thermal transitions in fe–ti–cr–c quaternary system used as precursor during laser in situ...

Materials Science and Engineering A 399 (2005) 318–325

Thermal transitions in Fe–Ti–Cr–C quaternary system used as precursorduring laser in situ carbide coating

Anshul Singha, Wallace D. Porterb, Narendra B. Dahotrea,c,∗a Department of Materials Science and Engineering, 326 Dougherty Hall, The University of Tennessee, 1512 Middle Drive, Knoxville, TN 37996, USA

b High Temperature of Materials Laboratory, Metals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USAc Materials Processing Group, Metals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA

Received 7 December 2004; accepted 2 April 2005

Abstract

The temperature range of thermal transitions within the quaternary system (Fe, Ti, Cr, and C) and the thermal stability of the evolved phaseswere studied with the help of differential scanning calorimetry (DSC). DSC studies indicated that the major exothermic reactions (formationof carbides) take place within 850–1150◦C. The evolved phases (TiC, M7C3, Fe–Cr, and Fe3C) were characterized using X-ray diffraction(XRD). This multicomponent powder mixture was used as a precursor for synthesizing a composite coating on the surface of steel via lasers evaluationo©

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urface engineering (LSE). The intended wear applications of the coating made thermal stability investigations vital. Experimentalf thermal stability of the phases formed was done.2005 Elsevier B.V. All rights reserved.

eywords:DSC; Carbides; Laser surface engineering; Precursor; Coating

. Introduction

The ductile fracture behavior, higher strength, highlastic modulus and thus better performance makes metalatrix composites (MMCs) attractive in comparison toetallic materials[1]. Carbide particles have been frequently

ncorporated within a metallic matrix by numerous processesith the aim of improving wear properties[2–6].The current study was initiated with the aim of forming an

n situ carbide composite coating on steel via laser surface en-ineering. The main impetus was towards forming ultra-fineultiple carbides (Ti- and Cr-based) within a ferrous matrix

o achieve a surface composite layer with superior tribolog-cal properties. In situ growth of fine carbide particles waschieved as a result of laser treatment. The microstructuralevelopments, phase evolution, the possible reasons for thehase evolution and enhancement in tribological propertiesue to carbide-phase formation have been already described

∗ Corresponding author. Tel.: +1 865 974 3609; fax: +1 865 974 4115.E-mail address:[email protected] (N.B. Dahotre).

elsewhere [7]. Furthermore, evolution of the primasolidified phases and their phase fields in this quate(Fe–Ti–Cr–C) system used as a precursor during lassitu coating have been computationally evaluated[8]. With acoating intended for wear applications, investigation of tmal stability of the phases evolved was important. Thisconsidered essential as phase evolution and its stabilityan important role in the wear characteristics of the coaA variation in evolved phases with laser processing po(heat input) at a constant scan speed was observed dX-ray diffraction (XRD) analysis. The effect of this behavon the hardness and wear properties has been reportedpast[7]. Hence, for process optimization and to tailor prerties according to application, the thermal transitions,temperature range for reactions (computationally evaluearlier[8]) needed to be experimentally investigated.

Differential scanning calorimetry (DSC) has beenlong established process for thermal investigations[9–13].Although the heating/cooling rates attained during LSEbe different than that are available for DSC runs, the prevstudies on laser in situ coating (with Fe–Ti–Cr–C precur

921-5093/$ – see front matter © 2005 Elsevier B.V. All rights reserved.oi:10.1016/j.msea.2005.04.009

