Mechanics of Materials
Mechanical Property and Fracture Characteristic of Ti–Cu–Ni–Alx Bulk Metallic Glasses under Different Strain Rates
2020 Volume 61 Issue 8 Pages 1607-1612
Details
2020 Volume 61 Issue 8 Pages 1607-1612
In this study, a Ti-based bulk metallic glass (BMG) was used to investigate the influence of the addition of Al on mechanical properties and glass forming ability (GFA). The experimental results show that the plastic flow stress value of the alloy will change with the change of strain and strain rate. As the strain rate increases, the plastic flow stress value increases. The fracture surface of the specimens can be observed through a scanning electron microscope. It can be seen that the fracture surface has the characteristics of a dimple-like structure. The appearance, distribution density, and grain density of the vein-like structure depend on the strain rate and the addition of the Al element.
From the Figure, it can be seen that there exists two stress during the fracture process, one is tensile stress and the other shear stress. Therefore, when specimen are subjected the compressive load, it will fracture along the maximum shear stress plane, leading the final shear failure and produce the shear zone. When specimen are fractured and separated, it will suffer the tensile stress and produce the tensile zone.
Amorphous alloys are formed by rapid solidification of a molten metal, so amorphous metals are also referred to as liquid metals or non-crystalline metals. Because of this lack of symmetry short-range order structure and glass structure is similar, it is also called glass metallic or metallic glass (MG).1) In the previous scientific basis of metal materials, the occurrence of plastic deformation is often explained using dislocation theory. However, amorphous materials only have a short-range orderly arrangement, and there is no fixed slip system for sliding, so there is also no defect of the general crystalline material, and the dislocation theory is not applicable to explain. Because of this special structure and excellent performance, it has also attracted the attention of the academic community and the industry, and has been widely studied.2,3) At present, the Ti-based MG has reached a strength of 2200 MPa, which is about twice the strength of traditional titanium alloys and has received much attention.4,5) In the development of Ti-based MG, the thin ribbons synthesized by melt-spinning are still dominant at early stages, such as Ti–Be,6) Ti–Ni,7) and Ti–Ni–Si8) etc. Bulk metallic glass (BMG) has many excellent properties such as large elastic limit, exceptionally high strength, good corrosion resistance, high yield strength, high hardness, and reduced sliding friction, which makes them considered as prospective candidates for engineering materials. Despite these excellent properties, the use of BMG as a structural material is limited. One of the major drawbacks of BMG compared to currently used crystalline alloys is their limited plasticity. Many studies has been performed to improve the plasticity of BMG. To enhance the plasticity, some inhomogeneities are introduced into the BMG matrix, atomic scale to micrometer (mm) scale, intended to enhance shear band nucleation or prevent shear band propagation. Ti-based BMG was explosively discussed until Inoue et al. added a small amount of tin to prepared a 6 mm diameter Ti–Cu–Ni–Sn BMG alloy by quench casting in 1988.5) And Sundaresanet et al. first use mechanical alloying to synthesize Ti–Cu–Ni MG with a clearly supercooled liquid region in 1988.8) Currently, scholars try to add zirconium, tin or niobium to improve the glass forming ability (GFA) of BMG.9–13) But it is difficult to exceed the thickness of 6 mm, the main reason is that titanium metal is easily reacting with the oxygen and nitrogen in the air. In order to improve the poor ductility of BMG, recent studies have proposed to improve the mechanical properties by adding a strengthening phase to precipitate in the base phase of a BMG alloy. The addition of this type of material also promotes the GFA of the amorphous alloy. This new type of alloy is called bulk metallic glass composites (BMGC). BMG-based composites improved the plasticity and work-hardening ability through the addition of crystalline phases, and the determination of the properties of the additives had a significant influence on the mechanical properties of BMGC, such as the ductile crystal addition will reduce the plastic deformation capacity of BMGC.14–19) However, in order to expand the field of application of BMG, more attention should be paid to the development of BMG based on ordinary metals such as Ni, Cu, Fe and Ti. Since Ti-based BMGCs have a low density that is almost matched to commercial Ti alloys, many Ti-based metallic glasses have been studied with the aim of improving the mechanical properties of Ti alloys.20,21) In 2005, Guo et al. reported that the Ti-based BMGC rod of 12 mm in diameter exhibits a large compressive plastic strain of 5% with a strength of 1800 MPa.22) In 2007, Lee et al. reported that add Nb as an alloying element can strongly stabilize the formation of the β phase of Ti-based BMGC.23) In addition, Hofmann et al. reported that aluminum exhibits partial solubility in body centered cubic phase titanium, which is evenly distributed between the glass and dendrites. Therefore, a small amount of aluminum can be added to improve the GFA of BMGs.24) In this study, the mechanical behavior and microstructure evolution of Ti-based BMG specimens (Ti44Cu40Ni16−xAlx (x = 2, 4, and 6)) at room temperature under strain rates ranging from 10−3 ∼ 3 × 10−3 s−1 were studied using a universal testing machine (MTS model) and compressive split-Hopkinson pressure bar (SHPB) system. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) analysis were used to examine the fracture morphology and crystalline properties of the Ti-based alloy system.
