Nuclear Instruments and Methods in Physics Reseanh B80/81 (1993) 386-389North-Holland

C. Jaouen, M.F . Denanot and J .P. RivièreLaboratoire de métallurgie Physique (UA LU CNRS), 40 Avenue du Recteur Pineau, 86022 Poitiers, France

M.O . Ruault and M . Salon(;SNSM, IN2P3 .

VRS, Bât. 108, 91405 O.-say, France

11 . Introduction

Some intemietaliic compounds undergo a crys-talline to amorphous phase transformation when sub-mitted to irradiation with energetic particles . Accord-ing to both experimental results [1-3] and to moleculardynamic simulations [4,5], the role of chemical disor-dering is now well established . It is suggested thatelastic softening related to a volume expansion occur-ring during the chemical disordering causes :, mechani-cal instability and so drives the amorphization [3,6] .However, results deduced from recent simulations showthat the buildup of point defects or defect complexes,increasing the system energy, is essential to trigger theprocess [7,8] .

AI 3Ti is an intermetallic compound exhibiting the1DO Z2 (body centered tetragonal) structure under equi-librium conditions. This structure is stabilized. relativeto the higher symmetry phase LI Z , by the stronger Ti dAl p hybridization and stronger directionality of thebonding [9]. Our interest for this system was motivatedby both its high ordering energy (no thermal order-dis-order transition is observed before melting) and by thefact that the two elements have similar atomic size(AR/R-1%). Thus, no elastic energy contributiondue to antisite defects is expected in this system apriori . In order to investigate the response of thissystem to ion irradiation, we have performed an in situtransmission electron microscopy (TEIA) study of the

2. Experimental

W68-5833X/93,106,(10 n 1993 - Elsevier Science Publishers B.V. All rights reserved

HHU 13Beam Inte-tions

wiéh Materla(sKAtoms

In situ TEM study of ion induced amorphization at low temperaturein A1 3Ti

Phase modifications induced at low temperature (T :5 40 K) in AI 3Ti by ion irradiation were investigated by in situ transmissionelectron microscopy (TEM). Irradiation with heasy ions induces a complete amorphization at a dose of approxi-ïately 1 dpa . Afterpartial chemical disordering, a crystalline phase change from the DO,-, structure into the LI Z structure is observed only after theonset of amorphization . These results suggest that a lattice strain release is produced by the amorphization pre"^ess allowing thetransition towards a higher symmetry phase . On the other hand, light ion irradiation leads to partial chemical disordering and tostrong lattice distortions, but the transition c-a is not completely achieved at doses higher than 2 dpa .

structural transformation in bulk AI - fi irradiated atlow temperature (below 40 K) with K and He ions .

Stoichiometric high purity A1 3Ti was used for thisstudy. Discs with 3 mm in diameter were spark-Gutted,then annealed at 1100 K for 12 h in evacuated silicatubes . Thin foils for transmission electron microscopywere prepared by jet electropolishing using a solutionof 10% perchloric acid in methanol at abort? - 20°Cand 20 V.

The samples were ion irradiated at a temperaturebelow 50 K within a Philips EM 400 linked to theOrsay ion implanter [10]. Irradiations were performedwith Kr and He ions . Energies of '_'50 Lt:d 40 kcV werechosen, respectively . In this way, rather homogeneousdamage profit?s were obtained. The characteristic pa-rameters of the damage were calcuiated using theTRIM simulation code [11]. The mean numbers ofdisplacements per atom (dpa) evaluated assuming anavera~,t: displacement threshold energy Ed of 28 eVwere 0 .26 and 0.24 dpa for fluences of 10 14 Krcm - Zand 10 1 `' Heem -Z. The in situ observations were per-formed for the Kr ion irradiation with a simple-tiltliquid-helium sample-holder [12] and for the He ionirradiation with the double-tilt liquid-helium sample-holder more recently built at Orsay by Salomé and

Biderman in collaboration with Raynaud from CEMESin Toulouse. The latter still has to be imprn" ,;d but italready permits to recover a given sample orientationby means of the classical ±35° first inclination and the± 15° second inclination, . The structural change of thematerial during irradiation was mainly followed bymeans of uiffraction patterns of a selected area

3. Results

3.1 . Krypton ion irradiation

The electron diffraction patterns obtained beforeand during irradiation at T< 40 K with Kr ions arepresented in fig. 1 . The initial spot pattern (fig. la)obtained wi,h the incident electron beam parallel to[100] reveals the fully ordered DO_ structure . Thesequence of visible ch,.nges begins with the appearanceof a faint halo for an ion fluence of 3 x 1013 Krem-2 .Simultaneously, both the high index and the superlat-tice reflections become weaker (fig. lb) . With furtherirridiation dose (up to 8 x 10 13 Kr cm -2 ), the haloincreases in intensity and a square figure typical -f afee (100) pattern stands out indicating the formation ofa higher symmetry fee phase (fig . le). This crystallinetransition towards a fee structure is finally confirmedby the pattern obtained at 10 14 Krcm -2 (fig . Id).However . observation of another area with [310] zoneaxis, at the fluence of 8 x 10 13 Kr cm - 2 , reveals unam-biguously the development of the metastable LI, or-dered structure (fig . le) preceding the formation of therelated disordered fee structure at higher fluences (fig .If). Finally, all Bragg spots disappear and !he typicaldiffuse ring pattern characteristic of an amorphousphase

is present only . At this point ((p >_ 5 x 10 14Kr cm - `, i.e . for doses higher than 1 .3 dpa), the irradi-ated area is considered to be completely amorphized(fig .

1 g).