A. Singh et al. / Materials Science and Engineering A 399 (2005) 318–325 319

involving XRD analysis of the laser-processed samples andcomputational evaluation with Scheil Gulliver assumptions(no diffusion in solid)[13,14] indicated the formation ofequilibrium phases (formed in equilibrium phase diagrams).This suggested that the range of laser processing parametersemployed in these efforts provided the thermal conditionsnot significantly different than the equilibrium conditions.It is, therefore, considered reasonable to conduct DSCstudies for identification and verification of phase transitionevents in the precursor powder mixture and correlatethem with the events occurred during LSE. The visionbehind investigations using DSC is that an insight into thetemperature range of various carbide forming reactions canbe achieved. Furthermore, the temperature ranges of otherthermal events during heating and cooling of the precursorpowder mixture, if any, can be captured. With any changein the heat capacity of the specimen, heat of transformationfor the reactions could be investigated. It is envisioned thatthis set of information would help in developing a thoroughunderstanding of the nature of phase transition in the precur-sor powder mixture (with a composition same as that usedas the coating precursor during laser processing) withoutconsulting ternary or higher order phase diagram and helpin tailoring properties according to the desired application.

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ersw ng onA bleF vinga gep C,M eri-m t.%C pre-c wderm ffer-e d ast derm per-i atinga pres-e , asb e seto pow-da ateda

weref werep lurryw amec anicb ited.

A 2.5 kW Hobart continuous wave Nd:YAG laser equippedwith a fiber optic beam delivery system was employed forlaser treatment. The laser scan speed was 150 cm/min and thelaser beam power used for processing was 1500 W. The lenseswithin the output-coupling module of the fiber optic deliverysystem were configured to provide a 3 mm× 600�m rect-angular beam in spatial distribution onto the sample surface.The focal distance of the lenses was 132 mm and argon wasused as cover gas. The detailed procedure for laser processingand the resulting coating has been described elsewhere[7].The resulting coating was shaved off the processed sampleusing a Buehler Isomet low speed saw. A part of this coatingwas used for DSC runs and heated at 40◦C/min from 25 to1525◦C. The 40◦C/min rate was chosen, as it was the limit ofcontrolled cooling rate of the DSC machine. The highest rateavailable with the DSC was chosen because the conditionsduring laser processing provide rapid heating rates.

XRD analysis was conducted for post-DSC run powdersamples on a Philips Analytical B.V. instrument to identifyand characterize the various phases evolved during DSC runs.The diffraction studies were done between 20 and 90◦. Sincethe amount of powder used in DSC runs was very small andfour different elements involved, the XRD runs resulted inspectra with a lot of noisy background. In an attempt to un-ravel this situation, two steps were taken. Firstly, the con-tinuous scans were done at a very slow rate with a step sizeo iablea g thepC den-t g fora ible.I wasl Vac-u lledw eds umo tem-p ns at8 Cr edo raredf

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. Experimental procedure

Since a mixture of elemental Fe, Ti, Cr, and C powdas used as a precursor during laser in situ carbide coatiISI 1010 steel, again a mixture of commercially availae (99.9% pure), Ti (99.5% pure), C (99.5% pure) (all haverage particle size <45�m), and Cr (99.8% pure, averaarticle size <10�m or less) powders, supplied by CERAilwaukee, WI was also used for the present DSC expents. The powder mixture contained 20 wt.% Ti, 20 wr, 5 wt.% C, and rest Fe (same composition used as aursor in laser coating). DSC runs were done on the poixture in a Netzsch 404 (Paoli, PA) high temperature dintial scanning calorimeter (HTDSC). Sapphire was use

he reference material. The weighed quantity of the powixture was heated in an alumina crucible. Prior to the ex

ments, the DSC cell was purged with argon and the hend cooling were done under constant argon flow. Thence of Ti and Fe made purging with argon imperativeoth of them are highly susceptible to oxidation. The onf runs (heating and cooling) was done on the elementaler mixture heated at a rate of 40◦C/min from 25 to 1175◦Cnd then back to 25◦C. The same sample then was rehet the same heating rate to the same temperature.

