Influence of the chemical composition of rapidly quenched amorphous alloys (Ni, Fe, Cr)-B-Si on its crystallization process

This paper presents results of research of the structure and phase transformations during the multistage crystallization of the metallic glasses with the compositions Ni71,5Cr6,8Fe2,7B11,9Si7,1 and Ni63,4Cr7,4Fe4,3Mn0,8B15,6Si8,5 labeled as AWS BNi-2 according to American Welding Society. Differential scanning calorimetry (DSC), X-ray diffraction analysis (XRD), scanning electron microscopy (SEM) and energy-dispersive X-ray microanalysis (EDX) were used as experimental research methods. The influence of the alloys chemical composition (boron, manganese and iron) on the temperatures and the exothermic heat effects of phase transformations, as well as on the phase composition of alloys at three stages of crystallization was analyzed. We present a thermodynamic explanation of the observed heat effects. It has been shown that manganese has the main influence on the phase transformations temperatures and heat effects in these two alloys. It is also assumed that at the final crystallization stage simultaneously with the formation of phases Ni3B and β1-Ni3Si should occur the nucleation of borides of CrB type with high Cr and low Si content.


Introduction
Alloys based on the ternary system Ni-Si-B (AWS BNi-2) in crystalline and amorphous states are widely used as a brazing alloys and corrosion-resistant coatings. It is known [1][2][3][4], that crystallization of amorphous BNi-2 alloys proceeds in three stages: the first stage of crystallization (450-500°С) is associated with the formation of solid solution crystals based on nickel (α-Ni). The second stage (500-550°С) is associated with the formation of a metastable τ-phase with M 23 B 6 -type structure. At the final stage (550-650°С), the formation of Ni 3 B and Ni 3 Si [2,4] or Ni 31 Si 12 [1] as a result of the decomposition of τ-phase was reported. It should be noted that other authors observed another phases in BNi-2 alloys [3]: Ni 4 B 3 , CrB, B 2 Fe 15 Si 3 , CrSi 2 and Ni 45 Si 2 B [5]; Ni 2 B, Ni 3 Si 2 and Fe 3 (Si,B). Data obtained for the phase composition of such alloys produced by crystallization from the amorphous state, as well as data on the temperatures and heat effects of the phase transformations are slightly different due to different chemical compositions of the investigated BNi-2 alloys.
Therefore, a purpose of this work is to explore the influence of the chemical composition of the alloys on their phase composition at all stages of crystallization, as well as on the temperatures and the exothermic heat effects of phase transformations.

