1 Introduction

Amorphous alloys of Fe-Si-B not only have excellent magnetic properties, such as lower coactivity and higher permeability, but also have distinct advantages over crystalline materials in respect of mechanical properties like strength and hardness. Therefore, researchers have developed a large number of amorphous alloys with breakthroughs in understanding the mechanical properties of ribbons and studies of their functional performance in the last ten years. However, the development in this area is limited by the tensile brittleness of Fe-Si-B amorphous alloys at room temperature, which is deemed as a key factor by researchers^[1-3]. Proposed methods for improving the brittleness of Fe-Si-B amorphous alloys at room temperature, such as selecting alloy elements which are favorable to improve toughness and selecting appropriate process parameters are given in the literature. Further, the study of the Fe-Si-B amorphous ribbon with certain plasticity has practical significance.

In this study, the microstructure and crystallization process of the Fe_81.50B_1.40Si_7.95Nb_7.37Cu_1.73P_0.05 alloy were analyzed and discussed, and the tensile test was performed. The tensile deformation behaviour and mechanism were investigated. The fracture morphology was observed by Scanning electron microscope (SEM) and the fracture mechanism of the amorphous ribbon was analyzed, which was helpful to supplement the basic mechanical properties data of the studied amorphous alloy.

2 Preparation of Materials and Experimental Method

Fe_81.50B_1.40Si_7.95Nb_7.37Cu_1.73P_0.05 amorphous alloys were prepared by using arc melting under a protective Argon atmosphere. Amorphous ribbons of 0.02 mm×0.02 mm were prepared by the standard procedure of rapid quenching by melting on a rotating disc (melt spinning method). Inductivity coupled plasma (ICP) analysis was used to check actual chemical composition. The amorphous nature of the ribbons was analyzed by using an X-ray diffractometer (Philips PW 1830) with Co-K 1.788 97 Å radiation^[4-5]. The surface morphology of the amorphous sample was observed by Atomic Force Microscope (AFM) and Zeiss's microscope.

The chemical composition of the Fe_81.50B_1.40Si_7.95Nb_7.37Cu_1.73P_0.05 alloy was measured by Inductivity coupled plasma (ICP). The major chemical components of the alloy are shown in Table 1.

Table 1 shows that there are 7.37% Nb element and 1.73% Cu element in the Fe_81.50B_1.40Si_7.95Nb_7.37Cu_1.73P_0.05 alloy in addition to Si-B.

The tensile properties of the Fe_81.50B_1.40Si_7.95Nb_7.37Cu_1.73P_0.05 alloy were tested by INSTRON E3000 electronic universal material testing machine. The amorphous ribbon sample was cut into 30 mm wide and 65 mm long; the sheared amorphous ribbon sample was prepared into a thickness of 25 μm. The tensile stress-strain curves, tensile strength at room temperature, elongation and tensile elastic modulus of amorphous samples were measured with reference to GB/T 228-2010, Metallic materials-tensile testing-Part 1: method of the test. The strain rate ε was less than 10^-3 mm/s. The tensile fracture morphology of the Fe_81.50B_1.40Si_7.95Nb_7.37Cu_1.73P_0.05 alloy was analyzed by JSM-6010LA Scanning Electron Microscope (SEM). In the process of the tensile test, the amorphous ribbons were installed on the chuck of the universal testing machine by rubber pad clamp because the amorphous ribbons were too thin and crushed. Rubber pad fixture can avoid skidding. The ribbon was tested by stretching measurement, and the sample was slowly loaded until the sample was broken^[6-7].

3 Results and Discussions 3.1 Characterization of the Fe_81.50B_1.40Si_7.95Nb_7.37Cu_1.73P_0.05 Alloy

X-Ray diffraction method is generally used to determine whether the amorphous sample is in an amorphous state^[8]. The X-ray diffraction (XRD) data of the Fe_81.50B_1.40Si_7.95Nb_7.37Cu_1.73P_0.05 alloy shows that there is a broad peak at 2θ=44.9° in Fig. 1 for the alloy and there is no sharp peak. The result reveals a complete amorphous structure of the Fe_81.50B_1.40Si_7.95Nb_7.37Cu_1.73P_0.05 alloy. In order to distinguish the phase evolution of crystallization, DSC scans were used to prolong the crystallization process of each step. The DSC curves of the Fe_81.50B_1.40Si_7.95Nb_7.37Cu_1.73P_0.05 alloy are shown in Fig. 2. Characteristic temperatures are associated with the onset temperature of primary crystallization (Tg, 515 ℃). The peak temperature (Tx1, 548 ℃) are indicated in Fig. 2. The crystallization was completed when the exothermic temperature reached the peak value of 569 ℃^[9-10].

3.2 Surface Morphology of the Amorphous Ribbons

The surface of the amorphous sample was tested by Atomic Force Microscope(AFM), as shown in Fig. 3.

The micrographs indicate that the free surface of the ribbon is much smoother than the surface of the roller. The surface of the roller is very rough with ribbon pits, small holes (or pores) and other defects. This may be due to the poor contact between the amorphous ribbon and the cooling roller during the spray process, and the nozzle distance not matching with the cooling roller distance also plays a part. The tensile strength of the alloy is affected by the surface defects and the gap between the free and the wheel surface. The surface roughness of the ribbons depends on the surface roughness of the cooling roller, but the opposite side is a free surface, so the surface roughness is difficult to control. The ribbon sticking to the cooling roller is smooth, but the opposite side is uneven and rough^[11].

The morphology of the surface defect was studied using Zeiss microscope as shown in Fig. 4.

It is evident from Fig. 4 that there are many small holes or pores on the surface of the Fe_81.50B_1.40Si_7.95Nb_7.37Cu_1.73P_0.05 alloy. In the case of tension, the formation and propagation of surface micro cracks lead to the overall fracture of the ribbons^[12].

