606 Materials Science and Engineering, A 133 (1991) 606-610

Electron microscopic study of Cr-Ti alloys

R. Prasad, R. E. Somekh and A. L. Greer Department of Materials Science and Metallurgy, University of Cambridge, Pembroke Street, Cambridge CB2 3QZ (U.K.)

Abstract

Our attempts to amorphize alloys of chromium and titanium by annealing the metastable b.c.c, fl-phase as suggested in existing literature have not been successful. We report our results on bulk and thin films of Cr-Ti and compare them with previous work. Of particular interest is the prominent diffuse scatter- ing seen in the electron diffraction patterns of the fl-phase. The stability of the fl-phase is discussed.

1. Introduction

The present work was motivated by the reports of a novel amorphization technique by Blatter and von Allmen [1-4]. It was found [1] that thin films of binary Cr-Ti alloys of 30-40 at.% Cr deposited on sapphire substrates formed a metastable body-centred cubic phase (r-phase) upon 50 ns pulsed laser-quenching (see ref. 5 for a phase diagram and the thermodynamics of the Cr-Ti system). Such single phase r-films were amorphized by annealing at 600 °C. Compared with the conventional solid state amorphization processes (see, for example, ref. 6), the novelty of spontaneous vitrification (the term used by the original authors) is that the starting material is a single phase crystal rather than a mixture of two crystalline phases. The thermodynamic possibil- ity of such an amorphization process has been considered by Greer [7] and Gallego et al. [8]. Considering the thermodynamic and kinetic factors underlying the process, von Allmen and Blatter [3] identified and successfully vitrified a number of alloys such as Co-Nb, Cu-Ti, Fe-Ti, Mn-Ti and Nb-Ti. Reasoning that neither long- range diffusion nor rapid heat extraction is essen- tial for spontaneous vitrification, Blatter et al. [2] concluded that the process might be possible in bulk. Indeed, they were successful in amorphizing 5 mm cube samples of Cr40Ti60. Our interest was primarily in the promise of the technique for the production of bulk amorphous alloys. But, as will be described in the following sections, despite several attempts we have not been able to vitrify

either bulk or thin films of Cr-Ti. We used trans- mission electron microscopy (TEM) to study various phases and their transformations in the Ti-Cr system.

2. Experimental techniques

Bulk alloys of chromium and titanium were prepared by arc-melting the pure elemental com- ponents in an argon atmosphere. The nominal compositions of the alloys were checked by energy dispersive analysis in the TEM and the SEM (scanning electron microscope). For anneal- ing, bulk samples were wrapped in tantalum foil and sealed in argon-filled silica tubes with tita- nium granules as getter. Thin films were prepared in an ultra-high vacuum planar d.c.-magnetron sputtering system using elemental targets (details in ref. 9). Rock salt cleaved in air, and glass slides coated with thin layers of vapour-deposited aluminium, were used as substrates for sputter- deposition. Alloys of selected compositions were deposited with and without external heating of substrates. Laser-quenching of thin films was per- formed using 5 ns pulses from a Nd-YAG laser. The thin film was removed from the substrate only after quenching.

X-ray diffraction experiments were performed in a Philips diffractometer. TEM characterization was carried out in a Philips EM400 operating at 120 kV and a Philips EM300 at 100 kV. Bulk samples for TEM were thinned down to less than 100/zm on 1200 grade SiC papers. These sam- ples were then electropolished using a twin-jet

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system with an electrolyte of 5 vol.% sulphuric acid in methanol held at - 5 0 ° C , as recom- mended for titanium alloys by Spurling [10]. Sputtered films were electron-transparent with- out thinning. They were floated off and picked on a TEM grid by dissolving the substrate in a suit- able solvent (distilled water for rock salt and dilute aqueous solution of NaOH for aluminium).

