Phases produced in mechanically alloyed powders of AlNiTi

Scripta METALLURGICA et MATERIALIA

Vol. 29, pp. 583-588, 1993 Printed in the U.S.A.

Pergamon Press Ltd. All rights reserved

PHASES PRODUCED IN MECHANICALLY ALLOYED POWDERS OF AI-Ni-Ti T. ltsukaichi,* M, Umemoto** and J. G. Cabafias-Moreno**~ * Graduate School and **Dept. of Production Systems Engineering, Toyohashi University of Technology, Toyohashi, AICHI, 441 Japan Permanent address: lnstituto Politecnico Nacional, ESIQIE, Apdo. Postal 75-874, Mexico, D. F. 07300, Mexico (Received March

29, 1993)

Introduction Mechanical alloying (MA) of the binary systems made of AI-N i, AI-Ti and N i-Ti has been the subject of a number of studies in recent years [e.g., 1-6]. These studies have found the existence of several metastable phases (amorphous phases, disordered intermetallic compounds, supersaturated solid solutions, etc.) in the as-mechanically alloyed powders fi-om these binary systems, as well as some interesting microstructural features in materials produced by the hot consolidation of such powders [e.g., 1,3,7-10]. Since the AI-Ni-Ti system is currently of interest in the development of structural intermetallic compounds [e.g., 11-I2], it seemed a natural choice to extend the MA studies to include this ternary system. Our first goal has been to characterize the general tendencies in the formation of phases in mechanically-alloyed AI-N i-Ti powder mixtures having a wide range of compositions, both in the as-milled condition and after hot-pressing by a fixed consolidation procedure. The results of this characterization are the subject of the present report. A more detailed account of the transformations and properties of the mechanically-alloyedmaterials will be published in the near future. Experimental Procedure Elemental powders of aluminum (99.9% purity, <180#m in size), nickel (99.9% purity, <63#m) and titanium (99.9% purity, < 150 #m) were used as starting materials. The nominal compositions of the powder mixtures are schematically indicated in Figs. la, b. These mixtures were milled in conventional ball mills, using austenitic stainless steel containers (128 mm in inner diameter and volume of 1.7/) and balls (9.6 mm diameter). The mills were loaded under argon atmosphere, with a ball-to-powder weight ratio of 100:l (36 g of powder) and the addition of 2 wt.% methanol (to prevent sticking of the powders to the container). The rotation of the mills was carried out at a speed of 95 revolutions per minute. The as-milled powders were consolidated by cold compaction into discs (1 mm thickness, 13 mm diameter), followed by vacuum hot pressing at 1173 K for 600 s under a pressure of 100 MPa. Subsequent cooling to room temperature occurred inside the hot pressing assembly. The phases in the as-milled powders and in the consolidated products obtained from them were characterized by x-ray diffraction (XRD), using a MacScience MXP18 diffractometer and CuKct radiation. Results The general nature of the phases found in the as-milled powder mixtures is indicated in Figs. la, b for milling times of 360 and 1800 ks, respectively. According to the results of XRD, the structure of the phases is classified as amorphous-like, A1Ni-type structure and solid-solution type. Some typical diffractometer traces from each of these structures are shown in Fig. 2. The amorphous-like phases readily appear in Ti-rich mixtures of the Ni-Ti binary system [5,10,13], and with increasing milling time they can be found over most of the ternary composition diagram (Figs. la,b), usually at the expense of the existence of the solid-solution type phases which are found at the corners of the composition diagram and near the AI-Ti binary compositions. Aluminum-rich and nickel-rich solid solutions clearly present less propensity to become amorphous than Ti-rich solutions. Only in mixtures with compositions near equiatomic AI-Ni we observe a strong tendency to form AlNi-type structures during MA.

583 0956-716X/93 $6.00 + .00 Copyright (c) 1993 Pergamon Press

Ltd.

584

MECHANICALLY

ALLOYED AI-Ni-Ti

Vol.

AI

29, No.

AI •

Amorphous-like s AINi structure Solid solutions

Ti

.

-

-

1,

.

