Formation of Nanocrystalline Matrix Composite during Spray Forming of Al83La5Y5Ni5Co2

Formation of Nanocrystalline Matrix Composite during SprayForming of Al83La5Y5Ni5Co2

V.C. SRIVASTAVA, K.B. SURREDDI, V. UHLENWINKEL, A. SCHULZ, J. ECKERT,and H.-W. ZOCH

In the present investigation, a multicomponent glass-forming Al83Y5La5Ni5Co2 (at. pct) alloywas spray deposited on a copper substrate to produce an 8-mm-thick plate. The substrate was30-mm thick and heated to a temperature of 160 �C prior to spray deposition. The temperatureof the substrate and the deposit was measured during and after deposition. The deposits as wellas oversprayed powders were characterized in terms of the microstructural features by opticalmicroscopy and scanning electron microscopy (SEM). The phase constitution and transfor-mation were studied by X-ray diffraction (XRD) and differential scanning calorimetry (DSC).The oversprayed powder revealed different microstructural characteristics showing crystalline,partially crystalline, and fully featureless particles. The spray deposit showed large fraction offeatureless regions with embedded dendrites of 1- to 10-lm size intermetallic phases. Theseregions were observed to have a nanocrystalline structure with an average grain size ofapproximately 100 nm. The XRD analysis also revealed the nanocrystallinity in terms of a haloand peak broadening. These microstructural features have been attributed to the deposition ofundercooled liquid on a highly conductive copper substrate and rapid heat extraction from thedroplets due to proper metallic contact between the deposit and the substrate. These results havebeen discussed in light of processing conditions and the microstructural evolution of droplets inflight and during deposition.

DOI: 10.1007/s11661-008-9737-5� The Minerals, Metals & Materials Society and ASM International 2009

I. INTRODUCTION

RECENTLY, the synthesis of Al-RE-TM-basedamorphous or nanocrystalline bulk materials haveattracted unprecedented interest due to their highspecific strength along with improved physical proper-ties.[1–6] In general, the bulk amorphous or nanocrys-talline material is synthesized by consolidation ofpulverized melt-spun amorphous ribbons or gas-atom-ized amorphous powders in the size range of 10 to20 lm.[7,8] The amorphization of Al-based alloysinvolves high cooling rate due to their low glass-formingability compared to the other systems, e.g., Cu-, Zr-, andFe-based alloys. Therefore, the melt-spinning processand the high-pressure gas atomization are the mostpreferred routes for producing amorphous Al alloys.However, the large number of processing steps and thesmall size of the product materials involved in theseroutes do not make these routes economically viable for

commercialization. In addition, it is difficult to achievehigh yield in atomization processes due to the fact thatthe standard deviation of powder size is generally verylarge, and the fraction of large size particles crystallizebecause of their slow cooling rate. Furthermore, a smalloperating temperature window DTx, which is the differ-ence between crystallization (Tx) and glass transition(Tg) temperatures, during hot consolidation limitsproper control of the process and mostly leads to thecrystallization of amorphous phases.[9] Therefore, it hasbecome extremely important to look for some otheralternative routes that can give rise to bulk amorphous/nanocrystalline material in a single step.In one of their works in 1991, Oguchi et al.[10]

demonstrated the ‘‘supercooled liquid-quenching’’method to produce 7-mm-thick Al84Ni10Mm6 (at. pct)amorphous sheet. This was carried out by high-pressure gas atomization of liquid metal into a sprayand its subsequent deposition on a rotating drum toachieve high cooling rate similar to melt spinning. Themajor aspect of the microstructural evolution in thistechnique is the rapid heat extraction from the highlyundercooled droplets during deposition. In the similarline of spray atomization and deposition, Afonsoet al.[11,12] attempted to develop Al-Y-Ni-Co alloys byspray forming at a very high gas to melt flow rate ratioof 10 m3/kg and found a considerable fraction offeatureless areas in the spray deposit. Similarly, Guoet al.[13,14] used liquid nitrogen-cooled substrate toachieve rapid heat extraction during the deposition ofAl89La6Ni5 and Al85Nd5Ni10 alloy and observed a large

V.C. SRIVASTAVA, Scientist, National Metallurgical Laboratory,Jamshedpur-831007, India, is AvH Fellow, Institut fur Werkstofftech-nik, Universitat Bremen. V. UHLENWINKEL and A. SCHULZ,Scientists, and H.-W. ZOCH, Director, are with the Institut furWerkstofftechnik, Universitat Bremen, D-28359 Bremen, Germany.Contact e-mail: [email protected] K.B. SURREDDI, DoctoralStudent, is with the Institute for Complex Materials, IFW Dresden.J. ECKERT, Director, Institute for Complex Materials, IFW Dresden,D-01171 Dresden, Germany, is Professor, Institute of MaterialsScience, Technical University Dresden, D-01062 Dresden, Germany.

