1 INTRODUCTION

For modern science and technology, studies on the effect of various combined actions on the structure and properties of promising materials, such as amorphous metal alloys (AMAs) [1–3] or metallic glasses remain relevant since products made of these materials are operated under various conditions, for example, in aggressive environment, electromagnetic field, and under cyclic loads. In this regard, it is of interest to study such a combined effect [4–8] on the structure and properties of AMA ribbons and products made of them. The fields of application for these materials are constantly expanding. The properties and structure of AMA ribbons have not been entirely studied [9–15], and there is a lack of information on bulk AMAs [16–20], which have been recently obtained.

Summing up the above, the study of the action of various external factors on the AMA’s properties is one of the relevant problems in physics and mechanics of solids. Within the framework of these problems, a comprehensive study of the mechanical properties of AMAs in areas of local exposure, in particular, to pulses of coherent radiation is of practical interest since the laser beam machining is one of the methods of heat treatment. One of the methods for studying the mechanical properties of AMA ribbons is Vickers hardness test. When performing indentation of AMA in the initial state, a deformation rosette is formed. As the annealing temperature rises, the deformation rosette is gradually replaced by a rosette consisting of a network of microcracks (ring and radial). Knowing the annealing temperature, each of them can be compared with the rosette type obtained as a result of indenter action. This approach has been used in this article to determine the heating temperature in the zones of laser radiation action on amorphous alloy ribbons.

2 STATEMENT OF THE PROBLEM

The purpose of this article is to compare changes in the mechanical characteristics of AMA ribbons under the action of annealing and laser radiation, as well as to study the combined effect of laser radiation and fatigue loading on AMA behavior. We also assess the crack resistance of alloys after various types of actions.

3 METHOD AND CONSTRUCTION OF THE SOLUTION

In the first part of the article, studies on the effect of laser radiation and fatigue loading on the mechanical behavior of AMAs have been performed.

For studying, we have used the AMA ribbons (AMAG-186: Co—85.5%; Fe—2.27%; Si—5.15%; Mn—4.07%; B—2%; Cr—1%). The working area of samples is 20 × 3.5 × 0.02 mm. Before carrying out fatigue tests, the samples have been exposed to laser radiation using LTA-4-1 setup. The radiation wavelength is 1064 nm. Samples have been irradiated by a single pulse in the center of working area. The pulse duration is 4 ms, the energy is 0.17 J. The area of the irradiation zone is ≈0.283 mm2.

Fatigue tests of samples have been carried out using a specially designed installation under a load varying over time from a maximum value of 750 MPa to minimum of 400 MPa under a condition of repeated stress cycle with a frequency of 2 Hz.

One of the technical parameters for predicting the mechanical behavior of high-strength and low-plastic constructional and tool materials is crack resistance determined via the critical stress intensity factor of the first kind K1c [21].

$${{K}_{{1c}}} = A{{\left( {\frac{E}{H}} \right)}^{{1/2}}}\frac{P}{{{{C}^{{3/2}}}}},$$
(3.1)

where A = 0.016 is the proportionality coefficient, E is the Young’s modulus, H is the Vickers microhardness, P is the critical load for the appearance of radial cracks, C is the length of the radial crack.

There are some difficulties in assessing the crack resistance for samples of AMA ribbons during their microindentation on substrates. Firstly, when AMA is indented on a solid substrate under significant loads P ≥ 160 g, one should not exclude the fact that cracks can form initially in the substrate and initiate coating destruction (AMA). Secondly, the prints on the AMA surface are surrounded by a system of cracks having different lengths—from a few micrometers to several millimeters.

To establish the cracking susceptibility for AMA, a method based on the determination of the relative crack resistance (χ) obtained by the formula (3.2) depending on the applied load is proposed. In this method, the relative crack resistance at load of 160 g is set equal to unit. At lower loads, crack resistance is determined as the average value over 10 measurements.

$$\chi = \frac{{K_{{1c}}^{i}}}{{K_{{1c}}^{{\max }}}}.$$
(3.2)

Here \(K_{{1c}}^{{\max }}\) is the critical stress intensity factor of the 1st kind that is determined for a load of 160 g, \(K_{{1c}}^{i}\) is the critical stress intensity factor of the 1st kind for a given load.

4 ANALYSIS OF THE RESULTS AND EXAMPLES

As a result of laser radiation action, a local reflow zone has been observed on the sample surface (Fig. 1a), in which four characteristic regions can be distinguished: internal region of fusion; dendritic growth area; area with unfinished growth of dendrites. The area not exposed to laser radiation is separated by a clear boundary (indicated by an arrow in Fig. 1a). For these areas, we have determined the critical stress intensity factor of the first kind and the relative crack resistance (χ). It has been found that the relative crack resistance (χ) determined via the critical stress intensity factor of the first kind K1c varies from 1 in the zone exposed to laser radiation and decreases monotonically to 0 when passing from the reflow zone to the region not exposed to laser radiation.

figure 1

Fig. 1.

