Further studies of ion mixing in binary metal systems

Nuclear Instruments and Methods in Physics Research B7/8 North-~olIand. Amsterdam

Section VIII.

547

(1985) 547-551

Ian Beam Mixing

FURTHER STUDIES OF ION MIXING IN BINARY METAL SYSTEMS Bai-Xin LIU Department

of Engineering

Physics, Tsrnghua Uniuerstty, Beijing

Chrna

Using free ener~-~m~sition diagram, a simple model is proposed for the fo~ation of amorphous alloys by ion mixing of metal layers. The basis of the model is the limited atomic mobility in such samples after ion mixing at a suitably low temperature. The model explains the formation of amorphous alloys that have been reported previously and those obtained in this study in the Zr-Ru and Ti-Au systems by ion mixing. These include phases with compositions in both two-phase and single-phase regions of the equilibrium phase diagram. In the Ni-Mo system, an unusual phase transition was observed by X-ray diffraction photos, i.e. an amorphous phase was formed after room temperature aging of an ion induced metastable crystalline phase (h.c.p. structure). Post-irradiation annealing of some ion mixed Ni-Mo amorphous alloys were performed at various temperatures. A schematic free energy diagram is proposed according to the phase evolution in the annealed samples upon annealing, and is used to discuss the ion induced phenomena in this system.

I. Introduction

2. Experimental procedure

In a previous paper [l], we formulated a so-called “structural difference rule” stating sufficient conditions for producing amorphous alloy films by ion mixing of multiple alternate metal layers: (i) the constituent metals have different structures, and (ii) the composition after uniform mixing lies within the two-phase region of the equilib~um phase diagram. Although the microscopic mechanisms underlying this rule are not well understood, a simple thermodynamic and kinetic interpretation has been proposed [2]. In this paper, we first elaborate the model by using the free energy-composition chart (or free energy diagram) and then discuss it with the previous data. It is found that the model can not only explain the formation of amorphous alloys so far obtained by this technique, but it also suggests other conditions for forming amorphous phases. To test these predictions, ion mixing experiments were conducted in two selected systems, i.e. the Zr-Ru and Ti-Au systems. Another system, i.e. the Ni-Mo system, was also chosen for further study in connection with the model, because this system covers various conditions for obtaining amorphous alloys and especially because an unusual transition (from crystalline to amorphous phase upon aging) has been observed in this system which requires an explanation in terms of free energy consideration. Post-irradiation annealing experiments were therefore performed with some ion-mixed Ni-Mo amorphous alloys with various impositions. The possible mechanisms of metastable phase formation and transformation are discussed.

Multilayered samples were prepared by alternate evaporation of two metals onto an inert substrate (SiO,) in an oil-free electron gun system. The vacuum during evaporation was about 5 x lo-’ Torr. The relative thicknesses of the two metals were designed to attain the desired compositions, and the total thickness of the deposited material was approximately equal to the projected range plus projected range straggling of the irradiation ion (300 keV Xe’). The samples were then irradiated at room temperature by Xe+ ions to doses ranging from 5 X lOI to 2 X lOI Xc/cm*. The vacuum in the target chamber during irradiation was better than 3 x lo-’ Torr. In post-irradiation annealing, ion mixed amorphous alloy films were consecutively annealed for 0.5 h periods at each temperature step. The vacuum level was about 1 x lo-+ Torr. All the samples were analyzed by X-ray diffraction (Read camera) to identify the structure of the ion induced phases, and some were analyzed by backscattering spectrometry of 1.5 or 2.0 MeV He+ ions.

0168-583X/85/$03.30 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

3. Results and discussion 3.1. Amorphous alloys formed in two-phase regions For a binary system with components having different crystal structures, it is impossible for the phase VIII. ION BEAM MIXING

Bai - Xin Liu / Ion mixing in binary metal systems

548

A

8 (a)

Phase

(b) Free

at

6

% -

dragram

energy

Fig. 1. Ion Induced amorphization

diagram

at T,

in an eutectic system.

diagram to be a continuous solid solution type [3], where only single phase covers the entire composition range. A diagram of another type, i.e. a simple eutectic diagram, is therefore chosen as being a representative (fig. la) for discussion of the amorphization mechanisms for this case, and the ~~espon~mg free energy diagram (fig, lb) is used for analysis,. If the overall composition of a multilayered sample is in the two-phase region shown in fig. 1, the formation of the amorphous phase by ion mixing in this sample is thought to proceed as follows: (i) atomic collisions induce prompt uniform mixing of the initial discrete multilayers of metals A