Manuscript submitted September 1, 2008.Article published online January 6, 2009

450—VOLUME 40A, FEBRUARY 2009 METALLURGICAL AND MATERIALS TRANSACTIONS A

fraction of glassy particle embedded in a crystallinematrix. These were referred to as hybrid composites.Similarly, Chang et al.[15] produced a 12-mm-thick bell-shaped deposit of Mg-Gd-Cu alloy by spray forming,which was partially amorphous. These reports indicatethe possibility of using spray forming process formaking bulk amorphous or nanocrystalline materials.In addition, the mechanism of microstructural evolutionduring spray forming of complex glass-forming alloys isnot yet understood, which may help to modify theparameters so as to achieve a large fraction of glassyphase. However, there are only a few reports, mentionedpreviously, on spray deposition of glass-forming alloys.The microstructural evolution mechanism during sprayforming of such alloys needs a better understanding so asto make it possible to design better experiments andachieve improved nanocrystallization/amorphization inthe processing of bulk materials.

Therefore, the present investigation is an attempt toproduce bulk plates containing nanocrystalline oramorphous phases of glass-forming composition ofAl-La-Y-Ni-Co alloy by spray forming. The processhas been designed so as to affect rapid heat extractionfrom the deposit before and after the deposition process.It is aimed to bring out a better understanding of thepossible phenomena of microstructural evolution ofdroplets during partial solidification in flight and thedevelopment of deposit microstructure in light ofdroplet solidification history.

II. EXPERIMENTAL DETAILS

A. Spray Forming

Spray forming of Al83Y5La5Ni5Co2 (at. pct) alloy wascarried out in one of the spray-forming plants (SK-2plant) of the Institute of Materials Science, University ofBremen.The details of the spray-forming setup can befound elsewhere.[16] The elemental materials weighing12 kg were melted together in an alumina crucible byinduction heating under high-purity argon atmosphere.The purity of materials was 99.9 pct Al, 99.6 pct Y,99.6 pct La, 99.9 pct Ni, and 99.9 pct Co. The melt waskept at a predetermined temperature for 30 minutes forhomogenization and subsequently poured into a tundishand atomized with nitrogen gas at a pressure of 0.5 MPausing a freefall nozzle. The gas flow rate and melt massflow rate were 1190 and 154 kg/h, respectively, which isequivalent to a gas to melt flow ratio (GMR) of 7.72.

A schematic of the deposition system for producingplates is shown in Figure 1, in mm. The spray ofatomized droplets was deposited on a copper substrate,the dimensions of which are shown in Figure 1, in mm.The spray cone was swivelled at a defined frequency andthe substrate was drawn, in the direction shown, as thedeposition progressed. The drawing speed of the sub-strate determines the desired thickness of the deposit.The temperature was measured using chromel-alumelthermocouple at different locations in the deposit (TD1to TD3) and substrate (TS1 to TS3) as shown(Figure 1). The thermocouples for deposits were kept

3 mm above the substrate surface, whereas the substratetemperature was measured 5 mm below the surface. Thesubstrate was resistance heated to 160 �C before thecommencement of the deposition process. Also,arrangements were made to cool the deposit with high-pressure nitrogen gas as soon as the deposited materialcame out from the spray cone. The deposit thusproduced was 8-mm thick and 250-mm wide. Thedeposition commences at the front of the substrateand ends at the back (Figure 1). Accordingly, thedeposit formed in the front, middle, and back of thesubstrate is denoted as front, middle, and back locationsin the deposit.

B. Materials Characterization

The microstructural characterization of the depositswas carried out both on optical microscopy andscanning electron microscopy (SEM). A dilute Kellersreagent was used as etchant to reveal microstructuralfeatures. The energy dispersive X-ray spectroscopy(EDAX) analysis was carried out to find out thecomposition of the matrix phase as well as the interme-tallic particles. The differential scanning calorimetry(DSC) analysis was carried out using a Perkin-Elmerdiamond calorimeter at a heating rate of 40 �C/minunder a continuous flow of purified argon.X-ray diffraction (XRD) measurements were done

on PHILIPS* PW 1050 diffractometer using Co-Ka

radiation (k = 1.7893 A) with 40 kV anodic voltage

Fig. 1—Schematic of substrate and deposit showing path of spraycone as substrate is drawn, direction of substrate movement, andthermocouple positions. Single spray cone scans the substrate as itmoves. TS (1 to 3) and TD (1 to 3) indicate position of thermocou-ples in substrate and deposit, respectively.

*PHILIPS is a trademark Philips Electronic Instruments, Mahwah,NJ.

METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 40A, FEBRUARY 2009—451

and 40 mA current. The average crystallite size wascalculated by using Scherrer formula[17] on measuredvalues of full width at half maxima (FWHM) for Al(111) reflections. Due to crystallize size, LaB6 was usedas a reference for estimating broadening.

The hardness measurements for different phases werecarried out on Vickers hardness tester (ShimadzuHMV2000, Shimadzu Europa GmbH, Germany) at aload of 0.25 N. A load of 1.96 N was used for measuringthe bulk hardness. The hardness values reported hereare the average of at least eight measurements.