3. R e s u l t s

3.1. Bulk alloys In accordance with the reported amorphiza-

tion [1, 2], compositions of 30 at.% and 40 at.% Cr were chosen for study. The behaviour of the alloy at both compositions was the same. X-ray diffraction of as-cast Cr30Ti70 (Fig. 1) shows a single phase b.c.c. (fl) structure of lattice param- eter a = 3 . 1 7 A_. We conclude that the cooling on a water-cooled copper hearth, after arc-melting, is sufficiently fast to retain the high-temperature fl-phase as a metastable phase at room tempera- ture. This result is at variance with that of Blatter et al. [2], who obtained a mixture of equilibrium phases, a-Ti (h.c.p.) and CreTi (cubic, C15), in the as-cast alloy. To obtain the fl-phase they annealed the as-cast samples in the fl-phase-field followed by water-quenching. This step was not essential for our samples. Various samples were annealed at 600 °C (the reported amorphization temperature [2]) for 0.5-24 h. In each case the final product, as characterized by X-ray diffrac- tion, was found to be the equilibrium mixture of a-Ti and CrzTi. To simulate the experimental conditions of the original report [2] we also made some runs with samples that were annealed in the fl-phase field and then quenched in water. The behaviour of the quenched fl-phase was no differ-

' 2 eJ

e.J

110

t 112

0 20 30 /+0 50 60 70 BO 90

28 (degrees)

Fig. 1. Cu Kct X-ray diffraction pattern from Cr3oTi7o. The peaks here are sharper than in reported amorphizing samples [4] (see Section 4).

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ent from that of the as-cast samples. We were thus entirely unsuccessful in achieving the spontane- ous vitrification of the bulk /3-phase either in Cr30Tiv0 or Cr40Ti60.

To investigate this outcome further we used TEM to characterize our samples in greater detail. Figure 2 shows a set of electron diffraction patterns from the as-cast r-phase, in which the major spots can be indexed to a b.c.c, phase, con- firming the results of X-ray diffraction. However, many arcs of diffuse scattering, most prominent in the case of the [110] zone (Fig. 2(b)) are seen in these patterns. Such diffuse scattering has been reported in many titanium alloys (e.g. see [11] for TiCr) and attributed to the mechanical instability of the r-phase prior to the ~0-transition (e.g. [12]). Here we characterize the distribution of diffuse scattering in reciprocal space. On the [110] pattern (Fig. 2(b)) diffuse scattering occurs as rings (not circular) centred on (i 11)* type loca- tions (i. e. around B2 superlattice reflections). The intensity is seen to vary along a given ring which usually shows four maxima at locations such as ~(222)* and 3(222)*. From cubic symmetry, these maxima lie at the corners of a cube. This diffuse intensity cube is located at the octahedral sites of the f.c.c, lattice in reciprocal space (correspond- ing to the real space b.c.c, lattice) and its orienta- tion is the same as that of the f.c.c, cube (Fig. 3). If we now try to reconcile the intensity maxima seen in the [100] diffraction pattern (Fig. 2a) (which are much weaker than those in the [110] pattern) we should also have intensities at the edge- centres of the diffuse scattering cube described above. These may be separate or may just be the tails of the intensity maxima at the cube corners.

3.2. Thin films Having failed to amorphize the r-phase in bulk

alloys, we tried the experiment on thin films. A range of alloy compositions from 30 at.% to 50 at.% Cr were investigated. On substrates that were not heated, as-deposited films on rock salt were mostly amorphous, whereas those on alu- minium-on-glass were usually an f.c.c, phase of lattice parameter a=4.3 A. No r-phase was obtained. On heated substrates, we found amor- phous, f.c.c., or equilibrium phases depending upon the substrate and its temperature, but the r-phase was still absent. For pure titanium, Vitta et al. [13] have reported an h.c.p, phase in the as- deposited film which transforms to a b.c.c, phase upon 5 ns pulsed laser-quenching. We therefore

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IK,

111

~ 3 1 3

(f> 001 101 Fig. 2. Electron diffraction patterns from as-cast fl-phase in Cr4c~Ti6,. Zone axes: (a)[100]; (b)[110]; (c)[l 11]; (d)[113]; (e)[315]. The relative orientation of these zone axes is shown in (f), a standard stereographic triangle for cubic crystals.

