Effect of mechanical and electrical activation on the combustionsynthesis of Al3Ti

K. Morsi • Pratik Mehra

Received: 12 August 2013 / Accepted: 28 March 2014 / Published online: 24 April 2014

� Springer Science+Business Media New York 2014

Abstract Titanium aluminides have attracted immense

interest as lightweight intermetallic compounds that pos-

sess good high-temperature mechanical and corrosion

properties. In the present work, titanium aluminides (Al3Ti)

have been reactively processed from elemental powder

using a combined mechanical and electrical activation

approach. The effect of mechanical activation and electric

current intensity on the ignition and phase development is

discussed. Ignition was not possible when powders were

milled for a short time, while prolonged milling resulted in

mechanical activation that promoted a self-propagating-

type reaction. The time to engulfment of the compact with

the reaction wave was found to decrease with an increase in

current intensity. A secondary reaction occurred at the

higher current intensity, which in turn increased the prod-

uct homogeneity.

Introduction

Combustion synthesis (CS) is a process where elemental

powder compacts are ignited, generating an exothermic

reaction that converts the elemental powders to prod-

uct(s) [1]. In the process, a significant amount of heat is

released, leading to a considerable rise in compact tem-

perature. CS is generally classified into two modes of

ignition: the first is termed thermal explosion or volume

combustion, where the whole powder compact is uniformly

heated to the ignition temperature. The second is self-

propagating high-temperature synthesis (SHS) where

powder compacts are ignited at one end, forcing a local

reaction, which converts reactants into products. The heat

released from this local reaction is conducted to the adja-

cent elemental layer which raises its temperature to the

ignition temperature and so on, such that a self-sustaining

reaction wave travels across the specimen converting

reactants into product(s). Although the CS processing of

titanium aluminide (AlTi and AlTi3) intermetallics [2–4]

has been investigated extensively, the reactive processing

of Al-rich lightweight Al3Ti intermetallic has not received

much attention. Moreover, there have been a number of

published studies that investigated the effects of combined

electrical and mechanical activation for various material

systems [5–13]. However, these studies apply significant

pressure during electrical processing thus promoting con-

solidation and/or do not quantify the exact current passing

through the specimens (due to some of the current (un-

quantified) traveling through the graphite die). In this

paper, Al3Ti has been processed via a combined mechan-

ical and electric current activation sequence using a con-

tainer-less electrical activation setup and only contacting

pressure. Such a setup can give rise to unique processing

conditions that promote the production of porous materials.

The effects of electric current intensity and mechanical

activation on the resultant reaction characteristics, micro-

structures, and properties are discussed.

Experimental procedures

Figure 1 shows the elemental powders used in the experi-

ments, which were Al (-325 mesh, Alfa Aesar, USA) and

Ti (-325 mesh, Atlantic Equipment Engineers, USA).

Initially, rotator mixing of the elemental powders in the

Al3Ti composition at 70 rpm for 30 min was carried out.

