The effect of high energy concentration source irradiation onstructure and properties of Febased bul
Applied Surface Science 374(2016)359–364
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Applied Surface
Science
j o u r n a l h o m e p a g e :w w w.e l s e v i e r.c o m /l o c a t e /a p s u s
c
The effect of high energy concentration source irradiation on structure and properties of Fe-based bulk metallic glass
Wirginia Pilarczyk ?
Silesian University of Technology,Faculty of Mechanical Engineering,ul.Akademicka 2A,44-100Gliwice,Poland
a r t i c l e
i n f o
Article history:
Received 23June 2015Received in revised form 18November 2015
Accepted 17December 2015
Available online 18December 2015
Keywords:Nanoindenter
Surface topography Bulk metallic glasses Mechanical properties
a b s t r a c t
Metallic glasses exhibit metastable structure and maintain this relatively stable amorphous state within certain temperature range.High intensity laser beam was used for the surface irradiation of Fe–Co–B–Si–Nb bulk metallic glasses.The variable parameter was laser beam pulse energy.
For the analysis of structure and properties of bulk metallic glasses and their surface after laser remelt-ing the X-ray analysis,microscopic observation and test of mechanical properties were carried out.Examination of the nanostructure of amorphous materials obtained by high pressure copper mold cast-ing method and the irradiated with the use of TITAN 80-300HRTEM was carried out.Nanohardness and reduced Young‘s modulus of particular amorphous and amorphous-crystalline material zone of the laser beam were examined with the use of Hysitron TI950Triboindenter nanoindenter and with the use of Berkovich‘s indenter.
The XRD and microscopic analysis showed that the test material is amorphous in its structure before irradiation.Microstructure observation with electron transmission microscopy gave information about alloy crystallization in the irradiated process.Identi?cation of given crystal phases allows to determine the kind of crystal phases created in the ?rst place and also further changes of phase composition of alloy.The main value of the nanohardness of the surface prepared by laser beam has the order of magnitude similar to bulk metallic glasses formed by casting process irrespective of the laser beam energy used.Research results analysis showed that the area between parent material and fusion zone is character-ized by extraordinarily interesting structure which is and will be the subject of further analysis in the scope of bulk metallic glasses amorphous structure and high energy concentration source.
The main goal of this work is the results’presentation of structure and chosen properties of the selected bulk metallic glasses after casting process and after irradiation process employing the high energy concentration sources.
?2015Elsevier B.V.All rights reserved.
1.Introduction
Laser as a source of heat has been widely and successfully employed to modify the composition and microstructure of the near surface region of a component to improve corrosion,oxida-tion and wear resistance of commercial materials.However,only a few studies have utilized laser surface processing to develop amor-phous materials [1–3].
Bulk metallic glasses (BMGs)are potential material for wide application because of their unique physical,chemical and mechan-ical properties [4–6].However,its industrial application is not widespread because of limited size.From the analysis of obtained data it can be concluded that the production of this type of Fe-based
?Corresponding author.Tel.:+48322372021;fax:+48322372281.E-mail address:wirginia.pilarczyk@polsl.pl
BMGs in the form of coatings can be an interesting technological achievement [4,7,8].
Another way to increase BMG application for industrial scale is the use of laser welding to enlarge the size of cast elements [9–15].
On the basis of investigation into the relationship between amorphous materials and laser treatment,Mojaver et al.[16]con-?rmed previous test results.The workers con?rmed that in pulsed laser process,overlapping phenomena has affected the structure and composition of Fe 49Cr 18Mo 7B 16C 4Nb 3laser treated zone.A high overlapping factor [16]can restrict amorphization.Therefore,to gain an amorphous phase this factor should be keep as low as pos-sible.The interdependence between laser beam speed,laser power and amorphous surface was studied.It has been concluded that the range of laser variables in pulsed Nd:YAG laser melting to produce amorphous phase is narrow.What is more,it is not easy to optimize the parameters of a process.
https://www.sodocs.net/doc/3914421599.html,/10.1016/j.apsusc.2015.12.136
0169-4332/?2015Elsevier B.V.All rights reserved.
360W.Pilarczyk/Applied Surface Science374(2016)359–364
Zhang et al.also[2]also conducted research in the Fe-based system.They obtained surface with amorphous phase,small grains and dendritic structure after laser melting.In this experiment,Nb and Si elements show obvious positive effects in improving glass forming ability of surface.As it was expected,the mechanical prop-erties increased when the amount of amorphous phase has been increased[2].