III. RESULTS

In this section, the initial results obtained from thespray forming of Al-Y-La-Ni-Co alloy plates are pre-sented. Figure 2(a) shows the spray-deposited plate on athick copper substrate. The thickness of the deposit isalmost uniform throughout the substrate surface and isaround 8 mm. A closer view of the deposit and itsbonding with the substrate is shown in Figure 2(b). Thespray-deposited material was highly brittle and it wasfound that the deposited plate cracked at severallocations after cooling. The bulk composition of thedeposit is given in Table I.

A. Microstructure of Oversprayed Powders

Figures 3(a) through (c) show the SEM micrographsof oversprayed powder particles. Figure 3(a) shows acrystalline particle of 25-lm diameter, the surface ofwhich is irregular dendritic. In contrast, the smaller

particles of size less than 10 lm reveal smooth surfacesindicating featureless structure (Figures 3(b) and (c)).Several particles were observed to have evolved throughdroplet collision in liquid state, and also a number ofsmall satellite particles were seen on large size particles.The cross-sectional microstructural features of particleswere observed under optical microscope after metallo-graphic polishing and etching (Figure 4). Figure 4(a)shows the microstructure of particles less than 25 lm.This indicates that most of the particles are featureless(indicated by 3). However, some of them show contrast,indicating varying fraction of primary phase formation(as indicated by 1 and 2, see the contrast difference).Figures 4(b) and (c) show the microstructure of

particles in the size range of 50 to 75 lm. It is distinctthat similar size particles also have different solidifica-tion morphologies. In Figure 4(b), particle 4 showsalmost featureless morphology of two collided droplets,whereas particle 5 is fully crystalline with large fractionof secondary intermetallic phases. The contrast of thematrix of the two particles is to be noted with whitecontrast in fully crystalline particles, and gray contrastin featureless particles. Similar feature is revealed inFigure 4(c) where a large size particle is almost feature-less, whereas a smaller one is fully crystalline (Fig-ure 4(c)). The partially crystalline particles arecharacterized by a large featureless area with primarydendritic phases, which is shown in the figure using anarrow.

B. Microstructure of Deposit

Figure 5 shows the microstructure of spray-depositedplate. A low-magnification micrograph (Figure 5(a))consists of fine primary intermetallic particles of size 1to 10 lm distributed in a light gray matrix. Thedistribution of particles is not uniform and gives animpression of a layered structure. A little considerationwill indicate that a viscous flow of liquid was predom-inant before the solidification. This also reveals somepresolidified particles with large volume fraction of thesecond phase. The volume fraction of the second phasein the deposit seems to be small compared to theirvolume fraction in fully crystalline particles shown inFigures 4(b) and (c). There is a large fraction offeatureless area in the white gray contrast as matrix.The composition of these areas is similar to that used forthe base alloy, which will be discussed subsequently. Thelocations in black contrast are pores. Figure 5(b) showslarge fraction of featureless areas and distinct layeredstructure. This figure also reveals the presence of apresolidified particle, which seems to be embedded in thefeatureless area. The solidification of the particle seemsto have started from some place of nucleation and the

Fig. 2—Photograph of deposit and substrate (a) full view of depos-ited material and (b) closer view of interface.

Table I. Bulk Composition of Spray-Formed Plate Deposit

Al La Y Ni Co

At. Pct Wt Pct At. Pct Wt Pct At. Pct Wt Pct At. Pct Wt Pct At. Pct Wt Pct

82.10 57.30 5.67 20.45 4.67 10.75 5.40 8.20 2.16 3.30


dendritic growth took place in the direction away fromthe nucleation area. However, no prior particle bound-ary is observed, which indicates mixing or diffusion ofboundary after deposition.

Figure 5(c) clearly shows featureless areas with finedendritic debris particles in white contrast. Almostfeatureless (indicated by an arrow) and partially crys-talline particles are seen in the micrograph. It has alsobeen observed that the featureless areas have distinctflow lines, which is indicative of highly viscous fluid flowcondition before solidification. However, this feature isnot delineated in the figure at this magnificationdistinctively. A large picture size does show the flowlines. Figures 5(d) and (e) are similar to that ofFigure 5(c), however, fully featureless particles or largefeatureless areas are seen clearly. This indicates that theparticles were highly undercooled before deposition andthe nucleation of the primary phase was completelyavoided or only a partial growth took place. Figure 5(d)also shows a featureless irregular shaped particle, shownby an arrow, indicating highly viscous flow of under-cooled droplet prior to solidification. Figure 5(f) showsan interesting feature where a partially solidified dropletseems to have deposited on the growing plate deposit,and the rapid heat extraction from the droplet did not

allow further dendrite growth. This led to featurelesssolidification of the remaining liquid in the droplet. It isobvious from the micrograph, considering the volumefraction of second phase in the surrounding areas, thatthese are the featureless regions.It was observed in some regions that the area just

