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2014 A combinatorial strategy for metallic glass design via laser deposition

A combinatorial strategy for metallic glass design via laser deposition Peter Tsai,Katharine M.Flores*

Department of Mechanical Engineering and Materials Science,Institute of Materials Science and Engineering,Washington University in St.Louis,

One Brookings Drive,Campus Box1185,St.Louis,MO63130,United States

a r t i c l e i n f o

Article history:

Received21May2014 Received in revised form

23July2014

Accepted24July2014

Available online23August2014

Keywords:

A.Metallic glasses

B.Alloy design

B.Glass forming ability

https://www.wendangku.net/doc/8912480335.html,ser processing and cladding

D.Microstructure a b s t r a c t

In this work,compositionally-graded Cu x Zr100àx specimens with x ranging61e76were fabricated by a direct laser additive manufacturing process.Topographically featureless surface regions,which suggest the possibility of a non-crystalline structure,were observed over the range62e67at%Cu using differ-ential interference contrast light microscopy.Electron diffraction and differential scanning calorimetry veri?ed that these regions were primarily amorphous.By varying the laser power and thereby the heating and cooling rate of the specimen,we show that the most stable glass-forming composition within the explored range is Cu64.7Zr35.3,in excellent agreement with the previously reported optimum composition of Cu64.5Zr35.5that was identi?ed by trial and error.

1.Introduction

Despite the attractive properties of metallic glass(MG)alloys, relatively few bulk-scale MG alloys(BMGs),which can be vitri?ed with dimensions of several millimeters or larger,have been iden-ti?ed.This limitation re?ects the lack of reliable criteria for pre-dicting good glass formers.For example,although the reduced glass transition temperature has been prevalently used as an indicator of high glass forming ability(GFA),its reliability is not universal[1,2], nor is it truly predictive of glass formation a priori since it requires knowledge of the glass transition temperature(T g).The same holds true for other parameters that are based on characteristic temper-atures[3e6].Recent structural models for MGs have identi?ed ef?cient packing conditions that are necessary,but not suf?cient, criteria for glass formation[7e10],and have primarily focused on binary and ternary systems,where determining cluster packing ef?ciency is a tractable problem.Without reliable predictive criteria,efforts to discover new,multicomponent BMGs still depend heavily on“trial and error”experimental methods.

Experimental evaluation of GFA is conventionally accomplished by casting discrete compositions,followed by application of diffraction techniques to verify amorphous structure.Because GFA is highly sensitive to chemical composition[11],a thorough investigation of multicomponent systems by casting,with each composition separated by no more than1at%,requires fabrication of an impractically large number of specimens.This shortcoming of conventional techniques has motivated the development of combinatorial approaches for BMG discovery and design[12e18]. For example,Ding et al.recently reported magnetron sputtering as a combinatorial tool for identifying chemical compositions with optimum GFA and thermoplastic formability in ternary alloy sys-tems[17,18].While the thin?lm libraries produced by sputtering techniques provide valuable information,notably through the ability to rapidly evaluate several properties using“lab on a chip”methods,the extreme quench rates achieved by such vapor depo-sition techniques result in atomic structures that are potentially quite different from those achieved by cooling from the liquid.

Alternatively,in this paper we demonstrate a novel,high-throughput combinatorial methodology for the identi?cation of BMG alloys by vitri?cation from the liquid,using a laser deposition technique.Previous studies have applied laser technologies to metallic glasses[19e27].Our group has also in prior studies depos-ited pre-alloyed powders of known Zr-,Cu-,and Fe-glass forming compositions[30,31].Continuous amorphous deposits were ach-ieved with varying degrees of a crystalline heat affected zone(HAZ) observed in the substrate.The extent of the HAZ was well-controlled by proper selection of the laser processing parameters.

In the present work,compositional libraries of the binary Cu e Zr system were fabricated using the Laser Engineered Net Shaping (LENS?)process.Glass-forming compositions were quickly identi?ed by observing the surface topography of the deposit with differential

*Corresponding author.Tel.:t131********.

E-mail addresses:ptsai23@https://www.wendangku.net/doc/8912480335.html,(P.Tsai),?oresk@https://www.wendangku.net/doc/8912480335.html,,?oresk@seas. https://www.wendangku.net/doc/8912480335.html,,km?ores72@https://www.wendangku.net/doc/8912480335.html,(K.M.