Also Zhu et al.and Zhu et al.[4,8]carried out a number of test in the Fe system alloy.The study shows that Fe–Co–B–Si–C–Nb amor-phous composite surface was produced by high power diode laser cladding and the fraction of amorphous phase is about87%.The supercooled liquid region equals51K and reduced glass transition equals0.61.These values con?rm high thermal stability of amor-phous composite surface[4].Zhu et al.[8]has been investigating Fe-based amorphous composite coatings which were synthesized from low purity of raw materials by small amount of elements addition using laser cladding.In the opinion of the authors the mul-ticomponent addition is proved to have positive effect on the glass forming ability and mechanical properties and multicomponent addition method is an effective factor to develop amorphous com-posite coatings with low purity raw materials by laser cladding.
The effect of surface laser treatment on wetting behavior and mechanical properties of Cu–Zr–Al metallic glasses was investi-gated by Fornell et al.[1].They have con?rmed the previous test results.
The primary objective of this work is to present the test results related to the structural examination and investigation of proper-ties of iron-based bulk metallic glasses before and after a fusion process performed using a highly concentrated energy source.
The tests will serve as the basis for developing a procedure used while adjusting parameters applied in the laser welding of iron-based bulk metallic glasses.
The alloy composition of Fe37.44Co34.56B19.2Si4.8Nb4was selected to tests based on its glass forming ability(GFA),producing facility,mechanical properties and our previous studies.
2.Material and methods
Fe-based master alloy ingot with compositions of Fe37.44Co34.56B19.2Si4·8Nb4was prepared by induction melting of pure Fe,Co,B,Si,Nb elements in argon atmosphere.
The investigated material was cast in form of plate with thick-ness1mm.From the master alloy,plate samples were prepared by the pressure die casting method in an argon atmosphere.The mas-ter alloy was melted in a quartz crucible using an induction coil and pushed in a copper mould by applying an ejection pressure.
Melting tests were performed using Tru-Laser Station5004.The parameters of the laser beam were selected on the basis of multiple tests.
Glassy structure was examined by X-ray diffraction(XRD)using a Seifert—FPM XRD7diffractometer with Co K?radiation at35
kV.Fig.1.X-ray pattern of Fe-Co-B-Si-Nb bulk metallic glass in the form of an as-cast
plate.
Fig.2.BF image and the diffraction pattern of the selected area of the as-cast plate tested.
The data of diffraction lines were recorded by means of the stepwise method within the angular range of30?to90?.The counting time in the measuring point was3s.
The microscopic observation of the cross-section of plate and laser beam molten surface were carried out by means of the Zeiss Supra35scanning electron microscope equipped with a chemical composition analysis detector EDS by Oxford.
Because of the amorphous character of the welded material and small dimensions of particular weld zones,TITAN80-300 high-resolution transmission electron microscope(HRTEM)and Hysitron TI950Triboindenter nanoindenter with the use
of Fig.3.Con?rmation of the amorphous structure of the Fe–Co–B–Si–Nb as-cast plate(SEM,cross-section,(a)magn.180×,(b)magn.5.00k×).
W.Pilarczyk/Applied Surface Science374(2016)359–364
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https://www.sodocs.net/doc/3914421599.html,ser beam molten surface with marked zones.
Berkovich‘s indenter were used for structure and nanomechanical analysis.
Preparation of lamellae of heat affected zones and fusion zone for microscopic observation with the use of FIB Quanta3D200i were carried out.Focused ion beam(FIB)allows specimens to be cut and modi?ed during viewing.
3.Results and discussion
The work presented below assumes that a fusion process should be accompanied by the fastest possible discharge of supplied heat with the smallest possible volume of molten material.Such an assumption is dictated by the necessity of preventing the forma-tion of crystallization nuclei.For this reason,the surface of bulk metallic glass was exposed to radiation emitted by a pulsed laser. In this process,a laser generates laser beam impulses,each of which causes single spot fusion.A time interval between impulses should be adjusted so that each successive spot fusion could take place after the solidi?cation of metallic liquid molten in the previous fusion process.Excessively high impulse repetition frequency could be responsible for a situation where the limited ability of heat discharge by an element previously subjected to laser radiation could accumulate heat,which,in turn,could result in the forma-tion of a liquid material pool moving along the direction of fusion. Therefore,the primary objective of the tests involving laser beam-triggered fusion was the application of selected parameters of a single impulse,i.e.maximum laser beam power,impulse time and impulse shape in order to ensure the obtainment of proper fusion and to determine the structure of the weld,and particularly of the heat affected zone(HAZ),in further tests.
A specimen used in the tests was a plate of amorphous structure; the type of structure was con?rmed by related X-ray examina-tions and microscopic tests.Fig.1presents the X-ray pattern of the surface of an iron-based bulk metallic glass plate.