The other set of samples used in DSC experimentsrom the laser in situ coated coupons. These samplesrepared as follows. Prior to laser processing, thick sas formed by blending together elemental powders (somposition) and suspending it in a water-soluble orginder. The slurry of precursor material was spray depos

f 0.02 and the per step time of 4 s. Secondly, apprecmount of sample, was produced by heating and coolinrecursor powder mixture (20 wt.% Ti + 20 wt.% Cr + 5 wt.%+ rest Fe) in an infrared furnace under the conditions i

ical to that observed during DSC runs. Furnace heatinll powders was carried out in an alumina–zirconia cruc

nto the infrared furnace, 15 g of powder in a crucibleoaded and a thermocouple was placed in the powder.um was pulled to 200 mTorr and the furnace was backfiith Ar to −5 psi. The backfilling procedure was performuccessively for three times followed by creation of vacuf 200 mTorr. The powders were heated at four differenteratures identified by the thermal events during DSC ru50, 1025, 1075, and 1175◦C with the same rate as in DSuns (40◦C/min). Thereafter, XRD analysis was performn all the samples for the phases evolved during these inf

urnace runs.

. Experimental results

.1. DSC analysis

The events observed during DSC run have been presn Fig. 1. Some small fluctuations were seen in the heaurve at lower temperatures (450–600◦C) (marked as ellips). These fluctuations could either be due to particleangement or surface degassing. These fluctuations ocery low temperatures and are not major events. Henould be assumed that these do not affect the proces

320 A. Singh et al. / Materials Science and Engineering A 399 (2005) 318–325

Fig. 1. DSC curves for heating and cooling of the precursor powder mixtureat the rate of 40◦C/min in argon atmosphere.

endothermic event was observed in the temperature range of650–800◦C. This event could possibly be a (�-Fe)→ (�-Fe)transition. In parallel efforts involving DSC runs for pure Fesuch endothermic event also occurred, however, it occurred ata higher temperature. This supported the earlier observationthat the endothermic event is the ferrite to austenite transitionof Fe. The range of temperatures on which this transforma-tion was observed, however, was much lower than the con-ventional transition temperature. This could be attributed tothe presence of other elements (20 wt.% Ti, 20 wt.% Cr, and5 wt.% C) within the powder mixture. This can further pos-sibly confirmed by a high temperature in situ XRD analysisof the precursor powder sample, which has been the ongoingeffort and will be presented separately. At higher tempera-tures (890–1150◦C), twin exothermic events were observed(marked as ellipse 2 inFig. 1). The presence of these exother-mic peaks may be attributed to the carbide forming reactions.

There were no events observed while cooling. This indi-cated that the entire powder has already been converted intoirreversible reaction product by 1175◦C without leaving be-hind any elemental Fe. Results for re-run (second run forpreviously run powder under similar conditions) have beenpresented inFig. 2. Again neither the heating nor the coolingcycle indicated any events till 1175◦C. Even the minor fluctu-ations corresponding to endothermic and exothermic eventswere absent for the powder mixture during this run. The tem-p firstr -v ic ore thei tionp uires eres thism 025,1 nsa

Fig. 2. DSC heating and cooling curves for re-run of Fe, Ti, Cr, and C powdermixture.

The DSC plots can be interpreted either by varying thesample parameters (like mass, composition) or by variationof operating parameters (rate of heating and atmosphere used)[15]. As the heating rate and the atmosphere used were keptconstant in the current case, the composition of the powdermixture was changed to study the evolution of thermal tran-sition events. The effect of absence of Fe in the mixture wasinvestigated. DSC runs on a powder mixture of elementalTi, Cr, and C in the same proportion as the original mixture(44.4 wt.% Ti + 44.4 wt.% Cr + 11.1 wt.% C, i.e.∼4:4:1) wasconducted and the corresponding plot is shown inFig. 3. TheDSC runs were again done at 40◦C/min in argon atmosphere.Exothermic events similar to the ones present for the coat-ing precursor powder mixture (Fig. 1) were observed. How-ever, no endothermic transition was observed. This confirmedthat the endothermic event in the precursor powder mixture(Fig. 1) was due to the presence of Fe and is (�-Fe)→ (�-Fe) transition. Another noteworthy point to be observed was