Flores).Contents lists available at ScienceDirect Intermetallics

journal homepage:www.elsev https://www.wendangku.net/doc/8912480335.html,/locat

e/intermet

https://www.wendangku.net/doc/8912480335.html,/10.1016/j.intermet.2014.07.017

Intermetallics55(2014)162e166

interference contrast(DIC)microscopy.By varying the laser process-ing parameters over a single deposited layer,the composition with the highest GFA was identi?ed.The results were consistent with previous “trial and error”experimental work in this system,demonstrating the viability of this technique.While the present work focuses on the Cu e Zr binary system for proof of concept,the technique is easily expandable to multicomponent systems.

2.Experimental methods

Compositionally-graded materials were fabricated via direct laser deposition using an Optomec MR-7LENS system.In the LENS process,pre-alloyed or elemental powders are added to a rapidly moving melt pool formed by a laser focused on the substrate sur-face.The substrate moves along a user de?ned path in three di-mensions,resulting in single or multilayer deposits with?exible geometries.Up to four individual powder feeders permit“on the ?y”changes in the deposited powder composition,enabling the use of LENS as a combinatorial tool for investigating compositional effects on structure and properties.

Copper and zirconium feedstock powders with minimum purities of99.9and99.2at%,respectively,were deposited on plates of grade 702Zr with approximate dimensions of3.5cm?4.0cm?0.5cm. Prior to deposition,the substrates were surface-ground with320grit SiC paper and cleaned with methanol to remove any surface con-taminants.To fabricate the graded Cu e Zr specimens,a square deposition geometry was chosen with dimensions of 25.4?25.4mm.Cu and Zr powders were simultaneously deposited with a250W laser traveling at8.5mm/s.To achieve compositional gradation,the delivery rate of both powders was varied in a stepwise manner from hatch to hatch.During deposition,the Cu delivery rate was gradually increased from3.51to4.88g/min while the Zr delivery rate was decreased from3.06to2.36g/min.After the initial deposit,a 200W laser with a travel speed of12.7mm/s was used to create9re-melted lines,spaced2.54mm apart,in the direction of the compo-sition gradient.The re-melting step facilitated adequate localized melting and mixing of the copper and zirconium powders.The alloyed lines were then re-melted a?nal time with different laser powers ranging from100to180W at a travel speed of16.9mm/s.

Preliminary identi?cation of potential glass-forming composi-tions in the compositionally-graded Cu e Zr specimens was accomplished using a Nikon light microscope equipped with DIC for enhanced imaging of the specimen's three-dimensional surface topography.This technique permits the easy differentiation be-tween crystalline regions,which appear rough,and primarily amorphous regions,which remain smooth when left unconstrained during cooling.It has been reported that DIC is capable of detecting asperities with diameters as small as40nm[28].

Following DIC imaging,the surface of the deposited specimen was polished to a mirror?nish with colloidal silica for further imaging in the scanning electron microscope(SEM),and compo-sitional measurements using energy dispersive x-ray spectroscopy (EDS).To improve accuracy,the EDS measurements were calibrated using three Cu e Zr specimens of known composition to correct for systematic errors.Finally,the amorphous structure of the material in the regions identi?ed by DIC was con?rmed by electron diffraction in the transmission electron microscope(TEM).The TEM sample was prepared by polishing and ion milling a strip of the topographically smooth region from the line re-melted by the 100W laser.An additional compositionally-graded specimen,also prepared using a?nal re-melt power of100W,was used to investigate the glass transition and crystallization behavior of the amorphous region with differential scanning calorimetry(DSC). The region was mechanically separated from the substrate and heated in the DSC at a rate of20K/s.3.Results

Fig.1provides the EDS compositional pro?le of the line re-melted by a100W laser.The pro?le was continuously graded from61e76at%Cu and is well-described by a linear trend line.The predicted compositional pro?le based on the powder delivery rates is also shown.The excellent agreement between the measured and predicted compositional trends con?rms our ability to accurately control the compositions of graded specimens by choosing appro-priate powder delivery rates.