The curve presents a wide fuzzy diffraction line present in the area of50–55?2 .It is not possible to observe any other re?exes formed as a result of improperly performed pressure casting or due to the heterogeneity of the material processed at the?rst casting stage.The result of the X-ray examination reveals the formation of an amorphous structure in the plate.The con?rmation of the X-ray analysis result required more accurate tests,performed using a transmission electron microscope.Fig.2presents a bright?eld image(BF image)and the electron diffraction pattern of the selected area.The wide fuzzy ring is the electron diffraction pattern of
the Fig.5.Surface morphology of the as-cast plate(1)heat affected zone(2)and fusion zone(3).(Hysitron TI950Triboindenter;gradient,forward direction,reverse direction).
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Fig.6.BF image,selected area diffraction pattern,HRTEM image and FFT from the crystalline—amorphous zone (2)for the laser beam parameters used.
selected area;the diffraction pattern con?rmed the presence of the amorphous phase in the Fe–Co–B–Si–Nb as-cast plate.
Fig.3(a)presents the cross-section of the molten surface of the specimen.Fig.3(b)presents the magni?ed scanning electron microscope image (SEM image)of the area presented in Fig.3(a).The thickness of the material tested amounted to 1mm.The cross-section clearly presents smooth areas of the fracture.The magni?ed area reveals a smooth grey homogenous surface,which is also char-acteristic of materials in which an amorphous phase is present.
The material prepared as described before was molten using highly concentrated energy.The tests were performed using var-ious values of laser beam impulse energy.This study presents selected,repeated and representative results of microscopic obser-vations.Fig.4presents a laser beam molten surface with marked zones.
The parent metal,i.e.a cast plate,without visible surface mor-phology changes,is designated as zone 1(1).Zone 3(3)is bulk metallic glass after being molten using a pulsed laser.The area sur-face morphology after melting reveals 6overlapping laser impulses.In turn,zone 2(2)is the area only indirectly affected by the laser beam.Heat supplied to metallic glass during the laser beam-induced melting of the surface affects the plate surface leading to the formation and growth of crystallization nuclei.The heat affected zone area is the subject of numerous tests and obser-vations.In order to deeply analyse this area it was necessary to perform tests using a nanoindenter.The test results precisely
present the plate surface topography after casting (Fig.5(a)),heat affected zone topography after laser beam operation affecting the surface of metallic glass (Fig.5(b))and the topography of the fusion zone (Fig.5(c)).
The topography of the as-cast plate surface in the area close to the heat affected zone reveals an entirely smooth area without any signi?cant characteristic peaks.The presence of single nanometric peaks can be ascribed to the presence of impurities during casting or laser beam operation or to the presence of nanometric areas char-acterized by heterogeneity resulting from varied substance density.
In turn,zone 3reveals a slightly corrugate surface.The delicate surface texture is the result of the direct effect of laser beam radia-tion on the amorphous material and of the subsequent fast cooling of the iron-based molten metallic liquid.
The surface topography in the zone between the molten area and the parent metal reveals the presence of slight fusion-related thickened areas and characteristic peaks indicating the presence of crystallites.The peaks were generated along the lines of overlap-ping laser impulses and formed a clearly visible boundary between the molten zone and the parent metal (Fig.5).The presence of the amorphous-crystalline structure in this zone was con?rmed by HRTEM-aided observations.
Fig.5presents a clearly visible boundary where crystallization nuclei are formed and grow.Fig.6presents a bright ?eld image,the diffraction pattern of the area marked by means of a circle and described as SAD 1(Selected Area Diffraction),HRTEM image
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Fig.7.BF image,selected area diffraction pattern and HRTEM image from the amorphous fusion zone(3)for selected laser beam
parameters.
https://www.sodocs.net/doc/3914421599.html,ser beam molten amorphous surface,(a)1200W,2.7J and(b)1400W,3.09J.
and FFT(Fast Fourier Transformation).The microscopic observa-tions and the electron diffraction solution revealed the presence of crystalline phase precipitates in zone(2)—(Fe,Co)23B6.
Fig.7presents the test results related to the laser beam-induced fusion of the plate surface in zone(3).The adjusted laser beam parameters and high cooling rate provided conditions preventing the formation of crystallization nuclei.The images presented con-?rm the presence of the amorphous zone after laser fusion affecting the plate in zone(3).
The nanohardness of the plate after laser treatment is character-ized by the following values:4.47GPa(zone1),4.30GPa(zone2) and4.36GPa(zone3).In turn,the value of the reduced Young mod-ule is restricted within the range of90.10GPa(zone3)—91.60GPa (zone1).The value of E r depends on the chemical composition of an alloy and on the atomic order.The results of the nanohardness tests and of the reduced Young module of the Fe37.44Co34.56B19.2Si4.8Nb4 bulk metallic glass after its exposure to laser beam radiation cannot be compared with information presented in scienti?c publications. Available reference publications reveal that the effect of laser beam radiation on bulk metallic glasses of the chemical composition pre-sented in this work has not been investigated before.In addition, laser beam parameters as those used in the presented experiment have not been previously subjected to analysis.As a result,it was not possible to compare the tests results concerning the mechanical properties of the laser processed surface an amorphous material.