ahead of the primary phase dendritic growth front giveswhite contrast in optical micrographs compared to thegray contrast for those of featureless area (Figure 6(a),indicated by arrows). This is also indicative of therejection of Al at the solidification front of the primaryphase, and due to sluggish diffusion kinetics in under-cooled droplets, no further growth could be possible.The white contrast of the matrix observed in a fullycrystalline particle in Figure 6(b) corroborates theassumption of the Al-rich phase in the crystallinematrix. The black areas surrounding the crystallineparticles indicate a large fraction of fine second phaseparticles. This may be due to the rapid crystallizationand growth in the undercooled liquid under the effect ofthe deposition of the high-temperature crystalline par-ticle.The described results show that there is a large

fraction of featureless areas in the deposit as well asthe widely-spaced debris of primary dendrites. Fully

Fig. 3—SEM micrographs of oversprayed powders showing (a) large size particle with dendrites visible at the surface, (b) 12-lm droplet withcollided small particle and satellites, and (c) smooth surface morphology of particles in the size range 5 to 6 lm.


crystalline, partially crystalline, and featureless particlesare also seen. The fraction of dendrite debris is small inthe deposit when compared with the fraction observedin the fully crystalline oversprayed powder particles ordeposited crystalline particles.

C. SEM, XRD, and DSC Analyses

As the optical micrographs do not indicate thestructural features within the featureless areas andbetween the debris particles of primary dendritic phases,SEM was carried out to see the microstructural featuresof these regions at a higher magnification. Figure 7shows the SEM pictures of the deposit (the locationsindicated by numbers in Figures 7(a) and (c) are therepresentative places where compositional analysis hasbeen performed). Figure 7(a) shows a low-magnificationmicrograph delineating two different regions (P and F)where P is the presolidified particle and F is thefeatureless area. The higher magnification micrographof the area (indicated by a rectangle) in Figure 7(a) isshown in Figure 7(b). This indicates two zones withdifferent microstructural scales. The zone A reveals thefeatures around 100 to 200 nm, whereas zone B showsfeatures close to 50 to 100 nm. The regions between the

debris particles of primary dendrites are shown inFigure 7(c), indicating their nanostructured features.This feature also indicates the debris particles embeddedin a nanocrystalline matrix, which might have crystal-lized or would have been glassy and devitrified duringheat accumulation in the deposit.The higher magnification picture (Figure 7(d)) of the

matrix shows that these regions have nanostructuredfeatures within a size scale of less than 100 nm. Inaddition, some area fraction in the featureless zone alsois observed without any nanocrystalline structure. It isexpected that these areas had very fine structure whichcould not be resolved by SEM. Further analysis of thesefeatures is required to confirm whether these areamorphous regions or not. However, fine primary phaseparticles of less than 10 lm embedded in the depositwere fully featureless compared to the matrix, indicatinga crystalline single phase.The compositional analysis of different locations (as

indicated in Figures 7(a) and (c) by numbers) in thefeatureless regions was carried out and the results aregiven in Table II. It clearly shows that the featurelessareas contain large amount of alloying elements, i.e., 4to 5 pct La, 3 pct Y, 5 pct Ni, and 2 pct Co (at. pct).This is indicative of solidification from undercooled

Fig. 4—Optical micrographs of cross section of powders (a) less than 25-lm particle size showing fully crystalline (1), partially crystalline (2),and featureless particles (3), (b) in size range of 50 to 75 lm indicating almost featureless (4) and fully crystalline (5) microstructures, and (c)showing large featureless particle with a small primary dendrite embedded, as shown by arrow.


droplets without any considerable microscale phaseseparation. Due to small size of the primary phasedendrite fragments, their exact composition could not beascertained. However, it was found that they are La-richphase with composition close to Al11La3.

The XRD patterns of samples from front, middle,and back of the deposit are shown in Figure 8. Allthree patterns are similar, indicating the macroscalestructural homogeneity of the deposit. A halo spreading

between 2h = 32 to 48 deg is noted in all samplesalong with evident peak broadening. This is indicativeof nanocrystallinity in the deposited material. Thestrongest peak comes from the intermetallic Al11La3phase, which forms as the primary phase duringdroplet solidification in the spray. The size of coher-ently diffracting structural domains (crystallites) in Alwas found to be approximately 30 to 40 nm usingScherrer formula.[17]

Fig. 5—Optical micrographs of deposit showing (a) through (e) large fraction of featureless areas with distribution of primary intermetallic den-drite fragments, and (f) large particle with crystallized as well as featureless areas. The white circles in (a), (e), and (f) show representative areasof hardness measurement as mentioned in Section III–C. Arrows in (c) and (d) indicate nucleation around a presolidified particle and viscousflow of undercooled droplet prior to solidification, respectively.


