Microstructural and Morphological Properties of AlNiCo and CoNi Alloys: An In-Depth Study Based on Low-Energy Mechanical Alloying

Abstract

This study focused on synthesizing AlNiCo and CoNi materials using a low-energy milling process. The aim was to explore the formation of low-energy phases in both systems, contrasting with the typical research on phases formed under high-energy conditions. In the Co-20 wt% Ni system, the phases ${\mathrm{Co}}_{0.75}{\mathrm{Ni}}_{0.25}$ and Ni were identified, as well as the FCC cubic phase of CoNi, using X-ray diffraction (XRD) with a molybdenum radiation source. The observed behavior aligned closely with the miscibility curve in the equilibrium phase diagram, which included a region of alloys with varying structures and similar compositions. A notable feature was the presence of a predominantly dispersed hexagonal Ni zone, consisting of nanoparticles. Transmission electron microscopy (TEM) was employed to observe the FCC CoNi phase, which displayed a specific arrangement. AlNiCo and CoNi alloys were successfully synthesized through mechanical alloying, incorporating equilibrium and non-equilibrium phases.

1. Introduction

The importance of magnetic materials in our daily lives is clear, as their applications have grown alongside advancements in science and technology. These materials are essential in various fields, including computers, consumer products in offices, automotive and transportation, electronic devices, and manufacturing equipment, as well as having medical and military uses. The behavior of magnetic materials is closely linked to the control of their phases and the use of nanoparticles. These factors allow for the creation of either soft or hard magnetizations, depending on the thermomechanical treatments applied during the different sintering processes that follow the formation of their initial phases.

A key property of interest in these phases is crystalline and magnetic anisotropy. Therefore, it is important to study how these phases emerge and evolve, investigate their anisotropic properties, and explore how they can be modified through mechanical deformations, external magnetic fields, and changes to their crystalline structure using various physical or chemical methods [1,2,3].

The development of applications for these materials is largely due to the different methods used to prepare micro- and nanostructured systems. Magnetic materials at micro and nano scales derive their properties from their crystalline structure, grain or particle size, and chemical composition. After preparation, these materials often form systems with different phases, where the alloys do not always show their best properties. Thus, it is crucial to analyze the relationship between the energy used in the preparation methods of magnets and the microstructural evolution of the phases in ferromagnetic materials. The resulting anisotropies during material fabrication depend on the material’s processing history.

Mechanical alloying (MA) is a widely used solid-state powder processing technique that involves repeated welding, fracturing, and re-welding of powder particles under high-energy ball milling conditions. This process allows for the creation of unique materials with novel properties that are often difficult to obtain using conventional methods. MA is particularly effective in producing composite materials, alloys, and metastable phases, offering a significant advantage in the design of advanced materials with customized properties. During the mechanical alloying preparation process, elemental powders undergo a series of deformations and processes, leading to the formation of distinct phases with specific orientations. Among the various types of magnets, those based on steels, ceramics, and non-ferrous alloys, such as AlNiCo and CoNi, have diverse applications in biomedicine and electromechanical systems [4,5].

A key characteristic of MA is its ability to induce the formation of fine, homogeneous microstructures with exceptional properties. It has been successfully used to produce nanostructured materials and improve the mechanical properties of alloys by refining their grain structure. In particular, MA enhances phase formation and promotes the dissolution of elements that would otherwise be immiscible under normal conditions. The use of different milling parameters, such as milling time, speed, and atmosphere, also plays a crucial role in determining the final properties of the material. Increased milling time, for example, results in more spherical powder morphology and enhanced microhardness, making MA an ideal method for modifying material properties such as hardness, wear resistance, and diffusion characteristics.

Additionally, MA can lead to a reduction in contamination, especially when conducted under controlled conditions [6,7]. This makes MA an attractive technique for producing materials with high purity and desirable properties, such as improved hardness, wear resistance, and enhanced diffusion characteristics.