The obtained samples have been subjected to fatigue loading. Fatigue failure of the sample can occur both in the irradiation zone and at some distance from it. In this case, if the sample failure occurs at a distance from the area exposed to laser radiation, then during load dropping, the reflow area is chipped (Fig. 1b) at the time of failure. This circumstance is explained by the high value of relative crack resistance (χ) determined via the critical stress intensity factor of the first kind K1c, in this region χ is equal to 1.

It has been established that laser action with the indicated parameters does not affect fatigue. The fatigue limit of the samples remains constant value and is equal to 428 MPa.

Fractographic studies of fracture in the zone exposed to laser radiation have been carried out. The characteristic features of the destruction are revealed.

Cracks can develop rectilinearly and there can be brittle fracturing. Shear bands are observed at the crack tip (Fig. 2). Also, cracks that develop with branching in the reflow zone and after passing through its boundary they propagate rectilinearly have been found (Fig. 3). Failure can occur in a plane parallel to the sample plane, which leads to chips (Fig. 4).

figure 2

Fig. 2.

figure 3

Fig. 3.

figure 4

Fig. 4.

In the second part of the article, a method for determining the surface temperature in the zone exposed to laser radiation is proposed.

Based on the statistical processing of experimental data obtained during the indentation of annealed AMAs, the common factors of their deformation and fracture are determined.

For each indentation, each sample has been repeatedly loaded by different loads starting from P = 40 g, at which cracks do not form and gradually increasing load in increments of 5–10 g until cracks appear. If cracks have formed as a result of indentation, then the relative crack resistance (χ) has been calculated by the formula (3.2), and if cracks have not formed, then the relative crack resistance (χ) is considered equal to zero.

Embrittlement of AMAs, as a rule, begins with the near-surface layers of amorphous ribbons. Embrittled layers are also in an amorphous state, however this condition differs from the state of the amorphous matrix. Annealing leads to a redistribution of metalloid atoms, as a result of which clusters with an increased concentration of metalloid atoms compared to the matrix are formed. These clusters are the cause of embrittlement (crack formation). Structural microinhomogeneities found in the near-surface layers of amorphous ribbons can be identified with such clusters of a new amorphous phase. The thickness of the embrittled layers of amorphous ribbons is a function of the chemical composition of the alloy and increases with increasing time and annealing temperature. Moreover, as a result of ten-minute annealing, cracks begin to form in the samples annealed at T = 773 K. Based on the obtained experimental data, the dependence of the relative crack resistance (χ) on the load (P) is constructed. Each point in the dependence corresponds to 10 experiments. The experimental results have been fitted by a linear approximation using dependencies of the form:

$$\chi = aP + b.$$
(4.1)

The values of the correlation coefficient at given temperature for t = 10 min are R = 0.96–0.99. Coefficients a and b are determined from experimental data.

The dependence of the relative crack resistance (χ) on the load applied to the indenter during microindentation is determined for the areas of thermal action of laser radiation, in which the critical stress intensity factor of the first kind K1c is determined.

This dependence is compared with the dependences χ(P) for furnace annealing at different temperatures that have been obtained by indentation on an identical substrate.

It has been found that heating the boundary region of the irradiation zone at different distances from the exposure epicenter (or different distances to the reflow zone) is equivalent to annealing in a furnace at different temperatures and annealing time.

Figure 5 shows the dependence of relative crack resistance (χ) on the load (P) applied to the indenter for the AMA sample subjected to pulsed laser radiation –2 and heat treatment in the furnace –1: between the melt-through and reflow zones, annealing temperature is Tan = 773 K. Line 1 shown in Fig. 5 is qualitatively and quantitatively consistent with a similar linear dependence 2 under action of laser radiation, which suggests equal heating temperatures.

figure 5

Fig. 5.

5 CONCLUSIONS

Thus, in the AMA samples, zones exposed to laser radiation are areas of local heterogeneity of the structure, which is associated with the formation of structural and thermal stresses. These stresses, combined with the fatigue loading stresses, cause the formation of radial and ring cracks in the area exposed to laser radiation, which leads to chipping. Determination of dependence for the relative crack resistance of the AMAs on the load applied to the indenter using the critical stress intensity factor of the first kind K1c allows us to estimate the equivalent heating temperature during laser beam machining, based on a comparison of the dependences χ (P) obtained for samples annealed in the furnace.

The relative crack resistance (χ) of AMA ribbons that is characterized using the cracking susceptibility is described by a linear law depending on the applied load.