The DSC analysis of the samples from middle andback of the deposit shows two predominant exothermicpeaks (Figure 9). The total heat evolved from the

transformations was 38.6 and 37.7 J/g for middle andback of the deposit, respectively. Although the peaktemperatures are similar for both samples, the heat

Fig. 6—Optical micrographs indicating (a) wide range of deposited particle microstructures (arrows show locations of primary phase formationand white contrast around it indicating solute lean areas) and (b) fully crystalline particle with large fraction of second phase in an Al matrix, inwhite contrast.

Fig. 7—SEM micrographs of deposit showing (a) featureless areas [F] and a fully crystalline presolidified particle [P], (b) high-magnificationmicrograph of the area marked in (a) indicating regions with two different (A and B) microstructural scales, (c) nanocrystalline matrix region,and (d) high-magnification picture of the area between dendrites. Numbers 1 through 5 in (a) and (c) indicate the location of compositional anal-ysis, which is summarized in Table II.


evolved in the first and second reactions differed. Theanalysis based on these data could not be done due tononavailability of a reference value. However, the highervalue of heat evolution in the first reaction is inagreement with the observation made by Louzguineand Inoue[18] on the amorphous Al85La4Y4Ni5Co2system.

The hardness values for the featureless areas(Figure 5(e)), the regions distributed with large primaryphase (Figure 5(a)), and areas with fine precipitates(Figure 5(f)), as indicated by white circles in figures,were found to be 4.2, 5.3, and 5.7 GPa, respectively. Thebulk hardness was 4.2 GPa. The hardness valueand their variation are similar to the variation reportedby Kamamura et al.,[23] for hot-pressed amorphousAl85Y8Ni5Co2 alloy with different phase constitution.It was reported that an increase in number and fractionof crystalline phases in amorphous matrix leads to anincrease in the hardness values.

D. Temperature Profile

The temperature profile of substrate and deposit isshown in Figure 10. In the figure, D1 and D3 are theprofiles for thermocouples TD1 and TD3, and S1 and S3are the profiles for thermocouples TS1 and TS3(Figure 1). The thermocouples for deposits are cooledunder the effect of a high-velocity gas jet, as the spraycone approaches the thermocouple. Therefore, thetemperature decreases in the initial stages and thenrecords an increase as the high-temperature droplets hitthe thermocouples. In contrast, the substrate experi-ences only a slight drop in the temperature due to gas

cooling. As the droplet deposition commences, thesubstrate temperature also starts increasing. The tem-perature of the deposit increases rapidly and thendecreases under the combined effect of convection andconduction through the substrate. Finally, the deposittemperature reaches the substrate temperature after theend of deposition process. The figure indicates that thefinal temperature of the deposit (D1 and S1) remainsaround 166 �C, whereas the temperature at D3 and S3comes close to 190 �C due to the accumulation ofincoming heat content to the substrate during theprocess and subsequent heating of the substrate. How-ever, the maximum temperature of the deposit could beseen to lie within 220 �C to 260 �C.In the subsequent section, the described results on

microstructural observations have been discussed inlight of the possible mechanisms of their evolution,taking into account the behavior of droplets in under-cooled state.

IV. DISCUSSION

The structural refinement of the aluminum-basedalloy system has been extensively studied, and the recent

Table II. Composition (Atomic Percent) of Featureless Areas

at Different Locations of Deposit, as Indicated in Figure 7

Location Al La Y Ni Co

1 85.70 4.18 3.07 5.09 1.962 85.15 4.76 3.02 5.06 2.003 85.15 4.88 3.08 4.88 2.014 85.08 4.80 3.24 4.94 1.945 85.52 3.95 3.07 5.34 2.12

30 40 50 60 70 80 90 100 110

Angle (2θ)

Inte

nsi

ty (

a.u

)

Front

Back

Middle

♦α-Al; + Al11La3; Al3Y; Al3Ni; ? unknown

♦♦ ♦ ♦ ♦

+

+

+ +

? ?? +

+ +

+ + +

Fig. 8—XRD patterns of samples from front, middle, and back partof the deposit.

250 300 350 400 450

Temperature (°C)

Hea

t fl

ow

Onset 327.6 °CPeak 342.5 °C∆H = -23.7 J/g

Onset 392.3 °CPeak 397.8 °C∆H = -14.9 J/g

Onset 328.6 °CPeak 343.1 °C∆H = -26.7 J/g

Onset 388.9 °CPeak 399.2 °C∆H = -11.0 J/g

Heating rate = 40 °C/min

Exo

Back

Middle

Fig. 9—DSC curves of samples from the middle and back part ofthe deposit.