Table 1 Amorphous

Zr-Ru

and B, and excite the mixture to a state of high free energy (point 1 in fig. lb), after which (ii} presumably some relaxation processes allow conversion to a state of lower free energy. Now the transition from the excited state to a state of two-phase equilibrium (point 3 in fig. lb) requires first chemical separation and then crystallization. Under usual conditions of ion mixing at suitably low temperatures, for which the atomic mobility is limited, this transition is severely inhibited and a polymorphic transition from point 1 to an amorphous state (point 2 in Fig. lb) is favored. This is because the latter transition requires neither a change of composition nor a rearrangement of the atoms into ordered configuration. In 20 previous reported amorphous alloys al1 formed in the system having metal pairs of different structures [1], 18 of them can be interpreted in the same fashion with a similar diagram. As similar free energy diagrams can equally well occur in a region bounded by two adjacent phases in an equilibrium phase diagram, an amorphous phase should also be obtainable according to this analysis. To test this prediction, a system with components of the same crystal structure is necessary. The Zr-Ru system was selected for this purpose, because Zr and Ru are both of h.c.p. structure, yet an intermetallic compound ZrRu (b.c.c.) divides the Zr-Ru diagram into two subdiagrams, each featuring a two-phase type. 300 keV Xe+ ion mixing was performed at room temperature with two multilayered samples with compositions of Zr,, Ru z5 and ZrzsR~,~ lying in the middle of Zr-ZrRu and ZrRu-Ru subdiagrams respectively, and two amorphous alloys were indeed formed after irradiating to certain doses (table 1) From the above discussion, the structural difference rule points out a thermodynamic condition just like that shown in fig. 1, which favors amorphous phase formation, hence the validity of the rule is extended to any two-phase region in a binary metal system. 3.2. A~or~ho~

alloys corned in si~gie-thee

regions

The NisoMoso amorphous alloy is an exception to the 20 alloys mentioned above, in that its composition is right at an equilibrium compound (&phase). The interpretation of the formation of this alloy is then different and is suggested to be as follows. After the Ni5,MoS0

alloys formed by 300 keV Xe ion mixing at room temperature

Alloy composition

Dose

X-ray diffraction

Phase formed

Zr,s RUZS WsRu7s

7x10’5 2 x 10’6

halo halo and weak lines

amorphous amorphous

and a trace of Ru

Bai- Xin Liu / Ion mixing in binary metal systems

549

600-630°C) among the amorphous alloys obtained in this system [l]. It is our contention that if the crystal structure of an equilibrium compound is sufficiently complex, ion mixing of a multilayers with the same composition as the compound can still make it amorphous, and the equilibrium phase will only result after subsequent thermal annealing. We have checked these ideas in another system, i.e. the Ti-Au system by choosing the average composition of the multilayers to be that of an equilibrium compound Ti,Au, which has the complex 8-W structure [5]. After irradiating these multilayers at room temperature to a dose of 5 x 1015 Xc/cm’, an amorphous phase was formed. Moreover, recent results reported by Hung et al. [6] are also in support of this explanation.

multilayers were mixed and hence excited to a highly energetic state, the transition to the equilibrium state and to the amorphous state are both polymorphic, i.e. both transitions require no composition change. But the &phase has a complicated orthorhombic crystal structure [4] with 56 atoms in a Bravais unit cell in a specific ordered configuration. To crystallize such a sophisticated structure obviously requires strict kinetic conditions (e.g. relatively high temperature and a long time). In our typical ion mixing experiments, the kinetic conditions are not sufficient for crystallization of the S-phase with the result that an amorphous phase is formed instead. The idea of the difficulty of growing the S-phase is also supported by the fact that Ni,,Mo,e amorphous alloy once formed is the most stable one (T, =

halo

(a) to

Diffraction

a dose

of 5x10’5

all other

lines

pattern

of

Xe/cm2

also

Ni,,MoS5

at RT.

from

MX phase

multilayers

Amorphous

(h,cp.)

irradiated

+ MX phase.

halo

(b) thermal Fig.

2.

Diffraction aging.

pattern

of

the

same sample

after

-I month

Amorphous.

X-ray diffraction photos showing the MX + amorphous + amorphous phase transformation. VIII. ION BEAM MIXING

Bai - Xin .Liu / Ion mixrng in binury metal systems

550

Experimental data offered thus far establishes that it is possible for an amorphous phase to be formed by ion mixing, when the average composition of the multilayers is the same as for an equilibrium compound with complicated structure. However, more studies are needed to clarify this issue. 3.3. Ion induced phenomena in the Ni-Mo

Table 2 Phase transformation of Ni-Mo amorphous alloys upon thermal annealing. Annealing time was 30 min at each temperature

step. Amorph.

Phase evolution with increase of

temperature

alloy -z4Wc Ni6+03,

+

K 6oooc Ni so MOSO

-D

< 5so’C Ni 35 M065

-D

h&er

SOOT

Amor.

-B Amor.+MX

-B Ni+ d-phase h&a

630-Z

Amor.

-B Amor.+ 600-Z

Amor.