On the other hand, AlNiCo alloys offer several advantages, including a wide operational temperature range, low temperature dependence of their magnetic properties, and excellent corrosion resistance. These alloys primarily consist of aluminum (Al), nickel (Ni), cobalt (Co), and iron (Fe). Typically, AlNiCo alloys undergo thermal treatments that yield high-energy products, primarily two nanophases formed due to spinodal decomposition: a Co(Fe)-rich isolated phase and a matrix of an AlNi-rich phase. Both phases possess a cubic crystalline structure, with slight tetragonal distortion, and are contained within individual grains [8]. According to phase diagrams for these systems at high energies, it is most common to obtain cubic and decagonal phases for AlCoNi and FCC cubic phases for CoNi [9,10]. However, in the same diagrams at low energy, binary and ternary phases are observed at the lower part, as published by Raghavan, featuring orthorhombic structures and others with an elongated c-axis exhibiting monoclinic phases [9]. These phase diagrams are generally developed for equilibrium systems produced through high-energy milling, making it of interest to explore the possibility of reproducing them for materials obtained through low-energy milling, as well as non-equilibrium phases [9,11]. Regarding the microstructure–magnetization relationship, it is known that these alloys tend to behave as magnetically isotropic if their grains lack preferred orientations, while they can exhibit magnetic anisotropy if their grains are or are not crystallographically oriented. These behaviors are linked to the potential for spinodal decomposition within the alloy [12].

Both CoNi and AlCoNi alloys have been synthesized using non-equilibrium preparation methods, particularly high-energy mechanical alloying [13,14,15], similar to many intermetallic systems. The binary CoNi alloy exhibits spinodal decomposition when transitioning from an FCC structure to an HCP structure, either through thermal treatment or mechanical stress [16,17,18,19].

The AlNiCo alloy system has been utilized in various compositions for magnetic materials and superalloys, particularly in compounds where the predominant phases are γ’Ni₃Al, identified as L$2_1$, and $\beta$(Ni,Cu)Al, identified as B2. The $\mathsf{\beta }$ phase often exhibits shape memory properties due to the thermally induced martensitic transformation from B2 to L$2_1$ (Figure 1) [20].

In these alloys, the compositions typically present a weight ratio of Al (28–30%), Co (25–69%), and Ni (0–70%), with limited analysis conducted on alloys where the percentages of Co and Ni are less than 20% by weight [20,21]. The literature presents various methods for producing AlNiCo and CoNi alloy systems. Most of these methods involve casting and reduction techniques, which produce high-energy products within the phase equilibrium, as shown in

Table 1. AlCoNi and CoNi alloys.

Phase	Structural Details	Preparation Method	Reference
AlCoNi	Cubic	High-energy Mechanical Alloying	[22]
AlCoNi	BCC	Casting	[23]
${\mathrm{Al}}_{13}$(Co,Ni)4	Martensitic	Casting	[24]
${\mathrm{Al}}_{13}{\mathrm{Co}}_4$	Orthogonal	Casting with Induction	[24]
${\mathrm{Co}}_{39}{\mathrm{Ni}}_{32}{\mathrm{Al}}_{18}$	$\beta$ + $\gamma$, ${\mathrm{L1}}_0$, tetragonal	Casting	[25]
AlCoNi	$\beta$ + $\gamma$, $\gamma$FCC	Casting	[26]
NiAl	${\mathrm{B}}_2$	Casting	[26]
(CoAl,NiAl)	$\beta$	Heat treatment	[27]
(Co,Ni)	$\gamma$	Heat treatment	[27]
(Ni,Co)3Al	${\mathrm{L1}}_2$	Heat treatment	[27]
NiAl	$\beta$’	Heat treatment	[27]
${\mathrm{Al}}_9{\mathrm{Co}}_2$	P2(1)/a	Casting	[28]
${\mathrm{Al}}_5{\mathrm{Co}}_2$	P6(7)mmc	Casting	[29]
AlCoNi	PmDm	Casting	[29]
CoNi	FCC	Casting	[30]
CoNi	FCC,HCP	Polyol reduction	[5]
CoNi	FCC, HCP	Casting	[31]
${\mathrm{Co}}_X{\mathrm{Ni}}_{100-X}$	FCC, HCP	Casting	[4]

.

An important behavior of the AlNiCo system is the formation of quasicrystals or quasiperiodic crystals obtained during thermal treatments, which generally form in Al-rich alloys along with one or more transition metals. In these cases, quasicrystals can form metastable phases whose physical properties are significant for electrical and thermal applications, in addition to friction anisotropies for electromechanical devices [20,32]. The AlCoNi and CoNi alloy systems are typically produced through high-energy milling, like SPEX and planetary mills [13].