100

140

180

220

260

300

0 100 200 300 400

Time (s)

Tem

per

atu

re (

°C)

TS1 (substrate-5 mm)TD1 (deposit)TD3 (deposit)TS3 (substrate-5mm)

Substratetemperature

S1

S3

D1D3

166 °C

190 °C

160 °C

Fig. 10—Temperature variation in deposit as well as substrate withtime. It is seen that a temperature of 160 �C to 200 �C remains for along time.


developments have focused upon achieving high densityof refined dispersoids in the matrix. Despite the presenceof several methods of producing ultrafine dispersoids,such as precipitation from supersaturated solution orsupersaturated intermediate phases, it has been realizedthat crystallization from an amorphous matrix does givehighest particle density.[19–21] It has been reported thatmulticomponent amorphous Al-RE-TM alloys show astrength of around 1000 MPa, however, partially devit-rified glass with high density of nanocrystalline Al leadsto an increase in strength to 1500 MPa.[22,23] It has beenreported by Senkov et al.[9] that consolidation of amor-phous powders of Al-Gd-Ni-Fe at 450 �C led to a grainsize of 700 nm and intermetallic particles showed anextensive plastic flow. However, the compressivestrength of semiamorphous material was 1030 MPa.

In view of the previous discussion, it seems that one ofthe most important factors in processing of these alloysis to obtain partially devitrified structure without largescale microstructural growth, in bulk. According to Daset al.[19] and Oguchi et al.,[10] this can be achieved bydisintegrating the liquid into small droplets and depos-iting them in undercooled state on a substrate so as toextract the heat from the droplet at a rapid rate. Thiskind of arrangement is possible by one of the mostversatile processes of spray forming. However, themicrostructural evolution during spray forming ofglass-forming alloys depends to a large extent uponthe size and distribution of droplets, thermal andsolidification condition of droplets prior to deposition,and the heat extraction rate from the deposit duringand after deposition. These factors are discussed inSection IV–A and IV–B.

A. Droplet Solidification

The spray-forming technique uses two distinct butintegral processes of liquid metal atomization into aspray of micron-sized droplets and its subsequentdeposition on a substrate. The spray of droplet gener-ated from disintegration of a liquid stream consists of awide size range of droplets ranging from 10 to 250 lm.These droplets travel away from the atomization zoneunder the influence of high-velocity gas jet and cools byforced convection. However, depending upon the size ofthe droplets, which in turn relates to specific surface areaof droplets, their cooling rate varies in the range of 103

to 105 K s�1. Therefore, a small size droplet experiencesa high cooling rate, whereas a large size droplet coolscomparatively slowly.[24–27] A high cooling rate ofdroplets engenders a high undercooling also. However,the maximum undercooling achieved for a given size ofdroplet also depends upon the purity of the melt and thecatalytic potency of heterogeneous nucleation sitesalready present in the droplet. Therefore, even a largesize droplet may experience a high undercooling inabsence of nucleation catalyst, compared to a relativelysmaller size droplet having potent nucleation sites.

Based on the analysis of Shukla et al.,[28] undercool-ing of droplets depends on the degree of volumeseparation of nucleants existing in the melt. A theoret-ical analysis of the possible number of nucleant-free

fraction (Fn) of droplets can be calculated using thefollowing relation:[29]

Fn ¼ exp � 1

6pNVd

3

� �

where Nv is the nucleant density in the bulk melt and dthe droplet diameter. The magnitude of nucleant densityhas been assumed to be of the order of 1012 m�3.[28]

However, the nucleant density depends upon factorssuch as the impurity content in the melt and theirpotential to act as heterogeneous nucleation sites. Basedon the this relation, the fraction of nucleant-freedroplets has been calculated and plotted against thedroplet size for different values of nucleant densities(Figure 11). It is obvious from the figure that thefraction of nucleant-free droplets strongly depends uponthe droplet size. Also, the fraction decreases withincrease in the nucleant density in the bulk melt. Thisanalysis indicates that a larger fraction of nucleant-freedroplets, or droplets solidifying homogeneously, can beachieved either by increasing the purity of the melt orreducing the mean droplet size. In other words, a largeundercooling in most of the droplets can be achieved inthis way. This is an analysis based on nucleant densitytaken from the literature,[30] but the plotted values maynot be true for the present study as the value of nucleantdensity in the present alloy in not known. It is importantto mention here that the mean particle size of theoversprayed powders in the present study was 90 lm.There can be surface nucleants or volume distributednucleants in the droplets.In general, surface nucleants come from interaction of

liquid droplets with gaseous environment in the form ofoxides or other reaction products, whereas the densityof volume-distributed nucleants depends upon the purityof the melt. The atomization technique does increase thefraction of nucleant-free droplets by distributing nucle-ants in small droplets, but it is difficult to control thesurface nucleation in the large processing systems.However, in the present study, only a small fraction ofparticles has been observed to have surface nucleated.In the present investigation, the oversprayed particles

show different microstructural features. A large fractionof smaller particles are featureless, whereas larger

0

0,2

0,4

0,6

0,8

1

0 40 80 120 160 200

Droplet diameter (µm)

Nu

clea

nt

free

fra

ctio

n

Nv=2x1012 m-3

Nv=4x1012 m-3

Nv=1x1012 m-3

Fig. 11—Plot showing variation of calculated values of nucleant-freefraction of droplets with droplet size.