F re at4

erature ranges in which the events took place during theun have been marked inFig. 2. Although the DSC plot proided the temperature range and the nature (endothermxothermic) of thermal transitions, they did not provide

nformation about the evolution of types of phases (reacroduct) during these thermal transitions. In order to acquch information, further detailed DSC investigations wupplemented with XRD analysis. For the purpose ofulti-technique analysis, specific temperatures (850, 1075, and 1175◦C) corresponding to the thermal transitiore identified and marked inFigs. 1 and 2.

ig. 3. DSC heating and cooling curves for Ti, Cr, and C powder mixtu0◦C/min in argon atmosphere.


the temperature at which the exothermic reaction took place(1350–1500◦C inFig. 3). This was much higher than the tem-perature range of reactions (890–1150◦C in Fig. 1) for thecoating precursor powder mixture (Fe, Ti, Cr, and C). Hence,it could be interpreted from the DSC curves that the presenceof Fe in the precursor mixture lowered the temperature forthe exothermic events (possibly carbide forming reactions).

3.2. XRD analysis

As mentioned earlier, the information from DSC analy-sis can be coupled with the results obtained by other struc-ture sensitive analytical technique such as X-ray diffractionfor complete and thorough understanding of the occurringevents[15]. Since no event was observed during cooling, itwas reasonable to assume that the phases evolved at high tem-perature during heating cycle of DSC run remained present

at room temperature. These phases can be identified usingXRD technique. XRD analysis of the DSC run samples hadstrong background signals, which made analysis susceptibleto errors. The strong background signals could be attributedto the presence of backing material on which in the smallamount of powder mixture (∼78 mg) was mounted. With theaid of an appreciable amount of powder (produced by heatingthe mixture of powder in the infrared furnace as described inSection2) along with a very slow X-ray scan, the backgroundsignal was minimized. The samples produced by heating thecoating precursor powder in the infrared furnace at the rate(40◦C/min) and the temperatures corresponding to the ther-mal transition events (Figs. 1 and 2) in DSC experiments. TheXRD spectra for all conditions have been presented togetherin Fig. 4. The XRD spectra revealed that no major reactionstook place till 850◦C (Fig. 4a) and most of the peaks (asmarked) are un-reacted powders in elemental state. The sam-

Fig. 4. XRD spectra for furnace heated samples: (a
) 850◦C, (b) 1025◦C, (c) 1075◦C, and (d) 1175◦C.


Fig. 4. (Continued).

ple heated to 1025◦C (Fig. 4b) indicated that carbide formingreactions had started to take place, but sufficient amount ofcarbides had not formed till this temperature. This confirmedanother fact that the infrared furnace heated samples (heatedunder identical conditions) followed the events of DSC plots.

Fe and Cr both have a bcc crystal structure and have nearequal atomic radii also (2.48 and 2.49A) and their charac-teristic peaks almost overlap each other. The powder mixtureheated to 1075◦C revealed the presence of various carbidesand the spectrum (Fig. 4c) indicated that the majority of thereactions had started to take place. The major phases presentwere TiC, Fe–Cr, M7C3 (M = Fe and Cr), Fe3C, and somegraphite. The powder mixture heated to 1175◦C (Fig. 4d) in-dicated the presence of similar phases (as that for 1050◦C).The DSC plots revealed that all the exothermic events (re-actions) were completed by this temperature. As no eventswere seen during cooling the same coating precursor mixture,it appeared that the phase transitions were irreversible. The

evolution of these phases was also found in agreement withthe phases predicted by Scheil Gulliver solidification model[8], which indicated that the terminal solidification tempera-ture as 1287◦C.