According to the literature, the energy of the mill used in the alloying process influences the types of phases obtained; among other reasons, this is because contamination in lower-energy milling is reduced, and milled powders are more homogeneous in size and shape [6]. Additionally, shorter milling times result in less amorphization of the alloys [13].

In this work, a detailed and in-depth analysis of the microstructural formation of the phases obtained through low-energy mechanical milling of the nanostructured alloy systems ${\mathrm{Co}}_{80}{\mathrm{Ni}}_{20}$ and ${\mathrm{Al}}_{80}{\mathrm{Co}}_{10}{\mathrm{Ni}}_{10}$ was conducted to determine the similarities or discrepancies with the systems obtained through high-energy milling and other methods reported in the literature.

2. Experimental Methodology

The samples were synthesized via mechanical alloying using high-purity (99.99%) elemental powders of Al, Ni, and Co supplied by Sigma-Aldrich (San Luis, MO, USA). The target compositions were ${\mathrm{Al}}_{80}{\mathrm{Co}}_{10}{\mathrm{Ni}}_{10}$ and ${\mathrm{Co}}_{80}{\mathrm{Ni}}_{20}$ in weight percent. For each alloy, 10 g of the powder mixture was loaded into a steel vial along with steel balls at a ball-to-powder ratio (BPR) of 5:1. The milling process was conducted under an inert Ar atmosphere and EtOH as a process control agent (PCA). The vial was placed in a low-energy ball mill (Spartan FRISCHT, Idar-Oberstein, Germany) operating at 180 rpm, with milling durations of 100 and 200 h to promote the formation of AlNiCo and CoNi phases.

The container was handled in an inert atmosphere to extract the samples necessary for microcrystalline identification. No contaminants due to exposure to the atmosphere were detected.

Phase identification of the milled powders was performed using X-ray diffraction (XRD, Karlsruhe, Germany) on a Bruker D8 Discover diffractometer with a Mo radiation source ($\lambda =0.7093\AA$). Particular attention was given to potential Fe and Cr contamination, which tends to increase with prolonged milling times. The crystalline phases identified in the samples were compared with diffraction patterns from the JCPDS database, part of the Powder Diffraction File (PDF-4), maintained by the International Centre for Diffraction Data (ICDD) [33]. The morphological characteristics of the alloys were examined using a field-emission scanning electron microscope (FE-SEM, Tokyo, Japan), JEOL JSM-6701F Tokyo, Japan, and high-resolution transmission electron microscopy (HR-TEM, Hillsboro, OR, USA) with a TITAN 80-300 instrument from FEI.

3. Results and Discussion

3.1. X-Ray Diffraction

In the X-ray diffractogram of the AlCoNi material after 200 h of milling (Figure 2), several distinct peaks are observed, indicating the formation of multiple crystalline phases. The reflections at $2\theta =15.{42}^{\circ }$, $18.{84}^{\circ }$, $28.7^{\circ }$, and $33.{68}^{\circ }$ correspond to the crystal planes of the ${\mathrm{Al}}_9{\mathrm{Co}}_2$ alloy, identified by JCPDS card No. 03656460. This phase exhibits a primitive monoclinic crystal structure with a space group of $P{2}_1/c$, 14. A peak at $2\theta =17.{45}^{\circ }$ corresponds to the formation of the ${\mathrm{Al}}_9{\mathrm{Co}}_2$Ni phase, associated with JCPDS card No. 520802, which crystallizes in an orthorhombic body-centered structure with a space group of $Immm$ (71). The reflections at $2\theta =17.{82}^{\circ }$, $23.{24}^{\circ }$, $33.1^{\circ }$, and $40.{76}^{\circ }$ are attributed to the ${\mathrm{Al}}_4{\mathrm{CoNi}}_2$ phase, identified by JCPDS card No. 654244. This phase exhibits a body-centered cubic (BCC) structure with a space group of $Ia\overline{3}d$ (230). The reflection at $2\theta =21.{37}^{\circ }$ is assigned to the ${\mathrm{Al}}_{70}{\mathrm{Co}}_{20}{\mathrm{Ni}}_{10}$ phase, referenced by JCPDS card No. 491653, which possesses a face-centered monoclinic structure with a space group of $C1$. Additionally, the CoNi phase is identified by a characteristic reflection at $2\theta =38.8^{\circ }$. A detailed summary of the crystalline phases observed in the ${\mathrm{Al}}_{80}{\mathrm{Co}}_{10}{\mathrm{Ni}}_{10}$ system after 200 h of milling is provided in

Table 2. AlCoNi phases after 200 h of milling.