+

&phase

+

&phase

higher

Amor. + MO *

MO+ d-phase

I

I

S-phase 1’ I’ I’

\ MO

t

II II

system

An unusual phase transition was observed after room temperature storage of two Ni,,Mo,, samples that had been irradiated at room temperature to 3 and 5 x lOI Xe/cm2. The structure of the samples were both two phase mixture: an amorphous and a metastable crystalline phase (MX). But after one month of aging at room temperature, the X-ray diffraction patterns of both these samples show only a diffuse band in each, and no sharp diffraction lines indicating the presence of crystalline phase. Fig. 2 shows two X-ray diffraction photos taken for the sample irradiated to a dose of 5 x 10’s Xe/cm2 directly after irradiation (fig. 2a) and after one month room temperature aging (fig. 2b). These indicate that a phase transformation has taken place: the MX phase has transformed to an amorphous phase upon aging. The MX phase has previously been identified to be h.c.p., the Ni concentration is greater than 65% (i.e. the average value of the sample), and the phase can be considered as a Hume-Rothery electron compound [7]. Consequently, the Ni concentration of the amorphous phase shown in fig. 2a should be less than 65%. To compare the relative energy levels of all the phases involved, three ion mixed amorphous alloys, Ni,,Mo,,, Ni,,Mo,, and Ni,,Mo,, (see fig. 3a, the Ni-Mo phase diagram), were therefore subjected to post-irradiation annealing at various temperatures. The annealing time at each temperature step was 0.5 h. Table 2 summarizes the sequence of the phase change observed in these samples upon annealing. One sees that Ni,,Mo,, and Nis5M06s alloys first decompose to the intermediate states (amorphous plus MX and amorphous plus MO, respectively) before going fully equilibrium.

,

I-

/I

II1 Ni

‘20 (0)

I

II

I

I

I

60 60 Eat %Ni-Mo phase diagram

MO

(+I

A.Go t-1

I ----

(b)

Free

A

highly

.

amorphous phase

energy

energetic phase

tronsformotion

diagram

at

To

state obtained

by RT

upon

ton mixing

post-annealing

Fig. 3. Proposed free energy diagram of Ni-Mo corresponding equilibrium phase diagram.

system and the

These observations lead us to speculate that the free energy curve of the amorphous phase is likely the one shown in fig. 3b. The free energy curve of MX phase is thought to be one similar to that of a compound. A schematic free energy diagram is thus proposed, as the free energy curves of the equilibrium phases can be drawn qualitatively according to the theory of thermodynamics of solids [3]. Also, the suggested paths followed in our experiments are included in this figure. The formation of these Ni-Mo amorphous alloys, two in two-phase regions and one in single-phase region, has been discussed in the previous sections, and can be seen clearer with the diagrams in fig. 3. Concerning the unexpected phase transition, it is of interest to note that the entirely amorphixed Ni,,Mo,, alloy changes to amorphous plus MX upon 480°C annealing, while the mixture of amorphous plus MX in-

Bai - Xin L.iu / Ion mixing in binaT

duced by low dose irradiation in Ni,,Mo,, sample changes to amorphous phase upon room temperature aging. The explanation for these is not clearly known. Nevertheless, if the free energy curves of MX and amorphous phases intersect each other along the composition axis (e.g. like those shown in fig. 3b), transitions of both types are possible. We have also observed a similar phase transformation in the Ni-Nb system. After irradiating a Ni,,Nb,, multilayered sample to a low dose of 5 X lOI Xe/cm2 at room temperature, X-ray diffraction revealed a mixture of amorphous plus MX (MX is also of h.c.p.), while after weeks of aging at room temperature, only the amorphous phase was seen by X-ray diffraction. We thus believe that this type of phase transformation is not peculiar to only a few systems. The major experiments were executed in 1982 when the author was in Professor M-A. Nicolet’s group at the California Institute of Technology. The author wishes first to thank M-A. Nicolet for his hospitality and

metal systems

551

helpful discussions, and to Drs. W.J. Johnson (Caltech), S.S. Lau (UCSD), and B.M. Paine (Caltech) for fruitful discussions and suggestions. The author also thanks R. Femandez (Caltech) for assistance in the preparation of the samples.

References M-A. Nicolet, and S.S. Lau, Nucl. Instr. and Meth. 209/210 (1983) 229 (Proc. 3rd. Int. Conf. Ion Beam Modification of Materials). PI Bai-Xin Liu, W.L. Johnson, M-A. Nicolet and S.S. Lau, Appl. Phys. Lett. 42(l) (1983) 45. in: Thermodynamics of Solids (Wiley, New [31 K.A. Swab, York, 1962). (41 C.B. Shoemaker and D.P. Shoemaker, Acta Cryst. 16 (1963) 997. VI Powder Diffraction Pattern File, No. 18-576. Fl L.S. Hun& M. Nastasi, J. Gyulai and J.W. Mayer, Appl. Phys. Lett. 42(S) (1983) 672. 171 Bai-Xin Liu, Phys. Stat. Sol. (a)75 (1983) K77.

111Bai-Xin Liu, W.L. Johnson,

VIII. ION BEAM MIXING