particles mostly show a fully crystalline structure.However, it was also seen that even large size dropletsshow considerable fraction of featureless areas and somesmall size particles show fully crystalline structure.A closer analysis of these observations suggest that thismay be due to the fact that the potent heterogeneouscatalyst in the droplets, either small or large sizedroplets, leads to a small undercooling prior to thecrystallization. Nucleation in the droplets withoutpotent nucleants takes place homogeneously in a highlyundercooled state.[31] A highly undercooled droplet ofmulticomponent glass-forming Al alloy (as is the case inthe present investigation) possesses high viscosity andthe atomic diffusion, even in the liquid state, becomesvery sluggish.[1] In such a situation, even after com-mencement of crystallization of primary phase in thedroplet, sluggish diffusion and the simultaneous highcooling rate does not allow further growth and thedroplet solidifies in undercooled state with some regionsof primary crystals.

In the case of smaller droplets, the cooling ratebecomes so high that the droplets solidify in totallyundercooled state giving an amorphous structure. As thedeposits are formed as a result of layer by layerdeposition of these droplets and solid particles, it canbe concluded that the deposit would receive the drop-lets/particles in the following solidification conditions:small or large size crystalline solid particles, smallamorphous particles, partially crystalline droplets witha large volume of highly undercooled liquid, andpartially crystalline droplets along with the remainingliquid at a high temperature, as a result of recalescence.However, the fraction of high-temperature liquid in thelast case would be very small as most of the growthprocess is expected to be complete by the time thedroplets impact the deposition surface. This would notbe the case with undercooled droplets due to sluggishdiffusion.

The undercooled droplets, which do not come to thesubstrate, get enough time for large number of nucleiformation and growth during flight. This may lead totheir full crystallization. However, the rapid heatextraction from the undercooled droplets upon theimpact on the substrate does not allow full crystalliza-tion and growth. This observation indicates that even ifthe oversprayed particles have crystallinity the depositmay show nanocrystallinity or amorphocity.

B. Microstructure Evolution in Deposit

The microstructure of the deposit clearly shows alarge fraction of featureless regions along with adistribution of embedded dendrite fragments. A com-parison of the deposit microstructure with crystallineparticles in Figure 4(b) or 5(f) indicates that the volumefraction of the second phase in Figures 5(a) through 5(e)is considerably small. This would mean that thesefeatureless regions have high solute content, if thefeatureless regions are crystalline, as is known fromthe composition of the alloy used in this study. Thetotal amount of alloying elements in these areas reach15 at. pct and it is difficult that this amount of solute

goes into solid solution. The featureless areas may becrystalline with high solute content, nanocrystalline, oramorphous. This conclusion is based on the observa-tions made from the optical microscopic studies wheresolute segregation is not visible at microscale. However,considering the high excess free energy associated withthe compositional metastability, it can be concluded thatthere would be structural metastability for which theexcess free energy is less.[32,33] Therefore, it is expectedthat the matrix phase in Figure 5 would be eitheramorphous or nanocrystalline. The high-resolutionSEM did reveal the nanocrystalline structure with anaverage grain size of 100 nm. As the semisolid dropletswith undercooled liquid impact on the depositionsurface at a high velocity, they are fragmented and thedebris of crystalline dendrites of primary phase isdistributed in an undercooled liquid. The solidificationof the undercooled liquid in this condition would giverise to an amorphous or a nanocrystalline structure.In Figure 5 there are two regions in the microstruc-

ture: (1) fully featureless areas with gray contrast andwithout second phase and (2) areas in gray contrast withdistribution of primary dendrite fragments. However, ithas been confirmed from Figure 7(c) that even betweenthe dendrites the structure is nanocrystalline and isfeatureless at optical resolution. Therefore, it can beconcluded that the areas with gray contrast, even withlarge fraction of dendrite fragments, are nanocrystalline.This statement became necessary at this stage because alittle consideration would indicate that the areasbetween the dendrites would be crystalline if a normalsolidification would take place.The XRD analysis also confirms the nanocrystallinity

of the deposits. However, the crystallite size of 30 to40 nm estimated from the peak broadening does notmatch with the average crystallite size of approximately100 nm observed in the high-resolution micrographstaken by SEM. The presence of only two peaks in theDSC analysis is similar to that reported by Louzguineand Inoue[18] for Al85La4Y4Ni5Co2 alloy. This is indic-ative of the presence of amorphous phase fraction in thedeposit. However, the peak temperature for the first andthe second peak was reported by them to be 319 �C and379 �C compared to 342 �C and 398 �C, respectively, inthe present study. This difference may be due to anincrease of 1.0 at. pct in La and Y content in the presentcomposition. Although the heat of transformation couldnot be compared, the presence of the two peaksindicates that in the spray deposit full transformationcould be avoided due to rapid heat extraction.The microstructural evolution in the deposit therefore