Fig.2(a)shows a DIC image of a possible amorphous segment within the line re-melted by the100W laser.The surface of the segment was topographically smooth with very few identi?able surface features.SEM micrographs of the polished specimen ob-tained from the middle of the segment also appeared featureless (Fig.2(b)),as expected of the uniform microstructure of metallic glass.In contrast to the smooth segment,the uneven topography of the material to the immediate left of the segment clearly indicates the presence of crystals in this region(Fig.2(c)).An SEM micro-graph obtained near the end of the smooth segment displayed crystallites of an unidenti?ed phase embedded within a featureless matrix(Fig.2(d)).

Fig.3shows the DSC heating trace for the strip of the featureless material that was mechanically separated from the line re-melted by the100W laser.Although the glass transition was weak,the curve displayed a prominent exothermic crystallization peak,sug-gesting that the topographically featureless segments contained a signi?cant volume fraction of amorphous material.T g was755K, and the onset temperature of the crystallization peak(T x)was 780K,consistent with the T g and T x of the best reported glass former,Cu64.5Zr35.5[29].TEM diffraction(not shown)resulted in diffuse rings,con?rming that the topographically featureless seg-ments in the re-melted lines were amorphous.

4.Discussion

In addition to the capability to rapidly synthesize compositional libraries,a complete combinatorial methodology for identifying new metallic glass alloys must include a means for high-

https://www.wendangku.net/doc/8912480335.html,positional pro?le of a Cu e Zr deposit line re-melted using a laser power of 100W and a travel speed of16.9mm/s.The measured pro?le is compared with the ideal pro?le predicted by the powder feed rates,assuming perfect deposition ef?ciency of both the Cu and Zr powders.

P.Tsai,K.M.Flores/Intermetallics55(2014)162e166163

throughput identi ?cation of amorphous material within the library.X-ray and electron diffraction are the most dependable techniques for verifying amorphous structure.However,conventional x-ray diffraction techniques are limited to interrogating relatively large (millimeter-scale)areas,while micro-XRD and electron diffraction often requires several hours to prepare and analyze a single composition.Therefore,while diffraction results provide the ?nal determination of the structure,they are not ideal for rapid identi-?cation of metallic glass-forming regions in large compositional libraries.

Ding et al.reported a high-throughput approach to investigate glass stability in the Au e Cu e Si ternary system by fabricating compositional thin-?lm libraries via magnetron sputtering [17].In their experiments,the libraries were melted inside an inert chamber and cooled at a controlled rate.A distinct change was subsequently observed in the ?lms'surface contrast upon

nucleation and growth of the primary crystalline phases.Their work demonstrated the possibility of rapidly distinguishing amor-phous from crystalline material by inspection of surface topog-raphy.A smooth,featureless topography is only indicative of glass formation if the surface is unconstrained by a solid mold during processing.Thus,identi ?cation of glass in cast specimens by in-spection of the surface is not feasible because the surface of the specimen adopts the surface features of the mold.However,the surfaces of the re-melted lines in our laser-processed specimens were unconstrained during solidi ?cation,thereby allowing crys-talline features to develop as visibly distinct asperities.

Fig.2shows the transition from a smooth,featureless deposit surface on the right to a rough,crystalline surface on the left.Be-tween the two distinct topographies was a narrow semi-amorphous region spanning a composition range of less than 1at %(Fig.2(a)and (d)).The narrowness of this transition region re-?ects the acute sensitivity of GFA to compositional deviations in the binary Cu e Zr system [34].

An important parameter to consider during laser re-melting is the laser line energy l ,a parameter quantifying the instantaneous heat input per unit area by the laser to the target substrate material.It is de ?ned mathematically as l ?P /(vD )where P is the (absorbed)laser power,v is the laser travel speed,and D is the diameter of the laser beam on the surface of the substrate,assumed to be inde-pendent of laser power and set to 1mm in our study.In laser deposition experiments with Zr 58.5Cu 15.6Ni 12.8Al 10.3Nb 2.8powder,an excellent glass former,Sun and Flores observed that for a ?xed laser power,reducing l below a threshold value by increasing the travel speed nearly eliminated crystallization in the HAZ [30,31].Chen et al.carried out laser surface melting experiments with Cu 60Zr 30Ti 10substrates and reported the same effect of laser travel speed [32].Additionally,they demonstrated that for a ?xed travel speed,decreasing the laser power below a threshold value was necessary for vitri ?cation of the substrate's surface.These obser-vations of maximum laser power and minimum travel speed as criteria for vitri ?cation suggest that a maximum value of l exists for a given alloy composition (l max ),above which the thermal burden becomes too large to bypass crystallization during the cooling of the melt pool.

