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FORMATION OF SUPERSATURATED (TI, AL)N SOLID SOLUTION BY

HIGH ENERGY BALL MILLING OF TIN-ALN POWDERS.

Radune M1,2,*

., Zinigrad M1. and Frage N

2.

1. Ariel University, Kiryat Hamada 1, Ariel 40700, Israel

2. Department of Materials Engineering, Ben-Gurion University of the Negev, P.O.B. 653 Beer-Sheva

8410501, Israel

Abstract In the present study (Ti,Al)N nano-size powders with various fraction of AlN were

synthesized from the mixtures of TiN and AlN powders by mechanical alloying using a planetary

high energy ball milling (HEBM). X-ray diffraction (XRD), energy-filtered transmission electron

microscopy spectrum imaging (EFTEM) and energy dispersive x-ray spectroscopy (EDS)

confirm the formation of the (Ti,Al)N supersaturated solid solution (up to 50 mole%AlN). The

crystalline size of the mechanically alloyed powders after 100 hours of milling is about 13nm.

Keywords Mechanical alloying, high-energy ball milling, supersaturated solid solution, titanium nitride,

aluminum nitride.

1. Introduction

Supersaturated titanium-aluminum nitride ( NAlTi xx1 ) is very attractive material for a wide

range of applications due to its high oxidation and wear resistance accompanied by high strength,

hardness, thermal conductivity and thermal shock resistance [1-4]. Currently, the applications of

NAlTi xx1 are limited to coatings, with a thickness less than m10 , obtained by physical or

chemical deposition [1, 5-7]. Bulk materials based on supersaturated NAlTi xx1 solid solution

was also fabricated by powder metallurgy approach, which involves preparation of NAlTi xx1

powder and its consolidation [8-10]. NAlTi xx1 powder may be synthesized by mechanical

alloying (MA) through a solid-gas reaction in a nitrogen atmosphere using high energy ball

milling (HEBM) approach [8, 9]. Ti powder and Ti50Al50 powder mixture were successfully

nitrided to form TiN and solid solution (Ti50Al50)N with NaCl-type structure [8]. In [9] Ti100-xAlx

(x≤50) mixtures were also nitrided completely to form a solid solution NAlTi xx1 . In [10] TiH2

and Al mixtures were used as starting materials for synthesis NAlTi xx1 powder under nitrogen

media using HEBM. Du et al.[11] have applied HEBM in order to treat AlN, Ti and ammonium

carbonate powders. Kim et al. [12] have tried to obtain NAlTi xx1 powder from mixtures of pure

nitrides using HEBM, however, milling at 250rpm for various durations (up to 40h) did not lead

to the formation of solid solutions. Despite this fact, in the present study we have tried to find the

parameters of HEBM for successful fabrication of NAlTi xx1 powder from the mixtures of pure

nitrides.

2. Materials and Methods

TiN and AlN both grade C (Starck H.C.) micron size powders, supplied by Starck H.C.

company, were used as starting materials. Various powder mixtures (10, 20, 30 and 50mole % of

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AlN) were treated in a planetary ball mill (Retsch PM 100, Germany) using chromium hardened

steel and sintered aluminum containers and balls (10mm diameter). The ball to powder weight

ratio was 20:1, rotational speed was 400 rpm and milling time was varied from 0 to 100 hours.

The milled powder was characterized by scanning electron microscopy (SEM), X-ray

diffraction (XRD), transmission electron microscopy (TEM), energy-filtered TEM spectrum

imaging (EFTEM) and energy dispersive x-ray spectroscopy (EDS).

The morphology and microstructure of ball-milled powders were observed by SEM (JSM-

6510LV, JEOL).

The XRD data were collected on Panalytical X'Pert Pro X-ray Diffractometer with KCu

radiation )0.154=( nm , operating at 40 kV and 40 mA. Data collection was performed by step

scanning of the specimen over the o8520:2 angular range in steps of o0.05 with 3 sec per

step. The XRD line profile parameters were then fitted with Rietveld procedure using the

PowderCell for Windows (PCW) program. The crystallite size and lattice strain of powder

during milling were determined from broadening of XRD peaks by Williamson-Hall (WH)

method [13].

TEM investigations were carried out using a JEOL JEM-2100F TEM operating at 200 kV.

The samples for TEM characterization were prepared by depositing a drop of ethanol suspension

of the milled TiN-AlN powder onto copper grid coated with the ultrathin carbon film (Cu, 400

mesh, Ted Pella cat.# 01824).

EDS analysis was performed using a JED-2300T Energy Dispersive X-ray Spectrometer. The

probe size during the analysis was set to 1 nm, to provide high sensitivity analysis at the nm-

scale.

Energy-filtered TEM (EFTEM) spectrum imaging applied for chemical mapping was

performed using Gatan imaging filter (GIF Quantum). The thickness of the samples was

measured with a zero-loss image using a 10 eV energy slit width. EFTEM images were obtained

by selecting only the inelastically scattered electrons corresponding to the inner shell losses of

the element edge of interest. We used three-window background subtraction to image the

distribution of Ti–L edge (456 eV), Al-K edge (1650 eV), N-K edge (401 eV) and the O–K edge

(532 eV). The spatial drift between images was corrected manually using Gatan Digital

Micrograph software.

3. Results and Discussion

The morphology of the starting and milled powders with 20mole% AlN (Fig.1) was observed

by SEM after the powders have been ultrasonic dispersed. Both TiN and AlN non-milled

powders (Fig.1a, b) consist of particles with various shapes (spherical, elongated and irregular).