K. Morsi (&) � P. Mehra

Department of Mechanical Engineering, San Diego State

University, 5500 Campanile Drive, San Diego, CA 92182, USA

e-mail: kmorsi@mail.sdsu.edu

123

J Mater Sci (2014) 49:5271–5278

DOI 10.1007/s10853-014-8215-2

This was followed by vacuum degassing at 150 �C for 3 h,

and then the powders were placed in a glove box under

argon atmosphere where they were weighed and then

placed in the compaction die between two punches. Fol-

lowing this, the die was taken to the compaction press, for

compaction. Due to the nonconductive oxide layers present

on the aluminum powder, green compacts of rotator-mixed

powder did not allow the passage of electric current during

subsequent electrical processing attempts. This problem

was resolved by breaking the nonconductive oxide layers

through mechanical milling and allowing metal–metal

contact during subsequent electrical processing. Hence,

two mixing procedures were investigated for the present

study. The first involved SPEX milling of the entire

3Al ? Ti mixture for 3 min (using a ball-to-powder-

weight ratio (BPR) of 5.3:1) in order to break the oxide

layer on powder surfaces and allow the passage of electric

current. Powders produced using this method are referred

to in this paper as MA3. The second involves milling the

powder mixture for a total of 60 min over two milling

stages. In the first stage, total titanium load is added to

75 wt% of the intended final aluminum load and milled for

45 min (here the BPR was 6.3:1); this was followed by

adding the remainder of the aluminum (25 wt%) and then

additionally SPEX milling for a final 15 min (with the

nominal 5.3:1 BPR). Powder produced using this method is

referred to in this paper as MA45/15. It should be men-

tioned that all SPEX milling carried out in this investiga-

tion was conducted at 1725 rpm. Due to the low oxidation

resistance of titanium, this method would allow the coating

of more titanium particles with aluminum. This would

prevent direct exposure of titanium particles to residual

oxygen (that may be present) during subsequent reactive

processing, and limit titanium oxidation. Specifically, the

mechanical milling procedure was conducted by placing

the powder in a hardened steel vial together with 1.5 wt%

methanol as a process control agent to prevent powder cold

welding and sticking. Steel balls of diameter 6.35 mm were

added to the vial to establish the intended ball-to-powder-

weight ratio (BPR). The vial was then closed under argon

atmosphere and the lid placed/sealed using an o-ring

arrangement, and placed in a SPEX mixer 8000 series and

operated for up to 120 min, predominantly with ‘on’

periods of 15 min and ‘off’ periods of 15 min to prevent

excessive heat buildup. Approximately, 10 g of powder

was then uniaxially compacted in a steel die to produce

cylinders of *19-mm diameter and *14-mm height (with

relative green density of *0.74, measured using compact

dimensions and mass). Zinc stearate was applied to the

inner die wall to act as a material/die interface lubricant

during compaction.

All compacts were then placed between two tungsten

carbide–cobalt cylinders and heated using direct electric

current of varying intensities (from 600 to 1000 A), using a

Power Ten, Model P63C-51000 power supply. The

experimental setup is shown in Fig. 2.

Fig. 1 Scanning electron micrographs of a aluminum, b titanium powders used in the experiments

Fig. 2 Experimental setup for electric current application

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For microstructural and property characterization,

specimens were ground and polished to 1 micron finish.

Phase and compositional analyses were performed using

X-ray diffraction (XRD) (Cu Ka, Panalytical Xpert Pro

diffractometer) and energy-dispersive X-ray spectroscopy

(EDXS), respectively. Electron microscopy was conducted

using a field emission electron microscope (FESEM quanta

FEG 450), and Rockwell hardness measurements (F-scale)

were conducted using a Wilson Instruments (Rockwell 574

Wilson instrument). Three indents were made for each

hardness measurement and an average calculated and

reported.

Results and discussion

Figure 3 shows electron micrographs of green compacts of

the MA3 and MA45/15 powders. It is clear that dividing

the milling into two stages for MA45/15 produces com-

pacts with larger titanium regions. This is understandable,

as during the initial 45-min milling, the aluminum content

is lower than that in the 3-min-milled samples, and hence

presents more opportunity for the cold welding of titanium

powder, thus increasing its size. The second stage allows

for the addition of the remaining aluminum powder. XRD

scans (Fig. 4) of the green compacts show only titanium

and aluminum peaks, indicating the absence of any inter-

metallic phases; therefore, no solid-state reactions occurred

between titanium and aluminum powder during milling,

which was also confirmed through electron microscopy.

During the electrically activated combustion synthesis

experiments, the MA3 powder compacts did not show any

signs of ignition, despite becoming red hot after exposure

to extremely high current (e.g., 1000 A) and temperatures

as high as 1200 �C. XRD investigations showed only

titanium and aluminum peaks following electrical pro-

cessing. This is clearly seen in Fig. 4, where the XRD scan

of the electrically processed MA3 compact is also super-

imposed on XRD scans of the mechanically milled MA3

and MA45/15 green powder compacts.

Fig. 3 Scanning electron micrographs of a MA3 and b MA45/15 green powder compacts

Fig. 4 XRD scans for MA3 and

MA45/15 green powder

compacts and electrically

processed MA3 compact

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However, upon further detailed examination of the

electrically processed MA3 compact using electron

microscopy and X-ray microanalysis, small medium gray

regions (Fig. 5) were detected with a composition of

76.1 at.% Al and 23.9 at.% Ti (obtained using X-ray

microanalysis) which is associated with Al3Ti. This was

not detected by the XRD scans due to the resolution limits

of XRD which is typically limited to *3 vol%.

It has recently been reported that Al3Ti is always formed

in the first phase between Al and Ti powders during the

combustion synthesis [14]. The results do, however, sug-

gest that under the present conditions, ignition of a com-

bustion synthesis reaction was not possible. On the

contrary, the MA45/15 powder compacts were ignited even

at electric current intensities as low as 600 A. This is lar-

gely enabled by the relatively prolonged mechanical mill-

ing stage and hence leading to mechanical activation of the

MA45/15 powder. It has previously been documented that

mechanical milling can increase the crystallographic defect

concentrations such as dislocations and point defects and

consequently increases the energy level of the powders and

hence activate them. Moreover, short circuit diffusion is

also promoted [15]. Hence in the present study, since the

MA45/15 powder is at an elevated energy state compared

to the 3-min-milled powder, reaction ignition is enabled.