To investigate the effect of l on our graded specimen,nine evenly-spaced re-melted lines were made in the direction of

the

Fig.2.(a)DIC image of a topographically smooth segment of the compositionally graded Cu e Zr deposit and (b)the corresponding secondary electron SEM image of the micro-structure,from a line re-melted using a laser power of 100W and travel speed of 16.9mm/s.The Cu content increases from left to right.(c)DIC image of crystalline material adjacent to the smooth segment.(d)The SEM micrograph recorded from the semi-amorphous region shows crystallites embedded within a featureless matrix.

Fig.3.DSC heating trace of the topographically smooth segment mechanically sepa-rated from the line re-melted using a laser power of 100W and travel speed of 16.9mm/s.DSC data was obtained at a heating rate of 20K/min.

P.Tsai,K.M.Flores /Intermetallics 55(2014)162e 166

164

composition gradient,with laser power increasing from 100to 180W in steps of 10W.Fig.4shows a DIC image of the complete specimen.The observed amorphous segment in each re-melted line is enclosed by dashed lines.The width of the segments de-ceases with increasing laser power or,equivalently,increasing l .Although the endpoints of the amorphous segments could not be sharply de ?ned due to the presence of the semi-amorphous tran-sition regions,they were approximated from the DIC image as the positions where the topographically featureless matrix was no longer clearly visible.

The chemical composition at the two endpoints of each amor-phous segment was measured by EDS,and the results are plotted versus l in Fig.5.To achieve statistical precision,each point on the plot represents the average of ?ve measurements at the same posi-tion in the specimen.The upper and lower bound data are each well-?tted by linear trend lines.From the intersection of these two lines it may be deduced that the optimum glass forming composition in the graded specimen was Cu 64.7Zr 35.3,in excellent agreement with the composition that has been reported in literature,Cu 64.5Zr 35.5[29,33].We further deduce that the maximum laser line energy for this optimal composition,above which a glass is not formed,is 10.9J/mm 2.While this agreement is remarkable,it should be noted that our predicted optimal composition is subject to inherent errors associ-ated with the accuracy of the EDS measurement technique,as well as “operator error ”associated with identifying the transition from amorphous (featureless)to crystalline at each endpoint.We estimate that each of these errors contribute ±1at%or less to each measured endpoint and to the predicted optimal composition.If additional precision is required,a more gradual compositional gradient may make the amorphous/crystalline transition more easily discernable,reducing the contribution of “operator error ”.

The linear narrowing of the range of glassy compositions with increasing laser line energy towards the optimum glass former implies a direct relationship between GFA and l max .High GFA is re ?ected in an alloy's ability to carry a larger thermal burden without crystallizing.

This ability to isolate and identify the best glass formers within a composition library is a marked advantage of laser deposition over

other combinatorial techniques such as sputtering.While smooth composition pro ?les could be obtained by magnetron sputtering,the exceptionally high quench rate of sputtering,and the resulting ease with which an amorphous structure is formed over a broad composition range,makes it impossible to directly differentiate one composition from another with respect to GFA without relying on “predictive ”parameters such as T rg [12,16,17].5.Conclusion

We have developed a novel combinatorial methodology via laser deposition that can be used to identify metallic glass alloys with high https://www.wendangku.net/doc/8912480335.html,positionally-graded Cu e Zr specimens were fabricated and the glass-forming regions were rapidly identi ?ed by observing the surface topography of the re-melted deposits.Furthermore,by processing with different laser line energies,we were able to deduce the best glass former,Cu 64.7Zr 35.3.For future investigations,this methodology could be extended to multicomponent systems with the goal of discovering new metallic glass alloys with high GFA and other desirable properties.Acknowledgments

This work was funded by the Defense Threat Reduction Agency,award number HDTRA1-11-1-0047.Copper powder was provided by the Materials Preparation Center,Ames Laboratory,US DOE Basic Energy Sciences,Ames,IA,USA.References

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