4. Discussion

Some key interpretations could be made from the resultsin the previous section. From the observations of the re-runof the mixture of Fe, Ti, Cr, and C (Fig. 2) and the XRDpattern (Fig. 4a) for the sample heated till 850◦C, it can beconcluded that the endothermic fluctuations in the DSC plotat lower temperatures (450–600◦C) did not represent anyreactions and was probably particle rearrangement or sur-face degassing. It can be inferred fromFigs. 1 and 3that theendothermic event at high temperature range (650–800◦C)was due to the elemental iron in the powder mixture. Since


there was no other reaction (event) observed till this temper-ature, it could be assumed that the powders had not reacteduntil this temperature. Once the powders reacted, to formcertain phases after the completion of the heating cycle, areverse transition was not observed during the cooling cycle(Fig. 1) or during the re-run (Fig. 2). This substantiates thefact that majority reactions had taken place and neither el-emental un-reacted powder was left nor the existing phaseswere reversible.

The twin exothermic events observed at higher tem-peratures (>850◦C) represented carbide forming events(Figs. 1 and 4) because the exothermic peaks observed inFig. 3 were due to the reaction between the elements, Ti,Cr, and C in the coating precursor powder without the pres-ence of any Fe. The amount of energy released as a re-sult of the reaction could be gauged from the general equa-tion for the formation of titanium carbide, Ti + C→ TiC(�G=−186,606 + 13.22T J/mol)[16]. The formation of thecarbides could be attributed to the strong carbide forming ten-dency of the elements. The Fe and Cr powders form a solidsolution Fe–Cr (bcc) with the lattice parameter of 2.88A andhence the Cr diffraction peaks (Fig. 4a and b) were no longervisible in the XRD spectra at higher temperatures (Fig. 4c andd). The key fact to note from the XRD results was that all theother phases, with the exception of Fe3C and graphite, hadalso been seen earlier in the XRD spectra for laser-processeds pre-c r thel ffortst thosei elyd riump ase,t ntiala n thel ropri-a velopa s andc

r el-e s ex-p f thec bidest ilitya ta mix-t werst 0( at-t e ofF

thets -a -p s

obtained during experiments, it was difficult to identify theexact temperature of the reactions due to partial overlap ofthe exothermic peaks. The overlap in the peaks also preventedDSC plots directly using for evaluating thermodynamic po-tential function (heat of reaction). In view of these constraints,de-smearing of the overlapping peaks was required. The ad-vanced peak separation software (NETZSCH peak separa-tion) was used to evaluate the data. By using interpretationsfrom DSC results and XRD analysis, it was possible to rep-resent the DSC plot as a superposition of peaks. The baselinewas made horizontal by subtracting the DSC profile fromthe plot for re-run sample (subtractingFig. 2 from Fig. 1).The Fraser–Suzuki profile was used for separation of peaks.It basically represents an asymmetric gauss profile, i.e. forasymmetry equal to zero; it corresponds to a gaussian profile[18]. The equations used for the calculation of signal and areaunder the peaks (for Fraser–Suzuki profile) is given by[18]:

yFraser= Ampl exp

×[− ln 2

[ln

{(1 + 2 Asym(x − Pos)/Hwd)

Asym

}]2]

AFraser= 0.5

√Π

ln 2Ampl Hwd exp

[Asym2

4 ln 2

]

w au eaks( halfw peakp

eb runs( Ther d- ectiono Thet

F rofilef

amples[7] with the coating synthesized using the sameursor powder mixture. This ascertained the fact that foaser processing parameters employed in the present ehe thermal transition events generated were same asdentified during DSC runs. The laser coating is extremynamic process and not necessarily purely non-equilibrocess. It is often[8] and also as observed in the present c

he analyses such as XRD indicated formation of substamount and number of near and/or equilibrium phases i

aser-processed region. It is, therefore, considered appte to run DSC at highest available scanning speed to dethorough understanding of the phase transition event

orrelate them with the events during laser processing.During laser processing, iron was used as the majo

ment in the coating precursor. The presence of iron iected to enhance the chemical compatibility (bonding) ooatings with 1010 steel substrate. Furthermore, the carhat are in situ grown in the coating have good compatibnd wetting characteristic with Fe[17]. The other significandvantage of using Fe in the coating precursor powder

ure deduced from this study was that its presence lohe temperature of the exothermic events by almost 30◦CFigs. 1 and 3). The reason for such a behavior could beributed to change in activity of Ti and Cr in the presence.