The particles size significantly decreases after 4 hours of HEBM and fine particles form

agglomerates. The particle size after prolonged treatment remains almost constant, while the

level of agglomeration increases considerably (Fig.1c, d and Table 1).

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Fig. 1 SEM images of starting TiN (a) and AlN (b) powders; NAlTi xx1 powder after 20h (c) and

100h (d) of milling Table 1 Mean powder sizes of the powder mixtures (TiN-20%AlN) after various milling

durations

Milling time, h Mean powder sizes, µm

0 1.2

4 0.18

10 0.15

20 0.14

30 0.15

100 0.14

XRD patterns of TiN-20%AlN powder mixtures after various milling durations are presented

in Fig.2. For the as-mixed powder (0h), all the expected sharp peaks of TiN and AlN exist. With

increasing milling time the TiN peaks become broader and shift to higher angles, while the

intensities of the AlN reflections gradually decrease and completely disappear after 100 hours of

milling. The presence of -Fe in the powder milled for 100 h was detected. It has to be noted that

a b

c d

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Fig. 2 The diffraction patterns of the powder mixtures (TiN-20%AlN) after various milling

durations

already after 30 hours of milling the powder became magnetic. Evidently, the origin of -Fe in

the powder is steel milling tool.

Similar results were obtained for all investigated compositions of mixtures. The XRD

patterns of NAlTi xx1 powders with various AlN contents after 100h of milling are shown in

Fig.3. The shift and broadening of the TiN peaks become more remarkable with increasing AlN

content.

Fig. 3 The diffraction patterns of the TiAlN milled powders with various AlN contents

The broadening of the peaks is attributed to the reduction of crystallite size and the presence of

lattice strain. According to the XRD data, the average crystallite size of the milled powder

decreases with milling time and after 100h of milling is about 13 nm, at the same time the lattice

strain increases from 0.36% to 0.61% (Fig.4).

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Fig. 4 Average crystalline size and lattice strain in the milled powder as a function of milling

time

The lattice strain accumulation during HEBM was discussed in details in [14, 15]. The shift of

TiN peaks to a higher angle indicates the reduction of the lattice parameter from Å4.24 for pure

TiN to Å4.20 for milled powder after 100 hours of milling (Fig.5). It seems that the change of

the lattice parameters and the absence of AlN peaks reflect the formation of NAlTi xx1 solid

solution. On the other hand, the reason for absence of AlN peaks may be its amorphous state as a

result of HEBM. In order to approve the formation of the solid solution the AlN and TiN

powders were milled separately for various milling durations under the same conditions.

According to the XRD pattern (Fig.6a), even after 100h of milling, AlN has crystalline

structure and the lattice parameter of TiN practically doesn’t change (Fig.6b). Thus, it may be

concluded that HEBM does lead to AlN dissolution in the TiN phase and to the formation of the

Fig. 5 Variation of lattice parameter of NAlTi xx1 with milling time

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Fig. 6 XRD patterns of AlN (a) and TiN (b) powder after various milling durations

supersaturetd NAlTi xx1 solid solution.

TEM analysis was also carried out to confirm the formation of NAlTi xx1 solid solution. As

mentioned above, the powder synthesized by HEBM in the hardened steel container, contains

iron contamination and its magnetic property complicates the TEM analysis. To overcome this

problem, the powder mixture was milled with aluminum oxide milling tools. Already after 30 h

of milling almost all AlN was dissolved in the TiN phase and the milled powder (Fig.7) consists

only NAlTi xx1 solid solution and alumina originated from the milling tools. The prolonged

HEBM leads to significant increasing of the alumina fraction in the milled powder. Thus, the

powder after 30h of milling was characterized by TEM.

The Al, Ti and N elemental maps (Fig.8a-c) and a combination of these individual maps

(color map) (Fig.8d) of the powder milled 30h show homogeneous distribution of Ti, Al, N over

the particles.

Fig. 7 The diffraction patterns of the TiN-20%AlN powder mixtures after various milling

durations, milled using an aluminum oxide milling tools

a b

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Fig. 8 EFTEM elemental maps for Ti (a), Al (b), N (c). EFTEM colour map(d) is a combination

of the individual maps of Ti, Al, N and O

The local composition of the selected regions (Fig.9) is presented in Table 2 and also

indicates that Ti, Al and N are uniformly distributed in the milled powder and their

concentrations are close to the starting composition of TiN-20 at% AlN.

Ti map

O map

a b

c d

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Fig. 9 The regions that were selected for EDS compositional analysis

Table 2 The local composition (at%) of TiN-20 at% AlN milled powder

Region Ti Al N O

A 23.78 8.00 58.27 10.01

B 19.30 10.43 50.64 19.62

C 17.60 5.78 55.09 21.53

4. Conclusions

Nanocrystalline powder NAlTi xx1 has been synthesized from TiN and AlN powders by

HEBM. The formation of solid solution was confirmed by SEM, XRD and TEM analyses. It was

found that after 100h of milling all AlN was dissolved into TiN leading to the formation of

NAlTi xx1 solid solution with NaCl structure. After 100h of milling the particle size is about

150nm, while crystallite size and the lattice strain are about 13nm and 0.63%, respectively.

Acknowledgements The authors would like to thank Dr. V. Ezersky for his assistance in TEM analysis, Dr. D.

Mogilyanski for his assistance in XRD analysis and N. Litvak for her assistance in SEM

analysis.

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