shows that large fractions of featureless zones arepresent, and this is due to the fact that a rapid heatextraction from undercooled liquid takes place after itsdeposition on a thick copper substrate. The efficient heatextraction could be attributed to the heating of coppersubstrate prior to deposition so that a close contact ismaintained between the deposit and the substrate, and ahigh conductive heat transfer could be affected.[34] Thethickness of the substrate was kept large enough so thata high heat capacity of the substrate does not allowheating of the deposit but in turn act as an efficient heat


sink throughout the process. This is in contrast to thegenerally accepted view of cooling the substrate withwater or liquid nitrogen to affect high cooling rate of thedeposit.[13] It has been observed[35] that the cooling rateof droplets on a heated surface is an order of magnitudehigher than that on nonheated surface, which is due to aproper adhesion at the interface. The initial adhesion ofsplats on the substrate is a key factor for a betterconductive cooling, which is not observed duringdeposition on a cold surface.[36] The splats on a coldsubstrate loose contact with the substrate at the initialstages of deposition and the possibility of high conduc-tive cooling is lost, leading to heat accumulation in thedeposit. This is why a larger fraction of featureless areasare found in the present investigation with deposition ona heated substrate acting as heat sink, compared toother studies.[11–14]

It is expected that heat accumulation in the presentstudy will be small due to rapid heat extraction,however, further solidification of deposits may lead toits temperature increase. The temperature profile inFigure 10 reveals that the temperature of the depositreaches to a maximum of 250 �C, which is well belowthe reported crystallization temperature of around300 �C for such alloys.[18] However, it has to bementioned here that the deposit temperature was mea-sured at 3 mm from the substrate/deposit interface. Theactual temperature in the middle of the deposit may behigher than that shown in Figure 10. However, as thedeposit thickness is only 8 mm and is also cooled fromthe top by the forced convection under the effect of high-velocity gas, the possibility of large heat accumulation inthe deposit is small.

At this stage, it is also important to mention that itmight be possible that the nanocrystalline featurelessareas observed in the deposit were amorphous duringdeposition and could have been crystallized after slightheat accumulation in the deposit. Once the crystalliza-tion stops, there remains no possibility of furthernucleation and growth. Only an increased temperaturecould lead to this phenomenon. However, as thearrangements were made to cool the deposited materialjust after coming out from under the spray cone, a hightemperature exposure for a long time could be avoided.The nanocrystalline structure also indicates toward thepossibility of formation of large number of nuclei in theundercooled state and their sluggish growth in a highlyviscous state at low temperature. It is also a fact that ifthe featureless areas are solidified from the liquid,without large scale compositional segregation, then itscomposition would be close to the composition of theliquid used for spray forming. The compositionalanalysis in the present study also led to the sameconclusion, which indicates that the featureless areascould retain a large fraction of alloying elements. A littledeviation in the composition of the featureless areas tothat of the bulk deposit may be attributed to theformation of intermetallic phases, which are rich inother alloying elements.

The previous discussion indicates that the rapid heatextraction from undercooled droplets leads to nano-crystallization or amorphization. Therefore, it is

concluded that the spray forming can be a viableprocess to design bulk nanocrystalline/hybrid compos-ite materials with controlled process parameters, suchas a narrow droplet size distribution to avoid a largevariation in the undercooling experienced by droplets,proper heat extraction arrangement during and afterthe deposition, and using high-purity materials. Thecomplex interplay of process parameters in the sprayforming process necessitates optimization to achievelarge fraction of highly undercooled droplets and theirdeposition onto the substrate kept at an optimizeddistance, where most of the heat from spray is alreadyextracted during flight.

V. CONCLUSIONS

The spray forming of bulk Al85Y3La5Ni5Co2 (at. pct)alloy gives rise to a large fraction of featureless regionsin the spray deposit with the distribution of primarydendrite fragments of size ranging between 1 to 10 lm,making the deposit a composite of intermetallic particlesin nanocrystalline matrix with an average size of100 nm. These microstructural features are attributedto the rapid heat extraction through conduction duringdeposition of undercooled droplets. The major fractionof the featureless regions in the deposit is formed due tothe deposition of partially solidified droplets with theremaining large fraction of undercooled liquid. How-ever, oversprayed particles do not show the largefraction of featureless areas, which is attributed to thelonger flight time that leads to recalescence and heatevolution in flight. The XRD analysis indicated thepresence of nanocrystallinity in the deposit with acrystallite size of 30 to 40 nm.

ACKNOWLEDGMENTS

The authors thankfully acknowledge the financialsupport from the German Research Foundation(DFG) under SFB-Transfer Program 58 of the Univer-sitat Bremen. Dr. Srivastava thanks the Alexander vonHumboldt Foundation (Bonn, Germany) for grantinga fellowship to support his stay in Germany and theCouncil of Scientific and Industrial Research (India)for granting him a leave to pursue this research work.Additional support through DFG Grant No. EC 111/16and DAAD is gratefully acknowledged.

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Formation of Nanocrystalline Matrix Composite during Spray Forming of Al83La5Y5Ni5Co2

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