/ . /

A¢' w w

Fig. 3. Schematic of locations of diffuse intensity maxima in the reciprocal space of fl-Cr40Ti60 as derived from Fig. 2. Large filled circles define the f.c.c, reciprocal lattice points of an ideal b.c.c, crystal. Comers and edge-centres, shown by open circles, of the smaller cube, centred at the octahedral site of the larger one, locate the diffuse intensity maxima. Also, there is one such small cube at each edge-centre of the larger cube but omitted for clarity.

Fig. 4. Laser-quenched thin film of amorphous Crs2Ti4s. (a) Micrograph showing b.c.c, islands (quenched regions) in the original amorphous/microcrystalline matrix; (b) diffraction pattern from the b.c.c, region; (c) diffraction pattern from the matrix.

tried laser-quenching and were indeed successful in producing a r-phase in some cases. Films con- sisting of crystalline fl islands (irradiated areas) in an amorphous/microcrystalline matrix (Fig. 4) were annealed in situ in the hot stage of the electron microscope by gradually increasing the temperature. This resulted in a complete trans- formation of the whole film, r-phase and the matrix, into the equilibrium product, a-Ti and Cr2Ti, at about 400 °C.

4. Discussion

We first discuss the probable reasons for our failure to amorphize the r-phase. Although there was no evidence of any equilibrium phase in either the X-ray diffraction pattern (Fig. 1) or the

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electron diffraction patterns (Fig. 2), the presence of very small amounts, which could act as nucleants for subsequent transformation, cannot be discounted.

The defect concentration of the r-phase (as revealed by the X-ray peak-widths) is considered by Blatter et al. [4] to be important in vitrification. The width A(20) of an X-ray peak is related to the coherence length L in the crystal via the well- known Scherrer formula L=O.9J . / [A(20 )cosO] where 0 is the Bragg angle and it the X-ray wave- length [14]. Blatter et al. [4] find that water- quenched and laser-quenched samples with a coherence length of about 10 nm vitrify, whereas splat-quenched samples with a coherence length of 35 nm fail to do so. In our case, measurements from Fig. 1 show that the width of the 011 peak corresponds to about 20 nm, i.e. twice that of reported vitrifying samples. The failure of our samples to vitrify can then be attributed to their larger coherence length. Surprisingly, the coher- ence length of 20 nm is much smaller than the grain size (of the order of 104-105 nm) of the samples as measured from the optical micro- graphs. Our measurement on the published X-ray diffraction pattern of Mizutani [15], who also failed in his attempt at spontaneous vitrification, again shows a coherence length of about 20 nm.

Blatter et al. [4] find that the samples capable of vitrifying show diffuse scattering, whereas the non-vitrifying samples do not. Yet, our samples show prominent diffuse scattering, but fail to vitrify. A direct correlation of the amorphizability and the presence of diffuse scattering does not, therefore, follow.

Or- laser-quenching work on thin films of CrTi followed by in situ annealing in a micro- scope shows that the r-phase transformed to the equilibrium phases. However, the composition of the laser-quenched film was - 52 at.% Cr, closer than Cr40Ti60 to the equilibrium compound Cr2Ti, and this could have made the formation of equilibrium phases easier.

We did not obtain a r-phase under any condi- tions by sputtering, although we could form stable and metastable crystals and an amorphous phase. This difficulty of forming a r-phase perhaps indi- cates its inherent instability. The diffuse scatter- ing and a coherence length smaller than the grain size also show that the ideal, undistorted r-phase is unstable. Such an instability, in principle, can be conducive to amorphization. But, our results show that the amorphization of the fi-phase by

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annealing is not straightforward. Perhaps special, as yet undetermined, conditions must be met for amorphization to occur, in particular to suppress nucleation and growth of equilibrium phases.

Acknowledgments

The authors thank Dr. A. Blatter and Dr. W. M. Stobbs for useful discussions.

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