One of the main aims of the study was to understandemperature range of reactions. The XRD spectra (Fig. 4)uggested that no reaction took place till 850◦C; some rections had begun by 1025◦C while the reactions are comlete between 1050 and1175◦C. But from the DSC plot

,

hereyFraserrepresents the signal andAFraserrepresents arender the separated peaks. Ampl the amplitude of the pdifference between the baseline and curve), Hwd theidth of the peaks, Asym the asymmetry, and Pos theosition.

An evaluation range of 850–1150◦C was chosen on thasis of XRD analysis and as no events during DSCreactions) took place at temperatures lower than this.esults of calculation have been presented inFig. 5. Sep-1 an2 represent the first and second separated peaks. The sf original DSC plot has also been shown in the figure.

ime and temperature scale were plotted on theX-axis while

ig. 5. Fitting with two peaks of Fraser–Suzuki (asymmetric gauss) por DSC twin exothermic event.


Table 1Evaluation of separated peaks obtained by de-smearing of twin exothermicDSC events

Peak Position (◦C) Area (J/g) Onset (◦C) Endset (◦C)

Sep-1 1003.48 169.30 889.82 1129.77Sep-2 1073.77 60.56 1049.93 1104.52

the calibrated DSC signal (heat flow, mW/mg) was chosenas theY-axis. The details of area under the peaks, onset andendset temperatures have been presented inTable 1. The peakpositions were found to be 1003 and 1073◦C. The onset andendset temperatures of these peaks were 889 and 1049◦C;1129 and 1104◦C, respectively. The events thus associatedwith the first peak (Sep-1), took place over a longer range oftemperature.

The temperature values match well with the results ob-tained by XRD analysis. The XRD analysis suggested thatno reactions were observed till 850◦C. Also, no TiC for-mation was observed till 1025◦C although some M7C3 wasfound to occur at this temperature. The next set of experimen-tal results, XRD spectra for 1075 and 1175◦C, suggested theformation of both TiC and M7C3 (Fig. 4c and d). Thus, itcould be interpreted that the wide peak Sep-1 represented theinitiation of formation of the M7C3 phase while the narrowpeak (Sep-2) represented the initiation of formation of TiC.The exact range of M7C3 cannot be marked as there are otherevolved phases also (Fe3C and Fe–Cr). But, it could be rea-sonably interpreted that for the current system, TiC forms inthe temperature range of 1050–1104◦C. In any case, it wasdeduced that the range of exothermic reaction was within889–1130◦C.

The re-run of the samples (Fig. 2) was done to investigatethe stability of the reaction products. The thermal stabilityo d forw bilityi waso ouldb table( par-t ted.S eredt tings[

ure,t ex-p at-i ls sr ), thet ty ofp ing theh to bes tionsc suchl 7

Fig. 6. DSC runs for laser-processed coating (processed at 1.5 kW) at40◦C/min in argon atmosphere.

[8]. Experimental results for the laser-processed sample in-dicate much higher thermal stability (1477–1527◦C).

