Deformation and fracture of heat treated ribbon of amorphous Co – Fe – Cr – Si – B alloy during indentation

Introduction

The process of amorphization, achieved by ultrarapid melt-quenching, allows for the production of homogeneous multicomponent amorphous alloys (AA). By varying the conditions of melt-quenching, it is possible to significantly expand the range of mutual solubility of elements, enabling a broader variation of AA properties compared to crystalline materials [1 – 4]. If heat treatment is also applied, a wide range of intermediate states can be obtained – from “purely” amorphous to amorphous-nanocrystalline and fully crystalline – enabling the creation of unique and diverse amorphous systems (metastable forms of binary, ternary, and multicomponent alloys) [5 – 7]. By adjusting the composition and applying controlled heating, it is possible to achieve useful and predefined physical properties, expanding the application of AA in engineering and industry as structural and functional materials [8 – 11]. The excellent magnetic properties of AA are complemented by other valuable characteristics such as high strength, hardness, electrical resistivity, corrosion resistance, and satisfactory plasticity. However, when applied in practice, issues such as thermal stability, embrittlement, and spontaneous crystallization at elevated temperatures are of particular concern [12 – 14]. To evaluate the structural-phase state of AA, a comprehensive set of research methods is used (e.g., transmission electron microscopy (TEM), X-ray structural analysis, differential thermal analysis, etc.). Some of these methods require multi-stage sample preparation and involve lengthy and labor-intensive phase identification processes. Moreover, when AA has been processed at temperatures below crystallization and remains in an amorphous state, there are significant challenges in studying the amorphous structure. Even high-resolution TEM and EXAFS-spectroscopy (Extended X-Ray Absorption Fine Structure) cannot definitively provide information about atomic arrangements and positional changes during relaxation. Studying structure-sensitive property responses (mechanical, electrical, chemical, magnetic) allows the analysis of structural relaxation processes [15 – 17].

Currently, the micro- and nanoindentation is an informative method for investigating the mechanical characteristics of melt-quenched amorphous alloys (hardness, Young’s modulus, fracture toughness) [18 – 22]. This method is also valuable for understanding the micromechanisms of their deformation, as it allows for the visualization of the deformation pattern and the assessment of the extent and shape of deformation zones in correlation with the structural state of the amorphous alloys. Indentation testing is an excellent tool for such studies, especially when only a small amount of material is required. Deformation during indentation is essentially stable, at least macroscopically, since the contact area between the indenter and the deformed material increases during indentation to accommodate any applied load. However, local loading of amorphous alloys has several specific features related to their unique geometry – small thickness of amorphous alloys ribbons and the qualitative difference between the contact side (adjacent to the quenching drum during production) and the free side of the ribbon [23]. Moreover, plastic flow in amorphous alloys under high stresses and low temperatures occurs non-uniformly, localizing in shear bands along planes of maximum shear [24; 25]. The formation of these shear bands is associated with localized atomic rearrangements correlated with regions of excess free volume [26 – 28]. Shear bands are crucial for the deformation behavior of amorphous alloys, and controlling them is equivalent to managing the plasticity and fracture of the material.

The aim of this work is to study the mechanical behavior of heat-treated thin melt-quenched amorphous alloy ribbons during microindentation, specifically to reveal the patterns of deformation zone modification and surface microfracture of amorphous alloys around the imprints formed by the Vickers pyramid, as the annealing temperature increases.

Materials and methods

The object of the study was an amorphous alloy Co_70.5Fe_0.5Сr₄Si₇B₁₈ (at. %), produced by melt-spinning process in the form of a ribbon with a thickness of 30 μm and a width of 20 mm. Alloy samples measuring 15×20 mm were subjected to vacuum annealing in the temperature range of T_an = 538 – 823 K for 10 min. Afterward, they were fixed using an elastic substrate – “Moment” glue (20.52.10.190 OKPD 2 / GOST 22345-77) with a thickness of approximately 1.0 – 1.5 mm on a steel plate. As a result, a composite blank was formed (Fig. 1, a).

Indentation was performed on the free side of the amorphous alloy ribbons across a wide range of loads (0.3 – 2.0 N) using a PMT-3M microhardness tester (LOMO). It is important to note that local loading on a hard substrate (for example, putty) presents a fundamentally unavoidable difficulty: cracks may initially form in the substrate and subsequently initiate the failure of the coating, i.e., the amorphous alloy.

The macroscopic plasticity behavior of the amorphous alloys was assessed by the U-method (by bending deformation). For this, the sample was placed between two flat parallel plates, and as the plates moved closer together at a constant speed, the distance d at which the bent sample fractured was determined (Fig. 1, b). The plasticity measure ε_f was taken as the ratio

\[{\varepsilon _f} = \frac{h}{{d - h}},\]

where h is the thickness of the ribbon.

The ductile-to-brittle transition temperature Т_f was calculated as the average temperature between Т₁ and Т₂, where Т₁ – is the highest annealing temperature at which ε_f  = 1, and Т₂ is the lowest recorded temperature at which a sharp decrease in plasticity occurs (ε_f  → 0).