Phase	Lattice Parameters	Structure	Space Group	2$\mathit{\theta }$	d (Å)	hkl	% Wt
AlCo	a: 2.86 Å	Cubic	Pm$\overline{3}$m	46.13	0.90	(310)	2.7
${\mathrm{Al}}_9{\mathrm{Co}}_2$	a: 8.55 Å, b: 6.29 Å, c: 6.21 Å	Monoclinic	${\mathrm{P2}}_1$/a (14)	15.42	2.63	(121)	44
				18.84	2.16	(12$\overline{2}$)
				28.74	1.43	(323)
				33.68	1.23	(12$\overline{1}$)
${\mathrm{Al}}_9{\mathrm{Co}}_2$Ni	a: 12.06 Å, b: 7.55 Å, c: 15.35 Å	Orthorhombic	Immm (71)	17.4	2.33	(125)	14.3
${\mathrm{Al}}_4{\mathrm{CoNi}}_2$	a: 11.39 Å	Cubic BCC	Ia$\overline{3}$d (230)	17.82	2.32	(422)	27.9
				23.24	1.75	(541)
				33.1	1.24	(842)
				40.76	1.015	(1051)
${\mathrm{Al}}_{70}{\mathrm{Co}}_{20}{\mathrm{Ni}}_{10}$	a: 12.12 Å, b: 4.06 Å, c: 7.64 Å, $\beta$: 105.${88}^{\circ }$	Monoclinic	C1	21.37	1.90	($\overline{2}$04)	5.13
CoNi	a: 3.53 Å	FCC	Fm$\overline{3}$m (225)	38.88	1.06	(311)	5.13

. These phases have not been reported in the equilibrium phase diagrams of the ternary alloy system [10], suggesting that the extended milling duration facilitates the formation of non-equilibrium phases. The high energy input accumulated during prolonged mechanical alloying is believed to have created the conditions necessary for such metastable phase formation [13]. It is proposed that the ${\mathrm{Al}}_4{\mathrm{Co}}_2$ phase may originate from the decomposition of the ${\mathrm{Al}}_9{\mathrm{Co}}_2$ phase, potentially driven by a rearrangement of slip planes under the severe plastic deformation conditions present during milling [9].

In the ${\mathrm{Al}}_{80}{\mathrm{Co}}_{10}{\mathrm{Ni}}_{10}$ alloy system, the formation of binary phases such as ${\mathrm{Al}}_9{\mathrm{Co}}_2$, AlCo, and CoNi appears to be more favorable during the early stages of alloying. Among these, ${\mathrm{Al}}_9{\mathrm{Co}}_2$ is the most abundant phase, as it corresponds to the lowest energy state. As the process progresses, the system evolves toward the formation of low-energy ternary phases, including ${\mathrm{Al}}_9{\mathrm{Co}}_2\mathrm{Ni}$, ${\mathrm{Al}}_4{\mathrm{CoNi}}_2$, and ${\mathrm{Al}}_{70}{\mathrm{Co}}_{20}{\mathrm{Ni}}_{10}$. In this context, the presence of Ni seems to promote an ordering effect within the crystal structure.

The X-ray diffractogram (Figure 3) corresponding to the ${\mathrm{Co}}_{80}{\mathrm{Ni}}_{20}$ sample subjected to 100 h of mechanical alloying reveals several Bragg reflections. Notably, peaks at $2\theta =18.{83}^{\circ }$, $20.{07}^{\circ }$, $21.{36}^{\circ }$, and $35.{95}^{\circ }$ correspond to the diffraction planes of the ${\mathrm{Co}}_{0.75}{\mathrm{Ni}}_{0.25}$ phase, which exhibits a hexagonal close-packed (HCP) structure with the space group $P{6}_3/mmc\left(194\right)$, as identified by JCPDS card 01717353.

