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.
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.
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.TEM diffraction(not shown)resulted in diffuse rings,con?rming that the topographically featureless seg-ments in the re-melted lines were amorphous.
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-
http://m.wendangku.net/doc/8a739c36910ef12d2bf9e7bf.htmlpositional 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.