The crystallization temperature was assessed using differential scanning calorimetry on a DSC 8271 analyzer (Rigaku). The transition from the amorphous to crystalline state during heat treatment was monitored through X-ray structural analysis using the ULTIMA IV multifunctional diffractometer (Rigaku).

Results and discussion

The experiment revealed that during the indentation of the Co_70.5Fe_0.5Cr₄Si₇B₁₈, amorphous alloys, as they transition from the amorphous to the crystalline state during annealing, there is a significant evolution in the micro-patterns of their deformation and fracture. Fig. 2 illustrates the variety of morphological modifications on the surface of the amorphous alloy following indentation with different loads.

As the alloy is heated, distinct patterns in the formation of deformation zones become apparent. In the temperature range T_room < 613 K, typical indenter imprints are observed at low loads, while at higher loads, the imprint is surrounded by shear bands (Fig. 2, a, b). Shear bands represent a phenomenon of plastic instability, where large shear deformations are localized within a relatively narrow band during material deformation. In the temperature range of 613 K ≤ T_an ≤ 748 K, straight radial cracks appear in the area of local loading on the amorphous alloy (Fig. 2, c). Some of these cracks may intersect with ring cracks (Fig. 2, d). Despite the embrittlement, plastic deformation of the amorphous alloy remains possible (with occasional shear bands) up to T_an = 748 K. Finally, in the heating range of 748 K < T_an ≤ 803 K, the imprints predominantly form a network of cracks oriented parallel to the faces of the indenter pyramid (Fig. 2, e). Additionally, ring cracks may form further away from the indentation zone (Fig. 2, f).

After indentation at the maximum load of P = 2 N, the annealing temperature T_cr, at which the first cracks formed and propagated in the amorphous alloy samples with a probability of no less than 0.5, was determined to be T_cr = 628 K [29]. The Tcr established on the elastic substrate effectively represents the ductile-to-brittle transition temperature of the amorphous alloy, closely matching the independent test data from U-bend testing of amorphous ribbons: T_f = 613 K (Fig. 3).

Subsequent attention was focused on calorimetric and structural studies to clarify the temperature ranges of phase transitions that the АA Co_70.5Fe_0.5Cr₄Si₇B₁₈  amorphous alloy undergoes. According to DSC data, the crystallization temperature of the studied amorphous alloy is T_crys = 803 K (Fig. 4).

According to structural studies, the formation of the first nanocrystals in the amorphous matrix, corresponding to α-Co with an HCP lattice (a = b = 2.514 Å, c = 4.105 Å), occurs after annealing above 688 K [30]. As the temperature increases, an intensification of crystallization processes and an increase in the volumetric fraction of crystalline phases are observed. Along with α-Co, the β-Co phase with an FCC lattice (a = b = с = 3.554 Å), appears, as well as Co₂Si, Co₄B, Co₃B, Co2B (Fig. 5).

By comparing the morphological atlas of the surface of the annealed Co–Fe–Cr–Si–B amorphous alloy after indentation (Fig. 2) with the results of its structural changes (Figs. 4, 5), three temperature ranges can be distinguished. The transition from one range to another results in changes in the characteristic deformation and fracture patterns, which are a consequence of ongoing structural relaxation processes and subsequent crystallization [31].

• At T_room < T_f , the amorphous alloy demonstrates its unique plasticity, where even the maximum load on the indenter can only cause highly localized plastic deformation in the form of shear bands around the imprint.

• The temperature range T_f  ≤ T_an ≤ T_sb is transitional, as no cracks form at lower temperatures (for Co70.5Fe0.5Сr4Si7B18 , the temper brittleness temperature is T_f  = 613 K), and at higher temperatures, shear bands are absent (the temperature of complete disappearance of shear bands for Co_70.5Fe_0.5Cr₄Si₇B₁₈ is T_sb = 748 K). In this temperature range, the amorphous alloy is still in an amorphous but embrittled state due to the ductile-to-brittle transition phenomenon. Therefore, large radial and ring cracks, as well as spalls, are observed.

• The range T_sb < T_an ≤ T_crys corresponds to the final transformation of the amorphous alloy into a crystalline state (for Co_70.5Fe_0.5Cr₄Si₇B₁₈ T_crys = 803 K). A distinctive feature of this range is the formation of relatively symmetrical fracture patterns consisting of square crack networks nested within each other.

Conclusions

Using the example of melt-quenched amorphous alloy ribbons of the Co – Fe – Сr – Si – B system, it was demonstrated that indentation on an elastic substrate with different loads enables an approximate express assessment of the structural state of the amorphous alloy. This is due to the fact that the deformation and fracture zones formed under the indenter (presence/absence of shear bands, cracks, their mutual arrangement) are highly structure-sensitive and exhibit characteristic features in specific annealing temperature ranges.

Overall, indentation testing is an attractive method for studying the deformation behavior of amorphous alloys for the following reasons. First, these tests are easy to perform and do not lead to macrofracture, as they are not specific to the sample size. Second, plastic deformation in the material is locally confined, facilitating the study of plastic flow in the zones surrounding and located under the indenter. Moreover, direct comparison of indentation results with responses, such as to bending or tension, further makes the indentation method an effective “probe” for understanding the physics of plastic deformation and fracture in amorphous alloys.