3.3 Test Results of the Fe_81.50B_1.40Si_7.95Nb_7.37Cu_1.73P_0.05 Alloy by Tensile Machine

The tensile stress-strain curve of the Fe_81.50B_1.40Si_7.95Nb_7.37Cu_1.73P_0.05 alloy is shown in Fig. 5.

From Fig. 5 and Table 2, it can be observed that tensile experience of the Fe_81.50B_1.40Si_7.95Nb_7.37Cu_1.73P_0.05 amorphous ribbons is almost completely in elastic phase, stress and strain exhibits a linear relationship. Plastic deformation is small, and there is lower plastic yield phenomenon. The maximum value of tensile strength reaches 1951.75 MPa. The mechanism of tensile failure of the alloy is the mainly brittle fracture. The plastic deformation is smaller^[13]. The maximum values of the elastic modulus reach about 70 GPa. The amorphous ribbons possess reasonable tensile elongation (2.46%). In addition, the ratio of elastic modulus of amorphous metal is found to be low, so its toughness is better. The elastic modulus reflects the binding energy between atoms. Compared to crystalline metal, the atomic spacing is larger due to the irregular accumulation of atoms.

3.4 Fracture Morphology of Fe_81.50B_1.40Si_7.95Nb_7.37Cu_1.73P_0.05 Alloy

Fig. 6 (a) shows that the fracture surface is severely uneven because of the highly complicated and uneven stress distribution inside the ribbons. The whole section is brittlest fracture zone with comparatively poor ductility. Fig. 6(e) shows the tearing ridge where ductile fracture and brittle fracture intersect.

The crack initiation area appears on the surface of the fracture, as shown in Fig. 6 (a). The surface crack begins with cleavage fracture, and the micro-fracture has tear lines. The shape of the crack is a radial river pattern.

The analysis of the fracture characterization is given as follows:

(1) Brittle fracture characterization:Fig. 6 (b), (c), (d) shows the characteristics of brittle fracture zone. This fracture zone is relatively smooth, and there are many crack sources. This crack source is generated from the ribbon surface and gradually evolves into multiple crack bifurcation. Fig. 6 (b) shows the crack source is coarse. In the process of crack diffusion, there are a lot of small branches.

These branches cover the entire section, and these intersections on the sections sometimes present molten droplets, as shown in Fig. 6 (d). This is due to the transformation of the deformation energy into thermal energy under quasi-static loading, which results in partial temperature rise exceeding the melting point, and the formation of the molten droplet morphology at the fracture surface of the alloy.

The source of the crack radially diffuses all around.The results show that the main reasons of fracture branches are small defects of the Fe_81.50B_1.40Si_7.95Nb_7.37Cu_1.73P_0.05 alloy, such as small pores. During the tensile process, the imbalance between the pores or the surrounding forces causes the alloy to break into the small regions.

(2) ductile fracture characterization:The fracture characteristics of ductile fracture are clearly seen from Fig. 6(e), (f). The figures show a clear plastic dimple structure with a smaller size and shallower depth. The dimple area indicates a certain local deformation, and the dimple has the same direction in the local range. The crack of ductile fracture zone and brittle fracture zone are in the same direction (Fig. 6(b)) ^[14-15].

The surface of the plastic deformation zone has natural characteristics of mutual tearing and viscous flow^[16]. The sample is subjected to positive stress, so that the slip plane is separated from each other, forming a rough fracture morphology. As shown in the arrow, the tear formation is not smooth, and its enlargement is shown in the upper right corner of Fig. 6(c), where there is natural mutual tearing and viscous flow.

The brittleness greatly affects the working efficiency and its industrial application^[17-18]. At present, its mechanical behavior cannot be completely explained by various models of the amorphous alloy deformation and fracture mechanisms. Some amorphous ribbons have brittleness and some have a ductile and brittle mixed characteristic, which is related to the composition of amorphous ribbons. For example, Song Hui et al.^[19] reported that the Fe₇₈Si₁₃B₉ ribbon is completely in an elastic deformation state and no plastic deformation occurs. Different kinds of the relative content of alloy elements seems to improve soft magnetic and mechanical properties. The ribbons Fe_81.50B_1.40Si_7.95Nb_7.37Cu_1.73P_0.05 can withstand 180° bending without breaking, which proves that it has certain plasticity. It is noted that enhanced ductility can be obtained in amorphous ribbons which contain a relatively high amount of Nb and Cu.

4 Conclusions

1) Analysis of the amorphous ribbons of Fe_81.50B_1.40Si_7.95Nb_7.37Cu_1.73P_0.05 alloy shows that it is a complete amorphous material and its surface is very irregular.

2) The maximum value of the Fe_81.50B_1.40Si_7.95Nb_7.37Cu_1.73P_0.05 alloy reaches 1951 MPa in tensile strength, which proves that the alloy can achieve high tensile strength. The value of tensile modulus of elasticity reaches about 70 GPa. The amorphous ribbons possess reasonable tensile elongation (2.46%).

3) The fracture zone of brittle fracture is relatively smooth. The crack originates from the surface of the samples. There are many crack sources, and a crack source starts from the surface of the alloy surface and gradually evolves into a plurality of crack sources.

4) The sample of the Fe_81.50B_1.40Si_7.95Nb_7.37Cu_1.73P_0.05 alloy has the characteristics of mixed fracture. The fracture appearance of the Fe_81.50B_1.40Si_7.95Nb_7.37Cu_1.73P_0.05 alloy is a mixed mode of ductile and brittle fracture which includes dimples and partial cleavage fracture similar to that of the crystalline materials.