What is important to note is that even under electrical

activation, the MA3 powder compacts still did not react in

the combustion synthesis sense, hence mechanical activa-

tion had a more pronounced effect on ignition than did

electrical activation under the present investigated

conditions.

As opposed to the MA3 compacts, the MA45/15 com-

pacts reacted at all investigated applied current intensities

ranging from 600 to 1000 A. The reaction was typically

characterized as an SHS-type reaction where the ignition

point was normally along the compact-pressure pad inter-

face, followed by rapid spreading of the reaction wave to

engulf the compact. Temperature–time profile measure-

ments were attempted, but it was determined that they were

not consistent and too unreliable, making the generation of

a temperature–time profiles unfeasible. The unreliability of

thermocouple readings in combustion synthesis due to

thermal contact and inadequate response time issues has

also been reported elsewhere [16, 17]: in our case, the

melting of aluminum (as will be explained later) and its

possible shorting of the thermocouple may also be a con-

tributing factor. The time it took for the specimen to be

totally engulfed with the reaction wave following the

application of current was roughly estimated by video

monitoring. It was determined that the time to reaction

decreased as the applied current intensity increased, rang-

ing from 18.3 s for 600 A to 12.5 s for 800 A, and then to

10.7 s for 1000 A. Although these measurements are not

highly accurate, it still does show an obvious trend of a

decline in reaction time with increase in current intensity.

XRD analysis reveals that major intermetallic phase

formation took place in all electrically processed MA45/15

samples, i.e., a combustion synthesis reaction did take

place at the microstructural level. The 600 and 800 A

conditions gave rise to residual titanium and aluminum in

the reacted samples, while the application of 1000 A

resulted in specimens with relatively smaller titanium and

aluminum peaks. SEM and X-ray microanalysis show that

the amount of the white phase (titanium solid solution, e.g.,

*97.41 at.% Ti–2.39 at.% Al) decreased as the applied

electric current is increased from 600 to 1000 A. This is

clearly seen in Figs. 6 and 7. It should be mentioned that

EDX results can typically yield accuracies within

1–2 at.%. Moreover, for fine-scale microstructural regions,

the electron beam-material interaction volume may be

Fig. 5 Scanning electron micrograph (backscattered mode) of electrically processed MA3 powder compacts showing largely unreacted Al and

Ti and evidence of Al3Ti formation a 9200 magnification and b 91000 magnification

5274 J Mater Sci (2014) 49:5271–5278

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comparable to the microstructural scale and hence may be

slightly influenced by surrounding regions, thus affecting

the final composition. Given the above, the three phases

shown in the figures are titanium solid solution (white),

Al3Ti (light gray, e.g., 76.64 % at.% Al), and aluminum,

e.g., 99.31 at.% Al, n.b. the Ti–Al phase diagram shows

almost no solubility of titanium in aluminum under 600 �C.

An interesting observation was that only for specimens

ignited under 1000 A, a second glow was observed in the

central region following the initial glow (Fig. 8). This glow

was not temporary but remained as long as the current was

applied. Taking into account the XRD and SEM results, it

seems that the second glow involves the consumption of

titanium and aluminum. The fact that the glow is not

temporary suggests that it is not totally dependent on a

reaction event, but the product microstructure is also con-

tributing to a sustained Joule heating effect under the

applied electric current conditions.

It is interesting to note that the electrical resistivity of

Al3Ti increases significantly with an increase in tempera-

ture to comparatively high values, e.g., *2.1 9 10-6 X m

at *1000 K [18]. This value is greater than that of liquid

aluminum (corrected for thermal expansion) at 1000 K

(2.48 9 10-7 X m [19]) or titanium (*1.75 9 10-6 at

1000 K [20]). Both Al3Ti and titanium (alpha and beta Ti)

have an electrical resistivity that increases with an increase

Fig. 6 Scanning electron micrographs (in the backscattered mode) of

the central region of the microstructure for the 600-A (a, d), 800-A (b,

e), and 1000-A (c, f) reacted specimens. The bottom row of images

taken at higher magnification [the dark gray color is still aluminum

(confirmed by EDX), but appears darker due to the fact that it is at a

deeper location]