Results and Discussion
The results of differential scanning calorimetry of 1301A and 1301 alloys are presented on figure 1 (curves 1 and 2 respectively). This figure also shows the DSC curve 3 for alloy of similar composition (Ni 64,4 Fe 4 Cr 4,9 Mn 2 B 16,2 C 0,5 Si 8 ), presented in [1]. It can be seen that curves differ significantly despite the similarity of the compositions of these three alloys. Primary crystallization of 1301 compared to 1301A alloy begins at a slightly higher temperature, while the temperature of formation of τ-phase is below it. Thus, the first and second peaks on the DSC curve of the alloy 1301 are partially overlapped. For the third alloy (Ni 64,8 Cr 4,9 Fe 4,0 Мn 2 B 16,2 Si 8 С 0,5 ) this overlap is considerably more visible.
Overlapping of the first and second crystallization peaks means that the formation of α-Ni and τ-phase crystals occurs substantially simultaneously.  Table 1 presents data on the temperatures of the three most high heat peaks (T 1 , T 2 , T 3 ) on the DSC curves for different stages of crystallization of alloys 1301 and 1301A in comparison with the published data for other amorphous alloys with similar chemical composition. Because the available DSC data were obtained at the various heating rates of the samples, the values were given for the one heating rate (20 °C/min). For this purpose the data on the crystallization temperature dependence of the heating rate for the amorphous alloy Ni 65 Cr 6,6 Fe 3 B 17,5 Si 7,6 C 0,3 for all three stages of the crystallization process were used [5].
Analysis of these data shows that at the beginning of the primary crystallization generally not boron content, but the amount of manganese and possibly iron affects temperature Т 1 . Indeed, alloys No. 2 6, which has the high boron content, but do not contain manganese and iron, has a primary crystallization temperature around 480 °C that is typical for the majority of alloys of this type. It also can be seen that alloys with manganese and possibly with increased iron content have the lower values of temperature Т 2 and the higher temperature Т 3 . Ni 65 Cr 6,6 Fe 3 B 17,5 Si 7,6 C 0,3 480 525 600 [5] Note that the relative and total heat effects for three crystallization stages of 1301 and 1301A alloys are approximately the same (table 2). As shown below the relative amount of phases in 1301A and 1301 alloys after the first and second stages are significantly different (the amount of α-Ni in 1301A is approximately 1.4-1.6 times higher, and amount of τ-phase is 1.1-1.3 times lower than in 1301). Thus, formation of the same amount of α-Ni phase in the alloy 1301A is accompanied by a less heat effect than in the alloy 1301, and a larger heat effect for τ-phase formation, respectively. These results of DSC can be explained in terms of thermodynamics. The process of crystallization of the metal glasses is always exothermic and reduces its free energy ∆G. At the first crystallization stage a metastable equilibrium between the amorphous phase and the α-Ni crystal phase is established. The compositions of these phases are determined by the common tangent to the curves of Gibbs free energy dependence on boron concentration ( figure 2).
It is known that manganese lowers the melting point of BNi-2 alloy (to 20°C at 2% of Mn) [1]. Therefore, the ∆G curve of the amorphous phase (supercooled liquid) should move down with the addition of manganese. Thus, increase of the manganese content in the amorphous alloy should lead to decrease of the thermodynamic stimulus of α-Ni phase nucleation and to the growth of the onset crystallization temperature.
Increase of the boron content in the amorphous alloy also reduces the thermodynamic factor. However, the crystallization temperature remains nearly unchanged. It can be explained by the fact that not only the thermodynamic, but also kinetic factor affects the nucleation rate. Kinetic factor is determined by the rate of the boron atoms transition through the boundary between crystalline and amorphous phase and increases with the boron concentration increase. Thermodynamic and kinetic  It should be noted that increase of the both boron and manganese concentration should reduce the heat effect of the first crystallization stage (per mole of α-Ni phase), because it reduces the difference of enthalpies of the crystalline and initial amorphous phase.
The estimation of the relative volume of the crystalline phase in the alloy 1301A after the first stage of crystallization was carried out. The XRD data from figure 3 and the formula = /( + ), where andintegrated intensities of reflections from the crystalline and amorphous phases, and α -constant (the Huang parameter), were used for this purpose. The values of α for similar alloys (Fe 73.5 Cu 1 Nb 3 Si 13.5 B 9 and Fe 73.9 B 13.2 Si 10.9 C 2 ) are known: α ≈ 1.05 and 0.77 [7]. The estimated volume and mole fractions of crystalline phase calculated using these parameters are (49-57) vol. % and (44-52) mol. %. Considering that the solubility of boron in α-Ni is negligible, the boron content in amorphous phase at the metastable equilibrium can be estimated as 21-25 at.%, that is close to τ-phase (M 23 B 6 ) composition. On the second stage of crystallization, the formation of τ-phase occurs by the polymorphic transformation without significant change of α-Ni phase amount (Figure 4). In the alloy Ni 64,4 Cr 4,9 Fe 4 Mn 2 B 16,2 Si 8 C 0,5 with the composition close to 1301 alloy significantly greater amount of τ-phase have been formed (65 vol. %) [1], which is in a good agreement with our data (figure 4).  It is also known that in the Ni-B system metastable τ-phase can be fixed by addition of Fe and probably, Cr, into its composition [1]. In the absence of Fe this phase does not form. Note that the formation of compounds of type M 23 (B,C) 6 is typical for such metals as Fe, Cr, Mn, but not Ni. Thus, these metals contribute to the stabilization of τ-phase by decrease of its free energy. This leads to the increase of temperature of τ-phase formation (Т 2 ) and to increase of its decomposition temperature Т 3 . Indeed, alloy 1301A completely decomposes at a temperature 600°C, while the alloy 1301 still contains τ-phase at temperatures up to 650°C ( figure 5). Annealing at 900°C led to the decomposition of this phase. The presence of two cubic phases (α-Ni and Ni 3 Si) with fcc-structure and different lattice parameters at the end of the crystallization process of alloys 1301 and 1301A was observed. Ni 3 Si-based phase can hardly be discriminated from α-Ni by XRD since it has the same structure, but the presence of the silicide phase is well observed in XRD pattern of alloy 1301. According to EDX analysis of alloy 1301 annealed at 900°C, in addition to phases mentioned above, formation of chromium boride of CrB type with low Si content can also be observed ( figure 6). Application of the EDX method in scanning electron microscope for alloys annealed at low temperatures is limited because of the small size of crystals (30-100 nm). XRD method did not detect the presence of CrB phase in the crystallized amorphous and massive cast alloys, although this phase was detected by EDX analysis [4,8,9].  Assuming that Ni 3 B does not contain silicon and Ni 3 Si does not contain boron, the concentration of silicon in α-Ni and Ni 3 Si corresponds to the Ni-Si diagram, and iron is not redistributed between the phases, the phase compositions of alloys were estimated from material balance equations. These estimations were carried out for two cases: considering of formation CrB and without it ( that even a small increase of the boron content in the alloy leads to a mentioned increase of the amount of Ni 3 Si and reduce of α-Ni. Note that the amounts of α-Ni and Ni 3 Si phases in the crystallized alloy 1301 are comparable as shown in figure 1 a, b. The observed high amount of α-Ni phase in the alloy 1301 and low amount of Ni 3 Si phase in the alloy 1301A after the complete crystallization cannot be explained without assumption of the formation of phase CrB.

Conclusion
The influence of the alloys chemical composition (boron, manganese and iron) on the temperatures and the exothermic heat effects of phase transformations, as well as on the phase composition of alloys at three stages of crystallization were analyzed. It is shown that the onset crystallization temperature is practically independent of the boron content in alloys, but it increases with the increase of manganese content. The manganese and iron stabilizes a metastable -phase, resulting in decrease of the onset temperature of the second crystallization stage (-phase formation) and increase of the onset temperature of the third stage (decomposition of phase). The thermodynamic explanation of these phenomena is proposed.
It is assumed that at the final crystallization stage simultaneously with the formation of Ni 3 B and Ni 3 Si phases the nucleation of borides of CrB type with high Cr and low Si content should occur.