The mechanical alloying process has been used to prepare nanostructured Fe31Co31Nb8B30 (wt%) alloy from pure elemental powders in a high energy planetary ball-mill Retsch PM400. Microstructural changes, phase transformation and kinetics were studied by X-ray diffraction, differential scanning calorimetry and M?ssbauer spectrometry. The crystallite size reduction down the nanometer scale (~8 nm) is accompanied by the introduction of internal strains up to 1.8% (root-mean square strain, rms). Further milling time leads to the formation of partially paramagnetic amorphous structure in which bcc FeCo nanograins are embedded. The kinetics of amorphization during the milling process can be described by two regimes characterized by different values of the Avrami parameter n1 = 1.41 and n2 = 0.34. The excess enthalpy due to the high density of defects is released at temperatures below 300°C. The glass transition temperature increases with increasing milling time.
Mechanical alloying (MA) is a non-equilibrium process tool which allows the elaboration of stable and/or metastable materials such as supersaturated solid solution, amorphous alloys, nanocomposite and intermetallic compounds [1-3]. Amorphization by MA has been suggested to be similar to that of solid-state amorphization (SSA) in alternate layered structures [
FeCo-based nanocrystalline (NC) alloys produced generally by devitrification of amorphous ribbons exhibit excellent soft magnetic properties. The presence of Nb in the parent amorphous alloy causes inhibition of grain growth resulting in a NC structure [
The purpose of this paper is to prepare the nanocomposite Fe31Co31Nb8B30 powder mixtures by MA, at room temperature, and to study the thermal stability and phase transformation kinetics as a function of milling time. To achieve this objective, emphasis has been given on microstructure characterization of ball-milled samples in terms of lattice imperfections (variation of lattice parameter, grain size, internal strains, etc.) and relative abundances of individual phases by analysing X-ray diffraction (XRD) profiles using the Rietveld refinement [
The Marquardt least-squares procedure was adopted for minimizing the difference between the observed and simulated powder diffraction patterns and the minimization was carried out by using the reliability index parameter, Rwp (weighted residual error), RB (Bragg factor) and Rexp (expected error). This leads to the value of goodness of fit, GoF [10,11]:
Refinement continue still convergence is reached with the value of the quality factor, GoF approaching 1. To simulate the theoretical XRD patterns containing boride phases and solid solutions, some considerations were taken into account as indicated in
Pure elemental powders of iron (particle size 6 - 8 µm>99%), cobalt (particle size 45 µm, 99.8%), niobium (particle size 74 µm, 99.85%) and amorphous boron (>99%) were mixed to give a nominal composition of Fe31Co31Nb8B30 (wt%). The milling process wa performed in a planetary ball-mill Retsch PM400, at room temperature, under argon atmosphere with a ball-to powder weight ratio of about 8:1. The rotation speed was 350 rpm. XRD measurements were performed by means of a Bruker D8 Advance diffractometer with Cu-Kα radiation (λCu = 0.15406 nm). The XRD patterns were refined using the Maud program [
The induced heavy plastic deformation into the powder particles during the milling process gives rise to the creation of a great amount of crystal defects such as dislocations, vacancies, interstitials and grain boundaries which promote solid state reaction at ambient temperature. Depending on the initial mixture, changes in structure of mechanically alloyed powders can occur as follows: grain refinement, solid solution diffusion and/or formation of new phases [
The disappearance of the fcc Co peaks in the early stage of milling (5 h) can be related to the allotropic transformation from fcc to hcp form. Indeed, fcc Co phase is metastable at room temperature and becomes unstable when an external mechanical or thermal energy is introduced [14,15]. It has been reported that the fcc → hcp Co allotropic transformation is faster for the high milling speed and short milling times. The fcc → hcp allotropic transformation can be confirmed by the decrease of the fcc Co volume fraction and the increase of the hcp Co one after 1 h of milling (
After 10 h of milling, one observes the increase of bcc α-Fe and orthorhombic Fe3B-type peaks intensity (see inset in
position of metastable Fe23B6 phase into orthorhombic Fe3B, tetragonal Fe2B and bcc α-Fe (
After 25 h of milling, the diffraction peaks of NC bcc FeCo and disordered FeB-type phases are superimposed to a broad halo typical of highly disordered like amorphous phase. Above this milling time, the XRD patterns present the same phases. An amorphous structure is achieved after 125 h of milling. The early investigators of the amorphization mechanism assumed that the powder particles melted because of the very high rate of plastic deformation and consequent rise in the powder temperature [