Ni

Ni

Ti

(a)

(b)

AI

i

A/kf,

A

i/k/k/k

...... Ti

v Am+NiTi

v

v

\ Ni

(c) Fig. 1 Nominal compositions of the powder mixtures and types of phases found in powders milled for (a) 360 ks and (b) 1800ks, and in (c) rapidly solidified ribbons. The predominant phases found in the hot-pressed products, made from powders milled for 1800 ks, are indicated in the diagram of Fig. 3. (Hot-pressing of powders milled for 360 ks did not substantially alter the results.) Phase nomenclature is taken mainly from Nash and coworkers [14,15] and is listed in Table 1. Notice that (i) some degree of solubility of the corresponding third element probably occurs in each of the binary compounds, and (ii) the phase denoted here as X is not included in the published phase diagrams [11,14,15]. In a number of cases one or two phases may be present in small quantities and an interrogation mark (?) is used to indicate that identification is not definitive; this is especially a problem with the identification of the o phase, which gives only one strong XRD peak. In other cases, a few peaks of low intensity present in the XRD patterns could not be accounted for by the phases indicated in Fig. 3. Some examples of this are shown in Fig. 4. Such additional peaks did not seem to be associated with oxides or nitrides which could be expected to form if contamination from the atmosphere had been important [1,16,17].

5

Vol.

29, No. S

M E C H A N I C A L LALLOYED Y A1-Ni-Ti

585

[Ah6Ni2sTi56 ] MA 360ks

~.~,,..'~,',~.~;~.~.~

1800ks .,',

{~A1 ~'Ni ~ T i (solid solution)

I AI6oNi~3Ti2, ]

. ~,~.~' ~ JJ"~%

360ks

1 800ks V !1 : ~. V ~

[ AlsoNi3sTi~51

, ~ , j A:I~\ r

-

.' .~. .

j

V

360k s

V

t' I"~ t ~ ' ~ ' ~ ' ~ ~ ' ~ ~ ' ~ v -~,.. '~'-'~,,.,,m~,~*,

I

30

VA1Ni

40

~.;'-~.,,.~,~**.~,~.,~'%,,.~.,~, 1800ks ,

I

I

I

I

I

50

60

70

80

90

20/deg. (Cu Kc0 Fig. 2 Typical XRD patterns from as-milled powders.

A!

A,,T,co,/',,

/'.+,N,

k-----'~+v+rt? ----~

/ \/~'~,/ \/\

k----------)~-o~ +~+~c--------)~---.---~ I Arri (z)~(

X ""--Tx : g,+~,+~+~2~-----~AINi ([M)

I XFx£%.

\

/ \~,'.\_L,'~..~'\i\~ x, .~ ~v _-x+~'ri2+x.+c.~-X-----~13t+~(.~

,/

/\

Ti

/ ~ / \ ./~m~@~? / X I X

NiTi2

NiTi~2)

+NiTi2+rl

IX

.~T~ ~,v

I\

Ni

Fig. 3 Phases identified in hot-pressed (1173 K) compacts made from mechanically alloyed (1800 k s) powders.

586

MECHANICALLY

ALLOYED

AI-Ni-Ti

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5

TABLE 1 Phases detected by XRD in mechanically-alloyed and consolidated A1-Ni-Ti mixtures. Phase Phase

.......

A13Ni A1Ni3 A1Ni A13Ni2 AI3Ti A1Ti AITi3 NiTi Ni3Ti NiTi2 ........

.......

.......

........

.......

;/ ......

.......

......

.......

"""

....

A16.sNiTi2.5 A12NiTi AINiTi A1Ni2Ti (unknown)] ..........

",i .............

;" .........

.........

g

............

i" ......

