Explosive consolidation of mechanically alloyed Ti-Al alloys

PIATERIAIS SCIENCE & ENGINEERING ELSEVIER

Materials Science and Engineering A226228

Explosive consolidation

(1997) 115-l 18

of mechanically

alloyed Ti-Al

A

alloys

E. Szewczak a**, J. Paszula b, A.V. Leonov c, H. Matyja a BDepartment of Materials Science and Engitzeekg, Warsaw Univemity of Technology, Narbutta SS, 02-524 Warsaw, Poland ’ Faculty of Ammnent and Aviatiotz Technology, Military Utzivemity of Technology, Kaliskiego 2, 01-489 Warsaw, Poland cDepartt?ient of Chemistry, Moscow State University, Vorobiovy Gory, 119889 Moscow-234, Russia

Abstract

Ti-Al intermetallicsare of great interestbecauseof their attractive mechanicalproperties,but they require advancedprocessing methods,due to their low ductility at room temperature. In this work, the following processes were employedin order to obtain bulk TM and Ti,Al intermetallics:(1) mechanicalalloying: ball milling of elementalTi and Al powderscausedformation of supersaturatedsolid solutions; (2) explosive consolidation: in this operation no phase transitions were observed and sample densitiesafter consolidation reached over 90% of theoretical values; (3) annealing:resulted in additional increaseof material density and causedphasetransitions leading to formation of TiAl and T&Al compounds.0 1997Elsevier ScienceS.A. Keywords: Explosive compaction; Mechanical alloying; Ti-Al

alloys

1. Introduction Intermetallic compounds are the class of materials which possess many attractive properties for potential high-temperature applications. Intermetallics found in Ti-Al system are particularly promising for aerospace applications because of their attractive combination of good high-temperature mechanical properties with good environmental resistance and low density [l-3]. A major barrier to the widespread use of these materials is that they suffer from the lack of room-temperature ductility [4]. For this reason, much research has been directed at trying to improve the ductility of Ti-Al compounds. A microstructural method which may lead to increased ductility is reduction of grain size [4,51. Mechanical alloying (MA) appears to be very promising technique, since it offers the opportunity not only for microstructural refinement but also for obtain-

ing material which is highly homogeneous in structure and composition [6]. It has been observed that there is a strong dependence between applied milling conditions and the properties of the obtained alloy [7]. There are many works concerning mechanical alloying and its influence on microstructure and phase composition, but only a few * Corresponding

author.

0921-5093/97/$17.00 0 1997 Elsevier Science S.A. All rights reserved.

- ___^^_ _ _^^^,^ _._^_^^ __

investigations relate to the influence of unique microstructure produced by MA on mechanical properties of alloys [7-91. This is because of the lack of suitable techniques of consolidation. Hard powders after MA are difficult to compact and require high pressures and temperatures. Moreover, the properties of consolidated powders can be affected by the conventional compaction methods like hot-pressing, because of prolonged heating which can lead to coarsening of the microstructure and can destroy chemical homogeneity. Prolonged heating can be avoided in the case of the explosive compaction technique [lo- 121. The high temperatures necessary for adequate metallurgical bonding are produced in a period too small to cause thermally induced microstructural changes. The basic process of this method can be visualised as a high-amplitude stress wave that compacts the powder as it propagates. Very high pressures, dynamically applied to powders can be achieved in this method [13,14]. The aim of this work is to present results of explosive compaction of mechanically alloyed Ti-Al alloys.