Ingot alloys with a master chemical composition of Ti44Cu40Ni16−xAlx (x = 2, 4, and 6 at%) were prepared via the vacuum arc-melting (shown in Fig. 1) of a mixture of high purity constituent Ti (99.94 mass%), Cu (99.97 mass%), Ni (99.99 mass%), and Al (99.99 mass%) elements in an Ar atmosphere. After all the elements have melted, a cylindrical rod is formed in the copper mold using suction casting. Furthermore, to make the specimen have more uniformly, we re-melted the specimens four times. At the same time, the rods are cooled with subzero liquid and the cooling rate is about 1.3 K/s. In order to ensure the homogeneity of the microstructure, the ingot was melted and recast four times, and a cylindrical sample with a diameter of 4 mm was prepared for mechanical testing using a vacuum suction casting method. The quasi-static compression test was performed by the MTS tester at the strain rate of 10−3∼10−1 s−1 at room temperature conditions. In addition, a dynamic compression test was performed using a compression SHPB system at strain rates of 5.1 × 103 s−1, 6 × 103 s−1, and 7.2 × 103 s−1. The X-ray diffraction (XRD) analysis is used to examine the crystal structure, the differential scanning calorimetry (DSC) is used to study the GFA of BMG with different Al content at a heating rate of 20 K/min, and the scanning electron microscope (SEM) is used to observe the fracture surface of various Ti-based MGs.
The diagram of vacuum arc-melting device.
Figure 2(a)∼(c) show the XRD patterns of the as-cast Ti44Cu40Ni16−xAlx (x = 2, 4, and 6) specimens with diameter of 4 mm. A peak occurs at about 42° and 76° at 2θ, and this characteristic peak is corresponding to crystalline CuTi2 phase,25) and no other component crystal peak appears. The peak of the brittleness phase is the strongest when 6% Al is added, and it can be inferred that the ductility of the 6% Al addition should be the worst. The Ti-based MGs with 4% Al addition shows that the peaks of brittleness phase are lower than 2 and 6% Al addition, so the ductility is the best of the three specimens According to the patterns above, we can know an amorphous structure with amorphous phase as a base containing a small amount of CuTi2 brittle phase can be successfully prepared via the vacuum arc-melting.
XRD patterns of the as-cast Ti-base MGs specimens with Al addition (a) 2, (b) 4, and (c) 6%.
Figure 3 shows the DSC curves of the as-cast Ti44Cu40Ni16−xAlx (x = 2, 4, and 6) specimens. It can be seen that there is a slight endothermic phenomenon in each component curve due to the amorphous phase structure transition at low temperature. The initial temperature of this phenomenon is defined as the glass transition temperature Tg. Then there will be a clear exothermic phenomenon, which means that the supercooled liquid state is successfully converted to the crystalline phase. The initial temperature of this phenomenon is defined as the crystallization temperature Tx. The supercooled liquid region is defined as ΔTx = Tx − Tg, a larger value of ΔTx indication of the thermal stability of BMG materials and also represents the better GFA. From the DSC curve, we can observe that when the addition of 2 and 4% of Al, the double peak of the crystallization temperature Tx of the amorphous alloy is particularly significant. The temperature characteristics of these three alloys will shift to high temperatures as the amount of addition increases, but it is not particularly significant. The values of Tg, Tx, and ΔTx of each specimen are summarized in Table 1. The value of ΔTx with 2, 4, and 6% Al addition are 54, 44, and 40 K, respectively. With the increase of the addition ratio, the thermal stability and the ability to form metallic glass are reduced.
The DSC curves of Ti-based MGs specimens with Al addition (a) 2, (b) 4, and (c) 6%.
Figure 4(a)∼(c) presents the stress–strain curves of the Ti44Cu40Ni16−xAlx (x = 2, 4, and 6) specimens under quasi-static strain rates and high strain rate. We found that when all specimens are subjected to different strain rate loads, all the plastic flow stress values increase with the strain rate. If the strain is fixed, the plastic flow stress value also increases with the increase of strain rate. At the same strain rate, the plastic flow stress value also increases with the increase of the strain, and the rising trend at high strain rates (5100 s−1, 6000 s−1, and 7200 s−1) is more obvious than at low strain rates (10−1 s−1, 10−2 s−1, and 10−3 s−1). It is speculated that the effect of strain rate enhancement at high strain rates is more pronounced than that at low strain rates.
Stress–strain curves of: (a) Ti44Cu40Ni14Al2, (b) Ti44Cu40Ni12Al4, and (c) Ti44Cu40Ni10Al6.