/

Discussion Phases in As-Milled Powders. The results in Figs. la, b show that amorphous phase formation was readily achieved in mixtures of composition near Ni33Ti67, corresponding to the intermetallic compound NiTi2. This has been similarly found by Aoki and coworkers [18] in rapidly quenched ribbons of the AI-Ni-Ti ternary system, as shown in Fig. lc. Nevertheless, prolonged bali-milling seems to produce amorphization over a much more extended composition range than that accessible by rapid solidification compare Figs. lb and lc. The AINi-type structure was the only one corresponding to one of the intermetallic compounds of the AI-Ni-Ti ternary system which was observed in the mechanically-alloyed powders (Fig. la, b). This is in contrast to the observations in rapidly solidified alloys [18], in which a large number of equilibrium intermetallic compounds has been obtained (Fig. lc). It has been suggested that amorphization in mechanically-alloyed powders may bc promoted by the absorption of oxygen during milling [e.g., 19]. However, we have not found evidence of major contamination in our as-milled materials. Chemical analyses of some of the powder mixtures have yielded contents of less than 1 wt% oxygen and even lower contents of Fe, Cr or Ni, elements which could be picked up from the milling media. In those cases in which leaks from the atmosphere can occur during milling, we have observed [1,16] that nitrogen rapidly combines with titanium and forms titanium nitride. No such cases occurred in the powders from which the results of Fig. la, b are derived. Therefore, it is not likely that oxygen plays a major role in the amorphization of mechanically-alloyed powders observed in this study. In addition, the results in Fig. lb suggest that the composition range over which amorphous phase formation was found might become even wider by increasing the milling times to longer than 1800 ks. Phases in Hot-Pressed Products. The purpose of the consolidation procedure used in this work was to obtain compacts with low porosity and small grain size; thus, based on previous experiences [1,10,13,16], the temperature and duration of the hot-pressing operation were kept relatively low (1173 K) and short (600 s), respectively. It is then not expected that equilibrium phase relationships have been fully attained in the hot-pressed compacts. Nevertheless, with possibly only one exception, the phases identified by XRD in the consolidated materials (Table 1) seem to be the same as those reported from equilibrium studies [11,14,15]. The only exception is the phase here denoted as the X phase. The ranges of composition over which the different phases were observed to exist in the hot-pressed compacts can be compared to the ternary sections assessed by Lee and Nash [14]. The most relevant data from their assessment corresponds to one complete ternary section at 1073 K and another incomplete section at 1173 K. Although the experimental conditions in our consolidation procedure were not optimal for achieving phase equilibrium, practically all the phases indicated in Fig. 3 are found within the bounds of their respective equilibrium phase fields. This will be shown in more detail in a later publication. Of course, having no counterpart in the equilibrium phase diagrams known to date, the X phase is not included in the above comparison. The X phase has a crystal structure similar to the E93 (fce) structure of NiTi2. This is more clearly shown in Fig. 4 by the XRD pattern corresponding to the mixture of nominal composition A125Ni25Tis0. In this pattern one can observe the diffraction peaks from the X phase simultaneously with those from the "regular" NiTiz phase, matching each other almost in a one-to-one fashion. However, some differences in relative peak intensities are apparent. Since these two phases were found to coexist in several alloys (Fig. 3), the X phase has been considered here as a phase different from NiTi2. Figure 5 shows that the calculated lattice parameters of the X phase are considerably higher than the ones corresponding to the well known NiTi2 structure. Nash and Liang [15] reported very small changes in the lattice parameter of NiTi2 by the dissolution of up to about 14at.% aluminum; therefore, the lattice parameter values given in Fig. 5 point to some structural and/or compositional difference between these two phases.

Vol.

29, No.

5

MECHANICALLY

o

~v~ ~

ALLOYED AI-Ni-Ti

587

Alx(NiTi2)loo.x [ O~. ONiTi2

~_.0~ . j ~

t

~ o

°i

v

~

x:70

0

,,d

~ not identified

~

v

~

o

~ - ~ - - ~ .

o

x=60

~

•

~k

•/~ z~, z .z v •

Ok

x=40 . ~

~ •

9,. 7~ • ~. o~. ~ , . a • ~

O

•

x

•

x=34

t_ | O

x:25

•

•

~

oo ~

o~.

?

oo9~_

a

1

I

I

I

I

30

40

50

60

70

80

20/deg. (Cu Kc0 Fig. 4

XRD patternsofsome hot-pressedsamples madefrom Alx(NiT~loo_xPOwders milled for 1800 ks.

1.150 II "-" 1.145 ~._..~

~

a'- 1.140 1135, o~

AIx(NiTi2)aoo-x

1.130 1.125 1.120 0

I

I

I

I

I

10

20

30

40

50

60

x (at.%) Fig. 5

Calculated lattice parameters of the NiTi2 and ?( phases in hot-pressed powder mixtures (1800 ks MA time) of various compositions. The shaded rectangle indicates the range of values reported in the literature.

588

MECHANICALLY

ALLOYED AI-Ni-Ti

Vol.

29, No.

5

In order to test the thermal stability of the )¢ phase, we have performed some annealing experiments on a powder mixture of composition A145Ni~0Ti4s, previously milled for 1800 ks. Annealing for 86.4 ks at 1373 K produced powders consisting almost entirely ofthe;~ phase alone. In contrast to this, powder mixtures which include the NiTi2 phase among the products found in sintered compacts (see Fig. 3) actually melted when heated to temperatures be~,een 1173 and 1273 K, which is consistent with the lowest temperature (1215 K) at which a liquid phase coexists with the binary NiTiz compound [14]. Thus, not only the lattice parameters, but also the physical properties of the )¢ phase seem to differ significantly from those of the NiT% phase. Further work is now in progress to characterize the structure and stability of the X phase, as well as those of the other phases observed in the consolidated compacts. Acknowledgments The authors acknowledge the Ministry of Education Scientific Research Fund to promote the research. JGCM acknowledges a sabbatical leave from COFAA, Instituto Politecnico Nacional (Mexico). References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19.

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