5. Conclusion

The information obtained from the present study is thusimportant for two reasons. Firstly, it can be used to tailor theprocess in terms of processing power and laser scan velocityto regulate the heat input and thus achieve required thermalfields to produce respective phases. Secondly, the coatingsynthesized for wear applications was found stable till ex-tremely high temperatures. Presence of Fe in the precursorpowder mixture reduced the temperature range of exothermicevents. The exothermic reactions were proven to be carbideforming reactions. The general temperature range over whichreactions take place was found to be 850–1150◦C. With theaid of experimentally deduced results and mathematical mod-els[8], the precise temperature range for exothermic reactionswas 889–1130◦C with the first exothermic event occurringover a wide range of temperatures in comparison to the sec-ond event. The phases evolved during furnace heating of thepowder mixture were TiC, M7C3, Fe–Cr, Fe3C, and graphite.The major phases correspond with the phases present in thespectra for laser-processed plain C steel samples. Second runon the powder mixture indicated no events thus proving thet olvedw allys

A

nal-y ergyE ARa tureM nalL .S.

f the carbides was a vital issue for coatings dedicateear resistant applications. This made the thermal sta

nvestigations via the re-run imperative. Since no eventbserved for during the re-run for the given system, it ce concluded that the evolved phases were thermally sirreversible) till this temperature. Also, since the carbideicles grew in situ, better bonding with the matrix is expecuch improved carbide particle matrix bonding is consid

o improve wear resistance of the laser-processed coa7].

In addition to DSC runs on the precursor powder mixthermal stability of the coating was investigated by DSCeriments on a 200�m thick slice of the laser-processed co

ng. The DSC plot for this run is shown inFig. 6. Thermatability was investigated till 1525◦C. When two surfaceub against each other (as in case of wear applicationsemperature at the point of contact rises. Hence, stabilihases was desired. No thermal event was observed dureating and cooling cycles and all the phases were foundtable until the temperature. The computational evaluaonducted earlier indicated that majority of the phases inaser in situ carbide coatings were stable till 1277–132◦C

hermal stability of the evolved phases. The phases evithin the laser-processed coating were found to be thermtable till 1525◦C.

cknowledgements

The authors would like to acknowledge the thermal asis (DSC) sponsored by the Assistant Secretary for Enfficiency and Renewable Energy, Office of FreedomCnd vehicle technologies, as part of the High Temperaaterials Laboratory User Program, Oak Ridge Natioaboratory, managed by UT-Battelle, LLC, for the U


Department of Energy under contract number DE-AC-05-00OR22725. We would like to express gratitude to Prof.Joseph Spruiell for permission to use the XRD facility in hislab. We would also like to express our sincere thanks to Mr.Fred Schwartz for assistance with the laser processing of thesamples. The help of Greg Engleman with infrared furnaceheating of samples is gratefully acknowledged.

References

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1621.[5] V.K. Rai, R. Srivastava, S.K. Nath, S. Ray, Wear 231 (1999) 265.

[6] D.B. Miracle, H.A. Lipsitt, J. Am. Ceram. Soc. 66 (1983) 592.[7] A. Singh, N.B. Dahotre, J. Mater. Sci. 39 (2004) 4553.[8] A. Singh, N.B. Dahotre, Metall. Trans. 36A (2005) 797.[9] L. Lu, M.O. Lai, C.W. Ng, Mater. Sci. Eng. A 252 (1998) 203.

[10] T. Fan, Z. Shi, D. Zhang, R. Wu, Mater. Sci. Eng. A A257 (1998)281.

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[12] N.B. Dahotre, M.H. McCay, T.D. McCay, C.R. Hubbard, W.D. Hub-bard, W.D. Porter, O.B. Cavin, Scripta Metall. Mater. 28 (1993)1359.

[13] E. Scheil, Z. Metallkd. 34 (1942) 70–73.[14] M. Hillert, B. Sundman, Calphad 14 (1990) 111.[15] G. Hohne, W. Hemminger, H.-J. Flammersheim, Differential Scan-

ning Calorimetry, Springer-Verlag, Berlin, 1996, p. 103.[16] C. Ragunath, M.S. Bhat, P.K. Rohatgi, Scripta Metall. Mater. 32 (4)

(1995) 577.[17] L. Ramqvist, Int. J. Powder Metall. 4 (1965) 1.[18] R.D. Fraser, E. Suzuki, Anal. Chem. 41 (1969) 37.

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