After PDF card comparison and SAED pattern calibration, it is determined that the precipitated phase of the slab sample is Be₁₂Ti when the casting temperature is 1073 K. In summary, when the casting temperatures are 1173 K and 1123 K, the slabs prepared by CC are amorphous alloys, and when the temperature is reduced to 1073 K, there will be crystallization-phase precipitation inside the slab.

The DSC curves of the slab samples prepared at different casting temperatures are shown in Figure 2d. It can be observed that with the increase in casting temperature, there is a trend of the glass transition temperature (T_g) and crystallization temperature (T_x) shifting towards higher temperatures. The thermal property parameters of the slab samples at different casting temperatures are listed in

Table 1. The thermal property parameters of the slab samples at different casting temperatures.

Casting Temperature/K	Tg/K	Tx/K	ΔTx/K	Trg
1073	633.0	683.5	50.5	0.678
1123	633.6	702.7	69.1	0.678
1173	633.4	703.4	70.0	0.678

. It can be seen that when the casting temperature is 1073 K, the T_x and ΔT_x of the slab samples are lower; when the pouring temperature is 1123 K and 1173 K, the thermal property parameters of the plate samples are essentially the same but slightly higher compared to the lower temperature. At the casting temperature of 1073 K, the lower casting temperature may lead to an uneven composition of the molten metal and a trend towards heterogeneous nucleation, thus reducing the crystallization energy barrier. This is also the reason for the decrease in crystallization temperature. The reduced glass transition temperature (T_rg) of the slab samples prepared by CC is basically the same when the casting temperature changes from low to high, which indicates that the casting temperature control has no significant effect on the GFA.

In order to further analyze the effect of the precipitation phase on the compression properties of BMG, the compression properties of slab samples were tested. Figure 3 shows the compressive stress–strain curves of slab samples prepared at different casting temperatures. The yield strength (σ_y), maximum compressive strength (σ_m), elastic strain (ε_e), and plastic strain (ε_p) of compressed slab samples prepared at different casting temperatures are listed in

Table 2. Mechanical properties of slab samples prepared at different casting temperatures.

Casting Temperature/K	σy/MPa	σm/MPa	εe/%	εp/%
1073	1877.837	1905.788	2.949	0.095
1123	1767.366	1952.139	3.199	0.842
1173	1807.829	1954.001	3.438	0.752

. The results show that the σ_m of the samples increases with the increase in casting temperature, but the σ_m at the casting temperature of 1073 K is significantly lower than that at the casting temperatures of 1123 K and 1173 K. When the casting temperature is 1073 K, ε_p is much lower than that at the casting temperatures of 1123 K and 1173 K, but there is little difference in ε_p between 1123 K and 1173 K casting temperatures. This phenomenon may be attributed to the fact that the precipitated phase Be₁₂Ti is a brittle material [30], leading to a decrease in both the σ_m and ε_p of the plate sample when the casting temperature is 1073 K. When the casting temperature increased from 1073 K to 1173 K, the σ_y first increases and then decreases. Combined with Figure 2d, it can be seen that the enthalpies of relaxation (ΔH) are calculated to be −0.638 J/g, −2.588 J/g, and −0.570 J/g for samples at casting temperatures of 1173, 1123 K, and 1073 K, respectively. According to the literature, the free volumes are affected by the enthalpies of relaxation [6]. The free volume increases with the increase in relaxation enthalpy, which results in the decrease in interatomic forces. Therefore, the slab sample has the lowest yield strength and the best plasticity, when the casting temperature is 1123 K. The free volume of amorphous slab samples is larger, which also leads to the ε_e of slab samples at casting temperatures of 1123 K and 1173 K being higher than that at a casting temperature of 1073 K.

Figure 4a–f show SEM images of the side view and the fracture morphology of slab samples at different casting temperatures. The angles between the compression section and the loading direction of the samples at different casting temperatures were 40.15° (1073 K), 40.31° (1123 K), and 43.55° (1173 K), respectively. The SEM images indicate that when the casting temperature is 1073 K, the fracture morphology of the sample is mainly a river-like pattern with a small amount of vein pattern. The samples’ fracture morphology between casting temperatures of 1123 K and 1173 K appears similarly. The dense vein-like patterns are distributed and some vein-like patterns are stacked with each other and show petal-like patterns. Several studies have revealed that the size of the vein-like patterns is small for the specimens with a large compression plasticity, and the size of the vein-like patterns is large for the specimens with a poor compressive plasticity [6]. The SEM images further indicate that the compressive plasticity of continuously cast slab samples is poor when the casting temperature is 1073 K.

In summary, when the CC speed remains constant, the casting temperature not only affects the length of the CC slabs, but also has a certain impact on the internal structure of the slabs. Lower casting temperatures can lead to the precipitation of brittle phases within the slab, resulting in a reduced plasticity of the slab samples.

4. Conclusions

In summary, the effect of casting temperature control on the microstructure and properties of continuously cast Zr_41.2Ti_13.8Cu_12.5Ni_10.0Be_22.5 bulk metallic glass slabs was studied. The results show that the length of casting slabs increases with the increase in casting temperature. At the same time, when the casting temperature is 1073 K, brittle grains of Be₁₂Ti precipitate inside the slab, and when the casting temperatures are 1123 K and 1173 K, the slab structure is amorphous. The glass forming ability of the slab at different casting temperatures is basically the same (T_rg = 0.678), indicating that casting temperature control does not significantly affect the glass forming ability of the continuously cast bulk metallic glass slab. Due to the precipitation of brittle grains, the plastic strain (ε_p) of the slab decreases from 0.752% at 1173 K and 0.842% at 1123 K to 0.095% at 1073 K. At the same time, the maximum compressive strength (σ_m) of the slab decreases from 1954.001 MPa at 1173 K and 1952.139 MPa at 1123 K to 1905.788 MPa at 1073 K. Overall, the Zr-based BMG slabs casting at temperatures of 1123 K and 1173 K exhibit good thermal physical and mechanical properties. In addition, the crawler-type continuous casting (CC) technology provides a new research direction for the production of bulk metallic glass with low cost and high efficiency. This research has a certain guidance for the selection of process parameters during the continuous casting of bulk metallic glass slabs. As the casting temperature decreases, the grains precipitate inside the slab and affect the mechanical properties of the slab. Reducing the pouring temperature is conducive to energy reduction, but the casting temperature should be properly controlled above 1073 K.