Additional reflections were assigned to the CoNi alloy with a face-centered cubic (FCC) structure and space group $Fm\overline{3}m\left(225\right)$, appearing at $2\theta =20.{019}^{\circ }$, $23.{156}^{\circ }$, $32.{97}^{\circ }$, and $38.{88}^{\circ }$. These observations are consistent with previous reports in the literature [34]. The HCP ${\mathrm{Co}}_{0.75}{\mathrm{Ni}}_{0.25}$ phase aligns with the system’s equilibrium phase diagram, representing a low-energy phase, whereas the FCC CoNi phase corresponds to a higher-energy product [35,36]. The close-up of the peak near $2\theta ={20}^{\circ }$, shown in Figure 3b, confirms the presence of two closely positioned peaks corresponding to hexagonal ${\mathrm{Co}}_{0.75}{\mathrm{Ni}}_{0.25}$ and cubic CoNi phases. These peaks exhibit sufficient resolution to be appropriately considered in the relative phase abundance analysis.

The diffraction peak at $2\theta =17.{97}^{\circ }$, shown in detail in Figure 3c, lies between reflections attributed to an HCP phase of Co and an HCP phase of Ni. This reflection is considered to indicate the formation of a hexagonal solid solution alloy with a higher Ni content than Co; thus, the notation NiCo was selected [13]. Additionally, several peaks were identified at $2\theta =27.{364}^{\circ }$, $31.{88}^{\circ }$, and $40.8^{\circ }$, corresponding to HCP Co.

Table 3. Phases of ${\mathrm{Co}}_{80}{\mathrm{Ni}}_{20}$ after 100 h of milling.

Phase	Lattice Parameters	Structure	Space Group	2$\mathit{\theta }$	d (Å)	h	k	l	% Wt
${\mathrm{Co}}_{0.75}{\mathrm{Ni}}_{0.25}$	a: 2.5 Å, c: 4.06 Å	Hexagonal	${\mathrm{P6}}_3$/mmc (194)	18.82	2.16	1	0	0	52.4
				20.09	2.03	0	0	2
				21.36	1.91	1	0	1
				35.95	1.14	1	0	3
CoNi	a: 3.53 Å	Cubic FCC	Fm$\overline{3}$m (225)	20.02	2.04	1	1	1	35.51
				23.15	1.76	2	0	0
				32.97	1.24	2	2	0
				38.88	1.06	3	1	1
				51.88	0.81	3	3	1
NiCo	a: 2.62 Å, c: 4.32 Å	Hexagonal	${\mathrm{P6}}_3$/mmc (194)	17.97	2.27	1	0	0	8.35
Co	a: 2.5 Å, c: 4.07 Å	Hexagonal	${\mathrm{P6}}_3$/mmc (194)	27.64	1.48	1	0	2	3.69
				31.88	1.25	1	1	0
				40.80	1.01	0	0	4

summarizes the morphological and phase characteristics of the ${\mathrm{Co}}_{80}{\mathrm{Ni}}_{20}$ system after 100 h of milling.

In the analysis of phase formation in the ${\mathrm{Co}}_{80}{\mathrm{Ni}}_{20}$ system, hexagonal alloys corresponding to low-energy phases were identified. The most abundant phase is ${\mathrm{Co}}_{0.75}{\mathrm{Ni}}_{0.25}$, followed by NiCo with an abundance of 8.35% and Co with 3.69%. These phases are located in the low-energy region of the phase diagram for this system.

A significant proportion (35.51%) corresponds to the FCC CoNi phase, which, according to the phase diagram, is classified as a high-energy phase [36]. The relative abundances of these phases were determined based on the intensities of the X-ray diffraction (XRD) results, using the Reference Intensity Ratios (RIR) method [6]. The Reference Intensity Ratios (RIR) method is a quantitative analysis technique widely used in X-ray diffraction (XRD) to determine the relative amounts of crystalline phases in a multiphase sample. The method relies on the comparison of the intensity of a characteristic diffraction peak of a phase in the sample to that of a known reference material under identical experimental conditions. The RIR value is defined as the ratio of the intensity of the strongest peak of a given phase to the intensity of the strongest peak of a standard reference material, typically corundum ($\alpha$-Al₂O₃). For a sample containing multiple phases, the relative weight fraction, $W_i$, of a phase i is calculated using the following formula:

$$W_i={\displaystyle \frac{\frac{I_i}{{\mathrm{RIR}}_i}}{{\sum }_j\frac{I_j}{{\mathrm{RIR}}_j}}}$$

(1)

where $I_i$ is the intensity of the peak for phase i, ${\mathrm{RIR}}_i$ is the Reference Intensity Ratio for phase i, and $W_i$ is the relative weight fraction of phase i [37,38,39].