2. Experimental

procedure

Aluminium of 99.8% purity and particle size 40-50 pm and titanium of 99.5% purity and particle size 150-200 pm were used as a starting powders. Elemen-

116

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et al. /Materials

Science

tal blends with compositions Ti-25at.%Al and TiSOat.%Al were mechanically alloyed in a vibratory ball mill, with a stainless steel vial and 12 mm diameter steel balls. The ball-to-powder weight ratio was 8: 1. The powders after mechanical alloying were consolidated by cold compaction under a pressure of 500 MPa. Compacted disks were canned in cylindrical copper containers closed with copper plugs. The experimental set-up for explosive compaction (Fig. 1) consisted of: inner copper tube with outer diameter of 15 mm in which the containers were put, steel tube filled with water and closed by steel plugs, momentum trap, plane-wave generator and primer. The outer steel tube is called the flyer tube. During detonation of the explosive, the steel tube transferred the pressure through the layer of water to the inner copper tube. This experimental set-up permits the generation of the plain shock wave in compacted powder. The consolidation was achieved by use of MWZA120 explosive. The detonation velocity of this explosive was 3000 m s-i and it gave a pressure 4.4 GPa and a temperature about 400°C in the compacted material. After consolidation, samples were annealed for 1.5 h at temperature of 700°C. The as-mechanically alloyed powders and the consolidated compacts were characterised by means of X-ray diffraction (XRD), optical microscopy and microhardness tests. XRD was performed using CuKa radiation. The profiles were corrected for the Kcc doublet by the Rachinger method. Microhardness tests were per-

and Engineering

A226-228

(1997)

115-118

as-milled .-tip - Ti(AI) o-fee-Al(Ti)

explosively compacted

annealed (7OU%/1.5 h)

o-TN

3

t

40

5

I

’

I

50 60 ?O 2 THETA (deg)

1

80

’

90

Fig. 2. XRD patterns of Ti,,Al,, sanlple.

formed in a ZWICK tester with a load of 200 G (HV0.2) applied for 15 s. The density was measured by the Archimedean method.

3. Results and discussion 3.1. Mechmicnl alloying

The XRD pattern of Ti,,Alw sample after 100 h of milling (Fig. 2) indicates the existence of three phases identified as [15]: 1l.c.p. Ti(Al) and f.c.c. Al(Ti) supersaturated solid solutions and amorphous phase. For T&Al,, after 100 h of milling, only peaks from h.c.p. supersaturated Ti(A1) solid solution are observed (Fig. 3). 3.2. Explosive compnction

Fig. 1. Cylindrical modified double tube system. (1) primer; (2) plane-wave generator; (3) MWZA120explosive;(4) PVC lube; (5) steel plugs;

(6) copper

tube;

(7) water;

(8) steel tube; (9) momentum

trap; (a) containers with compacted material; (b) copper bar.

After explosive consolidation, the phase composition of the T&Al,, sample did not change (Fig. 3). In the case of the T&Al,, sample, one can observe a small change in the relative intensities of f,c.c. and h.c.p. peaks, which indicates a phase transition of some part of the alloy (Fig. 2). The compacts exhibit two types of cracks: l circumferential cracks observed at the transverse section of the compact; this kind of crack can result from the reflection of the radially expanding compressive wave at the external walls of the cylinder

UOI; l

intergranular microcracks indicating that interparti-

cle bonding is not sufficient.

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111

115-115

as-nilled

.- kp - Ti(pI)

annealed (7&/l

40 40

.5 h)

i-h do 7b 80 I 90 ZTHETA

(degl

Fig. 3. XRD patterns of Ti,,Al,,

sample.

Samples are not fully dense (Table l), but densities exceed 90% of theoretical values. Failure to achieve fully dense samples can result not only from porosity of compacts but also from strongly defected microstructure. In Figs. 4 and 5, optical micrographs of cross-sectioned and etched samples are shown. It can be observed that etchant attack on boundaries of powder grains is very strong, while the microstructure inside grains is not well revealed. We can expect a high density of lattice defects and small crystallite size. To give a visual indication of the character of defects in the sample, the Williamson-Hall plot of B cos 8 versus sin S1 where B is the diffraction peak breadth, was made. The slope of this plot depends on strain, and the y-intercept varies as the reciprocal of the grain size. Stacking faults generated in the alloy during MA also cause diffraction peaks broadening, which is dependent on diffraction line index and leads to non-linear Williamson-Hall plots [ 161. Williamson-Hall plots for the TiT5AlZ5 sample after milling, consolidation and annealing are shown in Fig.