Fig. 7 XRD scans of cross

sections of the green compact,

600-, 800-, and 1000-A

electrically processed MA45/15

specimens

J Mater Sci (2014) 49:5271–5278 5275

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in temperature. This results in a higher Joule heating effect

at the higher current intensities (and therefore expected

higher compact temperatures), which is not necessarily

significant at the lower current intensity values (with

expected lower initial compact temperatures). Hence, the

sustained glow is possibly a result of three factors, an

initial high compact temperature at 1000 A leading to

increased Al3Ti resistivity, an increased amount of Al3Ti

relative to the lower current specimens, and an increased

level of fine porosity as seen in Fig. 6c, f. It should be

mentioned that initially some grain pullout was experi-

enced during grinding/polishing, and therefore subse-

quently great care was exercised during preparation to

minimize this effect. Image analysis of the central regions

of the reacted compacts shows that the % residual porosity

increases from 33 to 44 % and then to 62 % under the

applied current conditions of 600, 800, and 1000 A,

respectively. Due to the possibility of some small level of

grain pullout, these figures cannot be viewed with a high

degree of accuracy, but they certainly do show a definite

increase in porosity with increase in current intensity. This

is also substantiated by a measured compact swelling fol-

lowing reaction, the diametral swelling ranged from 11–15

to 15–31 and 26–36 % for current intensities 600, 800, and

1000 A, respectively. The increase in porosity with

increase in current intensity is believed to be a result of

generation and coalescence of excess vacancies at the

contact points between powder particles/small contact

points. Such porosity has been previously shown to

increase with an increase in current density [21, 22]. The

extent to which this porosity occurs prior or subsequent to

the combustion reaction is the subject of future work. The

second glow largely emanates from the central region of

the compact (due to heat losses at the top and bottom

regions to the pressure pads, and reduced aluminum con-

tent as will be explained later). Microstructural and EDX

analyses of the top and bottom regions of the reacted

compacts reveal two interesting findings. First, the top and

bottom regions appear more consolidated than the central

regions, but more importantly, there is a deficiency in

aluminum compared to the central regions.

A closer look at frames from the high-speed videos

(Fig. 9) reveals that at the interface between specimens and

the tungsten carbide pressure pads, electric sparks were

seen and aluminum droplets were seen running down the

specimens; this was also accompanied by the appearance of

smoke (presumably aluminum vapor) which continues

during the reaction.

SEM and EDX analyses of the central top regions

(Fig. 10) reveal a reduced aluminum content compared

with the central regions (Fig. 6). XRD analysis (Fig. 7)

also shows a more reduced aluminum peak intensity under

the condition of 1000 A.

Rockwell hardness measurements of the central regions

reveal a slight increase in hardness from 33.2 to 36.2 fol-

lowed by a drastic drop to 17.5 HRF for the 600-, 800-, and

1000-A reacted samples, respectively. The drastic drop at

1000 A is due to the significant increase in porosity.

Fig. 8 Initial glow and second glow during electrically activated combustion synthesis of Al3Ti at 1000 A

5276 J Mater Sci (2014) 49:5271–5278

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Conclusions

A number of conclusions can be drawn from the present

work:

1. Under the investigated processing conditions, electri-

cal activation alone (i.e., in the absence of mechanical

activation) is not sufficient for the initiation of a

combustion synthesis reaction for 3Al ? Ti powder

compacts, even at currents as high as 1000 A.

2. Mechanical activation is needed to trigger an SHS-type

reaction for all electric current intensities investigated.

3. The reaction time where compacts become engulfed

with the reaction wave was found to decrease with an

increase in applied current intensity.

4. In all mechanically and electrically activated com-

pacts, Al3Ti was the major phase with minor phases of

titanium and aluminum, the 1000-A specimens had the

lowest amounts of titanium and aluminum of all,

indicating a more complete reaction. This was attrib-

uted to the second heat generation and homogenizing

mechanism.

5. Compact porosity was found to increase with an

increase in current intensity, with the 1000-A reacted

compacts possessing the highest porosity (*62 %)

which consequently resulted in the lowest hardness.

Acknowledgements The authors wish to thank the National Sci-

ence Foundation for their support under Engineering Research Center

grant (No. 1028725) and major research instrumentation (MRI) Grant

DBI-0959908). Thank you also to Dr. Steve Barlow and Ms. Joan

Kimbrough for their assistance with SEM and XRD work,

respectively.

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