These observations suggest that the phase formation sequence begins with the stabilization of hexagonal phases and progresses toward the formation of the cubic phase, as described in the following sequence:

$$\begin{array}{c}{\mathrm{Co}}_{0.75}{\mathrm{Ni}}_{0.25}\\ \mathrm{Hexagonal}\\ \text{52.4\%}\end{array}\to \begin{array}{c}\mathrm{NiCo}\\ \mathrm{Hexagonal}\\ \text{8.35\%}\end{array}+\begin{array}{c}\mathrm{Co}\\ \mathrm{Hexagonal}\\ \text{4\%}\end{array}\to \begin{array}{c}\mathrm{CoNi}\\ \mathrm{FCC}\\ \text{35\%}\end{array}$$

The transition from the hexagonal phase to the FCC phase during mechanical milling occurs as mechanical energy accumulates and gradually converts into heat, providing sufficient energy to drive the phase transformation.

3.2. Scanning Electron Microscopy (SEM)

In Figure 4a, the ${\mathrm{Al}}_{80}{\mathrm{Ni}}_{10}{\mathrm{Co}}_{10}$ alloy is shown after 200 h of low-energy mechanical milling. The image reveals flake-shaped particles formed by clusters of grains within a size range of 3 to 16 nm, with an average size of 11.23 nm. The average grain size is 6.153 nm. These grains are nanocrystalline regions that exhibit uniform sizes throughout the sample. The measurement of grain lengths was performed using ImageJ V1.54g (2024) software (see Figure 4b).

Figure 5a shows the SEM micrograph of the Co₈₀Ni₂₀ sample after 100 h of milling. Analysis of this image using ImageJ software (Figure 5b) determined that the grain size diameters range between 6 and 20 nm. A total of 200 grains were measured, and the average size is 10.484 nm.

3.3. High-Resolution Transmission Electron Microscopy

A detailed study was performed using high-resolution transmission electron microscopy (HRTEM) and Fast Fourier Transform (FFT) analysis on the ${\mathrm{Al}}_{80}{\mathrm{Ni}}_{10}{\mathrm{Co}}_{10}$ and ${\mathrm{Co}}_{80}{\mathrm{Ni}}_{20}$ samples to elucidate the phases identified through X-ray diffraction (XRD).

Figure 6 and Figure 7 display clear-field micrographs highlighting various phases present in the ${\mathrm{Al}}_{80}{\mathrm{Ni}}_{10}{\mathrm{Co}}_{10}$ sample, which was subjected to 200 h of milling. Figure 8 and Figure 9 present the bright field micrographs of the ${\mathrm{Co}}_{80}{\mathrm{Ni}}_{20}$ sample.

The predominant phase determined by XRD analysis was ${\mathrm{Al}}_9{\mathrm{Co}}_2$, exhibiting a monoclinic crystal structure, as shown in Figure 6. The micrograph reveals the (004) plane of this phase, characterized by the reflections [031] and [022]. As indicated in Table 2, ${\mathrm{Al}}_9{\mathrm{Co}}_2$ constitutes the phase with the highest weight percentage (44%). This flake-like morphology is composed of well-ordered nanometric atomic layers. Graphs A and B illustrate the graphical measurement process utilizing DigitalMicrograph software [40]. This software enables the determination of interplanar spacings in multiple directions within the micrographs, allowing for correlation with crystallographic planes through the relevant JCPDS cards. Utilizing this information, it is feasible to assign a zone axis and calculate the lattice parameters for each detected phase.