Fig. 4. Optical micrographs of (a) T&Al,, compaction.

and (b) Ti,,Al,,

after

6. Since plots are non-linear (stacking faults influence), precise calculation of crystallite size and strain from this plot is not possible, but plots can be used for visual indication of the character of defects changes. One can see that slopes of plots are different for alloy after MA and after compaction. It can indicate that strains change in the alloy after explosive consolidation. Also, points deviation from a linear plot differ in values for sample after MA and after compaction, which indicates change of stacking fault density. The crystallite size of the Ti,,A& sample after consolidation, estimated from y-intercept of straight-line approximated WilliamsonHall plot, is about 400 nm.

Table 1 Densities (g cme3) of compacts after cold compaction, explosive compaction and annealing Technique

Density

Cold compaction Explosive compaction Annealing

2.1 3.3 3.4

2.9 3.8 3.9

Fig. 5. Optical micrographs of (a) T&Al,, annealing.

and (b) Ti,,A&

after

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115- 1 IS

4. Conclusions

0.014

A - MA - 1OOh 0.012 --B - Compaction C - Annealing

.’

o.-;:: . q _ &’

,*

,’ .’ _.*,h.. ‘- . -

#L. .-a

... A . .‘m

I. \B ‘0

’

a.;

0.25

0.3

0.35

0.4

0.45 sin theta

Fig, 6. Williamson-Hall plots for the Ti,,Al,, consolidation and annealing.

0.5

0.55

0.6

sample after milling,

3.3. Annealing

Annealing resulted in additional increase of material density and caused phase transitions. XRD patterns of samples after annealing are shown in Figs. 1 and 2. The pattern of the Ti,0Al,0 sample indicates formation of AlTi-L,, compound, while annealing of the T&AlZ5 sample leads to transformation of hexagonal disordered phase into hexagonal ordered phase identified as T&AI (JCPDS 09-0098). The increase of sample densities was caused first of all by phase transitions. The WilliamsonHall plot for the T&Al,, sample after annealing (Fig. 6) is almost linear and its slope is considerably less than before annealing, which indicates substantial decrease of microstrains and of stacking fault density. The estimated crystallite size of this sample is about 1000 run. In optical micrographs (Fig. 5), very slightly revealed grain boundaries inside powder grains can be observed. For both samples, change of hardness after annealing is the result of several factors: phase transitions, crystal size, microstrains and dispersoids introduced during MA. In the case of the A150Ti50 sample, the increase of microhardness caused by annealing is probably due first of all to phase transition from solid solutions into considerably harder L,, phase. The decrease of T&Al,, sample microhardness can be first of all the result of a decrease in the density of lattice defects and an increase in the grain size (Table 2). Table 2 Microhardness values of compacts after explosive compaction and annealing Technique

Explosive compaction Annealing

Microhardness&S.D.)

568 (48) 614 (69)

592 (72) 506 (64)

Mechanical alloying of elemental powder mixtures of Al and Ti leads to the formation of supersaturated solid solutions. The presence of amorphous phase in the T&Al,, sample was also observed. Explosive consolidation of samples after MA allows bulk material with grain size of about 400 nm to be obtained and does not cause transitions of metastable into stable phases. An insignificant change of amount ratio of h.c.p. Ti(A1) and f.c.c. Al(Ti) phases can be observed in the case of the Ti,,Al,, sample. Annealing of the compacts leads to the formation of stable phases: TiAl compound in the case of Ti50A150 composition, and T&Al compound in the case of Ti,,Al,S. Samples are not free from impurities such as T&AlN. Explosive consolidation causes little change in the structure of lattice defects produced by MA. A considerable change in this structure is observed after annealing.

Acknowledgements This work was supported by the grant ‘New Materials’ of the Warsaw University of Technology.

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