In the NC materials, the crystallite size and lattice strain are very important since the phase constitution and transformation characteristics are dependent on them. High density of defects can be generated with prolonged milling duration leading to an increase in micro-hardness of the powder particles as well as to an increase of internal micro strains (
The average crystallite size of α-Fe decreases rapidly to about 18 nm after 25 h, and remains rather milling time independent (
deformation [
The evolution of α-Fe lattice parameter during the milling process increases up to 10 h, decreases between 10 and 25 h, then remains nearly constant on further milling time (
The lattice distortion is also confirmed by the DebyeWaller factor changes during the milling process. The Debye-Waller parameter (DWP) is a measure of the dis
placement of atoms from their ideal positions. It consists of static (BS) and thermal (BT) components. According to the diffraction theory derived by Krivoglas [
ture, the BT of the Fe is fixed to 0.35 Å2 [20,21]. BS increases with microstrain (
For milled samples subjected to severe plastic deformation by impacts of balls, dislocations are the main defects besides grain boundaries of which density ρD can be represented in terms of d and by [
where b is the Burgers vector of dislocations and is equal to 0.24824 nm for Fe. The dislocations density, ρD, increases from 0.051 × 1016/m2 to about 1.52 × 1016/m2 after 40 h of milling (
[
In the case of Co where the Burgers vector is equal to for the direction of the hcp structure and for the direction of the fcc structure, the dislocations density, ρD, increases from 0.025 × 1016/m2 to about 0.812 × 1016/m2 after 16 h of milling for the hcp structure (
5 and 10 h of milling, respectively. Stacking faults are so abundant and the hcp phase can be described as a random sequence of close packed stacking planes.
The increase of the Nb lattice parameter to about 0.3432 nm after 5 h of milling (
The lattice parameter a of the Fe2B boride phase increases to about 0.5604 nm while c remains constant and close to that of the pure phase, c0 = 0.4250 nm. The same behaviour was observed in the FeAlB alloy where the lattice parameter a of the Fe2B phase increases from 0.5110 to 0.5150 nm [
The phase transformation kinetics can be followed by the evolution of the volume fraction of the amorphous phase as a function of milling time. The transformed fraction variation as a function of milling time can be described by the Johnson-Mehl-Avrami (JMA) kinetics formalism since the milling process occurs at a constant temperature:
where n is a power law exponent that can be used to distinguish the dimensionality of the growth process; x is the transformed fraction of the highly disordered phase (amp-type); t is the milling time and k is the rate constant. The kinetics parameter values, n and k, can be deduced from the double logarithmic plot versus lnt. Two regimes can be clearly observed as shown in
diffusion process in the MA process is controlled by both the thermal and mechanical energies. Repeated fracturing and cold welding during MA enable different particles to be always in contact with each other with fresh surfaces. The diffused layers between the powder particles are continuously fractured leading to the minimization of the diffusion distance. With the progress of the milling process, the effective diffusivity can be increased by decreasing crystallite size. The heavy plastic deformation induces a local fusion which supports the formation of alloys by a mechanism of fusion and/or diffusion at relatively high temperatures. This might explain somewhat the low value of the Avrami parameter (n2 = 0.34). This later is comparable to those obtained in the spinodal decomposition mechanism of the solid solution [
Thermal analysis is widely used in the reaction study of the mechanically alloyed powder particles because of the obtained metastable disordered structures. Hence, thermal annealing leads to the relaxation of the introduced stresses during the milling process.
The DSC scans of nanostructured Fe31Co31Nb8B30 powders milled for 50, 100 and 125 h are shown in
the atomic reordering and in the grain growth are comparable in value [
The glass transition temperature, Tg, of the ball-milled
powders, deduced from the DSC scans, is plotted versus milling time in
The XRD pattern of of the powders milled for 125 h and heated at 1023 K (
phases, in addition to the residual amorphous phase (~12%). The microstructural parameters are listed in
During the first 10 h of milling, the Fe31Co31Nb8B30 powders are subject to heavy plastic deformation accompanied by lattice defects, resulting in an enhancement in the solid state reaction between the diffusion couples of the reactant materials leading to the formation of metastable Fe-borides. On further milling, the Feborides are subjected to continuous defects that lead to a gradual change in the free energy. Hence, the powder mixture is completely transformed into an amorphous structure after 125 h of milling which is confirmed by the
appearance of the glass transition temperature at about 693 K.