Nanocrystallization kinetics and glass forming ability of the ${\mathrm{Fe}}_{65}{\mathrm{Nb}}_{10}{\mathrm{B}}_{25}$ metallic alloy

The crystallization kinetics of glassy ${\mathrm{Fe}}_{65}{\mathrm{Nb}}_{10}{\mathrm{B}}_{25}$ melt-spun ribbons is studied by differential scanning calorimetry in the mode of continuous heating and isothermal annealing and by x-ray diffraction and transmission electron microscopy. Continuous heat treatments of the ribbons show the presence of multiple exothermic peaks before melting. The low-temperature peak corresponds to the precipitation of nanoscale ${\mathrm{Fe}}_{23}{\mathrm{B}}_6$-type crystalline metastable phase, and further annealing leads to its transformation into the metastable ${\mathrm{Fe}}_3\mathrm{B}$ phase and subsequent formation of $\mathrm{bcc}\text{−}\mathrm{Fe}$, ${\mathrm{Fe}}_2\mathrm{B}$, and FeNbB stable crystalline phases. The nucleation frequency and the growth rate are determined at selected temperatures from the analysis of the microstructures that emerge during the ${\mathrm{Fe}}_{23}{\mathrm{B}}_6$-type nanocrystallization. The master curve method is used to obtain the apparent activation energy and the Avrami exponent at the nanocrystallization onset. The nanocrystallization kinetics is explained in the framework of the Kolmogorov-Johnson-Mehl-Avrami theory. The rejection of insoluble alloy atoms during primary crystallization, the formation of diffusion layers around the crystals, and the decrease in the nucleation frequency caused by alloy enrichment of the residual disordered matrix is modeled through a soft impingement factor. Estimated values for the interfacial energy that provide a satisfactory agreement between experiments and modeling are derived considering that homogeneous nucleation frequency and interface-controlled grain growth are dominant at the onset of the nanocrystallization. Consequently, the time-temperature-transformation diagram is also drawn and the critical cooling rate estimated for this glass forming alloy.

Figure 1

TEM micrographs and corresponding SAED patterns of the sample (a) as-quenched and (b) after primary crystallization.

Figure 2

DSC scan at $10\mathrm{K}∕\min$ up to $1200\mathrm{K}$. $P_1$, $P_2$, and $P_3$: first, second, and third crystallization peaks. $T_g$, glass transition temperature; $T_x$ temperature onset of the first crystallization peak; $\Delta T_x$, supercooled liquid interval.

Figure 3

X-ray diffraction pattern after continuous heating at $10\mathrm{K}∕\min$ up to (a) $990\mathrm{K}$, (b) $1103\mathrm{K}$, and (c) $1273\mathrm{K}$.

Figure 4

X-ray diffraction patterns after isothermal annealing at $893\mathrm{K}$ during (a) $5\min$, (b) $7.5\min$, (c) $20\min$, and (d) $60\min$.

Figure 5

DSC continuous heating scans as measured (dashed and solid lines, two different scans) and fitted values using the constant activation energy KJMA kinetic model (symbols).

Figure 6

DSC isotherms at temperatures of 898, 903, and $908\mathrm{K}$. Solid lines: as measured Dashed-cross lines: base line drift at long time range. Symbols: as fitted using the constant activation energy KJMA kinetic model.

Figure 7

TEM micrographs of the sample annealed (a) at $873\mathrm{K}$ during $30\min$ and (b) at $883\mathrm{K}$ during $30\min$.

Figure 8

Transformation rate under continuous heating as measured (dashed and solid lines, two different scans) and fitted values using the nucleation/growth KJMA formalism (symbols).

Figure 9

Master curve (black line) and experimental curves (gray thin lines) at an equivalent heating rate of $10\mathrm{K}∕\min$.

Figure 10

Plot of $P^{-1}\left(x\right)$ versus $x$ obtained from the experimental continuous heating DSC data using the master curve (solid line) and Avrami fit between $2\%<x<8\%$ with $n=4$ (dashed line).

Figure 11

$G\left(x\right)$ function obtained by integration of $P\left(x\right)$ (squares). The solid line corresponds to the asymptotic fit of the $P\left(x\right)$ function at the onset of the transformation.

Figure 12

Maximum diameter $D_{\max }$ as measured (symbols) and as fitted using the constant activation energy KJMA kinetic model at $873\mathrm{K}$ (dashed line) and at $883\mathrm{K}$ (solid line).

Figure 13

Temperature dependence of the viscosity as fitted using the Fulcher-Vogel-Tammann equation (solid line). Estimated value of the liquid (symbol).

Figure 14

Temperature dependence of the crystal-melt interfacial energy.

Figure 15

Calculated values for the nucleation frequency and the interface-controlled crystal growth (solid lines) and the corresponding asymptotic values obtained using the constant activation energy kinetic model (dashed lines) versus temperature. Symbols correspond to experimental data.

Figure 16

The TTT diagram obtained using the constant activation energy kinetic model (dashed lines, $\alpha ={10}^{-6}$) and the nucleation and growth KJMA formalism (solid lines, $\alpha ={10}^{-6}$, ${10}^{-3}$). Symbols: as fitted for $\alpha ={10}^{-3}$ using the constant activation energy KJMA kinetic model. The dot-dashed line indicates the thermal path for a cooling rate of $8\times {10}^2\mathrm{K}{\mathrm{s}}^{-1}$.

Figure 17

Critical cooling rate as a function of the reduced glass transition temperature. Dashed area: experimental values reported in Ref. 1. Symbol: calculated value for ${\mathrm{Fe}}_{65}{\mathrm{Nb}}_{10}{\mathrm{B}}_{25}$.