The crystallite(Fe,Co)23B6precipitates identi?ed in the amor-phous phase(using a high-resolution scanning transmission microscope)can strengthen zone2,which can lead to a slight increase in the value of nanohardness.However,it should be emphasized that the differences in the values are comparable.
In order to understand the essence of amorphous–crystalline phase formation when the surface of bulk metallic glass is affected by laser beam radiation,it is necessary to take into consid-eration the chemical composition of the material,the speci?c character of a melting process utilizing a pulsed laser and,in particular,laser beam parameters as well as the phenomenon of mass transport in the zone directly affected by the laser beam. Figs.3and8demonstrate that the specimen molten using a pulsed laser is supplied with energy of overlapping impulses.In rela-tion to this phenomenon,various zones of the molten surface of Fe37.44Co34.56B19.2Si4.8Nb4bulk metallic glass were exposed to various thermal cycles.Partially overlapping impulses can divide molten metallic liquid into zones,in which the liquid can undergo multiple remelting.A greater number of overlapping impulses translates to greater energy supplied to the surface of molten metallic liquid,thus extending its solidi?cation time.The primary condition of glassy structure formation consists in reaching the critical cooling rate,which can be achieved by adjusting appro-priate impulse repetition frequency.Increased impulse repetition frequency is responsible for the longer stabilization of the molten area.The extended solidi?cation time and the repeatability of phenomena trigger the primary segregations and precipitations in the zone exposed to higher temperature for the longest time.In order to ensure the formation of an amorphous structure on the surface of metallic glass after being processed by a laser beam,it is necessary to adjust impulse repetition frequency and impulse energy in a manner providing the molten liquid with suf?cient time for vitri?cation after being affected by a successive laser impulse.
The fusion of metallic glass surface using a laser beam is fre-quently accompanied by the formation of cracks.Fig.8(a)and(b) presents the area subjected to pulsed laser radiation.
The fusion parameters applied included the impulse peak power of1200W and1400W,the impulse energy of2.7J and3.09J,the impulse duration of3.33ms,the diameter of a laser beam on the material of0.3mm,the impulse repetition frequency of2Hz and the head travel rate of0.36mm/s,adequately.It was observed that the use of excessively high impulse peak power was accompa-nied by increased impulse energy supplied to the material,which generated spatters and burn-throughs in the laser beam-affected area.It was also observed that the metallic glass was highly sensi-tive to laser beam operation.This resulted in the development of cracks in the fusion zone and the propagation of the cracks to the
364W.Pilarczyk/Applied Surface Science374(2016)359–364
parent metal,which in some tests led to the defragmentation of the specimen tested.
4.Conclusions
The primary objective of this work was to present the test results related to the structural examination and the investigation of prop-erties of Fe37.44Co34.56B19.2Si4·8Nb4bulk metallic glass before and after a fusion process performed using a laser beam emitted in a pulsed mode
The article presents test results demonstrating the effect of laser beam impulse energy on the amorphous material.
Directly after being pressure cast into a copper mould,the mate-rial revealed the amorphous structure both on the specimen surface and on the cross-section of the plate tested.
The presence of the amorphous phase was con?rmed by related X-ray phase analysis and by microscopic observations.
The detailed topographic analysis of the surface of the laser-processed iron-based amorphous material revealed that the heat affected zone contained a fusion-related thickened area and peaks characteristic of the crystalline phase.The peaks were formed along the lines of overlapping laser impulses and constituted a clearly visible boundary between the fused zone and the parent metal.The presence of the amorphous-crystalline structure in this zone was also con?rmed by tests conducted using a high-resolution electron transmission microscope.In turn,in the fusion zone and in parent metal,it was possible to observe the slightly corrugated texture of the surface tested.
Different parameters of laser beam and chosen research results of strictly speci?ed parameters were analyzed.Two extreme exam-ples were presented,two different energy values to show the in?uence of laser beam parameters on bulk metallic glass to present the reaction of the amorphous material on high energy concentra-tion sources.
Laser beam impulse energy equal to3.09J was big enough to create burn-throughs and metal spattering.Whereas next remelt-ing made with the use of laser beam with energy equal to2.7J caused a small crystallization in HAZ,which can be observed in the presented pictures.
The main value of the nanohardness of the surface prepared by means of a laser beam has the order of magnitude similar to bulk metallic glasses formed by casting processes.In turn, the reduced Young module value is restricted in the range of 90.10GPa—91.60GPa and varies depending on the chemical com-position of an alloy and on the atomic order.
Acknowledgements
This project was funded by the National Science Centre allocated on the basis of the decision number DEC-2011/01/D/ST8/07327and by the Faculty of Mechanical Engineering of Silesian University of Technology statutory grants in2015.
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