3 2. Helit m ion irradiation

C. Jaouen ti al ,/ TEM study of ion induced amorphizat- in A!3 Ti

The progressive changes in the [100] diffractionpattern with the ion flucncc are shown in fig. 2 . Up tc6 x 10 15 Hecm - '-, no important modification occurs.One can only notice (fig. 2b) a broadening of thescattering intensity distribution around the differentreflections . At 10 11' He cm - Z, an important diffusionaround the central spot is visible (fig . 2c) and finallyabove 1 .5 x 10 16 and up to 8 x 10 1 HeCM2 , one cansimultaneously note the appearance of a weak diffusering typical of an amorphous phase, the developmentof diffusion streaks and important deformations of thereflection spots indicating the existence of strong dis-tortions generated by the defect accumulation . Nocomplete amorphization is obtained at higher doses.

4. Discussion and conclusions

387

Fig. 1 . Sequence of electron diffraction patterns of AI 3Tiduring in situ irradiation at T :540 K with 280 keV Kr'(a)-(d) : [100] zone axis ; (e)-(f): [3101 zone axis): (a) unirradi-ated ; (b) ?x1013 Kr /cnm = ; (c), te) ;x10 13 Kr/cm 2 ; (d). (f)1014 Kr/em`; (g) 5X1014 Kr/cm = (i .e . 1 .3 dpa) . Note theappearance of the crystalline LI, structure aftc. the onset ofamorphizaiion and the related fee disordered phase in thepatterns (c), (d), (e), (f) and the complete amotphization

in (g).

We have shown that the intermetallic compoundAI jTi can be amorphized by ion irradiation at lowtemperature M540 K). Two main results have been

Ilc. METAL MODIFICATION (c)

,',t88

Fig. 2. Progressive changes in [100] zone axis diffractionlp~tterns of AI,Ti during in situ irradiation at T 5 40 K with40 ke`J He' : (a) unirradiated- (hl 6x 10 15 He/cm 2; (c) 10 16

He/C.2 ; (d) 8 x 10 16 He/cm 2 (i.e. 1 .9 dpa) .

d.duced from this experimental study of ion irradia-tion induced amorphization:

i) complete amorphization is obtained only by heavyion irradiation but not with light ions at equivalentdoses and above (up to 2 dpa) ;

ii) when the amorphization process starts (duringthe Kr ion irradiation) a simultaneous crystalline phasechange of the initial DO 22 structure into the L1 2 isobserved .

The fact that complete amorphization is obtainedonly by heavy ion irradiation suggests the existence of aspike process inside the displacement cascades as al-ready eviii°.need in other experiments [13-16] . How-ever, it this Al-rich compound, the deposited energydensity is low and a local heating until melting appearsdifficult to consitici . :t is important to stress thatirradiation is perfumed at a temperature near 40 K. Inthis range, a debility of free point defects can occur.Effectively, during He irradiation, point defects areproduced randomly essentially as single Frenkel's pairs .If the motility is high, their mutual recombination andthe radiation enhanced ordering are important effectspreventing defect accumulation and chemical disorder-ing to take place. On the other hand, during Kr ionirradiation most of the point defects are producedinside displacement cascades and in that case it iski,own that the great majority of defects are clustered;the moLihty � f dusters being lower, there is no radia-tion enhanced chemical ordering at the irradiation

C.Jaouen et al. / TEMstudy ofion induced ainorphl'zadon in,413Ti

temperature . Nevertheless, to conclude on this point, itwould be interesting to investigate in detail the role ofthe irradiation temperature .

Due to the lack of atomic size mismatch betweentire cwo species, an elastic instaoility during the antisitedefect generation is not c.tpected in A1 3Ti . Howeve -,the crystalline collapse towards an amorphous state isobserved under heavy ion irradiation . Moreover, thediffraction pattcins obtained at low -on fluences revealimportant elastic strains of the lattice . This c,Ȋ<z~aisthat tiie inset Lion of anti~irr :1Gfccts in the DO22 struc-ture introduces strong laical distortions and probablythe formation of defect complexes . These observationsa e a strong indication tl.rd both chemical disorder andpoint defects are involved? in the amorphization processof Al 3Ti.

The analysis of the diffraction patterns obtainedduring heavy ion irradiation has revealed the formationof the metastsble LS-: phase, th^,n of the related feedisordered structure just after the beginning of theamorphization proce,,s. One can expect that chemicaldisordering by breaking the strong directional bondingsbetween Al and Ti atoms introduces local distortionsand consequently destabilizes energetically the low-symmetry D0 22 structure. A similar behavior is ob-tained for the Ni 2 Al 3 alloy [171 but in this case, thecrystalline transformation (135, 3 ;o 132) occurs beforeamorphization. Here the transition 130222 to 1-1 2 re-quires important modifications of the lattice and it isworth noting that this is observed experimentally onlyafter the onset of the amorphization process . Such afeature suggests that amorphization provides a latticestrain relaxation favouring the transition. Such an in-terpretation is supported by various experimental ob-servations sugf;esting also the gradual strain relaxationresulting from the amorphization process . In Zr3 A],this was reflected by anomalies (abrupt changes) in thelattice dilatation and shear constant, appearing afterthe onset of amorphization [3] . In NiAl, the strainedmartensite phase produced at the beginning of heavyion irradiation reverts into the initial B2 structureabove a critical ion fluence corresponding to the for-mation of an importani amorphous fraction (- 60%).This also suggests that amorphization favours a rel-.x-ation of the crystalline lattice [18,19] . Similarly, duringthe implantation of tretalloid in metals, a sudden strainrelease, as indicated by both static Debye-Walter fac-tor and homogeneous strain data, was also observed asthe amorphization started [20,21].

Acknowledgement

The auth-r .like to thank A . François (LEM, ON-ERA) for providing the A1 3Ti alloys used in this study.

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IIc. METAL MODIFICATION (c)