Using the same software, it was possible to identify additional phases with significant weight percentages: ${\mathrm{Al}}_9{\mathrm{Co}}_2\mathrm{Ni}$ and ${(\mathrm{Ni},\mathrm{Co})}_3{\mathrm{Al}}_4$, with values of 14.3% and 27.9%, respectively, alongside particles from the less prevalent ${\mathrm{Al}}_{70}{\mathrm{Co}}_{20}{\mathrm{Ni}}_{10}$ phase. Figure 7 illustrates the region where these phases aggregate into spherical granular clusters. In this area, the nanometric grains of the ${\mathrm{Al}}_9{\mathrm{Co}}_2\mathrm{Ni}$ phase are predominantly present.

Table 4. AlNiCo phases after 200 h of milling.

Zone	${\mathrm{d}}_1$ (nm)	${\mathrm{d}}_2$ (nm)	Phase	Structure	h	k	l
1	0.253	0.335	${\mathrm{Al}}_9{\mathrm{Co}}_2$Ni	Orthorhombic	−6	−6	6
2	0.481	0.704	${\mathrm{Al}}_{70}{\mathrm{Co}}_{20}{\mathrm{Ni}}_{10}$	Monoclinic	−3	−3	0
3	0.464	0.211	${\mathrm{Al}}_9{\mathrm{Co}}_2$Ni	Orthorhombic	−4	−12	4
4	0.240	0.311	${\mathrm{Al}}_9{\mathrm{Co}}_2$Ni	Orthorhombic	6	0	−6
5	0.211	0.396	${\mathrm{Al}}_9{\mathrm{Co}}_2$Ni	Orthorhombic	2	18	−6
6	0.424	0.380	${\mathrm{Al}}_9{\mathrm{Co}}_2$Ni	Orthorhombic	4	0	0
7	0.453	0.735	(Ni,Co)3${\mathrm{Al}}_4$	Cubic	−1	3	−1
8	0.819	0.576	(Ni,Co)3${\mathrm{Al}}_4$	Cubic	0	16	−16
9	0.311	0.408	${\mathrm{Al}}_9{\mathrm{Co}}_2$Ni	Orthorhombic	0	6	0
10	0.916	0.479	${\mathrm{Al}}_9{\mathrm{Co}}_2$Ni	Orthorhombic	12	−2	−4
11	0.606	0.580	${\mathrm{Al}}_9{\mathrm{Co}}_2$Ni	Orthorhombic	−3	−15	3
12	0.535	0.296	${\mathrm{Al}}_9{\mathrm{Co}}_2$Ni	Orthorhombic	6	0	0

summarizes the phases detected in the micrograph presented in Figure 7. Interplanar distances were measured for each grain in two orientations, corresponding to zones 1 to 12. These measurements are detailed in Table 4, where the coexistence of the ${\mathrm{Al}}_9{\mathrm{Co}}_2\mathrm{Ni}$ and ${\mathrm{Al}}_{70}{\mathrm{Co}}_{20}{\mathrm{Ni}}_{10}$ phases, previously identified through X-ray diffraction (XRD), is evident. It is likely that the other identified phases are homogeneously distributed throughout the sample. The remaining crystalline structures of the phases observed in this sample are also listed in Table 4, aligning with the results from the XRD analysis.

Of particular interest is the alignment of the zone axes of the ${\mathrm{Al}}_9{\mathrm{Co}}_2\mathrm{Ni}$ grains, which appear to align with an initial plane, potentially the (600), indicative of an easy magnetic axis for the material. To elucidate this phenomenon, simulated diffraction patterns generated using Fast Fourier Transform (FFT) are presented in Figure 7. The zone axes of the orthorhombic crystals of the phases displayed are arranged in a manner suggesting a specific inclination of the (600) plane. For magnetic materials, if we assume this zone axis is parallel to a vector of magnetization within the grain, then the magnetic field gradient promotes these inclinations to achieve a state of equilibrium in the material, thereby mitigating repulsions associated with the dipolar behavior of the grains [41,42,43,44].

Table 4 provides a detailed description of the analyzed zone axes for the phases present in the sample depicted in Figure 7.

For the ${\mathrm{Co}}_{80}{\mathrm{Ni}}_{20}$ sample after 100 h, shown in Figure 8, larger grains measuring 10–20 nm were identified as different phases within the CoNi system. Notable phases include Ni FCC and Co hexagonal, along with CoNi BCC and ${\mathrm{Co}}_{0.75}{\mathrm{Ni}}_{0.25}$ hexagonal alloys. The identification was performed similarly to that in Figure 6, utilizing the interplanar distance measurement tool in DigitalMicrograph software. The characteristics of the phases analyzed in the micrograph are presented in

Table 5. Identification of ${\mathrm{Co}}_{80}{\mathrm{Ni}}_{20}$ phases after 100 h by HRTEM.