Form the literatures,25–29) they reported that the BMG materials deformed as a viscous flow behavior and depend on strain rates. In this study, it also very clear to see that the deformation behavior of Ti-based BMG materials also depend on strain rates. Whatever the content percentage of Al element, the Ti-based BMG materials obviously present viscous flow during the deformation. The more detail explanations will discuss on the fracture analysis in next section. On the other hand, we also found that the fracture strain will decrease with the increase of the strain rate, this is due to the earlier occurrence of adiabatic shear bands at high strain rates. Furthermore, the decrease in free volume at the high strain rate induce the high temperature within the specimens, and it is expected to cause rapid decrease in viscosity. It caused the fracture earlier at high strain rate. Figure 5 shows the fracture strain properties of these specimens under high strain rate and static conditions. It was found that when the Al content was 6%, the fracture strain has a minimum value, indicating that the ductility was the worst in all the specimens. This also proves that when Al is added to 6%, the XRD analysis of the brittle phase peak is the strongest. Although the ductility was not effectively improved by adding Al element, there was a slight increase in strength.
Fracture strain of Ti-based MGs with different Al additions under (a) quai-static strain rate; (b) high strain rate.
In order to investigate the strain rate effect on the mechanical property of Ti44Cu40Ni16−xAlx BMG alloy, Figs. 6(a) and 6(b) show the variation of the flow stress with the logarithmic strain rate as a function of at true strains of 0.05 and 0.1. It can be seen that for both values of the true strain, the flow stress increases gradually with increasing strain rates. At the same rate, the flow stress also increases with true strain.
True stress as function of strain rate at true strain of 0.05 and 0.1 under (a) quasi static strain rate (b) dynamic strain rate.
We observe and analyze the surface morphology of fracture specimens after static compression and dynamic high-speed experiments. We can find that all fracture specimens are cracked along a plane oriented at 45° to the loading direction. Because this direction is the location of the maximum shear stress, the material fracture occurs in this direction. Figure 7 shows the failure morphology of the amorphous alloy after compressive stress deformation of the specimen. Through the observation of the fracture surface, we can find that the fracture surface is usually dominated by two deformation mechanisms, so the fracture surface is usually a combination of two different surface topographies, as shown in Fig. 7, indicating that the fracture surface is mainly composed of a shear surface and a tensile surface. In the shear area, it is caused by the shear stress, and in the tensile area, it is caused by the tensile stress. The fracture morphology of the amorphous alloy surface can be found in vein-like structures. The vein-like structure and fracture surface feature are different under quasi-static and dynamic experimental conditions, as shown in Fig. 8 and Fig. 9. The fracture surface of the Ti-Based BMG materials are exhibited vein-like feature within the shear bands, which was attributed to local viscous flow during the deformation.30–35) We can find that at the low strain rate deformation, the wider distribution of vein-like pattern and higher density, but at high strain rate deformation the vein-like pattern is smaller and denser, and the morphology is flatter.
SEM micrograph of Ti44Cu40Ni10Al6 fracture surface.
SEM fractographs of Ti44Cu40Ni14Al2 specimens deformed at strain rates of (a) 10−1 s−1, (b) 10−2 s−1, and (c) 10−3 s−1; Ti44Cu40Ni12Al4 specimens deformed at strain rates of (d) 10−1 s−1, (e) 10−2 s−1, and (f) 10−3 s−1; Ti44Cu40Ni10Al6 specimens deformed at strain rates of (g) 10−1 s−1, (h) 10−2 s−1, and (i) 10−3 s−1.
SEM fractographs of Ti-based MGs deformed at strain rate of 5.1 × 10−3 s−1: (a) Ti44Cu40Ni14Al2; (b) Ti44Cu40Ni12Al4; and (c) Ti44Cu40Ni10Al6.
We know the relationship between the fracture strain and the vein-like structure by SEM observation. When the BMG material is fractured by a load impact, a vein-like structure is produced, and the phenomenon of melting droplets or fluid characteristics is found. This is because the large amount of heat generated in the plastic deformation area inside the material cannot be transmitted from the inside of the material within a short time, so that the temperature in the area is instantaneously increased, and the fluid appears inside when the material is deformed, as shown in Fig. 10. Localized small-scale fluid properties or melted droplets can be found at low strain rates, while large areas of fluid characteristics or melted droplet at high strain rates propagate in the direction of the shear band. Slipenyuk and Eckert36) also reported that the free volume is proportional to the heat released during structural relaxation. Meantime, a small change in free volume will induce a dramatic effect on plastic flow behavior of BMG materials on high strain rate deformation. It means that the high strain rate deformation will cause the adiabatic deformation and have a smaller free volume and structural relaxation phenomenon.37,38)
SEM image of Ti44Cu40Ni12Al4 with melted droplet and fluid properties at Strain Rate of 10−1 s−1. (Scale bar = 10 µm)
The authors acknowledge the support in part by the Ministry of Science and Technology, Taiwan under grant number MOST 106-2628-E-992-302-MY3.