Zone	${\mathrm{d}}_1$ (nm)	${\mathrm{d}}_2$ (nm)	Phase	Structure	h	k	l
1	1.8	1.275	Ni	Cubic	0	0	4
2	1.248	1.7	CoNi	Cubic	0	0	4
3	2.14	1.211	Co	Hexagonal	0	−3	0
4	2.152	1.2025	Co	Hexagonal	0	−3	0

. Figure 9 provides a more detailed view of a grain identified as the ${\mathrm{Co}}_{0.75}{\mathrm{Ni}}_{0.25}$ hexagonal phase, with its zone axis represented by the vector [040] in this region.

As observed in the results obtained from X-ray diffraction and HRTEM, there is consistency in the phases identified in the ${\mathrm{Co}}_{80}{\mathrm{Ni}}_{20}$ and ${\mathrm{Al}}_{80}{\mathrm{Co}}_{10}{\mathrm{Ni}}_{10}$ systems.

In the case of the ${\mathrm{Al}}_{80}{\mathrm{Co}}_{10}{\mathrm{Ni}}_{10}$ system, the phases found in this research, specifically ${\mathrm{Al}}_4{\mathrm{CoNi}}_2$ with a BCC phase and Al(Co,Ni) with a cubic phase, have been reported by other authors under high-energy conditions. However, the ${\mathrm{Al}}_9{\mathrm{Co}}_2$Ni phase had not been previously observed, possibly because it requires a specific initial chemical composition and lower energy. At higher energy levels, it may dissolve or transform into a higher-energy phase, or it may only form at certain initial compositions [20,22].

In the case of the ${\mathrm{Co}}_{80}{\mathrm{Ni}}_{20}$ system, the formation of a CoNi alloy phase with an FCC-type crystalline structure was expected. However, it was found that the initial Co had not fully dissolved, as indicated by peaks associated with the hexagonal phase, which is very similar to the work of Olvera et al. [4]. It was anticipated that the hexagonal phase would transform into a cubic phase due to the creation of stacking faults; thus, its presence may be attributed to the excess Co initially present.

4. Conclusions

Through the low-energy mechanical alloying process, nanostructured alloy compounds of the ${\mathrm{Co}}_{80}{\mathrm{Ni}}_{20}$ and ${\mathrm{Al}}_{80}{\mathrm{Co}}_{10}{\mathrm{Ni}}_{10}$ systems were obtained at low energy. The average size of the nanostructures obtained for ${\mathrm{Al}}_{80}{\mathrm{Co}}_{10}{\mathrm{Ni}}_{10}$ after 200 h was 10–16 nm, while the nanostructures formed in the ${\mathrm{Co}}_{80}{\mathrm{Ni}}_{20}$ alloy after 100 h exhibited sizes ranging from 8 to 19 nm. The phases identified in the ${\mathrm{Co}}_{80}{\mathrm{Ni}}_{20}$ alloy after 100 h included ${\mathrm{Co}}_{0.75}{\mathrm{Ni}}_{0.25}$, CoNi, NiCo, and elemental Co. For the ${\mathrm{Al}}_{80}{\mathrm{Co}}_{10}{\mathrm{Ni}}_{10}$ material after 200 h, the binary phases identified were AlCo and ${\mathrm{Al}}_9{\mathrm{Co}}_2$, with ${\mathrm{Al}}_9{\mathrm{Co}}_2$ being the most abundant and exhibiting a monoclinic structure. The ternary phases observed included ${\mathrm{Al}}_9{\mathrm{Co}}_2$Ni, ${\mathrm{Al}}_4{\mathrm{CoNi}}_2$, ${\mathrm{Al}}_{70}{\mathrm{Co}}_{20}{\mathrm{Ni}}_{10}$, and CoNi, with ${\mathrm{Al}}_4{\mathrm{CoNi}}_2$ being the most abundant phase, displaying a body-centered cubic (BCC) structure. It was observed that the addition of Ni in the alloys promoted the formation of more ordered structures.